Darwinism designates a distinctive form of evolutionary explanationfor the history and diversity of life on earth. Its originalformulation is provided in the first edition ofOn the Origin ofSpecies in 1859. This entry first formulates‘Darwin’s Darwinism’ in terms of six philosophicallydistinctive themes: (i) probability and chance, (ii) the nature, powerand scope of selection, (iii) adaptation and teleology, (iv) theinterpretation of the concept of ‘species’, (v) the tempoand mode of evolutionary change, and (vi) the role of altruism andgroup selection in the explanation of morality. Both Darwin and hiscritics recognized that his approach to evolution was distinctive oneach of these topics, and it remains true that, though Darwinism hasdeveloped in many ways unforeseen by Darwin, its proponents andcritics continue to differentiate it from other approaches inevolutionary biology by focusing on these themes. This point isillustrated in the second half of the entry by looking at currentdebates in the philosophy of evolutionary biology on these sixthemes.

• 1. Introduction
• 2. Darwin and Darwinism
  • 2.1 Darwin’s Life
  • 2.2 Darwin’s Darwinism
  • 2.3 Philosophical Problems with Darwin’s Darwinism
• 3. The Six Core Philosophical Problems Today
  • 3.1 The Roles of Chance in Evolutionary Theory
  • 3.2 The Nature, Power and Scope of Selection
  • 3.3 Selection, Adaptation and Teleology
  • 3.4 Species and the Concept of ‘Species’
  • 3.5 Tempo and Mode of Evolutionary Change
  • 3.6 Evolutionary Ethics, Altruism, and Group Selection
• 4. Conclusion
• Bibliography
  • References
  • Further Reading
• Academic Tools
• Other Internet Resources
• Related Entries

1. Introduction

Scientific theories are historical entities. Often you can identifykey individuals and documents that are the sources of newtheories—Einstein’s 1905 papers, Copernicus’ 1539De Revolutionibus, Darwin’sOn the Origin ofSpecies. Sometimes, but not always, the theory tends in popularparlance to be named after the author of these seminal documents, asis the case with Darwinism.

But like every historical entity, theories undergo change throughtime. Indeed a scientific theory might undergo such significantchanges that theonly point of continuing to name it afterits source is to identify its lineage and ancestry. This is decidedlynot the case with Darwinism. As Jean Gayon has put it:

The Darwin-Darwinism relation is in certain respects a causalrelation, in the sense that Darwin influenced the debates thatfollowed him. But there is also something more: a kind of isomorphismbetween Darwin’s Darwinism and historical Darwinism. It is asthough Darwin’s own contribution has constrained the conceptualand empirical development of evolutionary biology ever after. (Gayon2003, 241)

Darwinism identifies a core set of concepts, principles andmethodological maxims that were first articulated and defended byCharles Darwin and which continue to be identified with a certainapproach to evolutionary questions.^[1] We will thus need to begin with Darwin’s Darwinism asarticulated inOn the Origin of Species in 1859. We will thenexamine these same themes as they have been discussed by evolutionarybiologists and philosophers of biology from the beginnings of theNeo-Darwinian Synthesis to the present.

Charles Darwin was not, as we use the term today, a philosopher,though he was often so described during his lifetime.^[2] Nevertheless, for an encyclopedia of philosophy what is needed is adiscussion of the impact of philosophy on Darwin’s Darwinism,and the impact of Darwin’s Darwinism on topics that both he, andwe, would consider philosophical. We focus here on the impact ofphilosophical discussions about the nature of science duringDarwin’s lifetime on Darwin’s scientific research,thinking and writing; and on the impact of that research, thinking andwriting on philosophy. Taking the time to do such philosophicalarchaeology stems from a conviction that if the concept of Darwinismhas legitimate application today, it is due to a set of principles,both scientific and philosophical, that were articulated by Darwin andthat are still widely shared by those who call themselves‘Darwinians’ or ‘neo-Darwinians’.

2. Darwin and Darwinism

2.1 Darwin’s Life

Charles Darwin was born February 12, 1809 and died April 18, 1882. Itwas a time of radical changes in British culture, and his familybackground put him in the midst of those changes. His grandfather,Erasmus Darwin, was a prosperous and highly respected physician livingin Western England, south of Birmingham. He was also a philosophicalradical, advocating Enlightenment ideas about human equality andliberty, including the liberty to think freely about the existence ofGod and about natural origins for the earth’s creatures. Hewrote a number of very popular works of natural history, some inverse, in which he defended views about progress that includedevolutionary speculations about the upward progress of living thingsfrom primordial beginnings.

Erasmus Darwin was an early member of an informal group of freethinkers self-styled the Lunar Society,^[3] that met regularly in Birmingham to discuss everything from thelatest philosophical and scientific ideas to the latest advances intechnology and industry. The Society included James Watt, JosephPriestley and Charles Darwin’s other grandfather, JosiahWedgwood. Wedgwood, like Erasmus Darwin, lived in Staffordshire andwas in the process of developing a family pottery works into a majorindustrial concern by applying new scientific and technological ideasto the production of ‘china’. The religious inclinationsof the group were ‘non-conforming’ and included a numberof Unitarians, a sect Erasmus Darwin referred to as ‘afeatherbed to catch a falling Christian’. Looked upon withsuspicion by High Church conservatives, they actively promoted inGreat Britain the revolutionary philosophical, scientific andpolitical ideas sweeping across Europe and the Americas. Most hadspent considerable time absorbing Enlightenment ideas in Edinburgh,Scotland.

Under the circumstances, it is not surprising that Robert Darwin,Charles’ father, should follow in his father’s footstepsand become a doctor, nor that he should end up marrying SusannahWedgwood, by all reports Josiah’s favorite offspring.Politically and philosophically engaged, Susannah worked to organizeher children’s education in the town of Shrewsbury, where sheand Robert took up residence. She sent her children to a day schooloperated by Unitarian minister Rev. George Case and this is whereCharles began his education. Unfortunately, Susannah died in 1817 whenCharles was only 8, and his father then transferred him to theShrewsbury School, operated by Dr. Samuel Butler, grandfather of thenovelist (and sometime satirist of Darwin’s work) of the samename. “Nothing could have been worse for the development of mymind than Dr. Butler’s school” Charles proclaimed in theautobiography he wrote for his family, and he escaped down the streetto his home whenever he could.

His older siblings took good care of him, under the Doctor’swatchful eye. Early letters indicate that he and his brother Erasmuswere enthusiastic amateur chemists, and after his brother went up toCambridge their letters were often full of possible experiments,orders to purchase chemicals and equipment for their‘laboratory’, and discussions of the latest discoveries.This was an obvious enough passion that his classmates nicknamed him‘Gas’. During summers he helped his father on his roundsto his patients, and when only 16 his father sent him and his brotherto Edinburgh for the best medical education Great Britain had tooffer. Erasmus needed to move from Cambridge to a proper medicalschool to complete his medical education, and young Charles was takenout of Shrewsbury School early to accompany his brother to Edinburgh,apparently being prepared to follow in his father’s andgrandfather’s footsteps in medicine. The two brothers arrived inEdinburgh in October of 1825. Erasmus left after the first year,leaving his brother on his own during his second year atEdinburgh.

Privately, Darwin early on decided he could not practice medicine. Buthis already serious inclination toward science was considerablystrengthened at Edinburgh both by some fine scientific lectures inchemistry, geology and anatomy and by the mentoring of Dr. RobertGrant. Grant certainly knew that young Charles was ErasmusDarwin’s grandson; Grant expounded evolutionary ideas derivedfrom Jean-Baptiste Lamarck and Charles’ grandfather. But hisprimary gift to Charles was introducing him to marine invertebrateanatomy and the use of the microscope as a scientific tool and as anaid to dissecting extremely small creatures dredged out of the Firthof Forth. Darwin joined an Edinburgh scientific society, the Plineansociety, of which Grant was a prominent member, and presented twolectures that reported discoveries he had made while working withGrant. This interest in marine invertebrates was to be a life longobsession, climaxing in his massive four-volume contribution to thecomparative anatomy and systematics of fossil and living Cirripedia or‘barnacles’ (Barrett & Freeman 1988, vols.11–13).

When he finally broke the news of his distaste for medicine to hisfather, he enrolled to take a degree in Divinity at Christ College,Cambridge University, from which he graduated in January of 1831. Aswith the Shrewsbury School and Edinburgh, his official course of studyhad very little impact on him, but while in Cambridge he befriendedtwo young men attempting to institute serious reforms in the naturalscience curriculum at Cambridge, Rev. John Henslow, trained in botanyand mineralogy, and Rev. Adam Sedgwick, a leading member of therapidly expanding community of geologists. Henslow and his wifetreated Darwin almost as a son, and through Henslow Darwin wasintroduced to the men whose ideas were currently being debated ingeology and natural history, as well as to men whom we look back on asamong the very first to take up the historical and philosophicalfoundations of science as a distinct discipline, Sir John Herschel andRev. William Whewell. As he wrote in his autobiography:

During my last year at Cambridge, I read with care and profoundinterest Humboldt’sPersonal Narrative. This work, andSir J. Herschel’sIntroduction to the Study of Natural Philosophy,^[4] stirred up in me a burning zeal to add even the most humblecontribution to the noble structure of Natural Science. No one or adozen other books influenced me nearly so much as these two.

In the next section we will discuss the influence of the philosophicalideals of Herschel and Lyell on Darwin.

Furthering his scientific training, Adam Sedgwick on two occasionstook Darwin on extended geological tours of England and Wales. Inaddition Darwin and a cousin, William Darwin Fox, a year ahead of himat Cambridge, developed what began as an amateur passion for bugcollecting into serious entomology.

2.2 Darwin’s Darwinism

His Edinburgh and Cambridge mentors were to shape Darwin’sphilosophical attitudes and scientific career decisively. It wasHenslow who was the final link to Darwin in a chain connected toCaptain Robert Fitzroy of H. M. S. Beagle. Fitzroy sought a gentlemancompanion who could also collect information on geology and naturalhistory during a proposed circumnavigation of the globe.Henslow’s note to Darwin, asking if he would be interested inbeing recommended for this post, arrived at the Darwin home,‘the Mount’, while Charles Darwin was on a geologicalsurvey of Northern Wales with Adam Sedgwick. After resistance from hisfather had been overcome, Darwin was offered the post and acceptedit.

The combination of meticulous field observation, collection andexperimentation, note taking, reading and thinking during what turnedinto the Beagle’s five year journey through a very widecross-section of the earth’s environments was to set the coursefor the rest of his life. During the voyage he read and reread CharlesLyell’s newly publishedPrinciples of Geology, athree-volume work that articulated a philosophical vision ofrigorously empirical historical science, oriented around five keyideas:

1. The geologist investigates both the animate and inanimate changesthat have taken place during the earth’s history.
2. His principal tasks are to develop an accurate and comprehensiverecord of those changes, to encapsulate that knowledge in generallaws, and to search for their causes.
3. This search must be limited to causes that can be studiedempirically—those ‘now in operation’, as Lyell putsit in the sub-title of hisPrinciples, on the assumption thatthey have always operated, into the deep past, at the same intensityat which they now operate.
4. The records or ‘monuments’ of the earth’s pastindicate a constant process of the ‘introduction’ and‘extinction’ of species, and it is the geologist’stask to search for the causes of these introductions and extinctions,according to the strictures note in 3., above.
5. The only serious attempt to do so according to the idea thatspecies are capable of ‘indefinite modification’, that ofJean Baptiste Lamarck, is a failure on methodological grounds. All theevidence supports the view that species variability is limited, andthat one species cannot be transformed into another.

This vision influenced Darwin profoundly, as he freely admitted. Whilehe became convinced by his observations and reading that the fossilrecord and current distribution of species could only be due to thegradual transformation of one species into another, he was determinedto articulate a theory that measured up to Lyell’s principles.The crucial event in convincing him that this was to be hislife’s work was likely a visit to Cape Town, South Africa duringthe Beagle’s return trip to England. John F. W. Herschel was inCape Town on a mission to do for the Southern Hemisphere what hisfather William had done for the Northern, namely to develop acomprehensive star map with the new powerful telescopes developed byhis father and aunt. As noted earlier, Darwin had been deeplyimpressed by Herschel’sPreliminary Discourse on the Studyof Natural Philosophy when it first appeared a year before theBeagle set sail, and in his private journal he referred to hismeetings with Herschel during a week long stop in Cape Town in June of1836 as among the most profound events of the entire voyage. Just fivemonths before meeting Darwin, Herschel had finished reading the2^nd edition of Lyell’sPrinciples. He sentLyell a long letter filled with detailed constructive commentary. Theletter opens by praising Lyell for facing the issue of the‘introduction of new species’—which Herschel calls‘that mystery of mysteries’—scientifically, and foradvocating that we search for ‘intermediate causes’ toexplain these ‘introductions’—code for natural, asopposed to ‘miraculous’, causes.^[5] This part of the letter was quoted in Charles Babbage’sBridgewater Treatise, published in 1837 while Darwin wasstruggling to develop just such a theory. Upon reading the Herschelquotation in Babbage, Darwin wrote in his private‘species’ notebooks:

Babbage 2d Edit, p. 226.—Herschel calls the appearance of newspecies. the mystery of mysteries. & has grand passage uponproblem.! Hurrah.—“intermediate causes”. (Barrett etal., 1987, 413; original punctuation)

He clearly recognizes that Herschel is here providing a philosophicaljustification for the project upon which Darwin was secretly working.And, in the very first paragraph ofOn the Origin of Species,Darwin looks back to this ‘Hurrah’, attributing the ideathat the origin of species is ‘that mystery of mysteries’to ‘one of our greatest philosophers’, without mentioningHerschel by name. The first mention of the possibility of anevolutionary solution to this problem is in hisOrnithologicalNotebooks, in a note written shortly after departing Cape Town.^[6]

Darwin’s theoretical task was, by the time he opened his speciesnotebooks, tolerably clear: the only process that could produce thesystematic patterns in the fossil record and the otherwise strangebiogeographic distribution of species he now understood so widely anddeeply was a process of slow, gradual transformation of species. Heneeded to come up with a natural, causal theory that would account forsuch transformations, and every element of that theory had to identify‘causes now in operation’, causes that could beinvestigated empirically. The problem, and the methodologicalconstraints, had been advocated by his geological hero, and now closefriend, Charles Lyell; and they had been defended philosophically byhis philosophical hero, Sir John Herschel.

Darwin, of course, expected, and got, outraged reactions fromreligiously conservative colleagues, such as his old geology teacherSedgwick, who in a review expressed his “deep aversion to thetheory; because of its unflinching materialism;–because it hasdeserted the inductive track,–the only track that leads tophysical truth;–because it utterly repudiates final causes, andtherby [sic] indicates a demoralized understanding on the part of itsadvocates.” What he had not expected was Lyell’s refusalto openly endorse his theory and Herschel’s decisive (if polite)rejection of its key elements. After we set out the theory in itsDarwinian form, we can consider these reactions from those whoapparently shared Darwin’s philosophical norms about scientifictheory, explanation and confirmation.

The theory can be set out as a series of causal elements that, workingtogether, will produce the needed transformations.

1. Species are comprised of individuals that vary ever so slightlyfrom each other with respect to their many traits.
2. Species have a tendency to increase in numbers over generations ata geometric rate.
3. This tendency is checked, to use the language of ThomasMalthus’On the Principle of Population, by limitedresources, disease, predation, and so on, creating a struggle forexistence among the members of a species.
4. Some individuals will have variations that give them a slightadvantage in this struggle, variations that allow more efficient orbetter access to resources, greater resistance to disease, greatersuccess at avoiding predation, and so on.
5. These individuals will tend to survive better and leave moreoffspring.
6. Offspring tend to inherit the variations of their parents.
7. Therefore favorable variations will tend to be passed on morefrequently than others and thus be preserved, a tendency Darwinlabeled ‘Natural Selection’.
8. Over time, especially in a slowly changing environment, thisprocess will cause the character of species to change.
9. Given a long enough period of time, the descendant populations ofan ancestor species will differ enough both from it and each other tobe classified as different species, a process capable of indefiniteiteration. There are, in addition, forces that encouragedivergence among descendant populations, and the eliminationof intermediate varieties.

It will be noticed that there is no element of this theory that isincapable of empirical investigation—indeed by now the publishedconfirmatory studies of this process would fill a small library.^[7] One can understand why devout and orthodox Christians would haveproblems; but why Darwin’s philosophical and scientific mentors?It would seem to be the model of Herschelian/Lyellian orthodoxy.

2.3 Philosophical Problems with Darwin’s Darwinism

The answer lies in six philosophically problematic elements of thetheory.

2.3.1 Probability and Chance

First, notice the use of the language of ‘tendencies’ and‘frequencies’ in the above principles. Privately, Darwinlearned, Herschel had referred to his theory as ‘the Law ofhiggledy-piggledy’, presumably a reference to the large elementplayed in its key principles by chance and probability. Darwin’stheory is, as we would say today, a ‘statistical’ theory.One cannot say that every individual with favorable variationv will survive or will leave more offspring than individualswithout it; one cannot say that no environment will ever support allof the offspring produced in a given generation, and thus that theremustalways be a competitive struggle. These arethings thattend to happen due to clearly articulated causes,and this allows us to make accurate predictions abouttrends,at the level of populations, but not to make absolute claims aboutwhat must happen in each and every case. Only well afterHerschel’s time did philosophers of science become comfortablewith the idea of a theory of this sort, and the proper philosophicalunderstanding of such explanations is still debated.

2.3.2 The Nature, Power and Scope of Selection

The core of Darwin’s theory is the concept of natural selection.Perhaps because of his use of the term selection, this core element ofhis theory apparently baffled nearly everyone. Could it be, as Lyell,Herschel and Darwin’s great American defender Asa Gray wouldask, an ‘intermediate cause’, i.e. a causal principleinstituted and sustained by God? Or is it, in its very nature, theantithesis of such a principle, as his old geology teacher Sedgwickbelieved? Could it possibly create species, or is it, by its nature, anegative force, eliminating what has already been created by othermeans? In one of his copies ofOn the Origin of Species,Alfred Russell Wallace crosses out ‘natural selection’ andwrites ‘survival of the fittest’ next to it. Wallacealways felt that ‘selection’ inappropriately importedanthropomorphic notions of Nature choosing purposefully betweenvariants into natural history. And, in a devastating review, FleemingJenkin happily accepted the principle of natural selection butchallenged its power to modify an ancestral species into descendentspecies, and thus limited its scope to the production of varieties. Anumber of reviewers, even some sympathetic ones, questioned thepossibility of extending the theory to account for the evolution ofthose characteristics that differentiate humans from their nearestrelatives.

2.3.3 Selection, Adaptation and Teleology

Moreover, because Darwin was very fond of describing natural selectionas a process that worked for the good of each species, Darwin’sfollowers seemed to have diametrically opposed views as to whether histheory eliminated final causes from natural science or breathed newlife into them. In either case, there was also serious disagreement onwhether this was a good thing or a bad thing.^[8]

2.3.4 Species and the Concept of ‘Species’

There is a fundamental philosophical problem with the idea that aspecies can undergo a series of changes that will cause it to becomeone or more other species. To illustrate it, look carefully at thefirst question that Charles Lyell wishes to address in the secondvolume of thePrinciples of Geology:

…first, whether species have a real and permanent existence innature; or whether they are capable, as some naturalists pretend, ofbeing indefinitely modified in the course of a long series ofgenerations. (Lyell 1831, II. 1)

Lyell pretty clearly assumes that to allow for evolution is to denythe reality of species. For a species to be ‘real’, itmust have ‘permanent existence in nature’, or as he putsit elsewhere , “…fixed limits beyond which thedescendants from common parents can never deviate from a certaintype…”. (Lyell 1831, II. 23) To accept evolutionarychange, on this view, you must become comfortable with a variety ofnominalism about species. And Darwin seems to have become so.^[9]

Hence I look at individual differences, though of small interest tothe systematist, as of high importance for us, as being the first steptowards such slight varieties as are barely thought worth recording inworks on natural history. And I look at varieties which are in anydegree more distinct and permanent, as steps leading to more stronglymarked and more permanent varieties; and at these latter, as leadingto sub-species, and to species. (Darwin 1859, 52)

Permanence, as applied to species, is for Darwin a relative concept,and there are no fixed limits to variability within a species. Givenenough time the individual differences found in all populations cangive rise to more permanent and stable varieties, these tosub-species, and these to populations that systematists will want toclass as distinct species. Moreover, he concludes theOriginwith very strong words on this topic, words bound to alarm hisphilosophical readers:

Systematists will be able to pursue their labours as at present; butthey will not be incessantly haunted by the shadowy doubt whether thisor that form be in essence a species. …In short, we will haveto treat species in the same manner as those naturalists treat genera,who admit that genera are merely artificial combinations made forconvenience. This may not be a cheering prospect; but we shall atleast be freed from the vain search for the undiscovered andundiscoverable essence of the term species. (Darwin 1859, 485)

Lyell, Herschel, Whewell, Sedgwick and many of Darwin’scontemporaries certainly would not find this a cheering prospect,since they were unrepentant realists about species.^[10] Members of a species possess a ‘type’ established in theoriginal parents, and this type provides ‘fixed limits’ tovariability. Lyell clearly feels this is an empirically verifiablefact—most of chapters 2–4 ofPrinciples Vol. IIis devoted to presenting the evidence that such ‘fixedlimits’ exist; and after theOrigin’s publicationthis evidence was canvassed again in Fleeming Jenkin’s review.If this is so, then species extinction is easy to accountfor—there are fixed limits to a species’ ability to trackenvironmental change. But a naturalistic account of speciesorigination is more difficult, since there will need to be, insexually reproducing species, a natural production of a new pair ofparents with a new type. On the other hand, to adopt the sort ofnominalism that Darwin seems to be advocating in the above quotationshas undesirable consequences as well.^[11] How are we to formulate objective principles of classification? Whatsort of a science of animals and plants will be possible if there areno fixed laws relating their natures to their characteristics andbehaviors? A good deal of chapter 2 of Darwin’sOriginis devoted to convincing the reader that current best practice amongbotanists and zoologists accepts a natural world organized as he isinsisting rather than as his opponents claim:

It must be admitted that many forms, considered by highly competentjudges as varieties, have so perfectly the character of species thatthey are ranked by other highly competent judges as good and truespecies. (Darwin 1859, 49)

From a Darwinian perspective, this is a predictable consequence of thefact that the organisms we today wish to classify as species aremerely the most recent stage of a slow, gradual evolutionary process.Organisms within a genus have common ancestors, perhaps relativelyrecent common ancestors; some naturalists may see ten species with afew varieties in each; others may rank some of the varieties asspecies and divide the same genus into twenty species. Bothclassifications may be done with the utmost objectivity and care byskilled observers. As systematists like to say, some of us are‘lumpers’, some of us are ‘splitters’. Realityis neither.

2.3.5 Tempo and Mode of Evolutionary Change

The question of nominalism versus realism regarding species pointstoward a final aspect of Darwin’s theory with which many ofthose otherwise sympathetic to him disagreed, his gradualism. Forapart from the question of whether his views entailed‘nominalism’ about natural kinds, they do seem to reflecta belief that the evolutionary process must be a slow and gradual one.It is perhaps here that we see the most lasting impact ofDarwin’s careful study of Charles Lyell’sPrinciplesof Geology while on H.M.S. Beagle. We stress slowandgradual, for it is clear that one could have aslow butnon-gradual evolutionary process (perhaps the long periods ofevolutionary stasis punctuated by geologically rapid periods ofspeciation postulated by Eldridge and Gould’s ‘punctuatedequilibrium model’ is such); and one could have arapid butgradual one (for example the process George Gaylord Simpsonlabeled ‘adaptive radiation’, where a population migratesto a location with a variety of unexploited niches, and rapidlyevolves to exploit them). Darwin stresses over and over again that heconceives of natural selection ‘adding up infinitely smallvariations’, and that he imagines the process of speciation totake place over a very long period of time.

2.3.6 Evolutionary Ethics, Altruism, and Group Selection

Despite Darwin’s effort to eschew discussion of human beings intheOrigin (famously writing only that “light will bethrown on the origin of man and his history”; Darwin 1859, 488),he clearly believed that an evolutionary account of the human“moral sense”—as Darwin described it, borrowing fromJames Mackintosh—could be offered. This account, as asub-species of what we now would call (though Darwin did not use theseterms) altruistic behavior in general (see the entry forbiological altruism), quickly brought Darwin into contact with a host of difficultproblems.

In theDescent of Man, he flirted with an explanation of themoral sense in terms of the characteristics not of moral individuals,who would seem to fare less well in the struggle for existence thantheir egoistic compatriots, but in terms of the characteristics ofgroups exhibiting moral virtues. In a case of struggle between twotribes of “primeval man,” he writes, the one with “agreater number of courageous, sympathetic, and faithful members, whowere always ready to warn each other of danger, to aid and defend eachother, this tribe would without doubt succeed best and conquer theother” (Darwin 1871, 1:162). Whether this involves a genuineappeal to what contemporary scholars would call “groupselection” (see the entry onunits and levels of selection), or whether this can be described solely in terms of individualsdesiring to help themselves and their relatives (i.e., in terms of kinselection) remains the subject of much discussion.

One of the strongest arguments for insisting that‘Darwinism’ as it is used today is isomorphic toDarwin’s Darwinism, as Gayon puts it, is that each of thesequestions is still hotly debated, and has been throughout thetheory’s history. With all of the amazing changes that have beenwrought by the genetic, biochemical, and molecular revolutions, withthe development of mathematical models of population genetics andecology, of sophisticated techniques for both field and laboratoryinvestigation of evolutionary processes, and of cladistic analysis insystematics, it nevertheless remains true that one can findevolutionary biologists who adhere to Darwin’s Darwinism, andare recognized as doing so by both themselves and their critics. Inthe next section of this article, we will develop a portrait ofcontemporary Darwinism around each of these contested features.

By the same token, however, Darwinism has evolved. As one example ofthis truth, think for a moment of contemporary debates about thenature of selection. The problems people had with natural selection inthe 19^th century continue to be problematic, but there area variety of problems that were either not discussed, or discussedvery differently, in the 19^th century. Can, and does,natural selection work at levels other than the level ofDarwin’s focus, individual organisms; is there a non-vacuous wayto formulate the theory abstractly; how are we to understand therelationships between the concepts of fitness, selection andadaptation? How strong are the constraints on the selection process,and what sorts of constraints are there? Are there other motors ofevolutionary change besides selection, and if so, how important arethey?

3. The Six Core Philosophical Problems Today

Theories need both essences and histories.
Stephen Jay Gould (2002, 1)

So reads the heading of the very first section of the first chapter ofGould’s monumentalThe Structure of EvolutionaryTheory. Opening with a subtle reading of an exchange of lettersin 1863 between paleontologist Hugh Falconer and Charles Darwin, Gouldeventually explains what he has in mind by this section heading:

In short, “The structure of evolutionary theory” combinesenough stability for coherence with enough change to keep any keenmind in a perpetual mode of search and challenge. (Gould 2002, 6)

Gould, of course, was both an unabashed admirer of Charles Darwin andone of the most outspoken critics of the ‘neo-Darwiniansynthesis’. We will be using both his account of ‘theEssence of Darwinism’ in Part I of this magnum opus and hisarguments for a ‘Revised and Expanded Evolutionary Theory’in its Part II as touchstones and targets.

In the preceding section of this essay, we organized our discussion ofthe problems that Darwin’s allies had withDarwin’s Darwinism around six issues: [i] the role ofchance as a factor in evolutionary theory and the theory’sapparently probabilistic nature; [ii] the nature of selection; [iii]the question of whether selection/adaptation explanations areteleological; [iv] the ontological status of species and theepistemological status of species concepts; [v] the implications ofDarwin’s insistence on the slow and gradual nature ofevolutionary change; and [vi] the impact of natural selection onethics and altruistic behavior. We claimed that one very good reasonfor continuing to characterize one dominant approach to evolutionarybiology, that represented by the so-called ‘Neo-DarwinianSynthesis’, as ‘Darwinism’ is that its proponentsside with Darwin on these issues, to the extent that Darwin had aclear position on them (and on many less fundamental ones besides).That in itself is remarkable, but it is the more so because theDarwinian position on each of these issues is under as much pressurefrom non-Darwinian evolutionary biologists today as it was in the wakeof theOrigin. It is not surprising, given the situation aswe have just characterized it, that historians and philosophers ofbiology have made significant contributions to the discussion,especially in pointing out the underlying philosophical issues andconceptual confusions and ambiguities that stand in the way ofresolving the issues at hand, and their historical origins.

It is our conviction that a full understanding of the underlyingphilosophical disagreements on these questions will only come from apatient historical study of how the ‘Synthesis’ positionson these various issues, and those of their critics, arose. That wecannot do here. Rather, in what follows we will simply be presupposingcertain answers to these questions of historical origins. The list ofreferences at the end of this essay includes a number of excellentpieces of work on this subject for those who share our convictionsabout its importance.

3.1 The Roles of Chance in Evolutionary Theory

Let us begin with the language Darwin uses when he first sketches histheory at the beginning of the fourth chapter of theOrigin:

Can it, then, be thoughtimprobable, seeing that variationsuseful to man have undoubtedly occurred, that other variations usefulin some way to each being in the great and complex battle of life,shouldsometimes occur in the course of thousands ofgenerations? If such do occur, can we doubt (remembering that manymore individuals are born than can possibly survive) that individualshaving any advantage, however slight, over others, would havethebest chance of surviving and of procreating their kind? (Darwin1859, 80–81, emphasis added)

Unlike Darwin’s contemporaries, and despite Darwin’s ownapparent hesitation, the founders of the synthesis of Mendeliangenetics and Darwinian selection theory, Sewall Wright, Ronald Fisherand J. B. S. Haldane, were entirely comfortable with a selectiontheory formulated in such terms. This was a substantial shift in thepresentation of evolution, from a reluctantly probabilistic picture toa thoroughly mathematized, statistical, and probabilistic theory,which occurred in the first several decades after the publication oftheOrigin (Gayon 1998; Pence 2022).

On this issue, contemporary Darwinism has adopted an approach everybit as ‘chancy’ as that of Darwin. Note one clearstatement of thePrinciple of Natural Selection from thephilosophical literature:

Ifa is better adapted thanb to their mutualenvironmentE, then (probably)a will have greaterreproductive success thanb inE. (Brandon 1990,11).

The theory trades pervasively in probabilities. Given the fact thatevolutionary biologists, especially in so far as they take their cuesfrom population genetics, deal with large populations conceived as‘gene pools’, and think of evolution as long run changesin the frequencies of different combinations of genes from generationto generation, it is clear that, in the sense of making use ofprobabilistic or statistical reasoning, chance permeates contemporaryDarwinism. The models of population biology provide a means ofassigning probabilities to various outcomes, given information aboutpopulation size, rates of mutation and migration (themselves given asaverages and estimates). That is, as Darwin notes, being relativelybetter adapted increases an organism’s ‘chances’,i.e. increases its probability, of leaving viable offspring. It doesnot guarantee it. Since natural selection is a stochastic process,Darwinians from Darwin to the present rightly characterize it in termsof influencing the ‘chances’ of a given outcome, givenvariables such as selection pressure, population size or mutationrates.

Conceptual confusion arises, however, from the fact that‘chance’ and ‘randomness’ are oftencontrasted, not with ‘deterministic’ or’non-probabilistic’ outcomes, but with‘selected’ outcomes. The evolutionary process, as Darwinunderstood it, involves both thegeneration of variation anda process producing a differentialperpetuation of variation.One way to think about chance in Darwinism is in relation to a logicalspace of alternatives, by means of the followingvariationgrid:

On this second sense of chance, what seems to make a theory‘chancy’ is the fact thatgeneration of variationandperpetuation of variation have both been, for some ofthese theorists, independent of future utility or fitness. As we seefrom the grid, a contrast on both scores is found in the evolutionaryphilosophy of Jean-Baptiste Lamarck. Lamarck’s is amaterialistic argument against the variation in nature beinga matter of chance. On the Lamarckian view, variations arise in anorganism as a direct response to environmental stress or demand,giving rise to a stimulus, which in turn elicits a physiologicalresponse, which finally can be passed on via reproduction tooffspring. Variations are not chance or random, since they are anappropriate response to an environmental stress. Here‘chance’ signals alack of relation or connection toadaptive needs.

The concept of ‘random variation’ is today often used as asynonym for ‘chance variation’ in precisely this lattersense. Here are two examples of this notion of chance or randomness asused by contemporary Darwinians.

…mutation is a random process with respect to the adaptiveneeds of the species. Therefore, mutation alone, uncontrolled bynatural selection, would result in the breakdown and eventualextinction of life, not in adaptive or progressive evolution.(Dobzhansky 1970, 65)

Thus the production of variations may be a ‘chance’process in that there are a number of possible outcomes withassignable probabilities, but it isalso a‘chance’ process in the sense that the probabilityassignments are not biased by ‘adaptive needs’ or‘fitness’.

Referring now to perpetuation rather than generation of variation,when John Beatty describes ‘random drift’ as‘changes in frequencies of variations due to chance’ inthe following passage, he presumably has something like a contrastwith changes in frequencies due to selection in mind.

In Darwin’s scheme of things, recall, chance events and naturalselection were consecutive rather than alternative stages of theevolutionary process. There was no question as to which was moreimportant at a particular stage. But now that we have the concept ofrandom drift taking over where random variation leaves off, we arefaced with just such a question. That is, given chance variations, arefurther changes in the frequencies of those variations more a matterof chance or more a matter of natural selection? (Beatty 1984, 196)

Notice that in the above quote we first get a substitution of‘random’ for ‘chance’ in the phrases‘random variation’ and ‘chance variation’, andthen at least the suggestion that the concept of ‘randomdrift’ can be characterized as ‘changes in frequencies ofvariations due to chance’, where the contrast class consists ofsimilar changes due to natural selection.

With respect to thegeneration of variation, chapter 5 ofOn the Origin of Species opens with the followingapology:

I have hitherto sometimes spoken as if the variations—so commonand multiform in organic beings under domestication, and in a lesserdegree in those in a state of nature—had been due to chance.This, of course, is a wholly incorrect expression, but it serves toacknowledge plainly our ignorance of the cause of each particularvariation. (Darwin 1859, 131)

Here Darwin is noting that, though to speak of ‘chancevariations’ may seem to be citing chance as the cause of thevariations, in fact it is simply acknowledging that they ‘appearto have no assignable cause’. But it is important to keephistorical context in mind here. Whether Darwin himself ever flirtedwith the idea of ‘directed’ variation or not, he wasacutely aware of two views from which his needed to be distinguished,very different from each other, but both holding to the view thatvariations arose for a purpose.^[12] The most widely shared alternative was that found in naturaltheology. To quote the Reverend William Paley’sNaturalTheology, regarding a beautiful instance of adaptation: “Aconformation so happy was not the gift of chance”. Likewise,among Darwin’s followers, the American botanist Asa Gray, in anessay entitled ‘Natural Selection and Natural Theology’,uses the same contrast to advise Darwin against the notion of‘chance variation’: “…we should advise Mr.Darwin to assume, in the philosophy of his hypothesis, that variationhas been led along certain beneficial lines.”

Gray is here insisting that, since Darwin admits that using the term‘chance’ merely signals ignorance of the true cause, andsince the pervasive adaptations in nature suggest design, Darwinshould avoid the suggestion thatvariations are due to chance inthe sense of ‘absence of design’. This introduces yeta third sense of ‘chance’ that has been instrumental inthe interpretation of evolutionary theory (Shanahan 1991, 264).

Darwin, in fact never refers to ‘chance variations’ in theOrigin, though occasionally he will note that if a beneficialvariation ‘chances [i.e. happens] to appear’, it will befavored by selection (see pp. 37, 82) What Darwin has in mind,however, is clear from his concluding remarks in his chapter onLaws of Variation:

Whatever the cause may be of each slight difference in the offspringfrom their parents—and a cause of each must exist—it isthe steady accumulation, through natural selection, of suchdifferences, when beneficial to the individual, that gives rise to allthe more important modifications of structure… (Darwin 1859,170)

Whatever the cause of thegeneration of a variation may be,the role of selection is toaccumulate those already presentvariations that happen to be beneficial, a process that, whileprobabilistic, is not at all independent of fitness (and hence not‘chancy’ in our second sense). As Beatty put it, thegeneration of variations and their selection are‘consecutive’ processes. But to call the generation ofvariation a ‘chance’ process is to use‘chance’ in either the second or the third sense, meaningeither that such generation is independent of the future utility ofvariations for the organism, or that it isnot by design,not for some end.

There are, therefore, at least three forms of ‘chance’ atplay in contemporary evolutionary theory: an invocation ofprobabilistic or statistical inferences, an invocation of processesthat act independently of current or future fitness, and an invocationof the absence of design. To these we might add, as mentioned above,chance as ignorance of causes, and chance as historical contingency,though we lack the space to discuss either of those notions furtherhere, bringing the total to five (see, e.g., Shanahan 1991,263–267). Eble (1999, 76) drops the notion of probabilistic orstatistical inference and adds the idea, less relevant in evolutionarycontexts, that ‘chance’ can refer to uncaused events (seeanalysis in Millstein 2000, 609–613).

One further example can illustrate how all this interacts in thebroader context of contemporary evolutionary theory (for more suchexamples, see the contributions to Ramsey and Pence 2016). Here, achampion of the neutral theory of molecular evolution characterizeshis position:

…the great majority of evolutionary changes at the molecular(DNA) level do not result from Darwinian natural selection acting onadvantageous mutants but, rather, from random fixation of selectivelyneutral or very nearly neutral mutants through random genetic drift,which is caused by random sampling of gametes in finite populations.(Kimura 1992, 225)

Here, it will be noticed, the focus is not on thegenerationof variations but on theperpetuation of variations. Thecontrast is between a random sampling of gametes that leads to thefixation of selectively neutral alleles and natural selection favoringadvantageous variations. That is, the contrast between‘chance’ and ‘fitness biased’ processes is nowbeing used to distinguish differentmeans of perpetuating certainvariations. We are contrasting two sampling processes. Driftsamples without concern for adaptation; selection samplesdiscriminately on the basis of differences in fitness. Both samplingsare ‘probabilistic’, of course, but that in no wayobviates the above contrast.

However, as Beatty has pointed out, it was quite common until fairlyrecently to characterize natural selection in such a way as to make italmost indistinguishable from random drift (cf. Lennox 1992, Lennoxand Wilson 1994). Numerous accounts of fitness characterized thefitness of a genotype as defined by its relative contribution to thegene pool of future generations—the genotype contributing thelarger percentage being the fitter. But of course that could easily bethe result of a ‘random’—non-fitnessbiased—sampling process; which organisms would be declared‘fitter’ by this method might have nothing to do withnatural selection. In order to provide a proper characterization ofthe role of chance in evolutionary change, then, it is critical toprovide a more robust and sophisticated account of fitness. (Forfurther information, see the entry onfitness.) This, in turn, requires that we discuss the conceptual network thatincludes the notions of adaptation and natural selection, to which wewill turn shortly.

For now, let us assume that there is a way of characterizing fitnesssuch that there is a substantial empirical question of what roleindiscriminate sampling of genotypes (or phenotypes) plays inevolutionary change. This issue was first placed squarely beforeevolutionary biologists by Sewall Wright in the early 1930s. As Wrightpointed out, genes that are neutral with respect to fitness can, dueto the stochastic nature of any process of sampling from a population,increase their representation from one generation to the next. Thelikelihood of this happening goes up as effective population size goesdown. Since Wright imagined that a quite typical scenario inevolutionary change was for species to be broken up into relativelysmall, relatively isolated, populations (or ‘demes’), withsignificantly more breeding within than between demes, the likelihoodthat such ‘neutral genotypes’ could become fixed atrelatively high levels was significant. Though he gradually toned downthis aspect of his work, a significant school of mathematicalpopulation geneticists in the 1960s and 70s took these ideas and ranwith them, developing a ‘Neutralist’ approach toevolutionary change. This is the position characterized by Kimura (oneof its most eloquent defenders) in the passage quoted above. Whetheror not such a process plays a significant role in evolution is not aphilosophical issue, but it is highly relevant to whether evolutionarybiology should be seen as predominantly Darwinian. For if any view iscentral to Darwinism, it is that the evolutionary process ispredominantly guided by the fitness-biasing force of naturalselection, acting on variations that arise by chance. It is to naturalselection and related concepts that we now turn.

3.2 The Nature, Power and Scope of Selection

The greatest number of females will, of course, fall to the share ofthe most vigorous males; and the strongest individuals of both sexes,by driving away the weakest, will enjoy the best food, and the mostfavourable situations, for themselves and for their offspring. Asevere winter, or a scarcity of food, by destroying the weak and theunhealthy, has had all the good effects of the most skilful selection.

The words of Charles Darwin? No; these are the words of John Sebright,penned inThe Art of Improving the Breeds of Domestic Animalsin 1809, the year of Charles Darwin’s birth and fifty yearsbeforeOn the Origin of Species was published. Darwin refersto this passage in Notebook C of his Species Notebooks.^[13] It will be noticed that Sebright isnot discussing domesticselection, but is quite clearly saying that processes leading todifferential survival and reproduction in nature will have ‘allthe good effects of the most skilful selection’. Darwin, then,did not need to read Malthus to see what is here so plainly andclearly stated—namely, that the struggle for survival in naturewill have the same ‘selective’ effects as the actions ofthe domestic breeder of plants and animals.

As this passage, and the argument of theOrigin, shows,‘natural selection’ began life as the product ofanalogical reasoning. Sebright sees clearly that the natural processeshe is describing will have the sameeffects as thebreeder’s selection, but he is not about to describe thoseprocesses as selection processes. Darwin took that step, and Darwinismhas followed.

Darwin himself consistently refers to natural selection as apowerof preserving advantageous, and eliminating harmful, variations.As noted in the last section, whether a particular variation isadvantageous or harmful is, in once sense of that term, a matter ofchance; and whether an advantageous variation is actually preserved byselection is, in another sense of the term, also a matter of chance.For Darwinism, selection is the force or power that biases survivaland reproduction in favor of advantageous variations, or to look aheadto the next section, of adaptations. It is this that distinguishesselection from drift.

Recent years have seen significant challenges to the idea that thisframework is sufficient to explain all evolutionary phenomena, or evento explain an important fraction of evolutionary phenomena ofinterest. On one side we find partisans of the so-called“extended evolutionary synthesis” (EES), who argue thatfeatures like niche construction, developmental bias, phenotypicplasticity, and non-genetic inheritance entail the existence of atheory that at least radically supplements, if not transcendsentirely, the Darwinian perspective (for an introduction, see Lalandet al. 2014; further references include Pigliucci and Müller2010; Uller and Laland 2019). On the other side we could put scholarslike George C. Williams, who has vigorously defended the explanatorysufficiency ofDarwinian selection theory (Williams 1992), ora number of proposals arguing that sufficiently reformulated conceptsfrom “traditional” evolutionary theory can allow it totake on the challenge of the EES without radical changes (e.g., thegene for Lu and Bourrat 2017, or adaptationism for Welch 2017; seealso the entries on thegene,adaptationism, andpopulation genetics).

We can distinguish two broad categories into which we might sort thesenon-Darwinian amendments: [i] proposedlimitations on naturalselection as an evolutionary force; and [ii]expansions ofthe scope of natural selection to include new ‘targets’,‘processes’ or ‘mechanisms’, and‘levels’. It will be noted that in neither case is itobvious that the theoryitself requires modification in theface of such challenges—in principle these might be nothing morethan challenges to the theory’srange of application.However, if it turned out thatmost evolutionary change couldbe explained without recourse to natural selection, this would begrounds for arguing that evolutionary biology was no longer Darwinian.And if it turned out that the theory of natural selection couldonly be integrated with our new understanding of theprocesses of inheritance and development by a wholesale modificationof its foundations, it might be best to see the new theory as amodified descendent of Darwinism, rather than Darwinism itself.Theories may need essences, as Gould claims; but if what isfundamental to the theory has changed, then so has its essence. Toborrow a phrase from Paul Griffiths, perhaps it is not that theoriesneed historiesand essences—perhaps what they need arehistorical essences.

Alfred Russell Wallace regularly urged Darwin to jettison the term‘selection’ as misleadingly anthropomorphic, andsubstitute Herbert Spencer’s ‘survival of thefittest’. Darwin went halfway—in later editions he added‘or Survival of the Fittest’ to ‘NaturalSelection’ in the title of chapter 4. As the theory developed inthe mid-20^th century, the expression ‘survival of thefittest’ was gradually eliminated from any serious presentationof Darwinian selection theory. On the other hand, the concept of‘fitness’ has played a prominent, and problematic, role.In the mathematical models used in population genetics,‘fitness’ refers either to the abilities of the differentgenotypes in a population to leave descendants, or to the measures ofthose abilities, represented by the variableW. Here is arather standard textbook presentation of the relevant concepts:

In the neo-Darwinian approach to natural selection that incorporatesconsideration of genetics, fitness is attributed to particulargenotypes. The genotype that leaves the most descendants is ascribedthe fitness valueW=1, and all other genotypes havefitnesses, relative to this, that are less than 1. … Fitnessmeasures the relative evolutionary advantage of one genotype overanother, but it is often important also to measure the relativepenalties incurred by different genotypes subject to naturalselection. This relative penalty is the corollary of fitness and isreferred to by the termselection coefficient. It is giventhe symbols and is simply calculated by subtracting thefitness from 1, so that:s = 1 −W. (Skelton1993, 164)

The problem lies in the fact that the concept of fitness plays dualroles that are instructively conflated in this quotation. For whenfitnesses are viewed as measures of differentialabilities oforganisms with different genotypes to leave different numbers ofoffspring, the language of fitness encourages us to suppose that‘fitness’ refers to the relative selective advantages ofgenotypes. On the other hand, if ‘fitness’ simply refersto themeasure of reproductive success, it is a quantitativerepresentation of small scale evolutionary change in a population, andleaves entirely open the question of thecauses of thechange. But then the assumed connections among the concepts offitness, adaptation and natural selection are severed.‘Selection coefficients’ may have nothing to do withselection; whatW represents may have nothing to do withselective advantage.

There is, however, a way of formulating the theory in its modern guisewhich maintains an essentially Darwinian character. Since there are anumber of confirmed ways in which natural populations can evolve inthe absence of natural selection, and since balancing selection, i.e.countervailing selection forces, may prevent a population fromevolving in its presence, it is clear that establishing, by measuringdifferent reproductive rates among its members, that the geneticmake-up of a population has changed does not establish that naturalselection was the source of that change; nor does the fact that nochange has been measured establish that natural selection is notoperative. Population genetics and its associated models should betreated as the ‘kinematics’, not the‘dynamics’ of evolutionary processes (on this distinction,see also Pence 2021). That is, it is a way of establishing that apopulation either is or is not in equilibrium, and it providessophisticated tools for measuring rates of change in a populationacross generations. Moreover, like the kinematics of any physicaltheory, if it establishes cross-generational change, it also tells usthat there are causes to be found—the detailed contours of thosemeasures may even provide suggestions as to where to look for thosecauses. What it cannot doon its own is provide knowledge ofthe forces at work. To use language introduced by Elliott Sober,fitness, unlike natural selection, iscausally inert. (Forfurther information, see the entry onpopulation genetics.)

That means that, as valuable as population genetics is, it should notbe equated with the theory of natural selection. Too often in bothbiological presentations of the theory and philosophical discussionsof it, this is forgotten. For example:

Most people are familiar with the basic theory of natural selection.Organisms vary in a heritable fashion. Some variants leave moreoffspring than others; their characteristics, therefore, arerepresented at a greater frequency in the next generation. (Wilson1984, 273)

This is a presentation of ‘the basic theory of naturalselection’ that makes no reference to natural selection atall!

Natural selection, if it is to resemble the Darwinian concept thatbears that name, must be reserved for reference to aninteractionbetween a variable, heritable feature of an organic system and theenvironment of that system. That interaction may or may notchange the proportions of those features across generations, and thoseproportions may change for reasons other than those interactions. Buta plausible natural selection hypothesis must posit some suchinteraction. (Whether this interaction is accurately described ascausal is another much-debated topic in recent years; see Pence 2021for a high-level summary.) On this issue we will give the last word toStephen Jay Gould:

…when we consider natural selection as a causal process, we canonly wonder why so many people confused a need for measuring theresults of natural selection by counting the differential increase ofsome hereditary attribute (bookkeeping) with the mechanism thatproduces relative reproductive success (causality). (Gould 2003, 619)

The concept of natural selection has to this point been presentedbroadly because of the other two critical questions surrounding thecontemporary Darwinian concept of natural selection that we mentionedearlier—questions having to do with possible limitingconstraints on natural selection and about the sorts of objects thatcan be viewed as appropriate organismic/environmental‘interactors’ in the selection process.

If we suppose that for Darwin natural selection was almost exclusivelythought of as an interaction between individual organisms and theirorganic and inorganic environments, then we can see two challenges toDarwinism today with respect tolevels of selection. Thereare those, such as G. C. Williams, Richard Dawkins (1976) and, morerecently, J. Arvid Ågren (2021), who argue that selection isalways and only of genes. Here is a clear statement:

These complications [those introduced by organism/environmentinteractions] are best handled by regarding individual [organismic]selection, not as a level of selection in addition to that of thegene, but as the primary mechanism of selection at the genic level.(Williams 1993, 16)

Dawkins’ preferred mode for making the same point is to refer toorganisms—or interactors—as thevehicles of theirgenes, in fact vehicles constructed by the genome for its ownperpetuation.

The original impulse for this approach, especially clear inWilliams’ classicAdaptation and Natural Selection(1966), was philosophical—it was to use a sort of Ockham’srazor strategy against group selection hypotheses, showing thatalleged group selection effects could be explained by explanationsoperating at the level of the genome (an approach more recently takenby the controversial Nowak et al. 2010). Throughout that book,selection is always said to be of individual alleles, regardless ofthe role environments at various levels may play in the process.

This view has been extensively challenged by philosophers of biologyon both methodological and conceptual grounds, though there are, amongphilosophers, enthusiastic supporters (cf. Dennett 1995). In all thegive and take, it is seldom noticed that defenders of this view claimto be carrying the Darwinian flag (Gayon 1998 and Gould 2003 areexceptions). Yet it is certainlynot a position that Darwinwould recognize—and not merely because he lacked a coherenttheory of the units of inheritance. It is not a Darwinian view becausefor Darwin it was differences in the abilities of organisms at variousstages of development to respond to the challenges of life that hadcausal primacy in the explanation of evolutionary change. Amongevolutionary biologists from the ‘neo-Darwinian synthesis’on, it is those who stress the role of organisms in populationsinteracting differentially to ever-variable ecological conditions incausing changes in the gene pools of those populations who are thecard-carrying Darwinians. Such a “return of the organism”(Nicholson 2014) in evolutionary explanations marks a profound linkbetween proponents of an extended evolutionary synthesis (e.g., Walsh2015) and Darwin himself.

Darwinism also has challenges from the opposite direction. In the1970s a number of biologists working in the fields of paleontology andsystematics challenged the Neo-Darwinian dogma that you could accountfor ‘macro-evolution’ by means of long term extrapolationfrom micro-evolution. Gould, in particular, opens Part II ofTheStructure of Evolutionary Theory (Towards a Revised andExpanded Evolutionary Theory), with a chapter entitled‘Species as Individuals in the Hierarchical Theory ofSelection’. That chapter title combines two conceptuallydistinct theses that connect debates about the fundamentals of naturalselection to patterns in macroevolution: first, the thesis defended byMichael Ghiselin (Ghiselin 1997) and championed and refined by DavidHull (Hull 2001), that species are, in a robust sense of the term,‘individuals’; and second, that there may well beselection among groups of organisms,qua groups (seesection 3.6). These debates over the importance of selective and non-selectiveprocesses and the relationship between these mechanisms of biologicalchange and broader patterns of diversification and adaptation comprisesome of the most important and heated discussions currently underwayin evolutionary theory.

3.3 Selection, Adaptation and Teleology

Early in the Introduction toOn the Origin of Species, Darwinobserves that the conclusion that each species had descended fromothers “even if well founded, would be unsatisfactory, until itcould be shown how the innumerable species inhabiting this world havebeen modified so as to acquire that perfection of structure andco-adaptation which most justly excites our admiration” (Darwin1859, 3). One might say this was the central promise ofDarwinism—to account for both phylogenic continuityandadaptive differentiation by means of the same principles; or as Darwinputs it, to integrate in one theory the supposed opposition betweenUnity of Type and Conditions of Existence.

But it is here that even the most sympathetic of Darwin’stheistic supporters were forced to qualify their support for thetheory of descent with modification by means of natural selection. InDarwin’s day the reactions of Asa Gray and John Herschel areperhaps the most interesting in this respect. Both men saw inDarwin’s theory a way to account for ‘that mystery ofmysteries,’ the regular appearance of new species by means ofnatural, or as they might say, ‘intermediate’ causes.However both instinctively recoiled from the irreducible and centralrole of ‘chance’ in the theory. They did not, but easilycould have, said ‘God does not play dice with theuniverse.’ But as Darwin stated repeatedly, if gently, toGray—if God ordained that variations should be along beneficiallines, natural selection would be redundant. Moreover, the evidencefrom the study of variation in domestic and natural populations putthe lie to any claim that God directs all or most variation alongbeneficial lines. Darwinian selection theory is a two-stepprocess—the production of variation unrelated to the adaptiverequirements of the organism, and differential perpetuation of thosevariations that serve adaptive needs. Again, a theory of evolutionthat could not be so described would not be a Darwinian theory.

The nature of ‘selection explanations’ is a topic to whichmuch philosophical attention has been devoted in recent years. Here wewant to focus on only one important question—to what extent isthe teleological appearance of such explanations simply that, anappearance masking a causal process in which goals play no role?

Theappearance of teleology is certainly present in Darwinianexplanations, and has been since Darwin spoke of natural selectionworking solely for the good of each being. The appearance ofteleology stems from the ease with which both evolutionary biology andcommon sense take it for granted that animals and plants have theadaptations they dobecause of some benefit or advantage tothe organism provided by those adaptations.

This is a hotly contested question, and we will here simply sketch acase that selective explanations of adaptations are robustlyteleological. The interested reader may want to refer to theliterature on this question referred to in the discussion and listedin the list of readings provided at the end of this entry. The seriousphilosophical issue can be put simply and directly: in selectionexplanations of adaptations, are the functions served by adaptations acentral and irreducible feature of the explanans in such explanations?If the answer is yes, the explanations are teleological.^[14]

A good place to begin is with a simple, yet realistic, example. Inresearch carried out over many years and combining painstaking fieldwork and laboratory experimentation, John Endler was able todemonstrate that the color patterns of males in the guppy populationshe was studying in rivers feeding into the southern Caribbean were aconsequence of a balance between mate selection and predatorselection. To take one startling example, he was able to test andconfirm a hypothesis that a group of males, with a color pattern thatmatched that of the pebbles on the bottoms of the streams and pondsthey populated except for bright red spots, have that pattern becausea common predator in those populations, a prawn, is color blind forred. Red spots did not put their possessors at a selectivedisadvantage, and were attractors for mates (Endler 1983,173–190). We may refer to this pattern of coloration as acomplex adaptation that serves the functions of predator avoidance andmate attraction. But what role do those functions play in explainingwhy it is that the males in this population have the coloration theydo?

This color pattern is an adaptation, as that term is used inDarwinism, only if it is a product of natural selection (Williams1966, 261; Brandon 1985; Burian 1983). In order for this to be true,there must be an array of color variation available in thegenetic/developmental resources of the species wider than thisparticular pattern but including this pattern. Which factors arecritical, then, in producing differential survival and reproduction ofguppies with this particular pattern? The answer would seem to be thevalue-consequences this pattern has compared to others available inpromoting viability and reproduction. In popular parlance (and theparlance favored by Darwin), this color pattern isgood forthe male guppies that have it, and for their male offspring, and thatis why they have it (Binswanger 1990; Brandon 1985; Lennox 2002). Thisanswer strengthens the ‘selected effects’ or‘consequence etiology’ accounts of selection explanationsby stressing that selection ranges overvalue differences.The reason for one among a number of color patterns having a higherfitness value has to do with thevalue of that patternrelative to the survival and reproductive success of itspossessors.

Selection explanations are, then, a particular kind of teleologicalexplanation, an explanation in whichthat for the sake ofwhich a trait is possessed,its valuable consequence,accounts for the trait’s differential perpetuation andmaintenance in the population.

3.4 Species and the Concept of ‘Species’

In listing the topics we would discuss under the heading ofneo-Darwinism, we distinguished the question of the ontological statusof species from the epistemological status of the speciesconcept. Though they are closely related questions, it isimportant to keep them distinct. As will become clear as we proceed,this distinction is rarely honored. Moreover, it is equally importantto distinguish thespecies concept from the categories offeatures that belong in a definition of species (Rheins 2011).Advances in our theoretical understanding may lead us to reconsiderthe sorts of attributes that are most important for determiningwhether a group of organisms is a species, and thus whether itdeserves to be assigned a name at that taxonomic level. It should notbe assumed that such changes constitute a change in the speciesconcept, though at least some such changes may lead us to restrict orexpand the range of taxa that are designated as species. In hiscontribution to the Neo-Darwinian Synthesis,Systematics and theOrigin of Species, Ernst Mayr titled chapter five ‘TheSystematic Categories and the New Species Concept’. Recall thatDarwin made a point of treating the species category as continuouswith ‘well-marked variety’ and ‘sub-species’,and made the radical suggestion that its boundaries would be just asfluid. Without explicitly acknowledging Darwin, Mayr takes the sametack, discussing ‘individual variants’ and‘sub-species’ as a preliminary to discussing the speciesconcept. Mayr notes that for someone studying the evolutionaryprocess, speciation is a critical juncture; “…hisinterpretation of the speciation process depends largely on what heconsiders to be the final stage of this process, the species.”(Mayr 1942/1982, 113) With this in mind, he offers the followingdefinition, the so-called ‘biological species concept’(BSC):

Species are groups of actually or potentially interbreedingnatural populations, which are reproductively isolated from other suchgroups (Mayr 1942/1982, 120; 1976, 518)

Mayr was well aware of the limitations of this definition, and treatedit somewhat as a ‘regulative ideal’. Dobzhansky in 1937gave what he claimed to be a definition of species, but which seems,as Mayr noted (Mayr 1976, 481), much more a definition ofspeciation:

…that stage of evolutionary process, “at which the onceactually or potentially interbreeding array of forms becomessegregated in two or more separate arrays which are physiologicallyincapable of interbreeding.” (312)

Simpson (1943) and others built even more historicity into theconcept. These are all, of course, intended asdefinitions ofthe speciescategory, and they attempt to provide a test (ora ‘yardstick’: Mayr 1976, 479) that in principle willpermit a researcher to decide whether a group of individuals shouldall be identified by a single species-level concept such as‘homo sapiens’. The test for species membership is thecapacity to interbreed; the test distinguishing two speciesisincapacity to interbreed. Dobzhansky makes the importanceof this test transparent—the transition from a singleinterbreeding population to two reproductively isolated ones is theprocess of speciation.

Now in each of these definitions, little attention is paid to theactual methods used by taxonomists and systematists in differentiatingbetween varieties of a species and distinct species, something towhich Darwin gave a great deal of attention. Darwin’s apparentnominalism regarding the species concept likely stemmed from his closeattention to his own taxonomic practices and those of otherspecialists.

Mayr, on the contrary, relates different approaches to the speciesconcept to the philosophical distinction between essentialism andnominalism (for the history of this argumentative move, see Witteveen2015; 2016). He associates what he calls essentialism (and what wecalled above “realism” about species) with the view that aspeciesconcept refers to a universal or type. This view ofthe referent of the concept leads to the Typological Species Concept,which he traces from Linnaeus back to Plato and Aristotle, and whichhe claims ‘is now universally abandoned’ (1976, 516). Itis worth noting that serious doubt has been cast both on thehistorical and the philosophical credentials of Mayr’s‘Typological Species Concept’ (see, e.g. Lennox, 1987;repr. in Lennox 2001b; Winsor 2001, 2006; Walsh 2006; Wilkins 2009).At the opposite extreme is nominalism, which combines the view thatonly individuals exist in nature and that species are conceptsinvented for the purpose of grouping these individualscollectively.

Mayr claims that his Biological Species Concept (BSC) is an advance onboth; individual species members are objectively related to oneanother not by a shared relation to a type but by causal andhistorical relationships to one another. He can thus be understood asarguing for a new, objective way of understanding the epistemologicalgrounds for grouping individuals into species. This new way ofgrouping stresses historical, genetic and various ecologicalrelationships among the individuals as the grounds for determiningspecies membership. His claim is that this is more reliable andobjective than similarities of phenotypic characteristics. This makessense of the importance he eventually places on the fact the BSCdefines species relationally:

…species are relationally defined. The word species correspondsvery closely to other relational terms such as, for instance, the wordbrother. … To be a different species is not a matterof degree of difference but of relational distinctness. (Mayr 1976,518)

Mayr has in mind that brothers may or may not look alike; the questionof whether two people are brothers is determined by their historicaland genetic ties to a common ancestry. Notice, however, that this is aclaim about which characteristics, among the many that they have,should be taken most seriously in determining the applicability tothem of the concept ‘brother’. That is, it is a defense ofa sort of essentialism.

A number of critics have pointed out that essentialism need not becommitted to ‘types’ understood asuniversalia inre; and on certain accounts of essences any species taxon thatmeets the standards of BSC does so in virtue of certain essential(though relational and historical) properties. At one extreme, MichaelGhiselin and David Hull have argued that this causal/historicalstructure of species provides grounds, at least within evolutionarybiology, for considering species to be individuals.^[15] Organisms are not members of a class or set, but ‘parts’of a phylogenetic unit. Taking a very different tack, Denis Walsh hasrecently argued that a form of ‘evolutionaryessentialism,’ bearing a striking resemblance to theessentialism of Aristotle’s zoological work, is implicit in thework of a number of evolutionary developmental theorists (Walsh,2006).

A critical issue in this debate over the account of the speciesconcept most appropriate for Darwinism is the extent to which theprocess of biological classification—taxonomy—should beinformed by advances in biological theory. Besides those alreadydiscussed, the moderate pluralism associated with Robert Brandon andBrent Mischler or the more radical pluralism defended by PhilipKitcher, argues that different explanatory aims within the biologicalsciences will require different criteria for determining whether agroup constitutes a species (perhaps, controversially, includingnon-epistemic value commitments; see Garnett and Christidis 2017;Conix 2019). Cladists, on the other hand, employ strictly definedphylogenetic tests to determine species rank (see Rheins 2011).

Unlike many of the other topics that define the history of Darwinism,there is no clear-cut position on this question that can be identifiedas ‘Darwinian’ or ‘neo-Darwinian’. In a recentcollection of papers defending most of the alternatives currentlybeing advanced (Ereshefsky 1992), our suspicion is that virtuallyevery author in that collection would identify himself as Darwinian.This may be because, as different as they are, a number of positionscurrently being defended have their roots in Darwin’s own theoryand practice (see Beatty 1985; reprinted in Ereshefsky 1992).

3.5 Tempo and Mode of Evolutionary Change

Contemporary debates over the tempo and mode of evolutionary changeoften travel with those concerning the role of“non-Darwinian” processes in evolutionary biology, asdiscussed insection 3.2. As was argued above, the classical Darwinist position on questions oftempo and mode is usually taken to be a strict gradualism, withnatural selection slowly pushing populations toward adaptive peaks(see the entry onadaptationism). From Darwin’s day to our own, a number of processes other thannatural selection that significantly impact the speed and direction ofpopulation change have been increasingly emphasized. The oldest amongthem wasgenetic drift, which draws our attention toward selectively neutral (sometimes, aswe saw above, described as “random”) change inpopulations. If one emphasizes processes of this sort, evolution mightstill be gradual, but it would not necessarily, or even not usually,be adaptively directed.

The same is true for the increasing interaction betweenevolution and developmental biology, or evo-devo. If processes like phenotypic plasticity, in which anorganism with a static genotype may exhibit radically differentphenotypes as a developmental response to environmental influences,are extremely prevalent, then the kind of gradual walk toward anadaptive peak which it is clear Darwin had in mind would be regularlypunctuated by fits and starts of various kinds, as new portions ofevolutionary space became available as a result of developmentalnovelty. In turn, this could lead to the non-gradual (potentially evennon-Darwinian) pattern ofpunctuated equilibrium, Stephen JayGould’s term for this oscillation between periods of stasis andrapid change across the history of the tree of life (see the entry onmacroevolution). Examples of this sort could be multiplied (e.g., biased mutation,epigenetics), though we lack the space to do so here.

To be sure, it is not clear that Darwin himself would have consideredany of these to be “anti-Darwinian” approaches. Ascounter-examples to Darwin’s gradualism accumulated in his ownday, especially those driven by (misplaced, we now know) concernsabout the age of the earth like those raised by William Thomson, LordKelvin, Darwin began to increase the importance of“sports” in later revisions of his works, large variationsthat could cause brief periods of rapid phenotypic change. That said,when a reference is made in contemporary work to the“Darwinian” position on tempo and mode, it is clearly hisearly, extreme gradualism that authors have in mind.

3.6 Evolutionary Ethics, Altruism, and Group Selection

In-depth discussions of the contemporary state of the field onevolutionary ethics and biological altruism would take us too farafield for our purposes here, and each is the subject of a separatearticle in this encyclopedia (seemorality and evolutionary biology;biological altruism). In short, ethical behavior seems to pose at least twoprima facie challenges for evolutionary explanations. First,how could genuinely altruistic behavior, which seems to involveorganisms making sacrifices for others, evolve under the strictoptimizing regime of natural selection? And second, what is therelationship between evolutionary explanations of our mental andperceptual capacities and our understanding of moral knowledge? Mustevolutionary theory undermine or “debunk” any claims totrue moral beliefs (see the entry onmoral epistemology)?

It is worth underlining here, however, that debate aroundDarwin’s own position on these issues has turned on whether ornot Darwin was genuinely offering us a “group-selection”explanation for moral traits in human beings (e.g., Ruse and Richards2016). This question, in addition to testing the limits of our abilityto interpret precious little source material found in Darwin’sown writings, is difficult also because of the host of issues thatmight be implicated in the effort to explicate just what we mean by“a group-selection explanation” of a particularphenomenon.

As we mentioned insection 3.2 above, in linking the question of species’ metaphysicalindividuality to the hierarchy of natural selection, Stephen Jay Gouldoffers us a window onto the conceptual complexity to which this debatecan lead. His title exemplifies one approach to groupselection—the unit of selection is always the individual, butthere are individuals other than individual organisms that are subjectto selection. A very different result emerges if one assumes thatgroups of organisms such as demes, kin-groups, or species, though notindividuals, are nevertheless subject to selection. Adding to theconceptual complexity, some researchers propose that the term‘group selection’ be restricted to the process wherebygroup-level traits provide advantages to one group over another, inwhich case there are strict conditions delimiting cases of groupselection. Others define group selection primarily in terms of groupleveleffects. Thus a debate analogous to that earlierdiscussed regarding the definitions of ‘fitness’ emergeshere—by group selection do we mean a distinct type of causalprocess that needs to be conceptually distinguished from selection atthe level of individual organism or gene, or do we merely mean atendency within certain populations for some well-defined groups todisplace others over time? (For further discussion, see Sterelny andGriffiths 1999, 151–179; Hull 2001, 49–90; and see theentry onlevels and units of selection.)

4. Conclusion

We hope that this survey has demonstrated, first and foremost, therich past and present of philosophical reflections both about andinspired by Darwin’s theory of evolution by natural selection.Furthermore, as we have seen, Darwin’s own positions and worksremain touchstones for such reflections, not only because he was thetheory’s first proponent, but also because his positions stilloffer us a useful frame of reference as well as a host ofsophisticated insights. To be sure, the philosophy and practice ofbiology have advanced significantly in the intervening nearly twohundred years since Darwin’s first notebooks on naturalselection. But if history is any guide, considering whether and howthese innovations depart from Darwin’s own views on the subjectwill be a fruitful enterprise well into the theory’s nextcentury.

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Further Reading

Charles Darwin’s Life

• Browne, E. J. 1995,Charles Darwin: A Biography. Vol. 1:Voyaging, Princeton: Princeton University Press.
• –––, 2000,Charles Darwin: A Biography. Vol.2: The Power of Place, Princeton: Princeton UniversityPress.
• Desmond, A. and Moore, J., 1992,Darwin: The Life of aTormented Evolutionist, New York: Norton.
• Herbert, S., 2005,Charles Darwin, Geologist, Ithaca:Cornell University Press.

Charles Darwin: Primary Sources

• Barrett, P. H. (ed.), 1977,The Collected Papers of CharlesDarwin, 2 Vols., Chicago: University of Chicago Press.
• Burkhardt, F. (ed.), 1985–2023,The Correspondence ofCharles Darwin, Volumes 1–30, Cambridge: CambridgeUniversity Press.
• Chancellor, G. and John van Wyhe (eds.), 2009,CharlesDarwin’s Notebooks from the Voyage of the Beagle,Cambridge: Cambridge University Press.
• Keynes, R. (ed.), 2000,Charles Darwin’s Zoology Notes& Specimen Lists from H.M.S. Beagle, Cambridge: CambridgeUniversity Press.
• Peckham, M. (ed.), 1959,The Origin of Species by CharlesDarwin: A Variorum Text, Philadelphia: University of PennsylvaniaPress. [1st Paperback edition, 2006]
• Weinshank, D. et al. (eds.), 1990,A Concordance to CharlesDarwin’s Notebooks, 1836–1844, Ithaca: CornellUniversity Press.

Charles Darwin’s Context

• Hodge, J. and Radick, G. (eds.), 2003,The Cambridge Companionto Darwin, Cambridge: Cambridge University Press.
• Owen, R., 1837/1992,The Hunterian Lectures in ComparativeAnatomy, May and June 1837 (Edited and with an Introductory Essayand Commentary by Phillip Reid Sloan), Chicago: Chicago UniversityPress.
• Rudwick, M., 1997,George Cuvier, Fossil Bones and GeologicalCatastrophes, Chicago: University of Chicago Press.
• Ruse, M., 1999,The Darwinian Revolution: Science Red in Toothand Claw (Revised edition), Cambridge: Cambridge UniversityPress.
• Ruse, M. and Richards, R. J. (eds.), 2009,The CambridgeCompanion to the Origin of Species, Cambridge: CambridgeUniversity Press.
• Snyder, L., 2010,The Philosophical Breakfast Club, NewYork: Broadway Books.

The Evolution of Darwinism

• Amundson, R., 2005,The Changing Role of the Embryo inEvolutionary Thought: Roots of Evo-Devo, Cambridge: CambridgeUniversity Press.
• Depew, D. and Weber, B., 1995,Darwinism Evolving: SystemsDynamics and the Genealogy of Natural Selection, Cambridge MA:MIT Press.
• Kohn, D. (ed.), 1995,The Darwinian Heritage, Princeton:Princeton University Press.
• Mayr, E., 1976,Evolution and the Diversity of Life,Cambridge MA: Harvard University Press.
• Ruse, M. (ed.), 2013,The Cambridge Encyclopedia of Darwin andEvolutionary Thought, Cambridge: Cambridge University Press.

Philosophy and Evolutionary Theory

• Brandon, R. N., 1996,Concepts and Methods in EvolutionaryBiology, Cambridge: Cambridge University Press.
• Burian, R. M., 2005,The Epistemology of Development,Evolution, and Genetics, Cambridge: Cambridge UniversityPress.
• Godfrey-Smith, P., 2009,Darwinian Populations and NaturalSelection, Oxford: Oxford University Press.
• –––, 2014,The Philosophy of Biology,Princeton: Princeton University Press.
• Hull, D. and Ruse, M. (eds.), 1998,The Philosophy ofBiology, Oxford: Oxford University Press.
• Lloyd, E., 1994,The Structure and Confirmation ofEvolutionary Theory, 2^nd edition Princeton: PrincetonUniversity Press.
• Pence, C. H., 2021,The Causal Structure of NaturalSelection, Cambridge: Cambridge University Press.
• Okasha, S., 2006,Evolution and the Levels of Selection,Oxford: Clarendon Press.
• –––, 2019,Philosophy of Biology: A VeryShort Introduction, Oxford: Oxford University Press.
• Sober, E., 1984,The Nature of Selection: Evolutionary Theoryin Philosophical Focus, Cambridge MA: MIT Press.
• ––– (ed.), 1994,Conceptual Issues inEvolutionary Biology, 2^nd edition, Cambridge MA: MITPress.
• –––, 2008,Evidence and Evolution: The LogicBehind the Science, Cambridge: Cambridge University Press.
• –––, 2024,The Philosophy of EvolutionaryTheory: Concepts, Inferences & Probabilities, Cambridge:Cambridge University Press.
• Smith, David Livingstone (ed.), 2017,How Biology ShapesPhilosophy: New Foundations for Naturalism, Cambridge: CambridgeUniversity Press.

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Other Internet Resources

Though there are an abundance of web sites on Darwinism, the threemost useful sites meeting the highest of academic standards are listedbelow. The first is the official site for the publication of materialin the extensive Darwin Archives at Cambridge University, but hasgrown to become the default site for Darwin texts and relatedliterature as well. The second is the official site for on-linepublication of Darwin’s extensive correspondence. The third siteis a very good starting point amd links to sites related to CharlesDarwin’s historical context.

• Complete World of Charles Darwin Online
• Darwin Correspondence Project
• Victorian Science: An Overview, The Victorian Web (funded by the University Scholars Program,National University of Singapore)

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