Evolution

First published Fri Aug 25, 2017; substantive revision Thu Sep 23, 2021

Evolution in its contemporary meaning in biology typically refers tothe changes in the proportions of biological types in a populationover time (see the entries onevolutionary thought before DarwinandDarwin: fromOrigin of Species toDescent of Man for earlier meanings). As evolution is too large of a topic toaddress thoroughly in one entry, the primary goal of this entry is toserve as a broad overview of contemporary issues in evolution withlinks to other entries where more in-depth discussion can be found.The entry begins with a brief survey of definitions of evolution,followed by a discussion of the different modes of evolution andrelated philosophical issues, and ends with a summary of other topics inthe philosophy of evolution focusing particularly on topicscovered in this encyclopedia.

1. Definitions of Evolution

The definition of evolution given at the outset of this entry is verygeneral; there are more specific ones in the literature, some of whichdo not fit this general characterization. Here is a sampling.

Although the work of Charles Darwin (see the entry onDarwinism) is usually the starting point for contemporary understandingsof evolution, interestingly, he does not use the term in the firstedition ofOn the Origin of Species, referring instead to“descent with modification”. In the early-mid 20thcentury, the “modern synthesis” gave birth topopulation genetics, which provided a mathematization of Darwinian evolutionary theory inlight of Mendelian genetics (see also the entry onecological genetics). This yielded a prevalent—probably the mostprevalent—understanding of evolution as “any change in thefrequency of alleles within a population from one generation to thenext”. Note, however, that this definition refers to evolutiononly in a microevolutionary context and thus doesn’t referencethe emergence of new species (and their new characteristics), althoughit is intended to underlie those macroevolutionary changes (see theentry onphilosophy of macroevolution).

In a popular textbook, Douglas Futuyma gives a more expansivedefinition:

[biological evolution] is change in the properties of groups oforganisms over the course of generations…it embraces everythingfrom slight changes in the proportions of different forms of a genewithin a population to the alterations that led from the earliestorganism to dinosaurs, bees, oaks, and humans. (2005: 2)

Note also that Futuyma’s definition, unlike the populationgenetics’ definition, does not limit itself to changes inalleles; John Endler’s definition is similar in thisrespect:

Evolution may be defined as any net directional change or anycumulative change in the characteristics of organisms or populationsover many generations—in other words, descent withmodification… It explicitly includes the origin as well as thespread of alleles, variants, trait values, or character states.(Endler 1986: 5)

Yet even this definition is not expansive enough; molecular evolutionfocuses on the molecular changes within macromolecules such as DNA andRNA.

In a very different vein, Leigh van Valen characterized evolution as“the control of development by ecology” (1973, 488); thisanticipates those who emphasize the importance of development inevolution, including proponents of “evo-devo” (see the entry onevolution and development). Today, some have called for an “extended evolutionarysynthesis” in light of developmental biology and other recentfindings in evolutionary biology.

Although this entry focuses on biological evolution, philosophers andbiologists have also sought to extend evolutionary ideas to thecultural realm. Figuring out how and whether to extend the definitionof evolution to this realm is part of the study ofcultural evolution.

In spite of this diversity of definitions, there has been very littlephilosophical analysis of the term “evolution” itself.This dearth forms a stark contrast to the voluminous literature in thephilosophy of evolution; indeed, for a long time thephilosophy of biology was focused almost entirely on evolution. Thankfully, that is nolonger the case, with philosophers turning their attentions to issuesingenetics,molecular biology,cell biology,ecology,developmental biology,microbiology, andmore. It may be, as Theodosius Dobzhansky famously said, that“Nothing in biology makes sense except in the light ofevolution” (1973: 125), but much of biology is not evolutionarybiology. Still, though, philosophy of evolution remains a growing andvibrant area within the philosophy of biology.

2. Modes of Evolution

It is essential to understand that biologists recognize many ways thatevolution can occur, evolution bynatural selection being just one of them, although it is often held to be the mostprevalent one. Evolution can also occur through genetic drift,mutation, or migration. It can also occur through sexual selection,which some consider to be a form of natural selection and othersconsider to be distinct from natural selection (the latter having beenDarwin’s 1859, 1874 view). Evolutionary theory, then, can betaken to be the study (including, but not limited to, mathematicalmodels) of these and other modes of evolution.

To see why it makes sense to think of multiple modes of evolution,consider again one of the definitions of evolution presented above,where evolution is understood as “any change in the frequency ofalleles within a population from one generation to the next”.With natural selection, the frequency of alleles that confer greaterfitness would tend to increase over those which confer lesser fitness.Sexual selection would be the same, but with fitness understoodstrictly in terms of mating ability. With genetic drift, a form ofevolution that involves chance (see the entry ongenetic drift for explanation), there could be an increase in the frequencyof alleles that confer greater fitness, an increase in the frequencyof alleles that confer lesser fitness, or an increase in the frequencyof alleles whose manifestation (if any) was neutral. If organismsmigrate from one population to another, it is likely that there willbe a change in the frequency of alleles in both populations. And ifthere is a mutation from one allele to another, then the frequency ofalleles in the population will likewise change, albeit by a smallamount. Distinguishing these different modes of evolution allowsbiologists to track the various factors that are relevant toevolutionary changes in a population.

The careful reader may have noted that the previous paragraph invokedprobabilistic language: whattends to happen, whatcould happen, what islikely to happen. Indeed,mathematical evolutionary models today (see the entry onpopulation genetics) are typically statistical models. This fact about evolutionarymodels has given rise to a debate in the philosophy of evolution overwhether natural selection and genetic drift should be be understood ascauses of evolution, as most biologists conceive them, or as merestatistical summaries of lower-level causes: births, deaths, etc. (Thenatural selection andgenetic drift entries give more information about this debate). It is for thisreason that this entry uses the more neutral phrase “modes ofevolution” so as not to beg any questions under dispute betweenthe causalist and the statisticalist.

Although there is widespread agreement that there are multiple modesof evolution, much contemporary work in biology and philosophy ofbiology has been focused on natural selection. Whether this focus is agood thing or not is in part what the debate overadaptationism is about. That is, do we have reason to think that natural selectionis the most prevalent or most important mode of evolution? Shouldscientific methodologies be geared toward testing natural selectionhypotheses or toward a variety of possible evolutionary modes? Thefocus on natural selection has also led to a large literature on theconcept offitness, given that population genetics’ definitions and otherdefinitions of natural selection typically invoke fitness; a naturalselection explanation of whyX was more successful thanY might invokeX’s higher fitness. What fitnessmeans, what entities it applies to (genes, organisms, groups,individuals, types), what sort of probabilities it invokes, if any,and how it should be calculated, are all under philosophical dispute.There is also a large literature on conceptually and empiricallydistinguishing natural selection from genetic drift. Migration,mutation (as a mode of evolution), and sexual selection have receivedless attention from philosophers of biology.

3. Other Topics in the Philosophy of Evolution

Some of the work in the philosophy of evolution deals withcontroversial issues. There is, of course, the debate overcreationism. The vast majority of philosophers agree that creationism hassignificantly less evidence in its favor as compared to the abundant evidence in favor of evolution. They also agree thatcreationism ought not to be taught in a public school science classroom,but they sometimes disagree over the reasons why. For example, isit because it fails some criteria of science? If so, which criteria?Or is it because of the lack of evidence? Or is it because of itsreligious basis? Debates oversociobiology andevolutionary psychology—areas that seek to explain human behavior and psychology as evolvedcharacteristics—have likewise stirred up controversy over theirscientific status. Proponents have also been accused of employing anexcessive and uncritical adaptationism and resting on sexist or otherproblematic biases (on the latter, see the entry onfeminist philosophy of biology).

Another nexus of topics in the philosophy of evolution involvesheredity andheritability. Although it was not explicitly emphasized in the definitions ofevolution given above, evolution is usually taken to be aboutheritable changes over time, i.e., characteristics that areable to be passed from one generation to the next. But there has beensome discussion over which entities can properly be said to beheritable. Genes are uncontroversial, but are seen as too limited bysome, who would consider phenomena such as learning and culturaltransmission, epigenetic inheritance, and ecological inheritance to beheritable as well. The term “heritability” can likewisecause confusion, as it is a technical term within evolutionary theory,and understanding the term and its implications is not trivial.Classically, heredity has been thought of in terms of thegenotype/phenotype distinction, with genotypes being seen as heritable and phenotypes being seen asnot heritable. But to accept that distinction seems to accept adistinction betweeninnate and acquired characteristics, and that distinction has been challenged, or at least shown to bemore complicated than it would seem at first glance. Heredity alsoraises questions aboutbiological information—do genotypes pass alonginformation, and if so, in whatsense?

Relatedly, for heredity to be a part of evolution, there must bereplication of entities, or at least reproduction (with the former being aspecial case of the latter that involves copying). In order to allowfor a more general theory of evolution, many authors will speak ofreplicators (or reproducers) and vehicles (or interactors) rather thanthe more limited and specific terms “genes” and“organisms”. With these terms in hand, one can more easilybegin to discuss (as many have) questions overunits and levels of selection: does selection occur at the level of the gene, the organism, thegroup, the species, or all of the above? These units of selection(replicators/reproducers or vehicles/interactors) are often taken tobe biological individuals (see the entry onthe biological notion of individual) as a necessary condition for being units of selection at all.

Interestingly, another major area in which biological individualityhas played a large role is in debates over the nature ofspecies. That is, many philosophers of biology maintain that species areproperly construed as individuals. Species, often referred to as“units of evolution”—groups of organisms that evolvein a unified way—are nonetheless rarely seen as units ofselection. In Elisabeth Lloyd’s terminology (see theentry onunits and levels of selection), this is presumably because species are rarely seen asreplicators/reproducers or vehicles/interactors but are commonly seenas beneficiaries of evolution by natural selection. In addition tosorting out whether species are individuals and what sort of units ofevolution (if any) they might be, there are many-decades’ worthof papers trying to characterize the species concept, whether in termsof interbreeding, phylogeny, morphology, ecology, or some other set ofcharacteristics. Here, as in many other areas of the philosophy ofbiology, there have also been arguments for a pluralisticapproach.

Yet another area of discussion isevolutionary game theory—an application of the mathematical theory of games to biological andother evolutionary contexts. It has provided a source of putativeexplanations for human and other behaviors; evolutionary psychology,mentioned above, is one area that frequently makes use of a gametheory approach. Among the more challenging behaviors thatevolutionary game theory has sought to explain isaltruism. With altruism, we again encounter questions about the level at whichselection is operating (organisms or groups) because of questionsabout which entities selection is benefiting or harming.

Bibliography

Darwin, Charles R., 1859,The Origin of Species by Means ofNatural Selection, or the Preservation of Favoured Races in theStruggle for Life, first edition, London: John Murray. [Darwin 1859 available online]
–––, 1874,The Descent of Man, and Selectionin Relation to Sex, second edition, London: John Murray. [Darwin 1874 available online]
Dobzhansky, Theodosius, 1973, “Nothing in Biology MakesSense Except in the Light of Evolution”,American BiologyTeacher, 35(3): 125–129. doi:10.2307/4444260
Futuyma, Douglas J., 2005,Evolution, Sunderland, MA:Sinauer Associates.
Endler, John, 1986,Natural Selection in the Wild,Princeton, NJ: Princeton University Press.
Van Valen, Leigh, 1973, “Book Review: Festschrift for GeorgeGaylord Simpson”Science, 180(4085): 488.doi:10.1126/science.180.4085.486

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Acknowledgments

Thanks to Melinda Bonnie Fagan and Jim Tabery for helpfulcomments.

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