Abstract

Evolution by natural selection is an explicitly genetic theory. Darwin recognized that a working theory of inheritance was central to his theory and spent much of his scientific life seeking one. The seeds of his attempt to fill this gap, his “provisional hypothesis” of pangenesis, appear in his notebooks when he was first formulating his evolutionary ideas. Darwin, in short, desperately needed Mendel. In this paper, we set Mendel’s work in the context of experimental biology and animal/plant breeding of the period and review both the well-known story of possible contact between Mendel and Darwin and the actual contact between their ideas after their deaths. Mendel’s contributions to evolutionary biology were fortuitous. Regardless, it is Mendel’s work that completed Darwin’s theory. The modern theory based on the marriage between Mendel’s and Darwin’s ideas as forged most comprehensively by R. A. Fisher is both Darwin’s achievement and Mendel’s.

Darwin and Mendel: The Not-Quite Connection

Pangenesis

InThe Variation of Animals and Plants under Domestication, Darwin proposed what he called “pangenesis,” a theory suggesting that the hereditary material was contained in “gemmules” that circulated through the body. Early in his evolutionary thinking, he had recognized that any natural selection–like theory required a component addressing the transmission of traits from one generation to the next. Jonathan Hodge (33) has traced the origin of pangenesis to Darwin’s notebooks in 1841, during the time he was formulating his evolutionary ideas (he produced the first short sketch of these ideas in 1842). Natural selection is indeed, in today’s terms, an explicitly genetic theory: its core is the differential probabilities of genetic variations being passed on. Today, knowledge of how those variations are passed on is woven into the theory. However, Mendel-less, Darwin was content to consign the mechanics of inheritance to a black box in which parental input was somehow converted into offspring output. All that was required for natural selection to work was that the black box generated offspring bearing their parents’ traits—that, in short, variations are inherited. Darwin addressed the inheritance issue in the first two chapters ofOrigin but ultimately rather finessed it, reckoning that the observations of both breeder and naturalist were a sufficient grounding for his theory: “No breeder doubts how strong is the tendency to inheritance: that like produces like is his fundamental belief” (ref.3, p. 12).

Darwin was, however, criticized for presenting natural selection without a model of transmission genetics. His work on pangenesis shows how he sought to address this criticism. The then-standard understanding of inheritance, that offspring are a blend of their parents, failed to explain some important observations. Some characters did not blend and indeed exhibited an inexplicable facility for reappearing in a second generation after disappearing in the first (cf. the snapdragon experiment referred to above). In essence, these were the occasional monogenic characters with simple dominance relations that Mendel studied. In 1866, Darwin wrote to A. R. Wallace on the subject (34):

I do not think you understand what I mean by the non-blending of certain varieties. It does not refer to fertility; an instance I will explain. I crossed the Painted Lady and Purple sweetpeas [Lathyrus odoratus], which are very differently coloured varieties, and got, even out of the same pod, both varieties perfect but not intermediate.

Then there was the problem implicit to blending inheritance: the inevitable loss of variation. In principle, assuming random mating and blending inheritance, a bell curve of variation of a given trait would progressively narrow over the generations, converging ultimately on the mean. The only way to preserve the extremes of the range of variation, after all, would be through a mating of two individuals with the same extreme phenotype. On average, extreme individuals would mate with individuals closer to the mean, producing, under blending inheritance, offspring half way between the two parents. Blending inheritance and the maintenance of genetic variation in populations were antithetical. This issue was a prominent feature of the 1867 critique of natural selection by Scottish engineer, Fleeming Jenkin (1833 to 1885), who pointed out that a beneficial new mutation would be diluted away over a few generations of blending inheritance (ref.35; for commentary, see ref.36). Darwin was impressed, writing to Wallace in 1869, “Fleming Jenkyn’s (sic) arguments have convinced me” (37), but as noted above, he had started to work on his solution to the problem that Jenkin described decades earlier.

For Darwin, pangenesis was a big departure from his normal style of science. He was a naturalist, an observer, but here he was taking a deep dive into the unseen: the field biologist becomes molecular biologist. He was uncomfortable doing this, saddling his idea with the qualifier “provisional,” when he first launched it as chapter 27 ofVariation that he titled “The Provisional Hypothesis of Pangenesis.” He had sought reassurance from T. H. Huxley earlier, sending him in 1865 a 30-page manuscript on the subject. Huxley’s response was not sufficiently enthusiastic: he pointed out the similarities of Darwin’s ideas to those of past thinkers wed to the inheritance of acquired characters. However, he nevertheless encouraged Darwin to publish: “Somebody rummaging among your papers half a century hence will find Pangenesis & say ‘See this wonderful anticipation of our modern Theories—and that stupid as, Huxley, prevented his publishing them’” (38).

Darwin never quite decided if his gemmules were capable of blending or not. However, at least they offered the possibility of particulate—nonblending—inheritance. Particles—nonblending ones—would not inevitably converge on the mean of a distribution but could retain their own discrete identities. Darwin suggested that some gemmules could remain latent for several generations (explaining the reappearance of characters that had apparently disappeared in intervening generations), while others could blend, and yet others could possibly pick up the effects of the external environment (allowing for the inheritance of acquired characters). In short, Darwin hoped that his provisional hypothesis would prove to be a general-purpose solution to many biological conundrums he had encountered in the breeding literature.

The reception was mixed. It is nicely typified by A. R. Wallace. He was initially extremely enthusiastic (ref.39, vol. 1, p. 422):

I am reading Darwin’s book (“Animals and Plants under Domestication”), and have read the “Pangenesis” chapter first, for I could not wait. The hypothesis issublime in its simplicity and the wonderful manner in which it explains the most mysterious of the phenomena of life. To me it issatisfying in the extreme. I feel I can never give it up, unless it bepositively disproved, which is impossible, or replaced by one which better explains the facts, which is highly improbable… I consider it the most wonderful thing he has given us, but it will not be generally appreciated.

However, the initial enthusiasm did not last. In 1905 (ref.39, vol. 2, p. 21), looking back, Wallace provided a post mortem on Darwin’s pet theory:

I never imagined that it could be directly disproved, but Mr. F. Galton’s experiments of transfusing a large quantity of the blood of rabbits into other individuals of quite different breeds, and afterwards finding that the progeny was not in the slightest degree altered, did seem to me to be very nearly a disproof, although Darwin did not accept it as such. But when, at a much later period, Dr. Weismann showed that there is actually no valid evidence for the transmission of such characters, and when he further set forth a mass of evidence in support of his theory of the continuity of the germ-plasm, the “better theory” was found, and I finally gave up pangenesis as untenable.

The experiments by Francis Galton (1822 to 1911) that Wallace refers to involved transfusing blood between differently colored rabbits to see if the transfused blood containing, supposedly, color-specifying gemmules affected the color of the offspring produced by the transfusion recipients. There was no effect. The recipient rabbits produced offspring completely lacking even a hint of the donor rabbits’ color. A period of family awkwardness ensued—Darwin and Galton were cousins—with Darwin publishing a response to Galton inNature (40) insisting that he had never specified that gemmules would be blood-borne as they traveled from every part of the body to the gonads: “I have not said one word about the blood, or about any fluid proper to any circulating system. It is, indeed, obvious that the presence of gemmules in the blood can form no necessary part of my hypothesis; for I refer in illustration of it to the lowest animals, such as the Protozoa, which do not possess blood or any vessels; and I refer to plants in which the fluid, when present in the vessels, cannot be considered as true blood.”

Heredity

Until the later 19th century, questions about heredity usually ran along lines separate from the problem of the origin of species. In the 1880s, for example, Galton, whose primary focus was heredity, proposed that offspring inherited equal portions of their heritable characteristics from each parent, who in turn had inherited equal portions from their parents, and so on back through the ancestral line. In more statistical terms, this meant that parents contributed one-half of the total heritage of the offspring; grandparents, one-quarter; and so on. This so-called law of ancestral heredity explained traits that ran in families as well as the appearance of only some of the parental qualities in successive generations. It proved a popular, commonsensical scheme and one, furthermore, readily amenable to statistical analysis. In the last decade of the century, evolutionary biologists W. F. R. Weldon (1860 to 1906) and Karl Pearson (1857 to 1936) merged these ideas with their own meticulous observations on variation and evolutionary change. They followed Darwin’s precepts, and more directly walked in Galton’s footsteps, in developing the field of biometry, in which they measured small, almost continuous changes in organisms, deeming these to be the material on which natural selection went to work.

Other leading biologists were at the same time exploring other ideas about inheritance that depended on the transmission of discrete material from parent to offspring. In 1883, August Weismann (1834 to 1914) famously proposed that the heritable material (“germ-plasm”) existed in what he called “germ-cells” that were entirely separate from ordinary somatic cells and transmitted independently and unchanged from generation to generation. He located these germ cells in the reproductive parts of the body, the gonads of multicellular organisms. Weismann rejected Darwin’s suggestion of pangenesis, but was nevertheless an advocate of Darwinian evolution by natural selection. By the end of the 19th century his theory of germ-plasm had become the most widely accepted way to understand heredity. In Weismann’s opinion the largely random process of mutation in the germ cells in the production of gametes supplied the only source of transmissible change for natural selection to work on.

The Dutch botanist Hugo De Vries (1848 to 1935) was prominent in bringing about this latter shift in perspective, emphasizing the importance of sports or macromutations in generating new species. His embrace of Darwinism led him to modify Darwin’s theory of pangenesis. He suggested that the inheritance of traits in organisms was specified by inherited particles that he called “pangenes,” a term that 20 y later was shortened to “genes” by Wilhelm Johannsen. Yet he was ambivalent about the power of natural selection to do all that was promised by Darwin and Wallace. In 1886 he discovered new forms of evening primrose (Oenothera lamarckiana) growing wild in an abandoned potato field near his research station in the Netherlands. Taking seeds from these spontaneous sports, he found that they produced many new varieties in his experimental gardens, some of them so different from each other and their parent as to appear to be distinct species. He defined these as mutations. In his two-volume 1901 to 1903 publicationDie Mutationstheorie (41), De Vries proposed that new species might appear more frequently from such large-scale changes than from gradualistic Darwinian processes.

This mutation theory did not stand the test of time.Oenothera, it turns out, is thoroughly atypical: unusual features of its chromosomal organization mean that it is particularly prone to meiotic disruptions. Many of the “new species” prized by De Vries were polyploids—individuals with extra sets of chromosomes. Not surprisingly, De Vries’s project was undone by its lack of generality: other organisms did not produceOenothera-type macromutations. By 1915, the mutation theory had begun to lose its grip on the biological community, and, by the time of De Vries’s death in 1935, it was almost completely abandoned (42). Yet, during the first decade of the 20th century it achieved an enormous popularity because it seemed to offer something that many sought: a Mendelian approach to understanding the evolutionary process.

Rediscovery

De Vries began a systematic program of plant breeding in 1892 to support his theory of pangenes, in which he hybridized varieties of several plant species. Unaware of Mendel’s work, he found that in peas the inheritance of characters occurred in a 3:1 ratio in the second generation. When it came time to publish his work some years later, he became aware of Mendel’s paper of 1866 and altered his terminology to match. He seems to have thought of Mendel’s work merely as an earlier empirical instance of what he was doing and that it confirmed his results. When he published the results of his own experiments in the French journalComptes Rendus de l’Académie des Sciences in 1900, De Vries neglected to mention Mendel’s work (43); only after criticism by Carl Correns did he concede Mendel’s priority—in a German translation of the same article published a few weeks later.

This moment can be formulated not just in terms of rediscovery but also as a case of disputed priority. Correns claimed to have discovered the 3:1 ratio in 1899 when he was doing crossing experiments in peas and maize. In his article, Correns thanked De Vries for sending him theComptes Rendus article and stated that he achieved the same result. He went on to say that Mendel preceded them both (44). He evidently came across Mendel from reading Focke (26), the handy list of all 19th century hybridizers that had been available to Darwin. Correns’s article in theDeutschen Botanischen Gessellschaft was therefore explicitly about bringing Mendel back into circulation. Weirdly, then, the fabled rediscovery was initially fueled by two biologists’ desire to one up each other: Mendel was just a weapon in the precedence war (45).

At much the same time, Erich von Tschermak-Seysenegg (known as Tschermak, 1871 to 1962) at the Academy of Agriculture in Vienna also became aware of Mendel’s work. Tschermak was an agronomist, and his hybridization experiments aimed at improving crops such as wheat, barley, and oats. Coincidentally, his maternal grandfather was Eduard Fenzl, who had been one of Mendel’s botany professors during his student days in Vienna. In January 1900, Tschermak wrote his PhD thesis on the inheritance of seed colors and shapes in pea hybrids (his “Habilitationschrift”). Apparently, he saw a reference to Mendel’s work and had the 1866 paper inVerhandlungen des Naturforschenden Vereines sent to him from the library of the University of Vienna. He found that Mendel’s work with the garden pea duplicated and, in some ways, superseded his own. His article (46), published in June 1900, was written after he had read both De Vries and Correns (ref.11, pp. 303–310). This calls into question Tschermak’s membership of the rediscovery club. Moreover, Tschermak’s place in the club has also been challenged on the grounds that he misunderstood the fundamentals of Mendel’s arguments and interpreted Mendelian phenomena within a pre-Mendelian concept of heredity (ref.42, p. 114).

Since the 1950s, questions have regularly arisen concerning both the chronology and the specific contributions of each of the three rediscoverers. Not only the independence but also the parallelism needs to be analyzed in the context of the individual research programs of all three. Also, perhaps the three-way focus needs to be reevaluated. Simunek et al. (47), for example, point out that Tschermak’s contributions were heavily influenced by his older brother, Armin von Tschermak-Seysenegg (1870 to 1952). These questions are important. We need to look carefully at the key overlapping influences operating in each case to understand what social, technical, and political factors conspired to bring Mendel so dramatically into the limelight.

The First Mendelians

The rediscovery ushered Mendel into a contested area of biology. There was an ongoing debate about the nature of variation—was it a seamless continuum of minor (even infinitesimal) differences, as Darwin had proposed, or did it occur predominantly as sports of nature? How did it arise, and how was it transmitted? Was it particulate? If yes, the suggestion was that the discrete particles could be freely mixed or recombined (not blended).

Mendel’s work was taken up by a group of biologists vigorously engaged in defending mutation theory, among them the Cambridge naturalist William Bateson (1861 to 1926), who believed that mutation was the most likely source of evolutionary innovation. Bateson’s first bookMaterials for the Study of Variation, published in 1894 (48), was written directly in opposition to the work of the evolutionists gathered around Pearson and Weldon in London. This latter group, who self-identified as “biometricians,” regarded themselves as the true apostles of Darwinism by exploring small, continuous variations. Pearson measured traits in animal populations and showed that they generally graphed as a smooth or continuous distribution. For 10 y or more, this intense debate—that began some years before Mendel’s paper resurfaced—juxtaposed individuals who, to a man, sincerely believed in evolution yet opposed each other over the question of whether it proceeds by jumps or gradually, via continuous variation.

Mendel’s rediscovered work accordingly stumbled into an ongoing controversy to which it seemed to speak directly. There is a story that, in May 1900, Bateson was on the train from Cambridge to London to give a lecture at the Royal Horticultural Society when he read Mendel’s work. Recognizing Mendel’s significance as support for his own saltationist point of view, Bateson immediately modified his speech to incorporate the new vision. Attractive as it is, this story is apocryphal (49). Regardless, Bateson rapidly became the most prominent proponent of Mendelian inheritance and soon published a translation of Mendel’s paper into English (50) andMendel’s Principles of Heredity: A Defence (19). Mendel was, for Bateson, a useful means of self-promotion as well: Bateson could style himself the noble champion of the neglected underdog. He suggested using the word “genetics” (from the Greek “to give birth”) to describe the science of variation and inheritance and first used the term at the Third International Conference on Plant Hybridization in London in 1906. At Cambridge University he led the first department of genetics in the country. Between 1900 and 1910 he directed a program of research at Cambridge into plant genetics mostly undertaken by young women trained at Newnham College, the first college for women at the university (51).

In effect, Bateson harnessed Mendel to promote his own world view and what he regarded as a new dawn in the study of variation. According to Arthur Koestler, he even named his son Gregory after him (ref.52, p. 54). Bateson and others cemented their self-perceived status as leaders of the new science in a wave of celebrations marking the revolution in biology. In 1909, in Cambridge, United Kingdom, Bateson was the leading figure in commemorating the 50th anniversary of the publication of Darwin’sOrigin of Species: Darwin’s sons attended, as did Hugo De Vries. Then, in 1910, Bateson delivered a speech at the international gathering in Brno to memorialize Mendel and erect a statue outside the abbey. A small Museum Mendelianum was set up in the abbey in 1922.

Darwin’s death in 1882 deprives us of the opportunity to learn what he would have made of these developments. However, we can gauge his likely response from that of A. R. Wallace, who lived until 1913. Darwin and Wallace disagreed on plenty of topics—most notably whether or not natural selection was sufficient to account for the evolution of our own species—but Wallace was nevertheless a keen defender of what he insisted modestly was Darwinian orthodoxy. We can observe two phases in Wallace’s response. First, prerediscovery, came the macromutational perspective of Bateson, as laid out in Bateson’s 1894 book,Materials for the Study of Variation Treated with Especial Regard to Discontinuity in the Origin of Species (48). In his review, Wallace pointed out that Darwin had already dealt with the issue (ref.53, p. 216): “Darwin, while always believing that individual differences played the most important part in the origin of species, did not altogether exclude sports or discontinuous variations, but he soon became convinced that these latter were quite unimportant, and that they rarely, if ever, served to originate new species.”

Wallace then supplied a number of objections to Bateson’s belief in the centrality of sports in the generation of evolutionary novelty, noting that despite the catalog of mutant forms that Bateson presented, the key consideration is that they tend not to survive in the wild, leading us to “conclude that all these irregularities and monstrosities are in a high degree disadvantageous” (ref.53, p. 221).

What of Mendelism? Again, Wallace had a response (ref.54, p. 137):

The curious phenomena that present themselves when these [mutants] are crossed with allied forms either domesticated or natural, though they may be of considerable interest as furnishing materials for the study of the theory of heredity, have absolutely nothing whatever to do with the origin or modification of species.

Wallace here clearly articulated the key disconnect: that the sciences of heredity and of evolution are in some way independent of each other. Wallace was pigeonholing the theory of heredity as some kind of special case involving the transmission of discrete characters; for him, it was distinct from the kind of inheritance that generates bell curves of individuals whose differences are infinitesimal. Wallace, like many of Darwin’s heirs, failed to recognize that Mendelian transmission is not a special case; rather, the special cases are those simple monogenic polymorphisms that permit the process to be observed directly.

Although there were plenty of suggestions during the first part of the 20th century that inheritance-writ-large and Mendelism were in fact the same thing, R. A. Fisher (1890 to 1962) deserves the credit for making the connection (55). The vaunted bell curve and Mendel’s wrinkled/round peas were in fact produced by the same processes: the bell curve, with its infinitesimal individual-to-individual gradations, was born of multiple Mendelian factors (genes) combining to affect a trait, along with input from the environment, which smears genotypic categories one into another.

Eugenics

The elephant in the room was the question of human heredity. The possibility of directing the future evolution of human beings through the application of eugenics had been discussed widely since 1883, when Francis Galton first articulated the term. With the arrival of Mendel’s insights, a new rigorous vision of eugenics beckoned: human traits, including inheritable medical conditions, could perhaps be understood in terms of the particulate inheritance of dominant and recessive Mendelian factors. Galton, whose original 1865 work had focused on the inheritance of hereditary genius in families, now had a new paradigm in which to continue his exploration (56). The project soon took on a life of its own. The fusion of Mendelian transmission genetics with eugenical ideas about the improvement or eradication of traits in humans was one of the most conspicuous aspects of biology in the first several decades of the 20th century. Eugenics was coconstructed, one might say, with the rise in biological determinism and was responsible for the troubling and ultimately disastrous trends in the application of biology to social policy (57).

By the 1920s, eugenics was a well-established field of inquiry in Europe and North America and would soon have a truly global reach (58). That identifiable traits like eye and hair color, or medical conditions such as albinism, apparently followed Mendelian patterns of inheritance bestowed credibility on the burgeoning social-cum-scientific movement. In the hands of early activists such as Charles Davenport, pedigree analysis of the transmission of Mendelian factors was trumpeted as the way to understand and control human biology. In 1904, Davenport became director of Cold Spring Harbor Laboratory, NY, where he founded the Eugenics Record Office in 1910. There, he and his team generated pedigree charts showing the transmission of scores of what they believed to be inheritable traits like alcoholism, pellagra (later shown to be due to a vitamin deficiency), criminality, feeblemindedness, bad temper, intelligence, and manic depression. The premise was simple: having identified the patterns of inheritance, society should prevent the “unfit” from passing on their flawed genetic heritage to the next generation. The extent to which the eugenic pedigree-builders overlooked the obvious impacts of rearing environments is remarkable. Davenport’s colleague Harry H. Laughlin analyzed the inheritance of boat-building skills in Rhode Island’s Herreshoff family. It did not occur to Laughlin and Davenport that being raised by a boat-builder might influence an individual’s decision to join that very trade. Laughlin also produced a chart showing the intellectual abilities of members of the Darwin–Wedgwood–Galton clan for display at the Third International Congress of Eugenics in 1932 at the American Museum of Natural History, NY. Galton, who was immensely proud of his connection to Darwin, would have loved it.

It was not just the Mendelians. Leading biometricians such as Pearson were closely connected to socialist eugenic groups in the United Kingdom. In fact, Pearson’s views on statistics, eugenics, race, empire, and evolution were irrevocably intertwined. Eugenics provided him with a language to express his racist and political views. Galton had in fact given Pearson the foundations of his statistical method, his commitment to evolution, and his belief in eugenic advance. In a way he was Galton’s intellectual heir.

However, when Galton died in 1911, he had no literal heir. Ironically, the father of eugenics had failed to contribute genetically to the human future. Who was the beneficiary? He left the balance of his considerable fortune to University College London, to finance a university eugenics department. Pearson was named the first Galton Professor in National Eugenics, a position that exists today as the Galton Chair of Genetics. Ronald Fisher was Pearson’s successor both as the Galton Chair of Eugenics at University College London and as editor of theAnnals of Eugenics. In 2022, University College London removed Galton and Pearson’s names from three of its buildings.

Conclusion

History tells us that great ideas are often not recognized or accepted in their own time. Mendel’s findings are a classic example of this, in the sense that his work is said to have been “forgotten” or “neglected” and then “rediscovered” independently by three biologists. Mendel’s modern fame is due to figures who were initially unaware of his work but then saw how it could be appropriated and turned to personal advantage in a post-Darwinian debate about the mechanism of evolutionary change. To be a Mendelian in the early years of the 20th century was to promote saltation and to downplay or even ignore the role of selection in favor of mutation and evolution by jumps. To others, such as the biometricians, Mendel’s findings were meaningless in a world where natural selection acted on minute distinctions. This surprisingly bitter impasse between saltationists and gradualists was only resolved with Fisher’s 1918 mathematical formalization (55) of the observation that a continuous distribution can readily be derived from underlying discontinuous variation at multiple contributing loci coupled with environmental variation.

It is impossible to overstate the significance of Fisher’s insight. It set the stage for the emergence of theoretical population and quantitative genetics, which in turn reestablished natural selection as the main driver of evolutionary processes, very much in the way that Darwin had envisaged. Building on the simple logic of Mendel and its application to populations, population genetics provided an understanding of the engine that drives the evolutionary process. Courtesy of Fisher’s bridging insight, the modern theory of evolution truly is a marriage of Darwin’s ideas and Mendel’s. Mendel himself was perhaps an early evolutionist (29), but unbeknownst to him, his role was much more central than that: he provided the crucial insight that subsequently transformed Darwin’s ideas from uncomfortably incomplete into a comprehensive theory of evolution by natural selection. Friar Mendel stands right there, front and center, beside Darwin in the pantheon of key contributors to evolutionary thought.

Footnotes

Data Availability

There are no data underlying this work.

References

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