The proximate/ultimate distinction in the multiple careers of Ernst Mayr

Ernst Mayr's distinction between “ultimate” and “proximate” causes is justly considered a major contribution to philosophy of biology. But how did Mayr come to this “philosophical” distinction, and what role did it play in his earlier “scientific” work? I address these issues by dividing Mayr's work into three careers or phases: 1) Mayr the naturalist/researcher, 2) Mayr the representative of and spokesman for evolutionary biology and systematics, and more recently 3) Mayr the historian and philosopher of biology. If we want to understand the role of the proximate/ultimate distinction in Mayr's more recent career as a philosopher and historian, then it helps to consider hisearlier use of the distinction, in the course of his research, and in his promotion of the professions of evolutionary biology and systematics. I believe that this approach would also shed light on some other important “philosophical” positions that Mayr has defended, including the distinction between “essentialism: and “population thinking.”     

Mayr's view of Darwin: was Darwin wrong about speciation?

We commonly read or hear that Charles Darwin successfully convinced the world about evolution and natural selection, but did not answer the question posed by his most famous book, ‘On the Origin of Species …’. Since the 1940s, Ernst Mayr has been one of the people who argued for this point of view, claiming that Darwin was not able to answer the question of speciation because he failed to define species properly. Mayr undoubtedly had an important and largely positive influence on the study of evolution by stimulating much evolutionary work, and also by promoting a ‘polytypic species concept’ in which multiple, geographically separated forms may be considered as subspecies within a larger species entity. However, Mayr became seduced by the symmetry of a pair of interlocking ideas: (1) that coexistence of divergent populations was not possible without reproductive isolation and (2) reproductive isolation could not evolve in populations that coexist. These beliefs led Mayr in 1942 to reject evidence of the importance of intermediate stages in speciation, particularly introgression between hybridizing species, which demonstrates that complete reproductive isolation is not necessary, and the existence of ecological races, which shows that ecological divergence can be maintained below the level of species, in the face of gene flow. Mayr's train of thought led him to the view that Darwin misunderstood species, and that species were fundamentally different from subspecific varieties in nature. Julian Huxley, reviewing similar data at the same time, came to the opposite conclusion, and argued that these were the intermediate stages of speciation expected under Darwinism. Mayr's arguments were, however, more convincing than Huxley's, and this caused a delay in the acceptance of a more balanced view of speciation for many decades. It is only now, with new molecular evidence, that we are beginning to appreciate more fully the expected Darwinian intermediates between coexisting species. © The Author. Journal compilation © 2008 The Linnean Society of London, Biological Journal of the Linnean Society, 2008, 95 , 3–16.     

The establishment of evolutionary biology as a discrete biological discipline

This is the second of two ‘Roots’ articles from Dr Ernst Mayr; the first appeared in the February issue. The article that follows is Dr Mayr's address on the 50th anniversary of the founding of the Society for the Study of Evolution, St Louis, USA, June 19, 1996.     

Ernst Mayr and the integration of geographic and ecological factors in speciation

Mayr's best recognized scientific contributions include the biological species concept and the theory of geographic speciation. In the latter, reproductive isolation evolves as an incidental by‐product of genetic divergence between allopatric populations. Mayr noted that divergent natural selection could accelerate speciation, but also argued that gene flow so strongly retards divergence that, even with selection, non‐allopatric speciation is unlikely. However, current theory and data demonstrate that substantial divergence, and even speciation, in the face of gene flow is possible. Here, I attempt to connect some opposing views about speciation by integrating Mayr's ideas about the roles of ecology and geography in speciation with current data and theory. My central premise is that the speciation process (i.e. divergence) is often continuous, and that the opposing processes of selection and gene flow interact to determine the degree of divergence (i.e. the degree of progress towards the completion of speciation). I first establish that, in the absence of gene flow, divergent selection often promotes speciation. I then discuss how population differentiation in the face of gene flow is common when divergent selection occurs. However, such population differentiation does not always lead to the evolution of discontinuities, strong reproductive isolation, and thus speciation per se. I therefore explore the genetic and ecological circumstances that facilitate speciation in the face of gene flow. For example, particular genetic architectures or ecological niches may tip the balance between selection and gene flow strongly in favour of selection. The circumstances allowing selection to overcome gene flow to the extent that a discontinuity develops, and how often these circumstances occur, are major remaining questions in speciation research. © 2008 The Linnean Society of London, Biological Journal of the Linnean Society, 2008, 95 , 26–46.     

J. B. S. Haldane,Ernst Mayr and the Beanbag Genetics Dispute

Starting from the early decades of the twentieth century, evolutionary biology began to acquire mathematical overtones. This took place via the development of a set of models in which the Darwinian picture of evolution was shown to be consistent with the laws of heredity discovered by Mendel. The models, which came to be elaborated over the years, define a field of study known as population genetics. Population genetics is generally looked upon as an essential component of modern evolutionary theory. This article deals with a famous dispute between J. B. S. Haldane, one of the founders of population genetics, and Ernst Mayr, a major contributor to the way we understand evolution. The philosophical undercurrents of the dispute remain relevant today. Mayr and Haldane agreed that genetics provided a broad explanatory framework for explaining how evolution took place but differed over the relevance of the mathematical models that sought to underpin that framework. The dispute began with a fundamental issue raised by Mayr in 1959: in terms of understanding evolution, did population genetics contribute anything beyond the obvious? Haldane’s response came just before his death in 1964. It contained a spirited defense, not just of population genetics, but also of the motivations that lie behind mathematical modelling in biology. While the difference of opinion persisted and was not glossed over, the two continued to maintain cordial personal relations.     

Why was Darwin’s view of species rejected by twentieth century biologists?

Historians and philosophers of science agree that Darwin had an understanding of species which led to a workable theory of their origins. To Darwin species did not differ essentially from ‘varieties’ within species, but were distinguishable in that they had developed gaps in formerly continuous morphological variation. Similar ideas can be defended today after updating them with modern population genetics. Why then, in the 1930s and 1940s, did Dobzhansky, Mayr and others argue that Darwin failed to understand species and speciation? Mayr and Dobzhansky argued that reproductively isolated species were more distinct and ‘real’ than Darwin had proposed. Believing species to be inherently cohesive, Mayr inferred that speciation normally required geographic isolation, an argument that he believed, incorrectly, Darwin had failed to appreciate. Also, before the sociobiology revolution of the 1960s and 1970s, biologists often argued that traits beneficial to whole populations would spread. Reproductive isolation was thus seen as an adaptive trait to prevent disintegration of species. Finally, molecular genetic markers did not exist, and so a presumed biological function of species, reproductive isolation, seemed to delimit cryptic species better than character-based criteria like Darwin’s. Today, abundant genetic markers are available and widely used to delimit species, for example using assignment tests: genetics has replaced a Darwinian reliance on morphology for detecting gaps between species. In the 150th anniversary of The Origin of Species, we appear to be returning to more Darwinian views on species, and to a fuller appreciation of what Darwin meant.     

The puzzle of sympatry. Recent evidence supports the notion of sympatric speciation--the rise of new species in the same location--but the reasons for this phenomenon remain elusive

Sympatric speciation—the rise of new species in the absence of geographical barriers—remains a puzzle for evolutionary biologists. Though the evidence for sympatric speciation itself is mounting, an underlying genetic explanation remains elusive.For centuries, the greatest puzzle in biology was how to account for the sheer variety of life. In his 1859 landmark book, On the Origin of Species, Charles Darwin (1809–1882) finally supplied an answer: his grand theory of evolution explained how the process of natural selection, acting on the substrate of genetic mutations, could gradually produce new organisms that are better adapted to their environment. It is easy to see how adaptation to a given environment can differentiate organisms that are geographically separated; different environmental conditions exert different selective pressures on organisms and, over time, the selection of mutations creates different species—a process that is known as allopatric speciation.It is more difficult to explain how new and different species can arise within the same environment. Although Darwin never used the term sympatric speciation for this process, he did describe the formation of new species in the absence of geographical separation. “I can bring a considerable catalogue of facts,” he argued, “showing that within the same area, varieties of the same animal can long remain distinct, from haunting different stations, from breeding at slightly different seasons, or from varieties of the same kind preferring to pair together” (Darwin, 1859).It is more difficult to explain how new and different species can arise within the same environmentIn the 1920s and 1930s, however, allopatric speciation and the role of geographical isolation became the focus of speciation research. Among those leading the charge was Ernst Mayr (1904–2005), a young evolutionary biologist, who would go on to influence generations of biologists with his later work in the field. William Baker, head of palm research at the Royal Botanic Gardens, Kew in Richmond, UK, described Mayr as “one of the key figures to crush sympatric speciation.” Frank Sulloway, a Darwin Scholar at the Institute of Personality and Social Research at the University of California, Berkeley, USA, similarly asserted that Mayr''s scepticism about sympatry was central to his career.The debate about sympatric and allopatric speciation has livened up since Mayr''s death…Since Mayr''s death in 2005, however, several publications have challenged the notion that sympatric speciation is a rare exception to the rule of allopatry. These papers describe examples of both plants and animals that have undergone speciation in the same location, with no apparent geographical barriers to explain their separation. In these instances, a single ancestral population has diverged to the extent that the two new species cannot produce viable offspring, despite the fact that their ranges overlap. The debate about sympatric and allopatric speciation has livened up since Mayr''s death, as Mayr''s influence over the field has waned and as new tools and technologies in molecular biology have become available.Sulloway, who studied with Mayr at Harvard University, in the late 1960s and early 1970s, notes that Mayr''s background in natural history and years of fieldwork in New Guinea and the Solomon Islands contributed to his perception that the bulk of the data supported allopatry. “Ernst''s early career was in many ways built around that argument. It wasn''t the only important idea he had, but he was one of the strong proponents of it. When an intellectual stance exists where most people seem to have gotten it wrong, there is a tendency to sort of lay down the law,” Sulloway said.Sulloway also explained that Mayr “felt that botanists had basically led Darwin astray because there is so much evidence of polyploidy in plants and Darwin turned in large part to the study of botany and geographical distribution in drawing evidence in The Origin.” Indeed, polyploidization is common in plants and can lead to ‘instantaneous'' speciation without geographical barriers.In February 2006, the journal Nature simultaneously published two papers that described sympatric speciation in animals and plants, reopening the debate. Axel Meyer, a zoologist and evolutionary biologist at the University of Konstanz, Germany, demonstrated with his colleagues that sympatric speciation has occurred in cichlid fish in Lake Apoyo, Nicaragua (Barluenga et al, 2006). The researchers claimed that the ancestral fish only seeded the crater lake once; from this, new species have evolved that are distinct and reproductively isolated. Meyer''s paper was broadly supported, even by critics of sympatric speciation, perhaps because Mayr himself endorsed sympatric speciation among the cichlids in his 2001 book What Evolution Is. “[Mayr] told me that in the case of our crater lake cichlids, the onus of showing that it''s not sympatric speciation lies with the people who strongly believe in only allopatric speciation,” Meyer said.…several scientists involved in the debate think that molecular biology could help to eventually resolve the issueThe other paper in Nature—by Vincent Savolainen, a molecular systematist at Imperial College, London, UK, and colleagues—described the sympatric speciation of Howea palms on Lord Howe Island (Fig 1), a minute Pacific island paradise (Savolainen et al, 2006a). Savolainen''s research had originally focused on plant diversity in the gesneriad family—the best known example of which is the African violet—while he was in Brazil for the Geneva Botanical Garden, Switzerland. However, he realized that he would never be able prove the occurrence of sympatry within a continent. “It might happen on a continent,” he explained, “but people will always argue that maybe they were separated and got together after. […] I had to go to an isolated piece of the world and that''s why I started to look at islands.”Open in a separate windowFigure 1Lord Howe Island. Photo: Ian Hutton.He eventually heard about Lord Howe Island, which is situated just off the east coast of Australia, has an area of 56 km² and is known for its abundance of endemic palms (Sidebar A). The palms, Savolainen said, were an ideal focus for sympatric research: “Palms are not the most diverse group of plants in the world, so we could make a phylogeny of all the related species of palms in the Indian Ocean, southeast Asia and so on.”…the next challenges will be to determine which genes are responsible for speciation, and whether sympatric speciation is common

Sidebar A | Research in paradise

Alexander Papadopulos is no Tarzan of the Apes, but he has spent a couple months over the past two years aloft in palm trees hugging rugged mountainsides on Lord Howe Island, a Pacific island paradise and UNESCO World Heritage site.Papadopulos—who is finishing his doctorate at Imperial College London, UK—said the views are breathtaking, but the work is hard and a bit treacherous as he moves from branch to branch. “At times, it can be quite hairy. Often you''re looking over a 600-, 700-metre drop without a huge amount to hold onto,” he said. “There''s such dense vegetation on most of the steep parts of the island. You''re actually climbing between trees. There are times when you''re completely unsupported.”Papadopulos typically spends around 10 hours a day in the field, carrying a backpack and utility belt with a digital camera, a trowel to collect soil samples, a first-aid kit, a field notebook, food and water, specimen bags, tags to label specimens, a GPS device and more. After several days in the field, he spends a day working in a well-equipped field lab and sleeping in the quarters that were built by the Lord Howe governing board to accommodate the scientists who visit the island on various projects. Papadopulos is studying Lord Howe''s flora, which includes more than 200 plant species, about half of which are indigenous.Vincent Savolainen said it takes a lot of planning to get materials to Lord Howe: the two-hour flight from Sydney is on a small plane, with only about a dozen passengers on board and limited space for equipment. Extra gear—from gardening equipment to silica gel and wood for boxes in which to dry wet specimens—arrives via other flights or by boat, to serve the needs of the various scientists on the team, including botanists, evolutionary biologists and ecologists.Savolainen praised the well-stocked researcher station for visiting scientists. It is run by the island board and situated near the palm nursery. It includes one room for the lab and another with bunks. “There is electricity and even email,” he said. Papadoupulos said only in the past year has the internet service been adequate to accommodate video calls back home.Ian Hutton, a Lord Howe-based naturalist and author, who has lived on the island since 1980, said the island authorities set limits on not only the number of residents—350—but also the number of visitors at one time—400—as well as banning cats, to protect birds such as the flightless wood hen. He praised the Imperial/Kew group: “They''re world leaders in their field. And they''re what I call ‘Gentlemen Botanists''. They''re very nice people, they engage the locals here. Sometimes researchers might come here, and they''re just interested in what they''re doing and they don''t want to share what they''re doing. Not so with these people. Savolainen said his research helps the locals: “The genetics that we do on the island are not only useful to understand big questions about evolution, but we also always provide feedback to help in its conservation efforts.”Yet, in Savolainen''s opinion, Mayr''s influential views made it difficult to obtain research funding. “Mayr was a powerful figure and he dismissed sympatric speciation in textbooks. People were not too keen to put money on this,” Savolainen explained. Eventually, the Leverhulme Trust (London, UK) gave Savolainen and Baker £70,000 between 2003–2005 to get the research moving. “It was enough to do the basic genetics and to send a research assistant for six months to the island to do a lot of natural history work,” Savolainen said. Once the initial results had been processed, the project received a further £337,000 from the British Natural Environment Research Council in 2008, and €2.5 million from the European Research Council in 2009.From the data collected on Lord Howe Island, Savolainen and his team constructed a dated phylogenetic tree showing that the two endemic species of the palm Howea (Arecaceae; Fig 2) are sister taxa. From their tree, the researchers were able to establish that the two species—one with a thatch of leaves and one with curly leaves—diverged long after the island was formed 6.9 million years ago. Even where they are found in close proximity, the two species cannot interbreed as they flower at different times.Open in a separate windowFigure 2The two species of Howea palm. (A) Howea fosteriana (Kentia palm). (B) Howea belmoreana. Photos: William Baker, Royal Botanical Gardens, Kew, Richmond, UK.According to the researchers, the palm speciation probably occurred owing to the different soil types in which the plants grow. Baker explained that there are two soil types on Lord Howe—the older volcanic soil and the younger calcareous soils. The Kentia palm grows in both, whereas the curly variety is restricted to the volcanic soil. These soil types are closely intercalated—fingers and lenses of calcareous soils intrude into the volcanic soils in lowland Lord Howe Island. “You can step over a geological boundary and the palms in the forest can change completely, but they remain extremely close to each other,” Baker said. “What''s more, the palms are wind-pollinated, producing vast amounts of pollen that blows all over the place during the flowering season—people even get pollen allergies there because there is so much of the stuff.” According to Savolainen, that the two species have different flowering times is a “way of having isolation so that they don''t reproduce with each other […] this is a mechanism that evolved to allow other species to diverge in situ on a few square kilometres.”According to Baker, the absence of a causative link has not been demonstrated between the different soils and the altered flowering times, “but we have suggested that at the time of speciation, perhaps when calcareous soils first appeared, an environmental effect may have altered the flowering time of palms colonising the new soil, potentially causing non-random mating and kicking off speciation. This is just a hypothesis—we need to do a lot more fieldwork to get to the bottom of this,” he said. What is clear is that this is not allopatric speciation, as “the micro-scale differentiation in geology and soil type cannot create geographical isolation”, said Baker.…although molecular data will add to the debate, it will not settle it aloneThe results of the palm research caused something of a splash in evolutionary biology, although the study was not without its critics. Tod Stuessy, Chair of the Department of Systematic and Evolutionary Botany at the University of Vienna, Austria, has dealt with similar issues of divergence on Chile''s Juan Fernández Islands—also known as the Robinson Crusoe Islands—in the South Pacific. From his research, he points out that on old islands, large ecological areas that once separated species—and caused allopatric speciation—could have since disappeared, diluting the argument for sympatry. “There are a lot of cases [in the Juan Fernández Islands] where you have closely related species occurring in the same place on an island, even in the same valley. We never considered that they had sympatric origins because we were always impressed by how much the island had been modified through time,” Stuessy said. “What [the Lord Howe researchers] really didn''t consider was that Lord Howe Island could have changed a lot over time since the origins of the species in question.” It has also been argued that one of the palm species on Lord Howe Island might have evolved allopatrically on a now-sunken island in the same oceanic region.In their response to a letter from Stuessy, Savolainen and colleagues argued that erosion on the island has been mainly coastal and equal from all sides. “Consequently, Quaternary calcarenite deposits, which created divergent ecological selection pressures conducive to Howea species divergence, have formed evenly around the island; these are so closely intercalated with volcanic rocks that allopatric speciation due to ecogeographic isolation, as Stuessy proposes, is unrealistic” (Savolainen et al, 2006b). Their rebuttal has found support in the field. Evolutionary biologist Loren Rieseberg at the University of British Columbia in Vancouver, Canada, said: “Basically, you have two sister species found on a very small island in the middle of the ocean. It''s hard to see how one could argue anything other than they evolved there. To me, it would be hard to come up with a better case.”Whatever the reality, several scientists involved in the debate think that molecular biology could help to eventually resolve the issue. Savolainen said that the next challenges will be to determine which genes are responsible for speciation, and whether sympatric speciation is common. New sequencing techniques should enable the team to obtain a complete genomic sequence for the palms. Savolainen said that next-generation sequencing is “a total revolution.” By using sequencing, he explained that the team, “want to basically dissect exactly what genes are involved and what has happened […] Is it very special on Lord Howe and for this palm, or is [sympatric speciation] a more general phenomenon? This is a big question now. I think now we''ve found places like Lord Howe and [have] tools like the next-gen sequencing, we can actually get the answer.”Determining whether sympatric speciation occurs in animal species will prove equally challenging, according to Meyer. His own lab, among others, is already looking for ‘speciation genes'', but this remains a tricky challenge. “Genetic models […] argue that two traits (one for ecological specialisation and another for mate choice, based on those ecological differences) need to become tightly linked on one chromosome (so that they don''t get separated, often by segregation or crossing over). The problem is that the genetic basis for most ecologically relevant traits are not known, so it would be very hard to look for them,” Meyer explained. “But, that is about to change […] because of next-generation sequencing and genomics more generally.”Many researchers who knew Mayr personally think he would have enjoyed the challenge to his viewsOthers are more cautious. “In some situations, such as on isolated oceanic islands, or in crater lakes, molecular phylogenetic information can provide strong evidence of sympatric speciation. It also is possible, in theory, to use molecular data to estimate the timing of gene flow, which could help settle the debate,” Rieseberg said. However, he cautioned that although molecular data will add to the debate, it will not settle it alone. “We will still need information from historical biogeography, natural history, phylogeny, and theory, etc. to move things forward.”Many researchers who knew Mayr personally think he would have enjoyed the challenge to his views. “I can only imagine that it would''ve been great fun to engage directly with him [on sympatry on Lord Howe],” Baker said. “It''s a shame that he wasn''t alive to comment on [our paper].” In fact, Mayr was not really as opposed to sympatric speciation as some think. “If one is of the opinion that Mayr opposed all forms of sympatric speciation, well then this looks like a big swing back the other way,” Sulloway commented. “But if one reads Mayr carefully, one sees that he was actually interested in potential exceptions and, as best he could, chronicled which ones he thought were the best candidates.”Mayr''s opinions aside, many biologists today have stronger feelings against sympatric speciation than he did himself in his later years, Meyer added. “I think that Ernst was more open to the idea of sympatric speciation later in his life. He got ‘softer'' on this during the last two of his ten decades of life that I knew him. I was close to him personally and I think that he was much less dogmatic than he is often made out to be […] So, I don''t think that he is spinning in his grave.” Mayr once told Sulloway that he liked to take strong stances, precisely so that other researchers would be motivated to try to prove him wrong. “If they eventually succeeded in doing so, Mayr felt that science was all the better for it.”? Open in a separate windowAlex Papadopulos and Ian Hutton doing fieldwork on a very precarious ridge on top of Mt. Gower. Photo: William Baker, Royal Botanical Gardens, Kew, Richmond, UK.     

Garland Allen,Thomas Hunt Morgan,and Development

Garland E. Allen’s 1978 biography of the Nobel Prize winning biologist Thomas Hunt Morgan provides an excellent study of the man and his science. Allen presents Morgan as an opportunistic scientist who follows where his observations take him, leading him to his foundational work in Drosophila genetics. The book was rightfully hailed as an important achievement and it introduced generations of readers to Morgan. Yet, in hindsight, Allen’s book largely misses an equally important part of Morgan’s work – his study of development and regeneration. It is worth returning to this part of Morgan, exploring what Morgan contributed and also why he has been seen by contemporaries and historians such as Allen as having set aside some of the most important developmental problems. A closer look shows how Morgan’s view of cells and development that was different from that of his most noted contemporaries led to interpretation of his important contributions in favor of genetics. This essay is part of a special issue, revisiting Garland Allen's views on the history of life sciences in the twentieth century.     

Georges Teissier (1900–1972) and the Modern Synthesis in France

This Perspectives is devoted to the ideas of the French zoologist Georges Teissier about the mechanisms of evolution and the relations between micro- and macroevolution. Working in an almost universally neo-Lamarckian context in France, Teissier was one of the very few Darwinians there at the time of the evolutionary synthesis. The general atmosphere of French zoology during the 1920s and the 1930s will first be recalled, to understand the specific conditions in which Teissier became a zoologist. After a brief overview of his joint work with Philippe L’Héritier on the experimental genetics of Drosophila, this article describes the ways Teissier, during the 1950s, conceptualized the mechanisms that could allow for macroevolutionary transitions.IT is usually acknowledged that France did not significantly participate in the elaboration of 20th century evolutionary theory, often designated The Modern Synthesis. In their classical book on the history of the synthesis, Ernst Mayr and William B. Provine devoted a whole—nonetheless small—chapter to this specific issue (Mayr and Provine 1998, pp. 309–328). Mayr clearly stated that “France is the only major scientific nation that did not contribute significantly to the evolutionary synthesis” (Mayr 1998, p. 309). In the absence of a French architect of the synthesis, Mayr and Provine asked Ernest Boesiger, a Swiss population geneticist and a former student of Georges Teissier, to tell the story of what had happened in French biology at the time of the evolutionary synthesis. Boesiger, who died in 1975, wrote a paper in 1974 that provided the firm basis of the chapter. In very strong terms, he depicted French biology as “a kind of living fossil in the rejection of modern evolutionary theories” (Boesiger 1998, p. 309). He insisted on the fact that, even in 1974, most French biologists and philosophers were still reluctant to accept Darwinism. As regards the period of the 1930s, Boesiger was able to think of only two exceptions: Georges Teissier and Philippe L’Héritier. He then referred to their joint research in population genetics, which was based on the new technique of the population cages with the species Drosophila melanogaster, and listed their contributions to this new discipline.If Teissier and L’Héritier’s works on Drosophila are nowadays more widely recognized than in 1974, due in particular to the efforts of Jean Gayon and Michel Veuille (Gayon and Veuille 2001), this recognition could have as an unintended consequence the reduction of both Teissier and L’Héritier to being simply the inventors of a useful technique, namely the population cages (see especially how Mayr presented their work in his other classical book, Mayr 1982, p. 574), or as the founders of a French school of population geneticists (Gayon and Veuille 2001). The aim of this article is to reevaluate the way Georges Teissier (1900–1972) conceived Darwinian natural selection not only as an important mechanism for evolution at the population level but more fundamentally as a general key for the unification of biology, exactly as Julian Huxley or Ernst Mayr did during the same period (1930–1970). However, starting in the early 1950s, Teissier went on to conceive a very specific understanding of the evolutionary synthesis.In this article, I will first describe the general atmosphere of evolutionary issues in French biology at the time when Teissier started working as a zoologist, to understand against what he developed his joint research program with L’Héritier and afterward his general conceptions about evolution. During the 1930s and the 1940s, only a very few scientists in France could be seen as Darwinians. In addition to Teissier and L’Héritier, one may also consider Marcel Prenant, Boris Ephrussi, and the mathematician Gustave Malécot. Building on Jean Gayon and Michel Veuille’s work, I will then give a quick overview of L’Héritier and Teissier’s most important achievements in the field of population genetics. In the third part, I will discuss the discovery made by Teissier and L’Héritier of a case of cytoplasmic inheritance in Drosophila. This unexpected finding led them into the field of non-Mendelian heredity. I will then develop in detail the way Teissier finally went on to conceive the relation between microevolution and macroevolution, in light of the general context of French biology and of the development of the field of cytoplasmic inheritance.     

The role of theories in biological systematics

The role of scientific theories in classifying plants and animals is traced from Hennig's phylogenetics and the evolutionary taxonomy of Simpson and Mayr, through numerical phenetics, to present-day cladistics. Hennig limited biological classification to sister groups so that this one relation can be expressed unambiguously in classifications. Simpson and Mayr were willing to sacrifice precision in representation in order to include additional features of evolution in the construction of classifications. In order to make classifications more objective, precise and quantitative, numerical pheneticists limited themselves to representing degrees of phenetic similarity. Finally, present-day cladists can be separated into phylogenetic cladists, who retain much of Hennig's theory of classification, and pattern cladists, who have stripped Hennig's system down to its bare essentials.     

Of lice and men: The return of the ’comparative parasitology‘ debate

The question of whether or not parasite phylogeny provides information about host relationships (‘comparative parasitology’) reached a peak in 1957 in a vigorous debate between Gunther Timmermann and Ernst Mayr. Timmermann argued that parasites were associated with their hosts by descent and that this produced congruent host and parasite phylogenies. In contrast, Mayr argued that parasites were often associated by colonization and that this led to incongruence between host and parasite phylogenies. To test these differing views. Adrian Paterson, Russell Gray and Graham Wallis derived a procellaniform phylogeny. This tree is here compared with Timmermann's tree based on the relationships of feather lice. Timmermann's tree is more similar to the seabird phylogeny than would be expected by chance. Thus, support is found for the ‘comparative parasitology’ approach.     

Explanations in evolutionary theory1

The theory of biological evolution is defined in many ways, leading to considerable confusion in its application and testing against objective empirical observations. Evolutionary change is usually defined as genetic which would exclude both cultural and template evolution; hence the qualifying adjective genetic should not be included in the definition of biological evolution. Darwin's theory, always described by him in the singular, is actually a bundle of five independent theories about evolution as advocated by Mayr. Furthermore only one of these theories, that of common descent, is historical, and the other four – evolution as such, gradualism, processes of phyletic evolution and of speciation, and causes of evolution – are nomological. Hence not all evolutionary theory is historical. Biological comparisons can be divided into horizontal and vertical ones and valid conclusions from one type of comparisons cannot be automatically extrapolated to the other. All phyletic evolutionary change, no matter how extensive it may be, never crosses species taxa boundaries; hence it is not possible to distinguish ‘trans‐specific evolution’ (= evolution beyond or above the level of the species) from evolution within the species level. Macroevolution does not differ from microevolution except in the scale of the overall change; no special causes or processes of macroevolution exist.     

Formalizing Darwinism and inclusive fitness theory

Inclusive fitness maximization is a basic building block for biological contributions to any theory of the evolution of society. There is a view in mathematical population genetics that nothing is caused to be maximized in the process of natural selection, but this is explained as arising from a misunderstanding about the meaning of fitness maximization. Current theoretical work on inclusive fitness is discussed, with emphasis on the author''s ‘formal Darwinism project’. Generally, favourable conclusions are drawn about the validity of assuming fitness maximization, but the need for continuing work is emphasized, along with the possibility that substantive exceptions may be uncovered. The formal Darwinism project aims more ambitiously to represent in a formal mathematical framework the central point of Darwin''s Origin of Species, that the mechanical processes of inheritance and reproduction can give rise to the appearance of design, and it is a fitting ambition in Darwin''s bicentenary year to capture his most profound discovery in the lingua franca of science.     

Regarding the confusion between the population concept and Mayr's "population thinking"

Ernst Mayr said that one of Darwin's greatest contributions was to show scholars the way to population thinking, and to help them discard a mindset of typological thinking. Population thinking rejects a focus on a central representative type, and emphasizes the variation among individuals. However, Mayr's choice of terms has led to confusion, particularly among biologists who study natural populations. Both population thinking and the concept of a biological population were inspired by Darwin, and from Darwin the chain for both concepts runs through Francis Galton who introduced the statistical usage of "population" that appears in Mayr's population thinking. It was Galton's "population" that was modified by geneticists and biometricians in the early 20th century to refer to an interbreeding and evolving community of organisms. Under this meaning, a population is a biological entity and so paradoxically population thinking, which emphasizes variation at the expense of dwelling on entities, is usually not about populations. Mayr did not address the potential for misunderstanding but for him the important part of the population concept was that the organisms within a population were variable, and so he probably thought there should not be confusion between population thinking and the concept of a population.     

A review of Gloger's rule,an ecogeographical rule of colour: definitions,interpretations and evidence

Gloger's rule is an ecogeographical rule that links animal colouration with climatic variation. This rule is named after C.W.L. Gloger who was one of the first to summarise the associations between climatic variation and animal colouration, noting in particular that birds and mammals seemed more pigmented in tropical regions. The term ‘Gloger's rule' was coined by B. Rensch in 1929 and included different patterns of variation from those described by Gloger. Rensch defined the rule in two ways: a simple version stating that endothermic animals are predicted to be darker in warmer and humid areas due to the increased deposition of melanin pigments; and a complex version that includes the differential effects of humidity and temperature on both main types of melanin pigments – eu‐ and phaeo‐melanin. The blackish eu‐melanins are predicted to increase with humidity, and decrease only at extreme low temperatures, while the brown‐yellowish phaeomelanins prevail in dry and warm regions and decrease rapidly with lower temperatures. A survey of the literature indicates that there is considerable variation/confusion in the way Gloger's rule is understood (based on 271 studies that define the rule). Whereas the complex version is hardly mentioned, only a quarter of the definitions are consistent with the simple version of Gloger's rule (darker where warm and wet), and most definitions mention only the effects of humidity (darker where wet). A smaller subset of studies define the rule based on other correlated climatic and environmental variables such as vegetation, latitude, altitude, solar radiation, etc., and a few even contradict the original definition (darker where cold). Based on the literature survey, I synthesised the qualitative (N = 124 studies) and quantitative (meta‐analytically, N = 38 studies, 241 effects) evidence testing the simple version of Gloger's rule (I found no tests of the complex version). Both lines of evidence supported the predicted effects of humidity (and closely linked variables) on colour variation, but not the effects of temperature. Moreover, humidity effects are not restricted to birds and mammals, as the data indicate that these effects also apply to insects. This suggests that the simple version of Gloger's rule as originally defined may not be valid, and possibly that the rule should be re‐formulated in terms of humidity effects only. I suggest, however, that more data are needed before such a reformulation, due to potential publication biases. In conclusion, I recommend that authors cite Rensch when referring to Gloger's rule and that they make clear which version they are referring to. Future research should concentrate on rigorously testing the validity and generality of both versions of Gloger's rule and establishing the mechanism(s) responsible for the patterns it describes. Since humidity seems to be the core climatic variable behind Gloger's rule, I suggest that the two most plausible mechanisms are camouflage and protection against parasites/pathogens, the latter possibly through pleiotropic effects on the immune system. Understanding the processes that lead to climatic effects on animal colouration may provide insights into past and future patterns of adaptation to climatic change.     

Population Genetics Theory: Another View

In this reply to Kempthorne's critique of current populationgenetics theory (this symposium), I describe the differencesin viewpoint and in choice of evidence which lead me to a morefavorable conclusion. Kempthorne's attacks on the general selectionmodel and on Fisher's Fundamental Theorem of Natural Selectionhave considerable justification; in both cases, however, qualificationis preferable to demolition. Furthermore, recent advances inselection theory really meet Kempthorne's well-founded objections.Finally, the most substantial point in Kempthorne's critiqueis not that current population genetics theory is inoperable,but that it is practically impossible to apply to life-historyproblems. Computers, properly programmed, may provide a valuableway around this impasse. In addition, the necessity and thedifficulty of manipulating one variable at a time have beenwith science for a long and exasperating time.     

A bird's-eye view on the modern genetics workflow and its potential applicability to the locust problem

Genetics is an immense science and the current developments in its methods and techniques as well as the fast emerging tools make it one of the most powerful biological sciences. Indeed, from taxonomy and ecology to physiology and molecular biology, every biological science makes use of genetics techniques and methods at one time or another. In fact, development in genetics is such that it is now possible to characterize and analyze the expression of the whole set of genes of virtually every living organism, even if it is a non-model one. Locusts are notorious for the damage they cause to the ecosystems and economies of the areas affected by their recurrent population outbreaks. To prevent and deal with these outbreaks, we now count on both biological as well as chemical agents that are proving to be successful in reducing the damage that otherwise locust population outbreaks might cause. However, a better, efficient and environmentally friendly solution is still a hoped-for target. In my opinion, the ideal future pesticide should be both environmentally friendly, risk free and species-specific. To reach the knowledge needed for the development of such species-specific anti-locust agent, deep and accurate knowledge of the locusts’ genetics and molecular biology is a must. Since genes and their expression levels lie at the bottom of every biological phenomenon, any species-specific solution to the locust problem requires a good knowledge of these organisms’ genes as well as the quantitative and spatio-temporal dynamics of their expression. To reach such knowledge, collaborative work is needed as well as a clear workflow that, given the fast development in the genetics tools, is not always clear to all research groups. For this reason, here I describe a genetics workflow that should allow taking advantage of the most recent genetics tools and techniques to answer question relating to locust biology. My hope is that the adoption of this and other work strategies by different research groups, especially when the work is a collaborative one, would provide precious information on the biology and the biological phenomena that these economically important organisms exhibit.     

Psychoontogeny and psychophylogeny: Bernhard Rensch’s (1900–1990) selectionist turn through the prism of panpsychistic identism

Toward the end of the 1930s, Bernhard Rensch (1900–1990) turned from Lamarckism and orthogenesis to selectionism and became one of the key figures in the making of the Synthetic Theory of Evolution (STE). He contributed to the Darwinization of biological systematics, the criticism of various anti-Darwinian movements in the German lands, but more importantly founded a macroevolutionary theory based on Darwinian gradualism. In the course of time, Rensch’s version of the STE developed into an all-embracing metaphysical conception based on a kind of Spinozism. Here we approach Rensch’s “selectionist turn” by outlining its context, and by analyzing his theoretical transformation. We try to reconstruct the immanent logic of Rensch’s evolution from a “Lamarckian Synthesis” to a “Darwinian Synthesis”. We will pay close attention to his pre-Darwinian works, because this period has not been treated in detail in English before. We demonstrate an astonishing continuity in topics, methodology, and empirical generalizations despite the shift in Rensch’s views on evolutionary mechanisms. We argue that the continuity in Rensch’s theoretical system can be explained, at last in part, by the guiding role of general methodological principles which underlie the entire system, explicitly or implicitly. Specifically, we argue that Rensch’s philosophy became an asylum for the concept of orthogenesis which Rensch banned from evolutionary theory. Unable to explain the directionality of evolution in terms of empirically based science, he “pre-programmed” the occurrence of human-level intelligence by a sophisticated philosophy combined with a supposedly naturalistic evolutionary biology.

Human Genetics and Politics as Mutually Beneficial Resources: The Case of the Kaiser Wilhelm Institute for Anthropology, Human Heredity and Eugenics During the Third Reich

This essay analyzes one of Germany’s former premier research institutions for biomedical research, the Kaiser Wilhelm Institute for Anthropology, Human Heredity and Eugenics (KWIA) as a test case for the way in which politics and human heredity served as resources for each other during the Third Reich. Examining the KWIA from this perspective brings us a step closer to answering the questions at the heart of most recent scholarship concerning the biomedical community under the swastika: (1) How do we explain why the vast majority of German human geneticists and eugenicists were willing to work for the National Socialist state and, at the very least, legitimized its exterminationist racial policy; and (2) what accounts for at least some of Germany’s most renowned medically trained professionals’ involvement in forms of morally compromised science that wholly transcend the bounds of normal scientific practice? Although a complete answer to this question must await an examination of other German biological research centers, the present study suggests that during the Nazi period the symbiotic relationship between human genetics and politics served to radicalize both. The dynamic between the science of human heredity and Nazi politics changed the research practice of some of the biomedical sciences housed at the KWIA. It also simultaneously made it easier for the Nazi state to carry out its barbaric racial program leading, finally, to the extermination of millions of so-called racial undesirables.