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

Abstract

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 J. Mallet 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.

Speciation slowing down in widespread and long-living tree taxa: insights from the tropical timber tree genus Milicia (Moraceae)

“…Most definitions of species are derived from two concepts: (1) the biological species concept (BSC) that defines species as reproductively isolated groups of living organisms (Mayr, 1942), and (2) the phylogenetic species concept (PSC) that groups in a given species individuals forming a monophyletic group (or a clade), according to the monophyly version (De Queiroz, 2007). Because testing for reproductive isolation is difficult and PSC requires many (molecular) traits, historical taxonomy has mostly been based on the morphological similarity of individuals, and ideally the occurrence of multiple diagnostic qualitative traits to define species (Hey, 2006;Mallet, 2010). Nowadays, when multilocus genetic markers are available, large-scale population sampling and Bayesian clustering analyses allow identifying groups of individuals with a shared recent common ancestry, groups that have been reproductively isolated (historically at least) and can be thought as candidate units corresponding to the BSC.…”

Ecological Speciation and the Intertidal Snail Littorina saxatilis

“…Or rather, it has come back in because the role of ecological processes in diversification dates back to Darwin, although some biologists of the 20th century gave a prominent role in speciation to geographical isolation (allopatry) [2,3]. This shift of view has been reviewed by Mallet [4]. It is our intention here to give an account of work on Littorina saxatilis (Olivi) over the last three decades, highlighting its contribution and promise to the study of speciation.…”

Alfred Russel Wallace and the Darwinian Species Concept: His Paper on the Swallowtail Butterflies (Papilionidae) of 1865

“…Wallace's paper also clearly demonstrates that Darwinists in the decade following the publication of 'the Origin' had a clear understanding of the nature of species (Mallet 2008b); it is difficult to sustain the frequent modern criticism (e.g., Mayr 1982;Coyne & Orr 2004) that Darwin and his followers misunderstood species, and that a better understanding had to wait until the mid 20 th Century. Quite to the contrary: today's evolutionary view of species, including Mayr's biological species concept, stem in part from this very paper of Wallace's, via Poulton, Rothschild, and Jordan, who all worked on the Papilionidae and had read Wallace's work carefully (Mallet 2004 is spiced with many extraordinary phenomena that bear further examination with today's modern tools of science, such as genetics and statistics.…”

“…Mayr (1942Mayr ( , 1963Mayr ( , 1982 and followers to accuse Darwinists of having failed to produce an adequate species definition. In fact, Darwin, as well as Darwinists such as Wallace and Bates, understood the nature of species very well, even in modern terms (Mallet 2004(Mallet , 2009b Poulton (1904). All three were informed by extensive knowledge of variation in Papilio butterflies, and they were in their turn among the most important influences on the species concept of Mayr and others (see Mallet 2004).…”