History of speciation

, a tree of lineages splitting to form new species It was not until 1906 that the modern term speciation'' was coined by the biologist Orator F. Cook. Darwin, in his 1859 publication, focused primarily on the changes that can occur within a species, and less on how species may divide into two. It is almost universally accepted that Darwin's book did not directly address its title. Darwin instead saw speciation as occurring by species entering new ecological niches. F. J. Sulloway contends that Darwin's position on speciation was "misleading" at the least and may have later misinformed Wagner and David Starr Jordan into believing that Darwin viewed sympatric speciation as the most important mode of speciation. Similar claims were promulgated by the mutationist school of thought during the late 20th century, and even after the modern evolutionary synthesis by Richard Goldschmidt. However, Mayr's view has not been entirely accepted, as Darwin's transmutation notebooks contained writings concerning the role of isolation in the splitting of species. Darwin also considered the second aspect of the origin of species. Darwin was perplexed by the clustering of organisms into species. Chapter 6 of Darwin's book is entitled "Difficulties of the Theory." In discussing these "difficulties" he noted "Firstly, why, if species have descended from other species by insensibly fine gradations, do we not everywhere see innumerable transitional forms? Why is not all nature in confusion instead of the species being, as we see them, well defined?" This dilemma can be referred to as the absence or rarity of transitional varieties in habitat space. Another dilemma, related to the first one, is the absence or rarity of transitional varieties in time. Darwin pointed out that by the theory of natural selection "innumerable transitional forms must have existed," and wondered "why do we not find them embedded in countless numbers in the crust of the earth." That clearly defined species actually do exist in nature in both space and time implies that some fundamental feature of natural selection operates to generate and maintain species. A possible explanation for how these dilemmas can be resolved is discussed in the article Speciation in the section "Effect of sexual reproduction on species formation." == Biogeographic influence ==

(1813–1887) Recognition of geographic factors involved in species populations was present even before Darwin, with many naturalists aware of the role of isolation in species relationships. In 1833, C. L. Gloger published The Variation of Birds Under the Influence of Climate in which he described geographic variations, but did not recognize that geographic isolation was an indicator of past speciation events. Other naturalists, such as Henry Walter Bates (1863), recognized and accepted the patterns as evidence of speciation, but in Bate's case, did not propose a coherent model. Edward Bagnall Poulton, the evolutionary biologist and a strong proponent of the importance of natural selection, highlighted the role of geographic isolation in promoting speciation, in the process coining the term "sympatric speciation" in 1904. Wagner and other naturalists who studied the geographic distributions of animals, such as Karl Jordan and David Starr Jordan, noticed that closely related species were often geographically isolated from one another (allopatrically distributed) which lead to the advocation of the importance of geographic isolation in the origin of species. He was the first to encapsulate the then contemporary literature in his 1942 publication Systematics and the Origin of Species, from the Viewpoint of a Zoologist and in his subsequent 1963 publication Animal Species and Evolution. Like Jordan's works, they relied on direct observations of nature, documenting the occurrence of geographic speciation. One of which was the concept of peripatric speciation, a variant of allopatric speciation (he has since distinguished the two modes by referring to them as peripatric and dichopatric). This concept arose by an interpretation of Wagner's Separationstheorie as a form of founder effect speciation that focused on small geographically isolated species. == Modern evolutionary synthesis ==

Many geneticists at the time did little to bridge the gap between the genetics of natural selection and the origin of reproductive barriers between species. Dobzhansky, a geneticist, published Genetics and the Origin of Species in 1937, in which he formulated the genetic framework for how speciation could occur. Both of their works gave rise, not without controversy, to the modern understanding of speciation; stimulating a wealth of research on the topic. In 1947, "a consensus had been achieved among geneticists, paleontologists and systematists and that evolutionary biology as an independent biological discipline had been established" during a Princeton University conference. This 20th century synthesis incorporated speciation. Since then, the ideas have been consistently and repeatedly confirmed. == Contemporary work ==

After the synthesis, speciation research continued largely within natural history and biogeography — with much less emphasis on genetics. Research on the influence of natural selection on speciation, including the process of reinforcement, has grown. Researchers have long debated the roles of sexual selection, natural selection, and genetic drift in speciation. Later, other biologists such as Hampton L. Carson, Alan Templeton, Sergey Gavrilets, and Alan Hastings developed related models of speciation by genetic drift, noting that islands were inhabited mostly by endemic species. Selection's role in speciation is widely supported, whereas founder effect speciation is not, Classification debate , parapatric, and sympatric) can exist within this continuum, as well as other non-geographic modes. Throughout the history of research concerning speciation, classification and delineation of modes and processes have been debated. Julian Huxley divided speciation into three separate modes: geographical speciation, genetic speciation, and ecological speciation. More succinctly, the modern classification of speciation is often described as occurring on a gene flow continuum (i.e., allopatry at m=0 and sympatry at m=0.5) This gene flow concept views speciation as based on the exchange of genes between populations instead of seeing a purely geographic setting as necessarily relevant. Despite this, concepts of biogeographic modes can be translated into models of gene flow (such as that in the image at left); however, this translation has led to some confusion of language in the scientific literature. while others argue for its merits. Proponents of non-geographic schemes often justify non-geographic classifications, not by rejection of the importance of reproductive isolation (or even the processes themselves), but instead by the fact that it simplifies the complexity of speciation. One major critique of the geographic framework is that it arbitrarily separates a biological continuum into discontinuous groups.—with allopatric and sympatric existing in the extremes. Kirkpatrick and Ravigné categorize speciation in terms of its genetic basis or by the forces driving reproductive isolation. Fitzpatrick and colleagues believe that the biogeographic scheme "is a distraction that could be positively misleading if the real goal is to understand the influence of natural selection on divergence." They maintain that, to fully understand speciation, "the spatial, ecological, and genetic factors" involved in divergence must be explored. Sara Via recognizes the importance of geography in speciation but suggests that classification under this scheme be abandoned. == History of modes and mechanisms ==

Sympatric speciation Sympatric speciation, from its beginnings with Darwin (who did not coin the term), has been a contentious issue. Today, sympatric speciation is supported by evidence from laboratory experiments and observations from nature. Øjvind Winge was the first to confirm allopolyploidy in 1917, and a later experiment conducted by Clausen and Goodspeed in 1925 confirmed the findings. Historically, zoologists considered hybridization to be a rare phenomenon, while botanists found it to be commonplace in plant species. Reinforcement in 1862 The concept of speciation by reinforcement has a complex history, with its popularity among scholars changing significantly over time. Wallace's hypothesis differed from the modern conception in that it focused on post-zygotic isolation, strengthened by group selection. Dobzhansky was the first to provide a thorough, modern description of the process in 1937, In 1930, Ronald Fisher laid out the first genetic description of the process of reinforcement in The Genetical Theory of Natural Selection, and in 1965 and 1970 the first computer simulations were run to test for its plausibility. and quantitative genetic studies were conducted showing that completely unfit hybrids lead to an increase in pre-zygotic isolation. A number of theoretical objections arose at the time. Since the early 1990s, reinforcement has seen a revival in popularity, with perceptions by evolutionary biologists accepting its plausibility—due primarily from a sudden increase in data, empirical evidence from laboratory studies and nature, complex computer simulations, and theoretical work. Roger Butlin demarcated incomplete post-zygotic isolation from complete isolation, referring to incomplete isolation as reinforcement and completely isolated populations as experiencing reproductive character displacement. Daniel J. Howard considered reproductive character displacement to represent either assortive mating or the divergence of traits for mate recognition (specifically between sympatric populations). Maria R. Servedio and Mohamed Noor consider any detected increase in pre-zygotic isolation as reinforcement, as long as it is a response to selection against mating between two different species. Coyne and Orr contend that, "true reinforcement is restricted to cases in which isolation is enhanced between taxa that can still exchange genes". == See also ==