The scientific study of speciation — how species evolve to become new species — began around the time of Charles Darwin in the middle of the 19th century. Many naturalists at the time recognized the relationship between biogeography (the way species are distributed) and the evolution of species. The 20th century saw the growth of the field of speciation, with major contributors such as Ernst Mayr researching and documenting species' geographic patterns and relationships. The field grew in prominence with the modern evolutionary synthesis in the early part of that century. Since then, research on speciation has expanded immensely.
The language of speciation has grown more complex. Debate over classification schemes on the mechanisms of speciation and reproductive isolation continue. The 21st century has seen a resurgence in the study of speciation, with new techniques such as molecular phylogenetics and systematics. Speciation has largely been divided into discrete modes that correspond to rates of gene flow between two incipient populations. Current research has driven the development of alternative schemes and the discovery of new processes of speciation.
Early history
Charles Darwin introduced the idea that species could evolve and split into separate lineages, referring to it as specification in his 1859 book On the Origin of 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.
Darwin's views Controversy exists as to whether Charles Darwin recognized a true geographical-based model of speciation in his publication On the Origin of Species. In chapter 11, "Geographical Distribution", Darwin discusses geographic barriers to migration, stating for example that "barriers of any kind, or obstacles to free migration, are related in a close and important manner to the differences between the productions of various regions [of the world]". 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. Nevertheless, Darwin never fully accepted Wagner's concept of geographical speciation.
The evolutionary biologist James Mallet maintains that the mantra repeated concerning Darwin's Origin of Species book having never actually discussed speciation is specious. The claim began with Thomas Henry Huxley and George Romanes (contemporaries of Darwin's), who declared that Darwin failed to explain the origins of inviability and sterility in hybrids.Mallet, James (2013). Darwin and species. In Michael Ruse (eds) The Cambridge Encyclopedia of Darwin and Evolutionary Thought, Cambridge University Press, Pp. 109–115. 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. Another strong proponent of this view about Darwin came from Mayr. Mayr maintained that Darwin was unable to address the problem of speciation, as he did not define species using the biological species concept. 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. Furthermore, many of Darwin's ideas on speciation largely match the modern theories of both adaptive radiation and ecological speciation.
Darwin's dilemmas
In addressing the question of the origin of species, there are two key issues: (1) what are the evolutionary mechanisms of speciation, and (2) what accounts for the separateness and individuality of species in the biota? Since Charles Darwin's time, efforts to understand the nature of species have primarily focused on the first aspect, and it is now widely agreed that a critical factor behind the origin of new species is reproductive isolation.Mayr, Ernst (1982). The Growth of Biological Thought: Diversity, Evolution, and Inheritance. Cambridge, Massachusetts: Belknap Press of Harvard University Press. ISBN 978-0-674-36445-5. LCCN 81013204. OCLC 7875904. p. 273 Darwin also considered the second aspect of the origin of species.
Darwin was perplexed by the clustering of organisms into species.Darwin, Charles (1859). On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life (1st ed.). London: John Murray. LCCN 06017473. OCLC 741260650. The book is available from The Complete Work of Charles Darwin Online. Retrieved 2015-09-12. 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
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. Another naturalist in 1856, Wollaston, studied island beetles in comparison to mainland species. He saw isolation as key to their differentiation. However, he did not recognize that the pattern was due to speciation. One naturalist, Leopold von Buch (1825) did recognize the geographic patterns and explicitly stated that geographic isolation may lead to species separating into new species. Mayr suggests that Von Buch was likely the first naturalist to truly suggest geographic speciation. 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.
In 1868, Moritz Wagner was the first to propose the concept of geographic speciation in which he used the term Separationstheorie. 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. Karl Jordan is thought to have recognized the unification of mutation and isolation in the origin of new species — in stark contrast to the prevailing views at the time. David Starr Jordan reiterated Wagner's proposal in 1905, providing a wealth of evidence from nature to support the theory, and asserting that geographic isolation is obvious but had been unfortunately ignored by most geneticists and experimental evolutionary biologists at the time. Joel Asaph Allen suggested the observed pattern of geographic separation of closely related species be called "Jordan's Law" (or Wagner's Law). Despite the contentions, most taxonomists did accept the geographic model of speciation.
Many of the early terms used to describe speciation were outlined by Ernst Mayr. 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. He described the three modes: geographic, semi-geographic, and non-geographic; which today, are referred to as allopatric, parapatric, and sympatric respectively. Mayr's 1942 publication, influenced heavily by the ideas of Karl Jordan and Poulton, was regarded as the authoritative review of speciation for over 20 years—and is still valuable today.
A major focus of Mayr's works was on the importance of geography in facilitating speciation; with islands often acting as a central theme to many of the speciation concepts put forth. One of which was the concept of peripatric speciation, a variant of allopatric speciationMayr, E. 1954. Change of genetic environment and evolution. In: Evolution as a Process (J. Huxley, A. C. Hardy & E. B. Ford, eds), pp. 157–180. Unwin Brothers, London.Mayr, E. 1982. Processes of speciation in animals. In: Mechanisms of Speciation (A. R. I. Liss, ed.), pp. 1–19. Alan R. Liss Inc., New York. (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. This model was later expanded and modified to incorporate sexual selection by Kenneth Y. Kaneshiro in 1976 and 1980.
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. Ronald Fisher proposed a model of speciation in his 1930 publication The Genetical Theory of Natural Selection, where he described disruptive selection acting on sympatric or parapatric populations — with reproductive isolation completed by reinforcement. Other geneticists such as J. B. S. Haldane did not even recognize that species were real, while Sewall Wright ignored the topic, despite accepting allopatric speciation.