Editing Speciation (section)

== Historical background ==
{{main|History of speciation}}

In addressing the origin of species, there are two key issues:

# the evolutionary mechanisms of speciation
# how the separateness and individuality of species is maintained

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 the critical factor behind the origin of new species is reproductive isolation.<ref>{{harvnb|Mayr|1982|p=273}}</ref>

=== Darwin's dilemma: why do species exist? ===

In ''[[On the Origin of Species]]'' (1859), Darwin interpreted biological evolution in terms of natural selection, but was perplexed by the clustering of organisms into species.<ref name="OofS">{{harvnb|Darwin|1859}}</ref> Chapter 6 of Darwin's book is entitled "Difficulties of the Theory". In discussing these "difficulties" he noted

{{blockquote|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?|''[[On the Origin of Species]]'' (1859), chapter 6<ref name="OofS"/>}}

This dilemma can be described as the absence or rarity of transitional varieties in habitat space.<ref>{{cite book |last=Sepkoski |first=David |title=Rereading the Fossil Record: The Growth of Paleobiology as an Evolutionary Discipline |chapter-url=https://books.google.com/books?id=jNfJyXKSDRcC&pg=PA9 |date=2012 |publisher=University of Chicago Press |isbn=978-0-226-74858-0 |pages=9–50 |chapter=1. Darwin's Dilemma: Paleontology, the Fossil Record, and Evolutionary Theory |quote=One of his greatest anxieties was that the "incompleteness" of the fossil record would be used to criticize his theory: that the apparent "gaps" in fossil succession could be cited as negative evidence, at the very least, for his proposal that all organisms have descended by minute and gradual modifications from a common ancestor.}}</ref>

Another dilemma,<ref>{{cite journal |last1=Stower |first1=Hannah |title=Resolving Darwin's Dilemma |journal=Nature Reviews Genetics |date=2013 |volume=14 |issue=747 |pages=747 |doi=10.1038/nrg3614 |s2cid=45302603 |quote=The near-simultaneous appearance of most modern animal body plans in the Cambrian explosion suggests a brief interval of rapid phenotypic and genetic evolution, which Darwin believed were too fast to be explained by natural selection. |doi-access=free }}</ref> 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.<ref name="OofS"/>

=== Effect of sexual reproduction on species formation ===
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It has been argued that the resolution of Darwin's first dilemma lies in the fact that [[out-crossing]] [[sexual reproduction]] has an intrinsic cost of rarity.<ref name="Bernstein85"/><ref name="Hopf85">{{cite journal |last1=Hopf |first1=Frederic A. |last2=Hopf |first2=F. W. |date=February 1985 |title=The role of the Allee effect in species packing |journal=[[Theoretical Population Biology]] |volume=27 |issue=1 |pages=27–50 |doi=10.1016/0040-5809(85)90014-0|bibcode=1985TPBio..27...27H }}</ref><ref name="Bernsteinbook">{{harvnb|Bernstein|Bernstein|1991}}</ref><ref name="Michod95">{{harvnb|Michod|1995}}</ref><ref name="Michod99">{{harvnb|Michod|1999}}</ref> The cost of rarity arises as follows. If, on a resource gradient, a large number of separate species evolve, each exquisitely adapted to a very narrow band on that gradient, each species will, of necessity, consist of very few members. Finding a mate under these circumstances may present difficulties when many of the individuals in the neighborhood belong to other species. Under these circumstances, if any species' population size happens, by chance, to increase (at the expense of one or other of its neighboring species, if the environment is saturated), this will immediately make it easier for its members to find sexual partners. The members of the neighboring species, whose population sizes have decreased, experience greater difficulty in finding mates, and therefore form pairs less frequently than the larger species. This has a snowball effect, with large species growing at the expense of the smaller, rarer species, eventually driving them to [[extinction]]. Eventually, only a few species remain, each distinctly different from the other.<ref name="Bernstein85"/><ref name="Hopf85"/><ref name="Michod95"/> Rarity not only imposes the risk of failure to find a mate, but it may also incur indirect costs, such as the resources expended or risks taken to seek out a partner at low population densities.{{Citation needed|date=November 2024}}

[[File:Flickr - Rainbirder - African pygmy-kingfisher (Ceyx pictus).jpg|thumb|[[African pygmy kingfisher]], showing coloration shared by all adults of that species to a high degree of fidelity.<ref>{{harvnb|Hockey|Dean|Ryan|2005|pp=176, 193}}</ref>]]

Rarity brings with it other costs. Rare and unusual features are very seldom advantageous. In most instances, they indicate a ([[Silent mutation|non-silent]]) [[mutation]], which is almost certain to be deleterious. It therefore behooves sexual creatures to avoid mates sporting rare or unusual features ([[koinophilia]]).<ref name="Koeslag, 1990"/><ref name="Koeslag, 1995"/> Sexual populations therefore rapidly shed rare or peripheral phenotypic features, thus canalizing the entire external appearance, as illustrated in the accompanying image of the [[African pygmy kingfisher]], ''Ispidina picta''. This uniformity of all the adult members of a sexual species has stimulated the proliferation of [[field guide]]s on birds, mammals, reptiles, insects, and many other [[taxon|taxa]], in which a species can be described with a single illustration (or two, in the case of [[sexual dimorphism]]). Once a population has become as homogeneous in appearance as is typical of most species (and is illustrated in the photograph of the African pygmy kingfisher), its members will avoid mating with members of other populations that look different from themselves.<ref name="Unnikrishnan"/> Thus, the avoidance of mates displaying rare and unusual phenotypic features inevitably leads to reproductive isolation, one of the hallmarks of speciation.<ref name="tutorial online"/><ref name=Maynard /><ref>{{harvnb|Mayr|1988}}</ref><ref>{{harvnb|Williams|1992|p=118}}</ref>

In the contrasting case of organisms that [[asexual reproduction|reproduce asexually]], there is no cost of rarity; consequently, there are only benefits to fine-scale adaptation. Thus, asexual organisms very frequently show the continuous variation in form (often in many different directions) that Darwin expected evolution to produce, making their classification into "species" (more correctly, [[morphospecies]]) very difficult.<ref name="Bernstein85"/><ref name="Koeslag, 1990"/><ref name="Koeslag, 1995"/><ref>{{cite journal |last1=Maynard Smith |first1=John |author-link=John Maynard Smith |date=December 1983 |title=The Genetics of Stasis and Punctuation |journal=Annual Review of Genetics |volume=17 |pages=11–25 |doi=10.1146/annurev.ge.17.120183.000303 |pmid=6364957|s2cid=3901837 |url=http://pdfs.semanticscholar.org/e479/e50bae660f043e7f6e5c1c91365776c17f72.pdf |archive-url=https://web.archive.org/web/20190305021759/http://pdfs.semanticscholar.org/e479/e50bae660f043e7f6e5c1c91365776c17f72.pdf |url-status=dead |archive-date=2019-03-05 }}</ref><ref>{{harvnb|Clapham|Tutin|Warburg|1952}}</ref><ref>{{harvnb|Grant|1971}}</ref>