Editing Speciation (section)

=== 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>