Evolution__3rd_Edition

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Summary
1 The evolution of a new species happens when one
population of interbreeding organisms splits into two
separately breeding populations.
2 Two theories of how reproductive isolation evolves
have been suggested: it evolves as a by-product of
divergence between two populations, or it evolves by
reinforcement.
3 The “by-product” theory is well supported by
experimental and biogeographic evidence.
4 Experiments have demonstrated that reproductive
isolation tends to arise incidentally between two populations
that are kept separate from each other and
allowed to evolve in different environments for a number
of generations.
5 Members of a species can be sampled from different
parts of the species’ biogeographic range, brought into
the laboratory, and crossed. Reproductive isolation is
often found between the individuals from distant parts
of the species’ range.
6 Prozygotic isolation evolves when it is genetically
correlated, by pleiotropy or hitch-hiking, with the
characters undergoing divergent evolution between
populations.
7 The Dobzhansky–Muller theory explains the genetics
of postzygotic isolation. When two populations
diverge, they may evolve new genes that are incompatible
when put together. Postzygotic isolation between
two species is usually caused by epistatic interactions
among multiple genes, and not by a single genetic
locus.
Further reading
CHAPTER 14 / Speciation 419
8 Haldane’s rule is a generalization about postzygotic
isolation. It states: “When in the F 1 offspring of the two
different animal races one sex is absent, rare, or sterile,
that sex is the heterozygous one.” Postzygotic isolation
evolves first in the heterogametic gender of the hybrid
offspring. The Dobzhansky–Muller theory can partly
explain Haldane’s rule.
9 Reinforcement is the enhancement of reproductive
isolation by natural selection: forms are selected to
mate with their own, and not with the other, type.
10 The theory and evidence for reinforcement are
both problematic. Reinforcement may contribute to
the evolution of reproductive isolation, but a compelling
case for it has not yet been made.
11 Many new plant species have originated following
hybridization of two existing species.
12 Speciation may occur in parapatric (that is,
geographically contiguous) populations. Parapatric
speciation begins with a stepped cline, and prezygotic
isolation then evolves between the forms on either side
of the step.
13 Speciation may occur in sympatry. The process
can begin with the establishment of a polymorphism,
and reproductive isolation then evolves between the
different forms. The shape of phylogenies, for instance
in lake-dwelling fish, provides evidence that sympatric
speciation has occurred.
14 Two current trends in research are: (i) to look
at the influence of sexual selection in speciation; and
(ii) to identify particular genes that cause reproductive
isolation between species.
The July 2001 issue of Trends in Ecology and Evolution (vol. 16, pp. 325–413) is a special
issue on speciation, and introduces most of the modern research trends. The opening
paper, by Turelli et al. (2001a), is an overview of the whole subject. Coyne & Orr (2003)
authoritatively review speciation at book length. Howard & Berlocher (1998) is a multiauthor
research-level book about speciation. Schiltuizen (2001) is a single-author,