Evolution__3rd_Edition

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Speciation often occurs as a byproduct
of evolutionary divergence
Reinforcement means ...
rule in at least some cases. The Dobzhansky–Muller theory may not be a universal,
complete theory of Haldane’s rule, but it is an important influence. Thus, the
Dobzhansky–Muller theory not only explains the multilocus, epistatic genetic control
of postzygotic isolation, it also helps to explain a general sex difference in the time
course of speciation.
14.5 An interim conclusion: two solid generalizations
about speciation
We have abundant evidence, from experiments and biogeographic observations, that
speciation evolves as a by-product when two geographically separate populations
evolve apart. For prezygotic isolation, we have some hypotheses, but few research
results, on the genetic changes that underlie it. For postzygotic isolation, we have extensive
theory and evidence about the genetic changes that underlie it. We can conclude
the chapter so far by saying that there are two solid results in the study of speciation:
reproductive isolation evolves as a by-product of allopatric divergence, and postzygotic
isolation is caused by epistatic interactions among multiple genetic loci.
These generalizations are worth keeping in mind as we move on. We are now going
to turn to some less solid, more controversial areas of research on speciation. If we concentrated
on these controversial areas alone, it might appear that little is known about
speciation and that it is a permanently confused area of evolutionary biology. John
Herschel, a senior scientific figure when Charles Darwin was starting in scientific
research, described the question of how a new life form could appear on Earth as “the
mystery of mysteries.” It is a haunting phrase, and Darwin remembered it. The theory
we have looked at so far does not provide a complete account of how all new species
evolve. But it does help to demystify the origin of species, and provides a clear scientific
searchlight for future research.
14.6 Reinforcement
CHAPTER 14 / Speciation 399
14.6.1 Reproductive isolation may be reinforced by natural selection
So far we have been looking at one of two main theories about how reproductive
isolation evolves: that it evolves as a by-product when natural selection favors different
genetic changes in separately evolving populations. The second theory suggests
that natural selection can act directly to increase the amount of isolation between
two populations. The process is called reinforcement and the general precondition for
reinforcement to operate is as follows. We assume there are two genetic types within a
population, and hybrids between them have lower fitness than the offspring of matings
within each type. The genetic difference between the two types could be in a multilocus
genotype, or in alleles at one locus, or in chromosomal form. The symbols A and A′ are
general, and stand for any of these kinds of genetic differences. The condition for reinforcement
is then: