Evolution__3rd_Edition

400 PART 4 / Evolution and Diversity
. . . natural selection to increase
reproductive isolation
The theory of reinforcement has
had supporters
Type 1 Hybrid Type 2
Genetic type AA AA′ A′A′
Fitness High Low High
The two types are partially, but not completely, postzygotically isolated from each
other. More formally, we could say the fitnesses of the two pure forms AA and A′A′ are
1, and the fitness of the hybrids is more than 0 but less than 1.
One way in which this set up could arise would be if two populations initially
occupied separate (allopatric) ranges and diverged, but their ranges then changed and
the two populations met up again. One population has one genetic type (A), the other
A′. These two populations might have evolved some postzygotic isolation by the
Dobzhansky–Muller process, but the isolation might not be complete. (We shall meet
some other ways in which the same basic set up can arise, in Sections 14.9 and 14.10,
when we look at parapatric and sympatric speciation.) What will be the next evolutionary
step?
Reinforcement is one possibility. Natural selection may increase the amount of
prezygotic isolation. If an AA individual mates with another AA individual, they produce
offspring with high fitness. If an AA individual mates with an A′A′ individual they
produce hybrid AA′ offspring who have low fitness. Natural selection favours individuals
who mate with others who are genetically like themselves a that is, assortative
mating. 1 The theory of reinforcement assumes that some postzygotic isolation exists,
and argues that prezygotic isolation will increase. Natural selection cannot, except in
strange circumstances, favor increases in postzygotic isolation. Natural selection favors
increased prezygotic isolation, because the individuals save themselves from producing
inferior hybrid offspring. But an increase in postzygotic isolation means that the fitness
of hybrids goes down. The hybrids become more likely to die. Natural selection cannot
favor genes that make their bearers more likely to die (except in special conditions
described in the theory of kin selection, Section 11.2.4, p. 298). Indeed the main effect
of natural selection on postzygotic isolation will be to decrease it, by favoring fitter
hybrids. Thus reinforcement is really only a theory of prezygotic isolation, not postzygotic
isolation.
How important is reinforcement in speciation? The initial condition for it looks simple,
and probably arises quite often. All we need is the evolution of two genetic forms
between which crosses are disadvantageous. The argument, that natural selection then
favors prezygotic isolation (or assortative mating), looks simple and inevitable. We
might therefore expect reinforcement to occur quite often during speciation, as a supplement
to the “by-product” theory we have looked at. Many evolutionary theorists,
from Wallace to Dobzhansky, have supported the theory of reinforcement. However,
when we look in more detail at the theory, and the evidence that has been put forward
for it, we find that the case is unconvincing. The arguments are well worth knowing,
1 Assortative mating means like mates with like. It can be contrasted with disassortative mating, in which
individuals preferentially mate with the other type from themselves, and with random mating. The theory of
reinforcement is usually concerned with evolution from random mating toward stronger and stronger assortative
mating. When assortative mating is absolute a an individual will never mate with someone of the other
genetic type a prezygotic isolation is complete and speciation has occurred.
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