Evolution__3rd_Edition

..
The theory of reinforcement has
problems with ...
. . . the loss of rare forms ...
. . . gene flow ...
. . . and recombination
however, because the theory of reinforcement has not been definitively falsified.
Reinforcement remains a topic of active research, and biologists hold a range of views
on how important it is in evolution.
14.6.2 Preconditions for reinforcement may be short lived
CHAPTER 14 / Speciation 401
The precondition for reinforcement is that two genetic types exist, and hybrids produced
by crosses between those types are disadvantageous. Natural selection favors
assortative mating. However, other evolutionary forces will also be acting, and may
remove the preconditions before reinforcement has increased reproductive isolation to
the point of full speciation.
1. Natural selection may eliminate the rarer genotype. The precondition for reinforcement
is inherently unstable. Imagine that 90% of the population are AA and 10%
are A′A′. Initially the two types mate randomly. For simplicity, we can asume that
AA and A′A′ individuals have equal chances of survival, and AA′ hybrids have a
much lower chance of survival. With random mating, an AA individual has a 90%
chance of mating with another AA individual and producing AA offspring. AA individuals
mate with A′A′ individuals only 10% of the time, producing inferior hybrids.
A′A′ individuals mate among themselves, producing high quality A′A′ offspring
only 10% of the time; they mate with AA individuals and produce low quality hybrid
offspring 90% of the time. The rarer genotype has an automatic disadvantage, and
natural selection acts to eliminate it. It may be driven extinct before full assortative
mating has evolved. (The precondition for reinforcement is an instance of positive
frequency-dependent selection: Section 5.13, p. 127.)
2. Gene flow merges the two genetic types. Imagine that the two genetic types in the
population are multilocus sets of genes. A might stand for one set of genes at five loci
(BCDEF) and A′ stand for another set (bcdef ). Hybrids (BCDEF/bcdef ) have low
fitness, but some recombinants will be formed (Bcdef, bcdEF, and so on), provided
hybrid fitness is more than zero. Over time the two distinct types will blur into a
continuous population. The rate of blurring will depend on the fitnesses of the
different gene combinations. Again, the precondition for reinforcement may disappear
before speciation takes place.
3. Recombination between gene loci may disrupt reinforcement. A model of reinforcement
usually has three gene loci (or three sets of gene loci). One controls adaptation:
that is, the A/A′ locus (or loci) in the case that we have been looking at. This locus
might control a digestive enzyme, with A types able to eat one kind of food, A′
another kind of food, and AA′ hybrids not able to eat either kind of food. A second
locus controls the degree of assortative mating. A third locus controls a character
that is used in mating decisions. Mating may be decided by coloration, and a color
locus may have two types, blue and green. Reinforcement can work if AA individuals
are usually blue and A′A′ individuals are usually green. Natural selection
favors assortative mating based on color. The problem is that recombination may
generate green AA individuals (and blue A′A′ individuals). A blue AA who mates
assortatively may now have an A′A′ partner, and produce inferior hybrid offspring.
The process of reinforcement will then collapse. Reinforcement requires tight linkage