Evolution__3rd_Edition

402 PART 4 / Evolution and Diversity
Evidence for reinforcement from
artifical selection is ...
. . . inadequate ...
. . . or negative
between the character used in mating decisions and the character influencing hybrid
fitness; but that requirement often may not be met.
These three objections considerably weaken the theory of reinforcement. But they
do not show that it is impossible, and counterarguments can be made. For instance, the
preconditions can be stabilized if the two genetic types are a polymorphism that is
actively maintained by natural selection (by any of the standard mechanisms of
Sections 5.11–5.14, pp. 121–33). If the rarer genotype can eat a food type that the common
type cannot, its advantage in feeding may balance its disadvantage in more often
producing hybrid offspring. The same process can prevent the two genetic types from
being blurred away by gene flow. Reinforcement then has more time to act. There are
also ways of configuring the various loci such that recombination does not disrupt
reinforcement (Schluter 2000, p. 192). The theory about reinforcement is therefore
inconclusive. We can identify weaknesses in the theory, but they are not enough to
show that it is impossible. We need to turn to the facts to find out how important reinforcement
has been in nature.
14.6.3 Empirical tests of reinforcement are inconclusive or fail to
support the theory
Two kinds of evidence have been used to test for reinforcement, one experimental and
the other biogeographic. The experimental evidence consists of artificial selection
experiments, in which the experimenter creates the preconditions for reinforcement.
For instance, Kessler (1966) put two closely related fruitfly species together in the
laboratory, in conditions in which they interbred. Over a number of generations, any
hybrids were prevented from breeding. The experimenter gave the hybrids low (indeed
zero) fitness. The tendency of the fruitflies to mate assortatively was measured, and it
increased over time (Figure 14.9). Natural selection favored assortative mating, which
duly increased. Many other experiments have obtained similar results. The problem
with these experiments, for our purposes here, is that arguably they do not test the theory
of reinforcement. Reinforcement is a process that drives speciation. But the experimenter
made hybrid fitness zero, meaning that speciation was effectively complete.
Gene flow between the lines was experimentally prevented. Rice & Hostert (1993)
called experiments of this kind “destroy the hybrids” experiments.
However, the experiments do have value. They show, for instance, how natural selection
can increase prezygotic isolation once postzygotic isolation is complete. But they
do not provide much of a test of reinforcement. A good test would make the hybrid
fitness low, but not zero, with some gene flow continuing during the experiment.
Hostert (1997) did this experiment and found no increase in assortative mating when
hybrid fitness was allowed to be anything above zero. But one experiment is not enough
to prove that reinforcement never works. Another species, in some other conditions,
might show a different result. However, at present the evidence from artificial selection
either fails to test, or fails to support, the theory of reinforcement.
The second main kind of evidence comes from biogeography. We require a special
biogeographic set up, in which two closely related species have partly overlapping ranges.
(This is the same set up we met in Section 13.6, p. 366, when looking at ecological
..