Evolution__3rd_Edition

384 PART 4 / Evolution and Diversity
Populations can be kept on
different resources ...
. . . and prezygotic isolation evolves
between them
The result is general, and
remarkable
14.3.1 Laboratory experiments illustrate how separately evolving
populations of a species tend incidentally to evolve
reproductive isolation
When two geographically separate populations are evolving independently, different
genes will be fixed in each, whether by drift or adaptation to different environments.
The theory of allopatric speciation suggests that two such populations will also, at least
sometimes, evolve some degree of reproductive isolation in consequence.
The idea has been tested experimentally. We can keep two populations apart, allowing
them to evolve indendently for a number of generations. Then we test whether they
have evolved any degree of reproductive isolation. Dodd (1989), for example, performed
the experiment with fruitflies (Drosophila pseudoobscura). The flies had originally
been caught in Utah and were then taken to a lab at Yale and divided into eight
populations: four of them were placed on a starch-based food medium; the other four
on a maltose-based medium. The populations were reared on these different resources
for a number of generations. After a while the flies had evolved detectable differences in
their digestive enzymes a differences that were almost certainly adaptations to the different
resources. Thus, the populations had diverged under the influence of selection to
live on different resources in the laboratory.
Dodd exploited these populations to see whether any reproductive isolation had
evolved as an incidental consequence of the divergence. She placed recently emerged
males and females from the starch and maltose populations in a cage, after marking all
the individuals of one of the populations. She then measured who mated with whom,
and found that the “starch” flies preferred a “starch” mate, and the “maltose” flies a
“maltose” mate (Figure 14.2). Some reproductive isolation had evolved a in this case it
is prezygotic isolation. It presumably evolved because the changes that had occurred in
the population influenced reproductive behavior in some way.
Dodd’s is only one experiment among many. Rice & Hostert (1993) listed 14 experiments
that measured whether prezygotic isolation emerged between populations that
had been experimentally isolated, and found that in 11 of them it did; in the other three
there was no significant change. The evolution of reproductive isolation is a general
result in experiments in which two populations are evolving separately, in different
environmental conditions. The result is quite striking because it might not have been
predicted. Would you expect that if you kept one population of humans on a starch diet
and another on a sugar diet for a number of generations, that at the end of the period
they would have evolved a mating preference for their own dietic type? The evolution
of adaptations to the food supply (starch in one population, sugar in the other) is
predictable. The evolution of reproductive isolation might not be, but it does seem to
happen. The result is also interesting because reproductive isolation is not, at least
directly, being selected for. The adaptations to the environment (such as diet) though
are being selected for, and the flies in Dodd’s experiment duly evolved appropriate
digestive enzymes. However, the reproductive isolation just “drops out,” as an incidental
consequence of the experimental procedure. The experiment did not selectively breed
from individuals that showed some mating preference, such that mating preferences
evolved upwards over the generations. The mating preference somehow evolves as a
correlated response when selection favors new adaptations to the environment.
..