Evolution__3rd_Edition

386 PART 4 / Evolution and Diversity
Divergence by drift alone may not
be enough
Two genetic factors may contribute
to the evolution of prezygotic
isolation, pleiotropy ...
Two other results of the experiments are worth noticing. One is that they suggest,
though they do not prove, that speciation normally requires natural selection; genetic
drift alone is not enough. Look at the controls in Dodd’s results, for instance (Figure
14.2). No reproductive isolation evolved between populations that were evolving separately
but in the same environment. These populations would have evolved apart
by drift, but not by selection. Reproductive isolation only evolved between lines kept
on different foods, and selection would have been acting differently between them.
Templeton (1996), however, has argued that this experimental design is inappropriate
for testing the influence of drift in speciation. Secondly, experiments have usually measured
the evolution of prezygotic, not postzygotic, isolation. This likely only reflects
what the experimenters happened to do. Postzygotic isolation would probably evolve
by the same process in experimental populations, but this has not been properly
shown. In conclusion for the experiments on allopatric speciation, we have strong evidence
that prezygotic isolation tends to evolve in populations that are kept separately,
in different conditions, for many generations.
14.3.2 Prezygotic isolation evolves because it is genetically correlated
with the characters undergoing divergence
In an experiment such as Dodd’s (Figure 14.2), the experimenter is not directly selecting
for reproductive isolation. The experimenter selects for an ecological adaptation:
some populations of flies are selected to live on one food type, other populations to
live on another food type. The prezygotic isolation between the populations evolves
somehow as a by-product. Here we shall look at the genetic reason. It is probably that
the characters influencing the ecological adaptation are genetically correlated with the
characters influencing prezygotic isolation. The correlation could exist for two reasons:
pleiotropy and hitch-hiking.
Pleiotropy means that one gene influences more than one phenotypic character of
the organism. Consider, for example, a gene that influences the shape of a bird’s beak.
Beak size is related to the food that the bird eats: smaller beaks are adapted to eat
smaller seeds, larger beaks to eat larger seeds (Section 9.1, p. 223). If two populations
occupied two islands with different-sized seeds, the populations would evolve apart as
the birds adapted to the local food supply. Beak shape, in this sense, is an ecological
adaptation.
Beak shape can also influence reproductive behavior. Some birds may choose their
mates by direct physical inspection of their beaks, but the influence may often be less
direct. Figure 14.3 shows an example, from the research of Podos (2001), in Darwin’s
finches. Beak shape is associated with the kind of song the bird sings. Species with large
beaks, for instance, do not produce rapid trills, whereas species with small beaks do.
This may be a direct physical consequence of the beak size as it may be physically harder
for a bird with a large beak to sing a rapid trill than for a bird with a small beak. Then,
when two populations adapt to different food supplies, their songs will change too.
Darwin’s finches partly choose their mates according to the songs they sing. Thus
a change in diet can incidentally cause a change in reproductive isolation. The genetic
mechanism is pleiotropy: a gene that is favored because it improves ecological
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