Evolution__3rd_Edition

414 PART 4 / Evolution and Diversity
Figure 14.14
Phylogenetic test between
sympatric and allopatric
speciation. The test has been
used most for lake-dwelling fish
species. (a) With allopatric
speciation, new species evolve
in separate lakes and for any
one species, its closest relative is
predicted to live in a different
lake. (b) With sympatric
speciation, new species evolve
alongside their ancestors.
Related species are predicted to
live in the same lake. Evidence
for some fish species, including
African lake cichlids, shows the
phylogenetic pattern expected
with sympatric speciation.
Sexual selection may contribute to
the evolution of prezygotic isolation
Time
Phylogeny
Sp. 1
Sp. 1
(a) Allopatric speciation (b) Sympatric speciation
Lake A Lake B Lake A Lake B
Sp. 2
Evolves
into new
species
Colonizes
Colonizes
Sp. 4
Sp. 3
Sp. 3
Lake A Lake B Lake A
Sexual selection is discussed in Section 12.4 (p. 327) and has two main components:
male competition and female choice. The mechanisms that females use to the choose
mates may influence speciation because they can contribute to, or even wholly determine,
prezygotic isolation.
The way natural selection acts on mate choice may help explain the evolution of
prezygotic isolation in both allopatric and sympatric populations. Consider again those
experiments in which some populations of a species are allowed to evolve in two environmental
conditions, such as a diet of maltose or of starch (see Figure 14.2). We saw
that prezygotic isolation evolves as a by-product and its genetic basis may be pleiotropy
or hitch-hiking. Now let us think some more about how natural selection will work in
each experimental population. On a starch medium, selection favors individuals who
can eat, digest, and thrive on starch. But it also favors female flies who choose as mates
those males that are better than average at living on a starch diet. Over time, females
may evolve a preference for males with adaptations for life on starch. In the maltose
population, females evolve preferences for males who are adapted for life on maltose.
Then, at the end of the experiment we give females a choice of males and find the
females from the starch lines prefer males from starch lines.
One way of seeing this argument is as providing an explanation for pleiotropy.
Prezygotic isolation evolves as a by-product when the character concerned with ecological
adaptation happens, perhaps coincidentally, also to be concerned with mate choice
Sp. 1
Sp. 3
Sp. 1 Sp. 2 Sp. 3 Sp. 4
Lake A Lake B
1 4 3 2 1 2 3 4
..