Evolution__3rd_Edition

..
Less interbreeding between species More interbreeding between species
+0.80
+0.60
+0.40
+0.20
0.0
–0.20
–0.40
–0.60
–0.80
0 2 4 6 8 10 12 14 16 18
Generation
Evidence from biogeography ...
. . . is consistent with reinforcement
...
CHAPTER 14 / Speciation 403
Figure 14.9
Artificial selection in female Drosophila pseudoobscura for
increased (low isolation, green line) and decreased (high isolation,
black line) tendencies to mate with male D. persimilis. The y-axis
is an index of frequency of mating with a member of the other
species (heterospecific mating). When the index is positive,
females are more likely to mate with heterospecific males than
are control females; when it is negative they are less likely to.
Redrawn, by permission of the publisher, from Kessler (1966).
character displacement.) For example, Drosophila mojavensis and D. arizonae are two
closely related species of cactus-eating fruitfly that coexist in Sonora, Mexico. But elsewhere
in the southwest, each species can be found living without the other: D. mojavensis,
for example, lives in Baja California where D. arizonae is absent, and D. arizonae
lives in other regions of Mexico without D. mojavensis (Figure 14.10).
The key result concerns the amount of prezygotic isolation between the species
in sympatric and allopatric populations. When a male of one species is put with a
female of the other, they are less likely to mate than are a pair from the same species.
Wasserman & Koepfer (1977) measured the degree of mating discrimination in populations
taken both from where the two species co-occur and from where only one of
the species lives. They found that discrimination against potential mates from the
other species was stronger in the flies from regions where both species are found
(Figure 14.10c).
The result is an example of character displacement. Character displacement occurs
when two species differ more in sympatry than in allopatry. The term can refer to
any character, and D. mojavensis and D. arizonae show reproductive character displacement,
or to be exact character displacement for prezygotic reproductive isolation.
These two species are one example among several in which this result has been
found.
One interpretation of reproductive character displacement is that prezygotic isolation
has been reinforced in sympatry. When the two species do not encounter each
other (that is, allopatrically), natural selection will not have favored discrimination
against mates from the other species. In sympatry, where interbreeding may produce
hybrids of reduced fitness, selection will have favored mechanisms to prevent crossbreeding.
Reinforcement has acted to increase prezygotic isolation only where the two
species coexist.