Evolution__3rd_Edition

404 PART 4 / Evolution and Diversity
(a) Simplified system to test for character displacement
Species 1
distribution
Allopatric
(species 1)
Sympatric
(both species)
(c) Isolation between D. arizonae and D. mojavensis
D. mojavensis female with
D. arizonae male
D. mojavensis female
Allopatry l = 0.3
Sympatry l = 0.94
D. arizonae male
Allopatry l = 0.54
Sympatry l = 0.6
Species 2
distribution
Allopatric
(species 2)
D. arizonae female with
D. mojavensis male
D. arizonae female
Allopatry l = 0.9
Sympatry l = 0.8
D. mojavensis male
Allopatry l = 0.78
Sympatry l = 0.92
Figure 14.10
(a) A study of character displacement requires two species with
partly overlapping ranges. (b) Distributions of Drosophila
mojavensis and D. arizonae in southwest America. They coexist
in part of Sonora, Mexico, and are found alone in other areas,
including large regions of Mexico for D. arizonae and Baja
California for D. mojavensis. The dots on the map for
D. arizonae are the collecting sites for the experiment in
(c), within a fairly continuous distribution. (c) Experimental
demonstration that reproductive isolation is higher between
the two species in sympatry than in allopatry. The experiments
give (i) a female of one species a choice of mating with males of
(b) Distribution of D. arizonae and
D. mojavensis in the southwest
California Arizona New Mexico
Baja
California
Sonora
D. mojavensis
D. arizonae
Rio Grande
Guatemala
either of the species, and (ii) a male of one species a choice of
mating with females of either species. In the experiment, the
number of matings with members of the same species (H s )
and with the other species (H o ), and total number of matings
(N ) was measured, and the isolation index was calculated,
I = (H s − H o )/N, as explained in Figure 14.2. In (c) the top left
number means that mojavensis females, taken from a place
where arizonae does not live (Baja California) a allopatric
D. mojavensis a when put with males of both species, show an
isolation index of only 0.3. The same applies for the other seven
conditions. Redrawn, by permission of the publisher: (b) from
Koepfer (1987) and (c) from Wasserman & Koepfer (1977).
The problem is that reinforcement is not the only explanation for the observations.
The same observations could also arise without reinforcement. The reason is that
sympatric species pairs with low levels of isolation may be lost, by fusion or extinction.
To see the problem, imagine that a number of populations, all descended from
one ancestral species, are evolving allopatrically. They will evolve various degrees
of isolation, depending on how the Dobzhansky–Muller process happens to influence
postzygotic isolation, and how pleiotropy and hitch-hiking happen to influence
prezygotic isolation. Some pairs of populations will evolve high isolation, other pairs
will evolve low isolation. We could measure the average amount of isolation between
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