Evolution__3rd_Edition

..
(a)
Space
(b)
(c)
Time
Geographic separation alone is not
reproductive isolation
CHAPTER 14 / Speciation 383
Figure 14.1
Three main theoretical types of speciation can be distinguished
according to the geographic relations of the ancestral species
and the newly evolving species. (a) In allopatric speciation the
new species forms geographically apart from its ancestor;
(b) in parapatric speciation the new species forms in a contiguous
population; and (c) in sympatric speciation a new species emerges
from within the geographic range of its ancestor.
extreme populations cut off from each other. Or a subpopulation may migrate (actively
or passively) to a new place, outside the range of the ancestral species, such as when a
few individuals colonize an island away from the mainland. Such a population, at the
edge of the main range of a species, is called a “peripheral isolate.”
One way or another, a species can become geographically subdivided, consisting of a
number of populations between which gene flow has been cut off. This is not, in itself,
an isolating barrier in the sense of Table 13.1 (p. 356). An isolating barrier is an evolved
property of a species that prevents interbreeding. When two populations are geographically
cut off, gene flow ceases but only because members of the population do not
meet. The two populations have not yet evolved a genetic difference. The evolution of
an isolating barrier requires some new character, such as a new courtship song, to
evolve in at least one of the populations a a new character that has the effect of preventing
gene flow. In the theory of allopatric speciation, the cessation of gene flow between
allopatric populations leads, over time, to the evolution of intrinsic isolating barriers
between the populations. Let us see what happens to the reproductive isolation
between these populations over evolutionary time.
14.3 Reproductive isolation can evolve as a by-product of
divergence in allopatric populations
We have two main kinds of evidence that reproductive isolation evolves when
geographically separate populations are evolving apart. One comes from laboratory
experiments and the other comes from biogeographic observations.