Evolution__3rd_Edition

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Reinforcement works in sympatry
European crows provide an
example of a hybrid zone
populations. Within one population, natural selection will not favor a genetic change
that is incompatible with genes at other loci.
Prezygotic isolation, however, does not require incompatible genetic change at several
loci. Prezygotic isolation can evolve as a by-product of divergence if the characters
that have diverged between populations are genetically correlated with characters causing
prezygotic isolation. This theory is less strongly tied to the theory of allopatric speciation.
The process can indeed occur between populations that are separately evolving
in different places. But adaptive divergence can also occur within one population, as we
shall see, and that at least raises the possibility that speciation could occur nonallopatrically.
The other theory was reinforcement. Reinforcement only occurs in sympatry.
Natural selection only favors discrimination among potential mates for the range of
mates that are present in a particular place. The theory of reinforcement is only weakly
tied to the theory of allopatric speciation. Indeed, it is hardly an allopatric theory of
speciation at all. Reinforcement was only used in the allopatric theory to “finish off ”
speciation that was incomplete in allopatry.
Thus, in the theories we have met so far, speciation in non-allopatric populations
is relatively unlikely. One well supported theory, the Dobzhansky–Muller theory, is
allopatric. Reinforcement is a sympatric process, but (as we saw) little supported by
evidence and problematic in theory. However, non-allopatric speciation has not been
ruled out, and in the next two sections we shall look some more at whether speciation
could occur parapatrically or sympatrically.
14.9 Parapatric speciation
CHAPTER 14 / Speciation 409
14.9.1 Parapatric speciation begins with the evolution of a
stepped cline
In parapatric speciation, the new species evolve from contiguous populations, rather
than completely separate ones, as in allopatric speciation (see Figure 14.1). The full
process could occur as follows. Initially, one species is distributed in space. The species
evolves a “stepped cline” pattern of geographic variation (Section 13.4.3, p. 363). The
stepped cline could exist because of an abrupt environmental change: one form of the
species would be adapted to the conditions on one side of the boundary, the other form
to the conditions on the other side of the boundary.
A hybrid zone is a stepped cline in which the forms on either side of the boundary are
sufficiently different that they can easily be recognized. The two forms may have been
given different taxonomic names, as subspecies or races, or they may be different
enough to have been classified as separate species.
The carrion crow (Corvus corone) and hooded crow (C. cornix) in Europe are a
classic example of species round a hybrid zone (Figure 14.13). The hooded crow is
distributed more to the east, the carrion crow to the west, with the two species meeting
along a line in central Europe. At that line a the hybrid zone a they interbreed and
produce hybrids. The hybrid zone for the crows was first recognized phenotypically,