Evolution__3rd_Edition

410 PART 4 / Evolution and Diversity
Many hybrid zones are tension
zones ...
. . . in which reinforcement may
operate
C. corone
C. cornix
Figure 14.13
Hybrid zone between the carrion crow (Cornix corone) and
hooded crow (C. cornix) in Europe. Here the two crows are
shown as two separate species, but some taxonomists classify
them as subspecies. Redrawn, by permission of the publisher,
from Mayr (1963). © 1963 President and Fellows of Harvard
College.
because the hooded crow is gray with a black head and tail, whereas the carrion crow is
black all over. The two species (or near species) are now known to differ in many other
respects too. The fact that the crows interbreed in the hybrid zone means that speciation
between them is incomplete. We shall meet some more examples of hybrid zones
in Section 17.4 (p. 497).
The conditions in a hybrid zone (or a stepped cline) are particularly ripe for
speciation if it is a tension zone. A tension zone exists when the hybrids between the
forms on either side of the boundary are selectively disadvantageous. (A hybrid zone
is not a tension zone if the hybrids have intermediate, or superior, fitness to the
pure forms.) For instance, if one homozygote (AA) is adapted to one environment,
and another homozygote (aa) to another environment, heterozygotes (Aa) will be
produced where the two environments meet up. If the heterozygotes are disadvantageous,
the meeting place is an example of a tension zone. Most known hybrid zones
are in fact tension zones (see, for example, Barton & Hewitt’s (1985) review of
170 hybrid zones).
In a tension zone, the conditions are exactly the preconditions for reinforcement
(Section 14.6.1). Matings within a type are advantageous, and matings between types
produce disadvantageous hybrids. Natural selection favors assortative mating. We can
therefore imagine a sequence where a stepped cline initially evolves, and then becomes
distinct enough to count as a hybrid zone. We are near the border of the origin of a new
species. Reinforcement could then finish speciation off, eliminating hybridization
from the hybrid zone. That sequence of events constitutes parapatric speciation.
The strong point of the theory of parapatric speciation is that the environment
“stabilizes” the preconditions for reinforcement. We saw that these conditions are
liable to autodestruct, as the two forms interbreed, or as one eliminates the other. But
if the environment varies in space, the clinal variation will be maintained. Parapatric
speciation could work, in theory.
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