Evolution__3rd_Edition

..
Most hybrid zones are due to
secondary contact
14.9.2 Evidence for the theory of parapatric speciation is
relatively weak
The theory of parapatric speciation has two main weak points in the evidence. One is
the evolutionary history of hybrid zones. Hybrid zones can be “primary” or “secondary.”
A hybrid zone is primary if it evolved while the species had approximately
their current geographic distribution. It is secondary if in the past the species was subdivided
into separate populations, where the differences between the forms evolved,
and the populations later expanded and met up at what is now the hybrid zone. Real
hybrid zones only illustrate a stage in parapatric speciation if they are primary. The
abundance of hybrid zones in nature would only be evidence that parapatric speciation
is a plausible process if those hybrid zones are mainly primary. If most hybrid zones are
secondary, the difference between the forms evolved allopatrically not parapatrically.
In fact the evidence suggests that most hybrid zones are secondary. Hooded and carrion
crows, for instance, have met up after their ranges expanded following the most recent
ice age. Indeed, range expansion following the ice age is a common explanation of
hybrid zones (Section 17.4, p. 497). Hybrid zones provide little support for the theory
of parapatric speciation.
Secondly, if reinforcement operates in hybrid zones, we predict that prezygotic
isolation will be stronger in the hybrid zone than between the two forms away from
the hybrid zone. The prediction is a special case of the general biogeographic test of
reinforcement (Section 14.6.3). The evidence does not support the prediction: we have
little good evidence that prezygotic isolation is reinforced in hybrid zones.
Thus, the process of parapatric speciation is possible in theory. The theory solves one
key problem in reinforcement. Most (but not all) stages of parapatric speciation can be
illustrated by evidence. But parapatric speciation lacks the solid weight of supporting
evidence and the theoretical near inevitability of allopatric speciation. Parapatric speciation
cannot be ruled out, and probably operates in some cases. But the case that it is
important has still to be made.
14.10 Sympatric speciation
14.10.1 Sympatric speciation is theoretically possible
CHAPTER 14 / Speciation 411
In sympatric speciation, a species splits into two without any separation of the ancestral
species’ geographic range (see Figure 14.1). Sympatric speciation has been a source of
recurrent controversy for a century or so. Mayr (1942, 1963) particularly cast doubt on
it, and in doing so has stimulated others to look for evidence and to work out the theoretical
conditions under which it may be possible.
In the theory of parapatric speciation, the initial stage in speciation is a spatial
polymorphism (or stepped cline). In sympatric speciation, the initial stage is a polymorphism
that does not depend on space within a population. For instance, two forms
of a species may be adapted to eat different foods. If matings between the two are disadvantageous,
because hybrids have low fitness, reinforcement will operate between