Evolution__3rd_Edition

..
. . . in exceptional conditions
In the Dobzhansky–Muller theory,
speciation is evolutionarily easy
We distinguish two theories of
hybrid fitness
Templeton 1996). Without entering into the details of these models, we can note that
they all invoke peculiar evolutionary mechanisms. Speciation requires the normal
action of selection and drift to be suspended. The inspiration of these ideas is that speciation
is a difficult process, because of the need for valley crossing. This is one view of
speciation.
The Dobzhansky–Muller model offers a different view of speciation. It has no valley
crossing. The fitness valley is generated as a consequence of the separate evolution of
the two species. In the Dobzhansky–Muller view, speciation happens as an almost
automatic consequence of ordinary selection and drift within a population, as each
population evolves in its own environmental conditions. Speciation does not require
special conditions, in which normal evolutionary processes are suspended.
The Dobzhansky–Muller theory applies only to postzygotic isolation, but a similar
argument can be made for prezygotic isolation. We saw earlier that prezygotic isolation
evolves as a by-product of normal evolutionary change, by genetic processes such as
pleiotropy (Section 14.3.2 above).
We have theories for both prezygotic and postzygotic isolation that are well validated
in fact and in theory, and in both cases the evolution of reproductive isolation does not
require valley crossing. Speciation instead is an almost automatic consequence of evolutionary
change. The special mechanisms proposed in the alternative, valley crossing,
view are little supported or unsupported by facts and are at best questionable in theory
(Turelli et al. 2001a). That could change in the future, but many evolutionists currently
prefer the view that speciation is an evolutionarily “easy” process, requiring no more
than the most commonplace of evolutionary mechanisms.
14.4.5 Postzygotic isolation may have ecological as well as
genetic causes
CHAPTER 14 / Speciation 395
The Dobzhansky–Muller theory is not the only theory of postzygotic isolation. In
Section 13.7.3 (p. 373), we looked at an ecological theory of postzygotic isolation,
closely related to the ecological species concept. We looked at an example from
Darwin’s finches (Table 13.2, p. 373): the fitness of hybrids between Geospiza fortis and
G. fuliginosa on Daphne Island in the Galápagos archipelago depends on the food supply.
When, in the 1982–83 El Niño event, the supply of small seeds increased, hybrid
fitness increased too. When small seeds were rare, hybrid fitness was lower.
The ecological theory of postzygotic isolation differs from the Dobzhansky–Muller
theory. In the Dobzhansky–Muller theory, hybrids are inferior because of incompatibilities
between their genes. The internal working of the hybrid’s body will be defective.
In the ecological theory, the internal workings of the hybrid’s body are just as good as
in any member of a pure species. Any hybrid inferiority is due to external conditions.
The ecological and Dobzhansky–Muller theories are potentially competing theories
in any one case; but they could also combine to explain the full amount of postzygotic
isolation. The ecological theory may work better for very closely related species that
repeatedly hybridize; their gene pools will contain few incompatible genes. The
Dobzhansky–Muller process may become more powerful as two species evolve separately
for a longer time. But it will take further research to establish the influence of each