5: The Prevalence of Stabilization Processes
(Continued from the previous page)
The previous section explained how saltation can occur via various well-known genetic processes. This section (i.e., Section 5), and the two that follow (Section 6 and Section 7) attempt to assess the evolutionary significance of saltation and explain why it is reasonable to suppose that the evolutionary production of new forms of life is typically saltatory, not gradual, as traditional evolutionary theory suggests.
According to neo-Darwinian theory, (1) reproductive isolation is an essential factor in the process that gives rise to differentiated populations; and (2) genetically distinct populations typically can arise only when a previously existing population is broken up into two or more reproductively isolated sub-populations. This idea is frequently encountered in evolutionary discussion. For example, in a recent article in Scientific American, Wong (2001) avers that
in order for one species to diverge into two, a population must be divided into two groups that cannot interbreed. Usually this reproductive isolation stems from genetic incompatibility, which can arise when a geographical barrier separates the groups, allowing them to drift apart genetically.
A mountain range rises up, continents drift apart, a river changes course — an endless variety of factors have been proposed as contributing to such break ups. But whether it be geographical, temporal, behavioral, or ecological, some isolation mechanism is generally posited that supposedly allows two populations, descended from a common ancestral population, to build up a distinctive set of traits by gradually accumulating favorable mutations over time.
The theoretical outcome of this process is two new populations intrinsically isolated from each other, that is, the isolation is based not upon external factors that prevent mating, but upon physiological incompatibilities preventing fertilization, or resulting in inviability or sterility of hybrids produced by the cross. Various authors have put forward mechanisms to explain how intrinsic isolation arises, but all these explanations are flawed (see discussion in Section 7).
It is well to remember, however, that it is a theoretical notion that populations have to be intrinsically isolated if they are to become and remain genetically distinct. We have already encountered evidence demonstrating that many populations treated as species are not intrinsically isolated in any strict sense. A wide variety of crosses between such populations are known to produce fertile, or at least partially fertile, hybrids in the wild (see Section 2). As we have repeatedly seen, many natural populations treated as species or subspecies are known to be of hybrid origin. NEXT PAGE >>
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5: The Prevalence of Stabilization Processes (Introduction) - © Macroevolution.net