Evolution__3rd_Edition

394 PART 4 / Evolution and Diversity
Fitness
Species 1 Species 2
Valley
Character state/genotype
Speciation might be thought to be a
difficult evolutionary process ...
. . . requiring drift ...
. . . or a genetic revolution ...
Peak
Figure 14.7
Valley crossing during speciation. The figure shows an adaptive
landscape (see Figure 8.7, p. 214): quality of adaptation, or fitness,
is on the y-axis; character state, or genotype, is on the x-axis.
Related species are adapted to somewhat different environments,
and each is well adapted to its own environment, Intermediate
forms are less well adapted with a fitness valley lying between
the two species. Natural selection acts against valley crossing.
If the landscape has the illustrated shape then speciation is an
evolutionarily difficult process, perhaps requiring special
conditions in which the action of natural selection is suspended.
Some theories of speciation, such as the Dobzhansky–Muller
theory, do not require valley crossing.
14.4.4 The Dobzhansky–Muller theory solves a general problem of
“valley crossing” during speciation
We began this section on the Dobzhansky–Muller theory by looking at a hypothetical
one-locus genetic model of postzygotic isolation. We can now look at a more general
version of that argument, and use it to explore a general question about speciation.
Is speciation an “easy” evolutionary process that follows almost automatically from
normal evolutionary change, or is it an evolutionarily “difficult” process that requires
extraordinary mechanisms?
The members of a species are usually fairly well adapted to their environments, and
the genes at different loci work well together a they interact well enough to produce
viable, fertile bodies. Species probably lie near, if not on, the peaks of an adaptive landscape,
and different species occupy different peaks (Figure 14.7). The problem in speciation
is that it seems to require “valley crossing.” For species 1 to evolve into species 2,
or vice versa, the population has to pass through a disadvantageous phase. The onelocus
model illustrates the difficulty a the fitness valley in the one-locus model corresponds
to the heterozygous hybrid (Section 14.4.1). However, many other genetic
models could also have a valley between two adaptive peaks.
It is hard, if not impossible, for a population to cross an adaptive valley. Natural
selection and random drift are the two main forces of evolution. Natural selection
almost always acts to drive species toward a peak on an adaptive landscape. Natural
selection opposes valley crossing as it requires genotypes of lowered fitness to somehow
spread through the species. Random drift is only a powerful force when the alternative
genotypes are selectively neutral. For drift to drive a population across a valley, it has to
work contrary to selection, and that is unlikely. Therefore, if speciation requires valley
crossing, speciation is a difficult evolutionary process and will not normally happen; it
will require some special conditions.
For instance, evolutionists have argued that speciation happens in small stressed
populations where a “genetic revolution” occurs (Mayr 1963, 1976). Or that it happens
by a special process of “peak shifts.” Or that it happens when the action of natural
selection is temporarily suspended, perhaps when a colonizing population exploits
abundant resources in the absence of competitors (the “founder flush” model: see
..