Evolution__3rd_Edition

390 PART 4 / Evolution and Diversity
Probably not
How about more than one locus?
Figure 14.5
The Dobzhansky–Muller
theory for the evolution of
postzygotic isolation. An
ancestral species splits into
more than one population,
between which gene flow is
absent. Each population adapts
to its local conditions by genetic
change. The genetic changes are
likely to be at different gene loci
in the different populations. If
the two populations later meet
up, the genetic changes in each
will probably be incompatible
and the hybrids sterile or
inviable. The genotypes shown
are for two loci. A and a are
alleles at one locus; B and b are
alleles at a second locus.
Here we have supposed that each species is fixed for a different allele at the locus. The
hybrids are heterozygotes, and if there is postzygotic isolation then those heterozygotes
must have low fitness. However, there is a theoretical argument to suggest that the
genetics underlying postzygotic isolation is unlikely to have this form. The problem is:
how could it have evolved in the past? Species 1 and 2 exist now, with genotypes AA and
aa. In the past, an ancestral species split into two to give rise to modern species 1 and 2.
What genotype did that ancestral species have for this locus? We do not know, but two
simple possibilities are that it had either AA or aa. Suppose, for instance, that the ancestor
had AA. The genotype has been retained in species 1. In the evolution of species
2, AA has evolved into aa. The allele a was, we might reason, advantageous in species 2.
The allele a appeared as a new mutation in the AA ancestor a creating an Aa heterozygote.
But we know that Aa heterozygotes are lethal or sterile (this is shown in the
postzygotic isolation between modern species 1 and 2). The one-locus model of postzygotic
isolation contains a paradox. The modern set up (with AA in one species and
aa in the other) must have evolved somehow. But evolution has to pass through a disadvantageous,
or even deadly, stage a which is improbable, if not impossible. The same
problem arises if the ancestor was aa. It also arises, in a more convoluted form, if the
ancestor had some third allele, such as A*. The paradox is unavoidable if postzygotic
isolation is controlled by one-genetic locus.
The solution to the paradox was suggested by Dobzhansky and by Muller in the
1930s, and is often called the Dobzhansky–Muller theory. They realized that postzygotic
isolation could evolve without difficulty if it was controlled by interaction among more
than one genetic locus. The simplest case has two loci (Figure 14.5): the ancestor has a
two-locus genotype such as AABB. It splits into two allopatric populations. In the environmental
conditions of population 1, the allele a is advantageous. Two copies of a are
better than one, and the population will evolve from AABB to AaBB to aaBB; natural
selection fixes the a allele. This is simple evolution by natural selection. In the environmental
condition of population 2, a change at the other locus is advantageous. Natural
selection drives the population from AABB to AABb to AAbb, and fixes the b allele.
Now suppose we cross members of the two populations. One is AAbb, the other
aaBB; the hybrid offspring will be double heterozygotes AaBb. These hybrids may have
low fitness, without creating the paradox we met in the one-locus model. The double
heterozygote has never existed before. The two new alleles a and b have never been
together in the same body. The a gene may be advantageous in a BB body but not in a
Two-locus genotype
AABB
Ancestral species
Population 1/environment 1
AaBB aaBB
AABb AAbb
Population 2/environment 2
Hybrid
AaBb
Sterile or
inviable
..