Evolution__3rd_Edition

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Hybrid fitness is influenced by many
loci
Two subtler predictions can be
made
CHAPTER 14 / Speciation 391
b-containing body. The a and b genes may be incompatible. In formal terms, the fitness
interaction between the two loci are epistatic (Section 8.8, p. 207). The fitness of a genotype,
such as Aa, depends on the genotype at the B/b locus. In informal terms, the a
and b genes cause some kind of genetic snarl-up when they are combined in the same
body. Therefore, postzygotic isolation can evolve by interactions between genes without
the paradox that we met with only one locus. The evolution of postzygotic isolation
is theoretically more likely to be caused by multilocus genetic interactions than by
single-locus ones.
14.4.2 The Dobzhansky–Muller theory is supported by extensive
genetic evidence
The main prediction of the Dobzhansky–Muller theory is that postzygotic isolation is
caused by multilocus interaction, not by single loci. The prediction is in principle easy
to test. Two closely related species can be forced to interbreed in the laboratory, and
classic genetic methods used to estimate the number of gene loci that contribute to
sterility or inviability of the hybrid offspring. Many such experiments have been performed,
particularly with fruitflies. Coyne & Orr (1998) reviewed evidence from 38
experiments on 26 pairs of species (or near species). Only in two species pairs was low
hybrid fitness due to a genotype at one locus. In the other 24 species the problems in
the hybrid were due to epistatic interactions at multiple loci. It is a well supported
generalization about speciation, that postzygotic isolation is due to multilocus gene
interactions.
Coyne & Orr (1998) also note that the Dobzhansky–Muller theory makes two subtler,
more specific predictions. One is that the amount of postzygotic isolation should
“snowball” as the number of loci differing between two population goes up. If new alleles
have evolved at only two loci, they may be incompatible and cause postzygotic isolation,
as shown in Figure 14.5. But, alternatively, a may not be incompatible with b and
the hybrids then do not have reduced fitness. Now suppose a third locus also undergoes
evolutionary change. Hybrids now contain three new genes, a, b, and c. The new a gene
was compatible with b, but the hybrid may now suffer because a is incompatible with c,
or b with c. The increase from two to three loci has increased the number of gene interactions
from one to three. The number of possible gene interactions (any one of which
may be incompatible) goes up faster than the number of gene loci that differ between
the species. In the Dobzhansky–Muller theory, postzygotic isolation is caused by gene
interactions. Hence the prediction that postzygotic isolation will “snowball” as two
populations diverge genetically.
A second subtle prediction is that there should be asymmetry in the gene interactions
in the hybrid. In Figure 14.5, low hybrid fitness is caused by the snarl-up between
the two new alleles a and b. If this is true, we can see that the other pair of alleles (A
and B) ought not to cause a problem. The reason is that the A and B alleles are the
ancestral combination, and the AABB ancestral organisms were good, functioning
creatures. Thus if we can identify which gene combinations are causing problems in
the hybrid, we can predict that the complementary sets of genes at those loci will not
cause problems.