Evolution__3rd_Edition

392 PART 4 / Evolution and Diversity
Metabolic pathways ...
. . . signal–receptor systems ...
These two subtler predictions of the Dobzhansky–Muller theory have not been as
extensively tested as the basic prediction that postzygotic isolation is caused by multilocus
interactions. However, the Dobzhansky–Muller theory has been rather neglected
until recently. Biologists are beginning to explore its rich implications for the genetic
changes that cause speciation, and these two subtle predictions are examples of the
kinds of hypotheses being tested now.
14.4.3 The Dobzhansky–Muller theory has broad biological
plausibility
The account of the Dobzhansky–Muller theory that we looked at in Figure 14.5 was
abstract. We considered two loci with two alleles each (A/a and B/b), but said nothing
about what the genes code for. We simply deduced that if the new alleles (a and b) were
incompatible, then there is postzygotic isolation. In reality, whether or not the alleles
are incompatible will depend on the biological details of what they code for. How general
the Dobzhansky–Muller process is will depend on how common it is for newly
evolved alleles in different populations to be incompatible.
A first concrete example concerns genes that interact in a metabolic pathway.
Imagine two genes (G 1 and G 2 ) coding for two enzymes (E 1 and E 2 ) that successively
process a substrate (S 1 → S 2 → S 3 ):
G 1
G 2
↓ ↓
S
E1
⎯→ ⎯ S
E2
⎯ ⎯→S
1
2
3
In the environment of one population, the food resources may differ from those in the
environment of the other population. The two populations will evolve different
enzymes to digest their differing local food supplies. We can symbolize the enzymes by
E 1 and E 2 for population 1 and E 1 * and E 2 * for population 2. The enzyme pairs work
within each population, as E 1 processes the substrate into a form that can be tackled by
E 2 . But in a hybrid there will be E 1 from one population and E 2 * from the other. E 1 may
process the substrate into a form that E 2 * does not bind to, resulting in a metabolic
inefficiency. Some biologists doubt whether multiple genes in metabolic pathways in
fact underlie postzygotic isolation (Orr & Presgraves 2000), but the example at least
illustrates how the Dobzhansky–Muller process could operate.
As a second example, consider genes coding for an egg receptor protein and a sperm
lysin in abalone (Swanson & Vacquier 1998). The sperm lysin breaks a hole in the egg
by binding to a particular receptor molecule on the egg membrane. The sperm lysin
gene and egg receptor gene evolve in concert within a species; the sperm can recognize
eggs of the right species. But different forms of the genes evolve in different species.
Now imagine that the ancestral abalone had an allele L 1 of the sperm lysin gene and R 1
of the egg receptor gene. Its genotype was L 1 L 1 R 1 R 1 . It split into two population, each of
which evolved different genotypes: L 2 L 2 R 2 R 2 in one and L 3 L 3 R 3 R 3 in the other. If we
cross members of the two populations the hybrid offspring are L 2 R 2 /L 3 R 3 . They may
have normal viability, but if they are bred their fertility may be approximately halved.
..