Evolution__3rd_Edition

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Fitness of hybrid offspring
Fitness of hybrid offspring
(a) It could have been …
Time
(b) … but in fact it normally is
Haldane’s rule zone
Time
Haldane’s rule describes a normal
stage in speciation, not a curiosity
Speciation
Speciation
XX hybrids
XY hybrids
XX hybrids
XY hybrids
CHAPTER 14 / Speciation 397
Figure 14.8
Haldane’s rule. (a) We might naively expect that the fitness of
male and female hybrid offspring would on average decrease in
much the same way as speciation proceeds. (b) In fact sterility or
inviability evolve first in the heterogametic hybrid offspring. This
gives rise to an intermediate stage in speciation, during which XX
hybrids have higher fitness than XY hybrids; this stage is where
Haldane’s rule operates. In fruitflies and mammals, males are XY
and females XX. In birds and butterflies males are XX and females
XY. (In (a) and part of (b) the fact that the XY line has been drawn
just below the XX line is insignificant: they are meant to be one
on top of the other. Also, the fact that the line is drawn straight in
(a) is insignificant. What matters is that it goes down the same for
male and for female hybrid offspring; whether the line goes down
smoothly or suddenly or in a curve is unimportant here.)
(or near species), that are in the gray area on the way to full speciation. They have I
between 0 and 1. In the simplest case, all the male hybrid offspring (in a mammal, for
instance) would be dead or sterile and all the female offspring would be perfectly
alright. Then I = 0.5, averaging over all offspring.
Now it might be thought that during speciation, the degrees of isolation would
increase in some fashion from 0 to 1 (Figure 14.8a). But Coyne & Orr (1989) found that
of 43 Drosophila species pairs with intermediate degrees of isolation (0 < I < 1), 37
showed a sex difference and fitted Haldane’s rule. It is a normal fact about speciation, at
least in fruitflies, that low male hybrid fitness evolves earlier than low female fitness.
The true course of speciation looks something like Figure 14.8b. Haldane’s rule is a
general property of speciation, not a curiosity.
Modern genetic techniques have been used to test Haldane’s rule in new ways. For
instance, snazzy genetic tricks can be used to introduce various numbers of genes from
one fruitfly species into another fruitfly species. True et al. (1996) introduced genes
from Drosophila mauritiana into D. simulans. They found that if they introduced the
same number of random genes into males and females, the males were six times as
likely to be sterilized. The result can be dramatized in an anthropomorphic analogy. It
is as if we introduced a certain number of chimpanzees genes into human males and
females, randomly scattering the chimp genes in the human DNA. True et al.’s result
would then imply that the men were more likely to be sterilized by the experience than
the women. The evolutionary interpretation is that male hybrid sterility evolves faster,
at a lower level of genetic divergence between species, than female hybrid sterility. That
is another way of expressing Haldane’s rule.
Haldane’s rule is a big generalization about speciation. Whatever the explanation
is for the rule, we can conclude that speciation often proceeds in the manner of