Evolution__3rd_Edition

396 PART 4 / Evolution and Diversity
Hybrids of the heterogametic sex
have lower fitness than hybrids of
the homogametic sex
theory. Meanwhile, the Dobzhansky–Muller theory has been extensively tested and
supported and has almost undoubtedly contributed to speciation, but only limited
work has been done on the ecological theory so its contribution is more uncertain.
14.4.6 Postzygotic isolation usually follows Haldane’s rule
In 1922, J.B.S. Haldane identified the following pattern in postzygotic isolation:
When in the F 1 offspring of the two different animal races one sex is absent, rare, or sterile,
that sex is the heterozygous one.
We should now say “heterogametic” instead of “heterozygous.” In mammals and in
fruitflies males are heterogametic (XY a whereas females are XX). In birds and in butterflies,
it is the other way round and the females are heterogametic. Haldane found
that in crosses in which one gender of hybrid offspring has lower fitness than the other
gender, the gender with lower fitness is male in mammals and fruitflies (and female in
birds and butterflies). Eighty years on, the facts continue to support Haldane remarkably
well (Table 14.1). His generalization has come to be called Haldane’s rule.
The rule has also gained an extra interest. As stated, Haldane’s rule only says that
when fitness differs between the sexes of hybrid offspring, the heterogametic sex has
lower fitness. But it could be that in most cases the sexes do not differ; the rule would
then only be something of a curiosity. However, we now know that in fact most speciation
events do go through a “Haldane rule” phase. Coyne & Orr (1989) quantified this
fact as follows. We can define the amount of postzygotic isolation (I) as the average
fractional reduction in fitness of the hybrid offspring in a cross between the two species,
or near species. I equals one minus hybrid fitness. Thus if we cross two members of a
species their offspring will have high fitness, and I = 0. If we cross individuals from two
different species, usually the hybrid fitness is zero, and I = 1. The isolation (I) increases
from 0 to 1 during speciation. For Haldane’s rule we are interested in pairs of “species”
Table 14.1
Support for Haldane’s rule. “Asymmetry” in the column “Hybridizations with asymmetry”
means that one sex is affected more than the other with respect to the trait such as fertility.
Many species of butterflies, moths, and mosquitos are also known to follow the same rule.
From Coyne & Orr (1989).
Hybridizations Number obeying
Group Trait with asymmetry Haldane’s rule
Mammals Fertility 20 19
Birds Fertility 43 40
Viability 18 18
Drosophila Fertility and viability 145 141
..