Faster male hypothesis
Another explanation of the Haldane rule is based on the existence of intense sexual selection amongst males and thus the anticipated greater rate of evolution of genes participating in reproduction amongst males than the corresponding genes in females (Wu & Davis 1993) (Fig. XXI.10). It is obvious that the faster male hypothesis can explain the existence of the Haldane rule only for species containing heterogametic males. The hypothesis could also explain only the differences in the level of fertility, but not in viability. While the sets of genes affecting the fertility of males and females mostly do not overlap, viability is affected by the same genes in males and females (Hollocher & Wu 1996; Turelli & Orr 2000). In species with heterogametic females, this effect has the opposite influence than the dominance effect; if it were to predominate, homogametic males would be affected more by the results of hybridization. Comparative studies confirm this theoretical conclusion. While, in Diptera insects, 81% of hybrid males have primarily affected fertility and only 19% viability, amongst butterflies and birds (where the females are heterogametic and thus the faster male effect acts in the case of genes for sterility against the effect of interactions of the sex chromosome with autosomes), 60% of cases of reduced fitness of hybrid males have lowered viability and only 31% have lowered fertility (Laurie 1997; Presgraves & Orr 1998). Further evidence for the faster male effect was provided by the results of studies performed on mosquitoes of the Aedes and Anopheles genera (Presgraves & Orr 1998). In the Aedes genus, the Y-chromosomes are characterized by an extensive pseudoautosomic area, i.e. an area that contains the same genes as the corresponding section of the X-chromosome. In contrast, members of the Anopheles genus have similar Y-chromosomes as humans and thus only a small fraction is formed by a pseudoautosomic area and contains normally expressed genes, while most of the Y-chromosome cannot recombine with the X-chromosome and contain only a minimum amount of expressed genes. In the Aedes genus, the X-chromosome should behave like an autosome in many respects, as a large part is located in two copies in the cells of both sexes. According to the dominance hypothesis, the Haldane rule should not be valid, either in reducing the viability or in reducing the fertility of hybrid males. In contrast, according to the faster male hypothesis, which, it will be recalled, explains only differences in fertility but not in viability, the Haldane rule should be manifested in a relative reduction in the fertility of hybrid males in the same way in the Aedes and Anopheles genera, because it should not depend on the size of the pseudosome areas. Experiments have shown that the reduced fertility of hybrid males is manifested in members of both genera, indicating that the effects described by the dominance hypothesis, i.e. effects connected with the presence of classical X-chromosomes, cannot be the only explanation of the reduced fertility of members of the heterogametic sex. The fact that, in accordance with the predictions following from the faster male hypothesis, the Haldane rule for inviability is not really valid for the Aedes genus, suggests that this hypothesis is another mechanism responsible for the Haldane rule.
As mentioned above, in species with heterogametic females, the faster male effect acts in the opposite direction to the dominance effect. The fact that members of the heterogametic sex are affected by reduced fertility in these species clearly indicates that the faster male effect is weaker than the dominance effect.