Selection for hybrid inviability through kin selection.

A model is considered in which a modifier allele that decreases the fitness of partially sterile hybrids is favoured by natural selection. Such genes can be favoured if there is sib competition and hybrid fertility is very low. They can be favoured through a process of kin selection because a reduction of hybrid viability brings about a reduction in the consumption of resources that are therefore available for utilization by non-hybrid siblings who are likely to also carry the gene for hybrid inviability. The present analysis differs from previous studies in that it treats the problem as a specific case of the evolution of altruism and considers the effect of the reduced viability of hybrids on their inclusive fitness. The model is analysed for monogamy and an extreme form of polygamy. In general, hybrid fertility must be lower with polygamy than with monogamy for modifier alleles to be favoured. However, the strength of selection on the modifiers is an order of magnitude greater with polygamy. With polygamy, provided that hybrid fertility is very low, modifier alleles will always be favoured irrespective of the degree of sib competition. However, for monogamy, if the individuals of more than just a few sib-groups compete for resources the modifier cannot be favoured. The effect of sib competition on preventing the spread of genes that increase hybrid viability is also briefly discussed.