HIERARCHICAL LEVELS OF SPATIAL STRUCTURE AND THEIR CONSEQUENCES FOR THE EVOLUTION OF SEX ALLOCATION IN MITES AND OTHER ARTHROPODS

Evolutionarily stable (ES) sex allocation strategies depend critically on how the population is structured with regard to competition for ma tes and resources. This structure is determined by spatial heterogenei ty, dispersal, and colonization behavior. In mating groups lasting one generation where mating precedes female dispersal, female-biased sex ratios will be favored by ''local mate competition,'' especially when the number of foundresses is small. When groups last several generatio ns before dispersal takes place (''haystack'' structure), relatedness among population members develops, and this can favor an extra female bias. However, when groups are permanent, with some dispersal in each generation (''island'' structure), relatedness builds up without an ef fect on sex ratio bias because most competition takes place within gro ups. Current models of evolution under a haystack structure generally assume unlimited growth for a fixed number of generations (and hence n o local density dependence), absence of substructure, and sex ratios t hat are inflexible over generations. These model limitations warrant s crutiny. Using extended versions of the haystack model, we show that, although local density dependence in the haystack population diminishe s the sex ratio bias, strongly female-biased sex ratios are still poss ible if foundress numbers are low. It is also shown that substructure in the haystack by subdivision into one-generation mating groups promo tes the female bias. Finally, it is shown that the ES sex ratio can ch ange radically with generations within haystacks. When population grow th is density dependent, the sex ratio of the last generation should b e more female biased than in the preceding generations. In the case of haystacks subdivided into local mating groups, the sex ratio in the f irst generation should be less female biased than in the following gen erations. It is argued that a haystack structure is frequently found a mong small arthropods with a colonizing lifestyle and that subdivision into one-generation mating groups may occur, for example, among plant -inhabiting mites. To illustrate these points, predatory mites of the family Phytoseiidae are considered in more detail. Some species are fo und to exhibit a stronger female bias in their sex ratios than expecte d from local mating competition alone. This extra bias may well stem f rom selection in a haystack or subdivided haystack structure. Other ph ytoseiid species have a lifestyle that leads to more permanent subpopu lations that have an island-like structure. As predicted, these specie s generally show less female-biased sex ratios.