ASSORTATIVE MATING BY SIZE - A METAANALYSIS OF MATING PATTERNS IN WATER STRIDERS

Assortative mating by size is a common mating pattern that can be gene rated by several different behavioural mechanisms, with different evol utionary implications. Assortative mating is typically associated with sexual selection and has been regarded as an attribute of populations , species, mating systems or even higher order taxa. In most animal gr oups, however, appropriate analyses of assortative mating at these dif ferent levels are lacking and the causes and forms of assortative mati ng are poorly understood. Here, we analyse 45 different population lev el estimates of assortative mating and non-random mating by size in se ven confamiliar species of water striders that share a common mating s ystem. A hierarchical comparative analysis shows that virtually all th e variance within the clade occurs among samples within species. We th en employ meta-analysis to estimate the overall strength of assortativ e mating, to determine the form of assortative mating and to further a ssess potential differences among species as well as the probable caus es of assortative mating in this group of insects. We found overall we ak but highly significant positive assortative mating. We show that an alyses of the degree of heteroscedasticity in plots of male versus fem ale size are critical, since the evolutionary implications of 'true' a nd 'apparent' assortative mating differ widely and conclude that the p ositive assortative mating observed in water striders was of the 'true ' rather than the 'apparent' form. Further, within samples, mating ind ividuals were significantly larger than non-mating individuals in both males and females. All of these non-random mating patterns were consi stent among species and we conclude that weak positive assortative mat ing by size is a general characteristic of those water strider species that share this mating system. We use our results to illustrate the i mportance of distinguishing between different forms of assortative mat ing, to discriminate between various behavioural causes of assortative mating and to assess potential sources of interpopulational variance in estimates of assortative mating, Finally, we discuss the value of u sing meta-analytic techniques for detecting overall patterns in multip le studies of non-random mating.