The correlation between anisogamy and sexual selection intensity : the broad theoretical predictions

Abstract

Darwin and Bateman suggested that precopulatory sexual selection is more intense on males than females, and that this difference is due to anisogamy (i.e., dimorphism in gamete size and number). While a recent paper apparently presents empirical support for this hypothesis, another appears at first sight to present evidence against it. We argue that this is partly due to lack of transparent theoretical predictions, and discuss and analyze sexual selection theory in relation to anisogamy evolution. On one hand, we find that there exists relatively little theory that can directly address all the tested predictions; on the other, the picture painted by current theory indicates that both sets of empirical results broadly match predictions about the causal link between anisogamy and sexual selection, thus reconciling the two apparently opposing claims. We also discuss in a very broad, general sense how anisogamy is expected to affect sexual selection.

One hundred and fifty years ago Darwin proposed that sexual selection, with rather rare exceptions, is more intense on males than females, typically causing males to be the sex that competes more intensely for matings. Defining males and females by anisogamy, that is, sperm and ova, an influential paper has shown Darwin's prediction to be true. However, a more recent paper on the same theme has shown that the degree of anisogamy fails to correlate with the intensity of sexual selection, casting doubt that anisogamy is responsible for the prevalence of male competitiveness for matings. Although at first sight this result seems to contradict long-standing theory, by analyzing existing theory we argue that in fact both findings are entirely compatible with theoretical expectations: while anisogamy is predicted to establish a general binary trend toward higher sexual selection intensity on males, the degree of anisogamy (across the typical range, where sperm vastly outnumber ova) is not predicted to be correlated with sexual selection intensity. Rather, though anisogamy should set a binary trend as Darwin predicted, the degree of anisogamy and variation in the intensity of sexual selection will be established by ecological, social, and other factors relevant to each taxon.