Anisogamy and the Darwin-Bateman paradigm

Abstract

The Darwin-Bateman paradigm advanced as the central concept to explain the evolutionary origin of sex differences. However, debates regarding its theoretical underpinnings persist, particularly with respect to the role of anisogamy in sexual selection. The theoretical work presented by Lehtonen and Parker suggests that the initial split in gamete production rate drives sex differences in sexual selection but that any further variation in the degree of anisogamy is not expected to alter the strength of sexual selection in males and females. Here, I discuss the historical background of a recently emerged controversy and present empirical data that corroborate the theoretical predictions. Lehtonen and Parker's contribution refines our understanding of the Darwin-Bateman paradigm by providing a broad theory for large-scale patterns of sex differences that can be observed in nature. Further understanding of how demographic and environmental factors influence sexual selection is essential to predict the vast diversity of sex differences across the tree of life, beyond the primordial impact of anisogamy.

Keywords: Bateman gradient; Bateman’s principle; gamete size; sexual selection.

Figure 1.

Degree of anisogamy and Bateman gradients across animals. Scatter plots show how Bateman gradients of males (blue) and females (red) relate to estimates of anisogamy in terms of gamete size bias (A) and gametic investment bias (B). Higher values indicate increased size of or investment in male gametes and are therefore representative of more isogamous species. Batmen gradients are shown as Pearson’s correlation coefficients (r) of the relationship between reproductive success and mating success. The radius of circles corresponds to the precision of the shown estimates of Bateman gradients (i.e., the inverse of the sampling variance in r). Phylogenetically informed meta-analyses suggest that estimates of gamete size ratio and gametic investment ratio are not related to male and female Bateman gradients, i.e., all shown slopes (dashed lines) are not significantly different from zero.