Male-male competition might be a driving factor of speciation in birds.
When I write that “sexual selection speeds up speciation”, you might nod your head in agreement. It sounds logical that sexual selection on certain traits (e.g., song or plumage color) can contribute to reproductive isolation, culminating in the origin of new species. However, several studies that approximated the strength of sexual selection as the degree of sexual dichromatism (i.e. the differences in color between males and females) found no supporting evidence for this hypothesis (see Huang & Rabosky 2014 and Cooney et al. 2017). There could be numerous explanations why these studies failed to detect a connection between sexual selection and speciation rates. Perhaps sexual dichromatism is not a reliable proxy for the strength of sexual selection? Or maybe certain environmental factors influence the relationship between sexual selection and diversification? Indeed, theoretical models suggest that sexual selection can speed up local adaptation in variable environments. A recent study in the journal Evolution revisited this evolutionary conundrum and investigated the link between sexual selection and speciation rates, taking into account environmental factors.
Measures of Sexual Selection
Justin Cally and his colleagues collected data on more than 5800 passerine species. They estimated sexual selection in two ways: (1) the degree of sexual dichromatism and (2) an index of male-biased sexual selection. This index is associated with information on sexual size dimorphism, social polygyny (i.e. male with multiple females) and the lack of parental care. For readers interested in the technical details, the index corresponds to the first principal component of a phylogenetic PCA that is correlated with the three features listed above. The two measures of sexual selection were then correlated with speciation rates across the phylogeny of the passerines. In addition, the researchers accounted for the possible effects of several ecological and environmental variables, such the seasonality of temperature and rainfall.
The analyses revealed that “the composite index of male-biased sexual selection, but not measures of sexual dichromatism, is correlated with the rate of speciation in passerine birds.” These findings are in line with previous studies that found no relationship between sexual dichromatism and diversification. The authors of the current study argue that this measure is not a good proxy for sexual selection, because males and females can evolve different plumage patterns for other reasons, such as adaptation to distinct ecological niches (see for example the Sulawesi Babbler, Pellorneum celebense).
A detailed look at the positive effect of the male-biased sexual selection index indicated that sexual size dimorphism was the main driver. Differences in size mostly evolve due to male-male competition in which males compete for access to females. Because body size is often associated with other morphological and ecological traits, it could speed up the divergence between young evolutionary lineages. In fact, studies on mammals suggested that the evolution of large body size allowed for the diversification of ecological strategies, leading to higher speciation rates. Similar processes might have occurred in some bird groups. These patterns highlight the potential role of male-male competition in generating diversity. Sexual selection can thus promote speciation, but maybe not in the way we might have expected.
Cally, J. G., Stuart‐Fox, D., Holman, L., Dale, J., & Medina, I. (2021). Male‐biased sexual selection, but not sexual dichromatism, predicts speciation in birds. Evolution, 75(4), 931-944.
Featured image: Superb Fairywren (Malurus cyaneus) © Benjamint444 | Wikimedia Commons