Researchers from the ELKH Centre for Ecological Research and the University of Debrecen have recently published a study in the international multidisciplinary journal Scientific Reports. They claim that the Darwin–Bateman paradigm, according to which sex differences originate in differences in size and/or function of gametes, can only be partially supported by life-history trait data and their results question a key assumption of the 150-year-old paradigm.
Sex roles, i.e. behavioural and physiological differences between males and females, are common in reproduction. The debates about the evolutional origin of sex roles date back to Charles Darwin, the father of the theory of evolution, writes ELKH.
150 years ago, he discovered that inter-and intrasexual competition is important for natural selection. In the process of intrasexual selection, same-sex animals compete to get the best or most mates. Intersexual selection results from interactions between the sexes, and it involves sex differences in parental care and morphological traits.
Sexual selection takes place at many levels, but the goal is always to produce more high-quality offspring by minimising energy investment. Therefore, most species try to get away with providing the least amount of care to their offspring. This often results in one parent, typically the male, providing less parental care.
According to the Darwin–Bateman paradigm, differences in size and/or function of gametes between the sexes (male gametes tend to be small and motile, while female gametes are usually larger and sessile) lead to biased sexual selection and sex differences in parental care and body size.
Because females invest a lot of energy in producing large eggs, it is much more important for them to have high-quality offspring. Males, however, tend to fight for better females but invest little energy in caring for the offspring. If this theory is true, it means that the degree of anisogamy predicts the intensity of sexual selection.
Some aspects of the Darwin–Bateman paradigm have already been extensively investigated. Nonetheless, the results of different studies often show inconsistency and the whole Darwin–Bateman paradigm has not been studied previously using life-history trait data.
Hungarian evolutionary biologists investigated the relationships between anisogamy, sexual size dimorphism, sex difference in parental care and intensity of sexual selection, analysing the dataset of 64 species. According to their results, the degree of anisogamy does not predict the intensity of sexual selection. However, researchers associated male-biased sexual selection with female-biased parenting, so parental care is related to sexual selection.
They believe that in the evolutionary past, anisogamy initiated sexual selection, but later other factors such as ecological factors, life history and demography had a stronger influence on sex roles. In conclusion, the study questions the first step of the Darwin–Bateman paradigm but supports another element, so it does not refute the Darwin–Bateman paradigm.
Source: elkh.org, nature.com