Sexual selection or non-random mate choice acts to ‘filter’ out less competitive/desirable phenotypes from a population. In the presence of small effect mutation loads, i.e. small fitness differences between a mutation-free population, and one with persistent deleterious mutations, sexual selection would be expected to purge mutation load. In other words, lineages under strong sexual selection will purge deleterious mutations from the population more efficiently, fathers would carry less deleterious mutations than all males (competing sex) in the population, and females (choosing sex) that are able to maintain reproductive fitness via sexual selection (against loci involved in sexual conflict), should also be able to purge deleterious mutations.
In a recent publication, Lumley et al. (2015) study the properties of sexual selection in purging mutation load in experimentally evolving lab populations of the red flour beetle, Tribolium castaneum. Two regimes of varying intensities of sexual selection were enforced under similar mutation load pressures (due to full-sib mating, or severe inbreeding) – (a) male- versus female-biased sex ratios, and (b) polyandry versus monogamy (no sexual selection) over >45 generations (~7 years).
Results suggest that populations under strong sexual selection resist extinction, and maintain fitness more efficiently in the presence of mutation load. Other key findings include (a) 44% longer survival of male-biased populations than female biased, (b) 37% longer survival of polyandrous versus monogamous populations, and (c) history of sexual selection is a significant predictor of time to extinction.
Our results indicate, as demands rise upon populations that have been depleted and fragmented, and displaced further from their fitness peaks via increasing anthropogenic stress, that sexual selection could be an important force protecting species or populations from the extinction vortex.
Reference: Lumley, Alyson J., et al. “Sexual selection protects against extinction.” Nature (2015). DOI: http://dx.doi.org/10.1038/nature14419