Sexual selection protects against extinction

Populations of the flour beetle Tribolium castaneum with histories of strong versus weak sexual selection purge mutation load and resist extinction differently. Mutational load minimized by sexual selection Sexual reproduction is more costly than the asexual kind because the genetic contribution of a given individual to the next generation is halved. A disadvantage of asexual reproduction is the accumulation of mutations, and it has been suggested that sexual selection, which acts when reproduction is achieved through competition and choice, might purge mutation. Matthew Gage and colleagues tested this theory in a study of populations of the flour beetle Tribolium castaneum raised for seven years under conditions of strong or weak sexual selection. Lineages from populations that had previously experienced strong sexual selection were resilient to extinction and maintained fitness, even with strong inbreeding, with some families continuing to survive after 20 generations of sibling–sibling mating. By contrast, lineages derived from populations that experienced weak or non-existent sexual selection showed rapid fitness declines under inbreeding, and all were extinct after the tenth generation. Reproduction through sex carries substantial costs, mainly because only half of sexual adults produce offspring 1 . It has been theorized that these costs could be countered if sex allows sexual selection to clear the universal fitness constraint of mutation load 2 , 3 , 4 . Under sexual selection, competition between (usually) males and mate choice by (usually) females create important intraspecific filters for reproductive success, so that only a subset of males gains paternity. If reproductive success under sexual selection is dependent on individual condition, which is contingent to mutation load, then sexually selected filtering through ‘genic capture’ 5 could offset the costs of sex because it provides genetic benefits to populations. Here we test this theory experimentally by comparing whether populations with histories of strong versus weak sexual selection purge mutation load and resist extinction differently. After evolving replicate populations of the flour beetle Tribolium castaneum for 6 to 7 years under conditions that differed solely in the strengths of sexual selection, we revealed mutation load using inbreeding. Lineages from populations that had previously experienced strong sexual selection were resilient to extinction and maintained fitness under in