“Female life history trade-offs and the maintenance of genetic variation in Drosophila melanogaster”
Devin Arbuthnott (Oct 2018)
Many processes and trade-offs work together to weaken selection, allowing populations to maintain genetic diversity
Why do we see so much variation within species, with individuals differing widely in traits that affect fitness? Natural and sexual selection should favour advantageous traits, and populations should become more homogeneous as selection increases the frequency of these traits. Yet substantial variation is observed in fitness-related traits in a wide variety of species. In this study, Arbuthnott tests a number of alternate hypotheses as to how variation is maintained using inbred lines of fruit flies. From dozens of trait measurements and experiments, the author finds that no one mechanism is strong enough to counteract selection and maintain variation on its own. However, the author finds a number of weak genetic correlations and trade-offs between different fitness-related traits, and suggests that several mechanisms may each weaken selection incrementally. When these weakening mechanisms are combined, they may act to slow the purging of disadvantageous traits, and maintain genetic variation within populations. This comprehensive study is the first to suggest that many weak processes may act together to maintain variation, which may have important implications for our understanding of genetic diversity and evolution.
Why do we observe substantial variation in fitness-related traits under strong natural or sexual selection? While there is support for several selective and neutral mechanisms acting in select systems, we lack a comprehensive analysis of the relative importance of various mechanisms within a single system. Furthermore, while sexually selected male traits have been a central focus of this paradox, female sexual traits have rarely been considered. In this study, I evaluate the contribution of various selective mechanisms towards the maintenance of substantial variation in female attractiveness and offspring production observed among Drosophila melanogaster genotypes. I tested for contributions from antagonistic pleiotropy, frequency-dependent selection, changing environments, and sexual conflict. I found negative genetic correlations between some traits (male attractiveness vs. female resistance to male harm, early offspring production and reproductive senescence), and genotype-specific changes in fitness between environments. However, no measurement found strong trade-offs among the fitness components of these genotypes. Overall, I find little evidence that any one mechanism is strong enough to maintain genetic variation on its own. Instead, I suggest that many mechanisms may weaken the selection among genotypes, which would collectively allow neutral processes such as mutation-selection balance to maintain genetic variation within populations.