The evolution of sexual dimorphism in life history traits

Many dioecious organisms exhibit some level of sexual dimorphism; the sexes can differ in phenotype, and can have different fitness optima for certain traits and have different reproductive strategies. Sex-specific selection, thus, can be highly diverging in both magnitude and direction. When selection for a trait differs in direction we have antagonistic selection; this often leads to sexual conflicts since the two sexes share most of their genetic architecture. Sexual dimorphism is often seen as a resolution to sexual conflict, a way to decouple the genetic basis of traits under conflict and allow sex-specific expression. One way to better understand the evolution of sexual dimorphism is using a life history theory framework; where traits linked to growth, reproduction and survival are studied and quantified. These quantitative traits, their genetic architecture and how they covary within and between the two sexes, can be further studied using a quantitative genetics approach – G and B matrices. These are essentially genetic variance-covariance matrices of all traits measured, pairwise comparisons that give a picture of how these traits coevolve within an individual (G), but also how different traits covary between the sexes (B). These tools allow us to understand the underlying genetic architecture of life history traits, and also how these traits change under different and sex-specific selection pressures. This, in turn, will aid our understanding of how sexual dimorphism evolves. I end this review by focusing on a particular model species for studying these questions; the nematode Caenorhabditis remanei.