The evolution of sex and two sexes

Sexual reproduction has proven so formidable a challenge for evolutionary biologists that it is commonly spoken of as “the paradox of sex.” Stated simply, individuals forsake one-half of their genetic representation in offspring by engaging in sexual versus asexual reproduction. There must be substantial benefits to compensate for so great a cost. Recent theory proposes that the primary benefit of sex is the tremendous diversity of genotypes produced via recombination during sexual reproduction that provides the raw material necessary to compensate for mutational erosion of mitochondrial genes. Another line of new thinking proposes that the reason that virtually all eukaryotes have two mating types rather than multiple mating types is that the existence of two mating types enables single mitochondrial genotypes to be vetted for compatibility with nuclear genotype. This chapter considers the implications and evidence for these new mitonuclear-based theories of key evolutionary ideas.