Inversions and Evolution

Abstract

Abstract Inversions are chromosomal rearrangements where the order of genes is reversed. Inversions originate by mutation and can be under positive, negative or balancing selection. Selective effects result from potential disruptive effects on meiosis, gene disruption at inversion breakpoints and, importantly, the effects of inversions as modifiers of recombination rate: Recombination is strongly reduced in individuals heterozygous for an inversion, allowing for alleles at different loci to be inherited as a ‘block’. This may lead to a selective advantage whenever it is favourable to keep certain combinations of alleles associated, for example under local adaptation with gene flow. Inversions can cover a considerable part of a chromosome and contain numerous loci under different selection pressures, so that the resulting overall effects may be complex. Empirical data from various systems show that inversions may have a prominent role in local adaptation, speciation, parallel evolution, the maintenance of polymorphism and sex chromosome evolution. Key Concepts Inversions are chromosomal rearrangements where the order of loci is reversed. Inversions can be under negative, positive or balancing selection. A key effect is that they reduce recombination in individuals heterozygous for the arrangement. They therefore facilitate the maintenance of ‘blocks’ of associated alleles. This is favourable under local adaptation, positive epistasis and frequency‐dependent selection favouring multiple different morphs in a population. Accordingly, inversions have been shown to contribute to the maintenance of within‐species polymorphism and between‐species divergence in various empirical studies. Inversions also contribute to speciation by coupling different barriers to gene flow. Finally, they also contribute to sex‐chromosome differentiation and degeneration. Identifying the targets of selection within inversion is challenging but possible, at least for old inversions. Knowing the evolutionary history of inversions is fundamental to understanding their influence in adaptation and speciation.