Chromosomal inversions are structural mutations that invert the orientation and thus the sequence of a chromosomal segment; in diploid heterozygous individuals, when one chromosome carries the inverted segment and the other homologous chromosome is noninverted, recombination is strongly or even completely suppressed. Most inversions are deleterious or neutral, but occasionally they are beneficial. Positive selection can establish a new, initially rare inversion via indirect (linked) selection (e.g. when the inversion captures a locally adaptive haplotype and then ‘hitchhikes’ with it) or via direct positive selection (e.g. when a beneficial mutation arises fortuitously at the breakpoints). After their establishment, adaptive inversions often seem to be maintained by balancing selection in a polymorphic state, that is, they are neither lost nor do they become fixed at 100% frequency. Such balancing selection acting on inversion polymorphisms might involve overdominance, associative overdominance, negative frequency‐dependent selection, spatially and/or temporally varying selection.
Key Concepts
Chromosomal inversions are structural mutations that reverse the segment of a chromosome.
In heterozygous state, inversions suppress recombination between inverted and noninverted (standard arrangement) chromosomes.
Most inversions are deleterious or neutral; rarely they are beneficial.
Positive selection can establish an initially rare inversion in a population despite drift.
If the inversion happens to capture a locally adapted haplotype, it can ‘hitchhike’ with it and increase in frequency; it has an advantage because it protects the haplotype from recombination and maladaptive gene flow.
Another possibility is that the inversion breakpoints directly and fortuitously generate an adaptive mutation.
Once established, balancing selection can maintain the inversion polymorphism; this might happen due to heterozygote advantage, frequency‐dependent selection, spatially and/or temporally varying selection.