These complex and highly coordinated processes are thoroughly checked by natural selection at each meiosis, and so the genetic unity of the species is preserved. The evolution of geographically isolated populations, however, leads to the fixation of novel chromosomal rearrangements and a divergence of the factors controlling DSB formation and the DNA sequences involved in homology search. In hybrids, karyotypic and genetic divergence can result in meiotic aberrations and variable degrees of infertility due to germ-cell death or the formation of unbalanced gametes.