The emergence of barriers to reproduction between two populations is one of the most important features of speciation. Among the mechanisms of reproductive isolation are incompatible interactions between gene products of the parental species that reduce the fitness of hybrid individuals. The accumulation of such incompatibilities is described by the Bateson–Dobzhansky–Muller model (BDM)  that provides a framework for understanding how genes can coevolve to stay compatible within populations and become incompatible between populations. Only a handful of such loci have been identified and characterized at the molecular level. In this issue of EMBO Reports, Jhuang and colleagues  show that BDM incompatibilities have accumulated between a nuclear‐encoded gene and a mitochondrial ribosomal RNA between two yeast species.
See also: H‐Y Jhuang et al
Central to the study of speciation is the identification of the barriers that prevent gene exchanges between species and that maintain their genetic independence. Various mechanisms are known to prevent gene flow between species. For instance, pre‐zygotic isolation could occur when one species is unable to recognize the opposite sex of the other species as potential mates, or when species are isolated in space. Other mechanisms act after zygote formation, for instance, through the maladaptation of hybrid traits that makes hybrid individuals unable to thrive in the environments available to them. Alternatively, hybrids may suffer from intrinsic developmental problems and fail to develop normally, leading to poor viability or impaired fertility. Such intrinsic incompatibilities may arise from large chromosomal rearrangements and inversions that prevent proper chromosome segregation  or the accumulation of genetic changes between two populations of incipient species, making the alleles of specific genes compatible within species but incompatible when reunited in a hybrid individual (Fig 1A). Because many loci can potentially evolve following this scenario, …
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