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Why do crosses between closely related species fail to
produce fertile hybrids? A new study shows that differences in the levels - not
necessarily the sequences - of certain key proteins are crucial in mediating
reproductive isolation.
Two individuals are defined as belonging to the same
biological species, if matings between them give rise to viable and fertile
offspring. Crosses between closely related, but already distinct, species
produce hybrid offspring that are either inviable or sterile, and thus cannot
give rise to a self-propagating hybrid lineage. In the early 20th century,
geneticists and evolutionary biologists developed a theoretical model to
explain why distinct species that share the same common ancestor soon diverge to
such an extent that their hybrids are unable to reproduce. This model
postulates that certain genes evolve more rapidly than others – and in a manner
which ensures that they function well within each of the diverging populations,
but interfere with one another when brought together in hybrid genomes.