Reproductive isolation develops with increasing genetic divergence

As pairs of species diverge genetically, they become increasingly reproductively isolated (Figure 22.5). Both the rate at which reproductive isolation develops and the mechanisms that produce it vary from group to group. Reproductive incompatibility has been shown to develop gradually in many groups of plants, animals, and fungi, reflecting the slow pace at which incompatible genes accumulate in each lineage. In some cases, complete reproductive isolation may take millions of years. In other cases (as with the chromosomal fusions of Rhogeessa described above), reproductive isolation can develop over just a few generations.

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Figure 22.5 Reproductive Isolation Increases with Genetic Divergence Among pairs of Drosophila species, the more the species differ genetically (x axis), the greater their reproductive isolation from each other (y axis). Each dot represents a comparison of one species pair. Such positive relationships between genetic distance and reproductive isolation have been observed in many groups of plants, animals, and fungi.

Question

Q: Why might some genetically similar species pairs exhibit high levels of reproductive isolation?

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Partial reproductive isolation has evolved in many strains of plants that have been artificially isolated by humans. In 1835, Thomas Drummond collected seeds of a newly discovered species of phlox in Texas and distributed them to nurseries in Europe. The European nurseries established more than 200 true-breeding strains of Drummond's phlox that differed in flower size, flower color, and plant growth form. The breeders did not select directly for reproductive incompatibility between strains, but in subsequent experiments in which strains were crossed and seed production was measured and compared, biologists found that reproductive compatibility between strains had been reduced by 14 to 50 percent, depending on the cross—even though the strains had been isolated from one another for less than two centuries.