What causes reproductive isolation due to Dobzhansky-Muller incompatibility?

Dobzhansky-Muller incompatibility refers to a specific type of reproductive isolation that arises when two populations that have been separated evolve independently. In this context, the genetic incompatibility happens because specific gene interactions that occur in one lineage are disrupted when genes from both lineages come together in a hybrid.

This means that the incompatibility arises not merely from the presence of different alleles, but rather from the functional interactions between those alleles from different evolutionary backgrounds. Essentially, if each lineage has adaptations to its environment or unique evolutionary paths, when they hybridize, the novel combinations of alleles can lead to malfunctions or reduced fitness due to negative interactions.

Thus, the correct answer highlights that reproductive isolation is caused by the failure of loci (gene locations) from distinct lineages to function together harmoniously in hybrids, leading to reduced viability or fertility, and thus preventing the two populations from successfully interbreeding.