What is one cause of structural (morphological) isolation?

Structural (morphological) isolation refers to the physical differences between species or populations that prevent them from mating or producing viable offspring. One of the primary causes of this type of isolation is differences in genitalia structure. When two species have evolved different reproductive structures, they may be physically unable to copulate even if they are in the same area and could potentially mate. This reproductive barrier ensures gene flow does not occur between them, contributing to speciation over time.

In contrast, variations such as different habitats within the same space could lead to behavioral isolation rather than structural. Similarly, incompatible timing of reproduction leads to temporal isolation, while variation in genetic drift pertains more to changes in allele frequencies within populations rather than direct physical barriers to reproduction. Hence, differences in genitalia structure stands out as a direct cause of structural isolation that facilitates speciation by preventing successful mating.