In ecological speciation, what is the role of divergent selection in marine versus freshwater environments?

• A. Reproductive isolation can occur purely by geographic barriers without ecological differences.
• B. Local adaptation cannot lead to isolation.
• C. Ecological divergence never influences gene flow.
• D. Divergent selection in marine versus freshwater environments leads to reproductive isolation and eventual speciation.

Answer: (D)

Divergent selection across different environments can push populations down separate evolutionary paths, building reproductive barriers as each group adapts to its own habitat. In marine versus freshwater settings, the distinct salinity, resources, and ecological pressures favor different traits, so individuals become better suited to their own environment and less fit in the other. Hybrids between these populations often have lower fitness in either habitat, which reduces successful gene flow and strengthens isolation. Over time, this ecological separation can lead to mating patterns that prefer locally adapted partners, driving reproductive isolation and ultimately speciation. The idea here is that the ecological differences themselves create the barriers that separate gene pools, not just physical separation or static local adaptation alone.