How does the presence of freshwater-adapted alleles in marine populations influence postglacial recolonization dynamics?

• A. Freshwater alleles have no effect on recolonization dynamics.
• B. Only new mutations drive recolonization.
• C. Marine populations with standing freshwater alleles can respond quickly to freshwater colonization, accelerating adaptation.
• D. Freshwater alleles slow down adaptation in marine populations.

Answer: (C)

The main idea is that standing genetic variation lets populations adapt quickly when the environment changes. Marine populations that already carry freshwater-adapted alleles have a ready-made set of beneficial variants to draw on as they encounter freshwater conditions during postglacial recolonization. Selection can immediately increase the frequencies of those alleles, helping individuals survive and reproduce in freshwater and speeding the establishment and spread of populations in those habitats. Relying on new mutations would take longer, whereas pre-existing variation provides a rapid route to adaptation. So, having freshwater-adapted alleles in marine populations accelerates recolonization by enabling a faster adaptive response.