How do ancestral marine alleles contribute to freshwater stickleback adaptation?

• A. They are always selected against in freshwater environments
• B. They have no role in adaptation
• C. Some freshwater populations retain ancestral marine alleles that can be selected against or repurposed, showing complex allele frequency changes.
• D. They always fix in freshwater populations

Answer: (C)

Ancestral marine alleles can supply standing genetic variation that freshwater sticklebacks draw on during adaptation. When marine and freshwater populations split and colonize new habitats, alleles that were present in the ancestral marine gene pool can persist in freshwater populations at varying frequencies. Selection in freshwater environments can act to increase, decrease, or repurpose these alleles. Some ancestral marine alleles may become beneficial in freshwater and rise in frequency, some may be neutrally stored or repurposed for new functions, and others may be maladaptive and drop in frequency. This leads to nuanced, population-specific allele frequency changes rather than a uniform pattern.

So the best answer reflects that freshwater populations can retain ancestral marine alleles that can be selected for or repurposed, producing complex changes in allele frequencies. These variants do play a role in adaptation, and they do not always fix in the population.