In population genetics, what concept explains why marine populations already carry freshwater-adapted alleles that facilitate colonization of freshwater?

• A. Standing genetic variation.
• B. New mutation.
• C. Genetic drift.
• D. Founder effect.

Answer: (A)

Standing genetic variation is the presence of existing genetic diversity in a population, including alleles that may be advantageous under different conditions. In marine populations, freshwater-tolerance alleles can already exist at low frequencies due to past environmental variation. When a population encounters freshwater, those pre-existing freshwater-adapted alleles experience positive selection and increase in frequency, enabling rapid colonization without waiting for a new mutation. This makes adaptation faster and more likely across populations than waiting for new beneficial mutations. The other processes don’t explain this pattern: a new mutation would need to arise after exposure and is slower; genetic drift is random and not specifically directional toward freshwater tolerance; founder effects describe reduced variation from founding a new population and don’t inherently account for pre-existing adaptive alleles.