Adaptation to freshwater environments in sticklebacks can proceed via standing genetic variation without requiring new mutations.

• A. Strongly disagree
• B. It sometimes requires new mutations
• C. It occurs only in marine populations
• D. True

Answer: (D)

Adaptation using standing genetic variation means that alleles already present in a population can be favored when the environment changes, allowing rapid evolution without waiting for new mutations. In sticklebacks, after glacial periods many marine ancestors repeatedly colonized freshwater habitats, and similar freshwater forms evolved by shifting the frequencies of preexisting alleles. A well-known example is an allele in the Eda gene that reduces armor plates; this variant exists at low frequency in marine populations and rises in frequency in freshwater where heavy armor is less advantageous. Another example involves regulatory variants at Pitx1 that affect pelvic reduction, which can also arise from standing variation in ancestral populations. Because these variants are already in the gene pool, freshwater adaptation can occur quickly and in parallel across many independent populations. While new mutations can contribute over longer timescales, the frequent and rapid freshwater changes in sticklebacks illustrate that adaptation can proceed via standing genetic variation without requiring new mutations.