What is the significance of 'standing variation' versus 'new mutations' in repeated stickleback adaptation?

• A. New mutations are always faster than standing variation.
• B. Standing variation allows rapid adaptation through existing alleles, while new mutations may contribute later; standing variation often explains rapid, repeated patterns.
• C. Only standing variation is relevant; mutations never matter.
• D. Standing variation causes slower adaptation than new mutations.

Answer: (B)

Standing variation means the genetic options for a trait already exist in the ancestral population before the environment changes. In sticklebacks, many traits that differentiate freshwater from marine forms—like armor plating—can be driven by alleles that were already present at low frequency in the marine population. When populations invade freshwater, natural selection can quickly boost the frequency of those pre-existing alleles, producing rapid, repeated shifts in many independent populations. That rapid, parallel adaptation is why standing variation often explains these repeated patterns. New mutations, while they can contribute later or in unique ways, arise randomly and are slower to accumulate, so they don’t usually drive the fast, repeated changes seen across many populations.