In a common garden experiment, if two populations still differ in gill raker length after rearing in the same environment, what can we conclude?

• A. Differences have a genetic basis.
• B. Differences are entirely environmental.
• C. Differences are due to mutation rate.
• D. Differences due to random chance.

Answer: (A)

In a common garden, you control the environment so any remaining differences in a trait come from genetics. If two populations still show different gill raker lengths after being raised in the same conditions, that means the trait is heritable and there are genetic differences between the populations for this characteristic. Environmental effects have been removed, so the persistent difference points to genetic divergence, possibly due to local adaptation, selection, or drift.

If differences were entirely environmental, they would disappear once both groups experience the same environment. Mutation rate wouldn’t reliably explain a stable, between-population difference observed after rearing under identical conditions, and random chance alone wouldn’t produce a consistent, population-wide difference.