If a PCA plot shows two clusters corresponding to ecotypes, how would you interpret it?

• A. It indicates random mating across ecotypes.
• B. It indicates genetic differentiation aligned with ecotypes; suggests limited gene flow and adaptation to different niches.
• C. It indicates no genetic differentiation between ecotypes.
• D. It reflects measurement error causing clusters.

Answer: (B)

PCA reveals patterns of genetic structure, and when two clusters line up with ecotypes, it shows that the ecotypes have distinct genetic backgrounds. This indicates genetic differentiation between the ecotypes, suggesting limited gene flow and that each ecotype has its own gene pool shaped by adaptation to different niches or environments. If there were widespread random mating across ecotypes, the clusters would mix rather than form clean, separate groups. If there were no genetic differentiation, individuals from both ecotypes would appear in a single, overlapping cloud. If measurement error were driving the result, you’d expect random scatter without a coherent pattern linked to ecotypes. So, the best interpretation is that the ecotypes are genetically distinct populations with limited gene flow and ecological divergence.