What is the primary scientific technique used to identify a signal of coevolution across different locales?

Answer

Correlating trait measurements across interacting populations.

Scientists look for spatial covariance in traits; if populations of species A in location X possess a trait differing from those in location Y, and species B's traits correlate inversely across those same locations, it suggests reciprocal selection.

What is the primary scientific technique used to identify a signal of coevolution across different locales?
speciesEcologyEcosysteminteractioncoevolution