The caveats

While the approach Arnold suggests is intriguing, there are a number of caveats that must be kept in mind in trying to apply it.

• This approach assumes that the $\bf G$ matrix remains constant.

• This approach cannot distinguish strong selection that happened over a short period of time from weak selection that happened over a long period of time.

• This approach assumes that the observed differences in populations are the result of selection, but populations isolated from one another will diverge from one another even in the absence of selection simply as a result of genetic drift.
  • Small amount of differentiation between populations within sites could reflect relatively recent divergence of those populations from a common ancestral population.

  • Large amount of differentiation between populations from inland versus coastal sites could reflect a more ancient divergence from a common ancestral population.