This type of variation may be adaptive. Models of geographic variation have shown that genetic differentiation among populations may be determined by gene flow and selection pressures on populations. Gene flow tends to decrease differentiation. Reduced gene flow among populations leads to increased geographic differentiation. Restriction of gene flow may occur in continuously distributed populations by distance alone.
Natural selection increases genetic differentiation among populations.
He provides such examples from natural populations. Adaptiveness of Geographic Variation Ecotypic Variation Demonstrated to be due to heritable differences. When different populations have different mean phenotypic values, determining whether the differences are the result of genetic factors, environmental factors, or both is difficult. If individuals from the populations can be grown in the same environment, we can make inferences about the nature of the variation we have observed. Such approaches are called common-garden experiments .