There was much debate in 1950's about its importance. It now is generally thought to be important in many populations. Non-genetic variation due to direct responses of the phenotype to environmental factors. 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.