Selection can produce uniformity in the absence of gene flow.

An example comes from work by Ochman et al on the snail Cepaea nemoralis (pictured opposite) in the Pyrenees. The snail rarely lives above 1500 m in the mountains because of the cold. In the Pyrenees, it lives in neighboring river valleys separated by mountains: where those mountains are higher than 1500 m, gene flow between valleys will be absent. If gene flow is required to maintain the integrity of the species, the populations in different valleys should have diverged.

Research showed that shell morphology had little geographic differentiation: the frequency distribution of color forms and banding patterns was much the same in all populations.

The frequencies of different protein forms had diverged between different areas, but in a pattern that transcended the mountainous barriers to gene flow. Ochman et al. recognized three main areas, within which protein form frequencies were relatively constant; but within each of the areas there are several valleys separated by mountains. Gene flow cannot explain the uniformity within each of the three areas.

Cepaea in the Pyrenees, therefore, is an example of selection maintaining uniformity between populations when there is no gene flow.

Figure: the next screen shows two maps of the Pyrenees. Top map: shell morphology (that is shell color) shows little geographic variation whereas in the bottom map protein polymorphism falls into three main areas. The map is for the four alleles of one enzyme, indophenol oxidase (Ipo-1). The similarity within mountainous areas is unlikely to be maintained by migration (gene flow). From Ochman et al. (1983).