Selection and gene flow are probably not usually opposed forces in nature.

Gene flow, tending to break down differences between two forms, and natural selection, working on reproductive behavior, should normally unify the ecological and reproductive species concepts.

The few examples we have seen may not be representative of nature; but they do show that natural selection can be powerful enough to overcome gene flow, and to produce integrity without the help of gene flow.

The checkerspot butterfly Euphydras editha pictured opposite provides yet another example of selection alone producing uniformity. Colonies of the butterfly scattered across California have almost no gene flow between them and yet it remains a good species, leading Ehrlich to conclude:

"There seems no possibility that gene flow 'holds together' widely scattered populations."