Much of the genetic variation within species is preserved as differences among members living in different places (populations). Populations often vary genetically because they are subjected to different selective pressures in different environments. Some individuals of the white clover (Trifolium repens) produce the poisonous chemical cyanide. Poisonous individuals are less appealing to herbivores—particularly mice and slugs—than are nonpoisonous individuals. However, clover plants that produce cyanide are more likely to be killed by frost, because freezing damages cell membranes and releases cyanide into the plant’s own tissues.

In European populations of Trifolium repens, the frequency of cyanide-producing individuals increases gradually from north to south and from east to west (Figure 20.18). A pattern of gradual change in phenotype across a geographic gradient is known as clinal variation. In the white clover cline, poisonous plants make up a large proportion of populations only in areas where winters are mild. Cyanide-producing individuals are rare where winters are cold, even though herbivores graze clovers heavily in those areas.