Genetic variation is maintained in geographically distinct subpopulations

Much of the genetic variation in large populations is preserved as differences among members in different places (subpopulations). Subpopulations often vary genetically because they are subjected to different selective pressures in different environments. Plant species, for example, may vary

The proportion of cyanide-producing individuals increases gradually along a gradient from colder to milder winters.

The proportion of cyanide-producing individuals increases gradually along a gradient from colder to milder winters.

These white lines connect points with equal January mean temperatures.

These white lines connect points with equal January mean temperatures.

White indicates proportion of plants not producing cyanide

Red indicates proportion of plants producing cyanide

23.21 Geographic Variation in Poisonous Clovers The frequency of cyanide-producing individuals in each population of white clover (Trifoliumrepens) is represented by the proportion of the circle that is red.

geographically in the chemicals they synthesize to defend themselves against herbivores. Some individuals of the clover Trifolium repens produce the poisonous chemical cyanide. Poisonous individuals are less appealing to herbi-vores—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 the toxic cyanide into the plant's own tissues.

In populations of Trifolium repens, the frequency of cyanide-producing individuals increases gradually from north to south and from east to west across Europe (Figure 23.21). Poisonous plants make up a large proportion of clover 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.

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Essentials of Human Physiology

Essentials of Human Physiology

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