A cline is one form of geographic variation in which characteristics of a species change gradually through the species’ geographic range.

Many plant and animal species have populations that differ in terms of their morphological, physiological, and biochemical characteristics. A species is generally defined as a group of organisms that have the potential to interbreed and produce fertile offspring.

A population is defined as a group of organisms which are actively interbreeding. The following example will clarify the relationship between species and populations and simultaneously introduce geographic variation.

Geographic Variation

The ponderosa pine (Pinus ponderosa) occupies a broad geographic range in western North America. Leaves (needles) of ponderosa pines in the Rocky Mountains are bundled into groups of two or three, and cones of these trees are more than 9 centimeters long.

In contrast, leaves of ponderosa pines in southern Arizona and northern Mexico are bundled in groups of five, and their cones are less than 9 centimeters long. These differences constitute geographic variation which has developed because reproduction between Rocky Mountain ponderosa pines and Arizona-Mexican ponderosa pines was restricted because of geographic separation.

Despite their differences, the two groups belong to the same species because they could produce fertile offspring if their geographic separation were overcome. However, they are members of different populations because they are not currently interbreeding. They are different populations of the same species.

Geographic variation
Geographic variation

Such geographic variation occurs in many species with broad geographic ranges and is often due to differences in the environmental conditions under which the separate populations exist.

The different environments select for different genetic adaptations, resulting in hereditary variation. If such geographic variation occurs gradually over the range of the species, it is clinal variation.

Clinal Geographic Variation

In the foregoing example, the geographic variation is too abrupt to be considered clinal. However, ponderosa pines in the Sierra Nevada of California do show clinal geographic variation.

The pines at the base of the mountains grow appreciably larger than the pines growing at the highest elevation on the mountains. The change in size is gradual; ponderosa pine trees become progressively smaller as elevation increases.

By taking seeds from trees at several elevations and planting them at the same elevation, scientists showed this size variation to be hereditary. Although all the trees were grown under the same conditions, the largest trees grew from the seeds collected at the base of the mountains, and tree size decreased as the elevation of seed origin increased.

The advantages to being small in the relatively harsh environment of the high mountains and tall at the mountain base were important enough to code tree size into the trees’ genes. The yarrow (Achillea lanulosa) and a number of other plant species show similar clinal variation with elevation in mountains.

In clinal variation, populations are not completely separated from one another, and individuals from adjacent populations do interbreed. However, reproduction between populations is not as common as reproduction between members of the same population. As a result, slight differences between adjacent populations are maintained.

Interestingly, in some clines members of the two extreme populations (the populations at the two ends of the cline) may not be able to interbreed and produce fertile offspring.

They are still considered to be members of the same species because they exchange genes through the intermediate populations. The seaside goldenrod (Solidago sempervirens) illustrates this. It grows along the Atlantic coast of North America and displays a cline in flowering time.

Canadian plants flower in August, plants in the middle Atlantic states flower in September and October, and those in Florida do so in November. These are genetically controlled flowering times, so even if grown together, the plants from Florida and Canada could not interbreed.

However, because Canadian plant flowering times overlap those in the northern United States (which overlap those in the central United States, which overlap those to their south, which overlap those in Florida), there is interbreeding between all adjacent populations and, indirectly, between the Canadian plants and the Florida plants.

If the cline were to be subdivided into two or more species, where would the separations be drawn without separating interbreeding organisms into different species? The simplest solution is to consider all members of the cline to be members of the same species.

Local Clinal Variation

White clover lawn
White clover lawn

Great distances are not always required to establish clines. White clover (Trifolium repens), a European native which has been introduced all over the world, affords an example. Some white clover plants release cyanide when parts of the plant are eaten by grazers, such as snails and slugs.

Others do not. The cyanide protects the plant from further grazing because it is toxic to the grazers. However, the cyanide-releasing form of clover suffers more frost damage than clover plants that do not release cyanide. Plants protect themselves from the cyanide by sequestering it into cellular compartments.

Frost damage occurs when cyanide is released into the plant cells after those compartments are ruptured by ice crystals. Cyanide-storing plants also grow more slowly than forms that do not store cyanide, because some energy that could otherwise be used for growth is required to sequester the cyanide.

Latitudinal and elevational temperature gradients generate clines in the production of cyanide, with greater cyanide production at lower elevations and latitudes. More frequent freezing results in more frequent cyanide damage, and low temperatures result in less grazing, because grazers are not as active.

Plants that do not go to the expense of storing cyanide are favored under those conditions. This is a classic geographic cline. However, changes in grazing pressure overmuch smaller distances also generate clines in cyanide storage.

Grazing pressure changes over meters when white clover grows in a garden protected by pesticides and in an adjacent, unprotected field. The result is a cline in cyanide storage by white clover very similar to the geographic clines discussed above but on a scale of meters.