Cline

Definition

A gradual, continuous geographic gradient in the frequency of a phenotype, genotype, or allele within a species, typically correlated with an environmental gradient such as latitude, altitude, or climate. Clines result from selection pressures that vary across space, gene flow that homogenizes populations, or a combination of both; they may be smooth (primary clines) or stepped (secondary clines where gene flow is restricted).

Etymology

From Greek klinein, 'to lean, slope'

Example

In the grasshopper Chorthippus brunneus, body size shows a latitudinal cline across Europe, with larger individuals in cooler northern populations due to Bergmann's rule and shorter growing seasons.

Synonyms

• character gradient
• geographic gradient
• ecocline

Related Terms

• Ecotype
• Bergmann's rule
• Allen' rule
• Gene flow
• local adaptation
• ring species
• Subspecies
• cline analysis
• isolation by distance

Usage Notes

Contrast with 'cline' as a taxonomic author (e.g., Cline, 2009 for nitidulid beetles) and 'ecocline' (environmental gradient without implying genetic response). Clines may be clinal (adjective) or stepped; 'cline' alone usually implies continuous variation. Not all geographic variation forms clines—discrete jumps indicate barriers or distinct populations. In entomology, clines are frequently analyzed for wing length, coloration, or diapause timing across altitudinal or latitudinal transects.