Geographical Variation


Geographical variation refers to differences among populations in genetically based traits across the natural geographic range of a species. Understanding the factors that give rise to and maintain geographical variation helps elucidate the causes and consequences of evolution. In the simplest case, we divide these factors into purely genetic versus environmental components to tease apart their relative contributions to observed phenotypic variation. Subsequent experiments allow us to partition further the genetic components of phenotypic variation towards a molecular understanding, hopefully to reveal the actual deoxyribonucleic acid (DNA)‐scale changes giving rise to adaptations we see at the population level. Studies of a select number of organisms – all of which begin with geographic variation – demonstrate this highly productive strategy. Given its relevance to differentiation among populations, geographic variation also has direct bearing on the question of species origins.

Key Concepts:

  • Geographical variation refers to differences among populations of organisms in genetically based traits across the natural geographic range of that species.

  • Understanding how initial population size, gene flow, mutation frequency, and environmental (abiotic) factors give rise to and maintain geographical variation can shed light on direction and rate of evolutionary change.

  • Clinal variation – a form of geographic variation – suggests past or present action of natural selection.

  • Reciprocal transplantation and ‘common garden’ experiments are two effective manipulations that can reveal genetically based phenotypic variation.

  • A number of studies of geographic variation, including those about sickle‐cell anaemia, warfarin‐resistance, and lepidopteran colouration, have shifted to a more molecular focus.

  • A number of other studies attempt to understand geographic variation in the broader context of community and ecosystem structure.

  • Given the multi‐faceted approach to understanding it, geographic variation can help address the question of species’ origins.

Keywords: adaptation; drift; clines; selection; speciation

Figure 1.

The effects of population size on random genetic drift. Allele frequencies for six different genes are plotted from the results of a Monte Carlo random simulation. Simulation models permit investigation of evolutionary change when random influences (e.g. genetic drift and founder effects) may be important. Although each run of the simulation model will produce slightly different results due to differences in random effects, overall mall populations lose genetic variation rapidly and are subject to rapid shifts in gene frequencies from one generation to the next. Large populations, in contrast, appear more resilient to the effects of random genetic drift. These models were generated using ‘Populus’ (, a simulation program available from Donald Alstad at the University of Minnesota. Initial allele frequencies were 0.5 for all genes, and simulations were run for 100 generations for population sizes of 20 (top) and 100 (bottom).

Figure 2.

Founder effects and population differentiation. Founded by one or a few individuals, ‘satellite’ populations have different allele frequencies from the more genetically heterogeneous source (and from each other) as a result of founder effects. Founder effects may play a role in rapid divergence and speciation because they sometimes lead to combinations of genes of superior fitness that could not persist in the larger source population.

Figure 3.

The effects of migration on genetic variation within and among populations. In clusters of small populations, migration among populations tends to eliminate genetic differentiation and increase genetic variation within populations (top). In the absence of immigration, genetic drift may lead to differentiation among populations and the loss of genetic variation within populations (bottom).

Figure 4.

Clinal variation in body size. A cline forms when a trait shows gradual change along an environmental gradient (e.g. latitude or altitude). Many endotherms (e.g. birds and mammals) show an increase in body size from south to north – a pattern of variation known as Bergmann's rule. In contrast, many ectotherms (e.g. insects) show the ‘converse to Bergmann's rule’, whereby body size decreases with latitude.

Figure 5.

Cryptic colouration in the peppered moth, Biston betularia. In pristine habitats, like Dorset (England), the carbonaria form stands out against the lichen‐covered tree bark. However, in polluted areas, such as Birmingham (England), the typica form appears more visible and, thus, more susceptible to visually guided predators, for example, insectivorous birds. The arrows point to the cryptic forms (from Kettlewell, ).

Figure 6.

Evidence for ecological character displacement in Darwin's finches, as plotted on histograms for beak depths for two species of finches (Geospiza fuliginosa, light bars, and Geospiza fortis, dark bars) on three different islands. Beak depths on Santa Cruz Island, where both finches co‐occur, appear markedly different. However, beak depths appear less extreme and have overlapping distributions where the two species occur alone, that is, on different islands.



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Further Reading

Endler JA (1977) Geographic Variation, Speciation, and Clines. Princeton, NJ: Princeton University Press.

Futuyma DJ (1998) Evolutionary Biology, 3rd edn. Sunderland, MA: Sinauer Associates.

Grant BR and Grant PR (1989) Evolutionary Dynamics of a Natural Population. Chicago, IL: University of Chicago Press.

Haliburton R (2004) Introduction to Population Genetics. Upper Saddle River, NJ: Pearson Prentice‐Hall.

Mousseau TA, Sinervo B and Endler JA (eds) (2000) Adaptive Genetic Variation in the Wild. New York: Oxford University Press.

Roff DA (1992) The Evolution of Life Histories. New York: Chapman.

Schlichting CD and Pigliucci M (1998) Phenotypic Evolution: A Reaction Norm Perspective. Sunderland, MA: Sinauer Associates.

Tauber MJ, Tauber CA and Masaki S (1986) Seasonal Adaptations of Insects. New York: Oxford University Press.

Zimmer C (2010) The Tangled Bank: An Introduction to Evolution. Greenwood, Colorado: Roberts and Company Publishers.

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Olvido, Alexander E, and Mousseau, Timothy A(Oct 2012) Geographical Variation. In: eLS. John Wiley & Sons Ltd, Chichester. [doi: 10.1002/9780470015902.a0001736.pub3]