Though clines were formally defined as recently as 1938 by Julian Huxley, the gradual change of characters within species has been observed by naturalists for centuries. Consequently, the number of published examples of clinal variation is staggering and include clines in morphology, physiology, behavior, and genetic loci. Some morphological clines are so common as to be cited as a 'rule' of nature. The oldest and most contentious of these clines is Bergmann's rule, which posits that body size increases with latitude. The pattern is widespread within and among species of mammals, birds, and some insect groups (e.g., Drosophilids; Figure 1). It appears likely that selection must act on the latitudinal cline in body size because the pattern has evolved within multiple lineages of organisms and on several continents. Further, some of these clines develop over extremely short periods of time. For example, the invasive populations of the fruit fly Drosophila suboscura in North America has evolved a latitudinal cline similar to that seen in native Europe in less than 20 years (Figure 1). As with many published clinal patterns, however, the selective mechanisms underlying Bergmann's rule are unclear. Air temperature sharply declines with latitude and represents the most obvious environmental factor operating on body size, but the exact manner in which temperature drives body size evolution remains unsolved. Other common morphological clines include Allen's rule (populations of homeotherms in colder climates have shorter appendages) and Gloger's rule (populations in more arid environments are paler in color). These and other clines are often discordant. For example, within 100 years of invading North America, house sparrows (Passer domesticus) evolved a north-south cline in body size (Bergmann's rule) and east-west cline in coloration (Gloger's rule).
Physiological traits also reveal clinal variation. For example, subpopulations of the vascular plant Anthoxanthum odoratum can grow on soil containing extremely high concentrations of zinc from mine waste. These plants are highly localized to an area less than 500 m across. When plants are tested in the laboratory, physiological tolerance for zinc of plants is positively related to the level of zinc in the soil. This pattern indicates selection for zinc-tolerance on contaminated soil and a cost to maintaining zinc-tolerance on normal soils. Other organisms reveal clinal variation in the physiological responses to temperature, salinity, and day length among other environmental factors. The best examples of clines in behavioral traits occur within hybrid zones between differentiated populations and species, where mating behavior for each of the parental types changes over space.
Clines at genetic loci (i.e., allozymes, microsatellites, protein-coding loci) are commonly under direct selection or tightly coupled to loci under selection. For example, the LdH of the killifish Fundulus heteroclitus shifts with latitude along the east coast of the United States. One allozyme is more efficient in warmer temperatures, while another is more efficient at cooler temperatures. This cline is also seen in humans across European and Middle Eastern populations. However, some clines may occur at loci that are neither under direct nor indirect
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Figure 1 Drosophila wing size cline in North America and Europe, as a proxy for insect body size. In less than 20 years, a wing size cline for North American females (see map) has evolved that is statistically indistinguishable from that in Europe (upper lines). Males, however, are different (lower lines). Whereas in Europe, the slope for males and females are similar, in North America, the slope of the male cline is very shallow. The error bars indicate ±1 standard error. Figures taken from Gilchrist GW, Huey RB, and Serra L (2001) Rapid evolution of wing size clines in Drosophila subobscura. Genetica 112-113: 273-286.
selection. In theory, these neutral clines must be the consequence of secondary contact (i.e., historically separated populations are reconnected) that are not at equilibrium. At equilibrium, neutral clines should have collapsed because of the ongoing introgression of alleles due to gene flow.
Much of our understanding of single-species clines comes from hybrid zones between two different species because the theory ofclines between and within species is broadly similar. One of the first applications of cline theory was by Szymura and Barton, who studied the hybrid zone between two Bombina toads in Poland (Figure 2). There is striking concordance between morphological and allozyme clines across the same geographic space in this system. This pattern suggests that the species evolved in allopatry and their alleles are currently introgressing into the cline. The high level of linkage disequilibrium - the nonrandom gametic association of alleles between two or more loci - also suggests that strong selection helps to maintain these clines in the face of ongoing gene flow.
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