T

10 km

Figure 2 The hybrid zone between two Bombina toad species in Poland across approximately 10 km. (a) Frequencies of B. variegata allozymes averaged across all loci. (b) Frequencies of seven morphological characters. (c) Standardized linkage disequilibrium, R, averaged across all pairs of loci. There is concordance of morphological and allozyme characters and the highest values of linkage disequilibrium within the hybrid zone. From Szymura JM and Barton NH (1991) The genetic structure of the hybrid zone between the Fire-bellied toads, Bombina bombina and B. Variegata: comparisons between transects and between loci. Evolution 42: 237-261.

narrow clines in poorly dispersed organisms can be maintained by weaker selection. The analytical power of clines is that if the magnitude of selection were known, this could be used along with the width of a cline to estimate dispersal. Conversely, if the magnitude of dispersal were estimated, this could be used to infer selection. The balance between selection and dispersal at the center of the cline is

Distance

Maximum gradient

Environment a WAA = 1-s

Environment A WAA=1+s

Maximum gradient

Distance

K a2

Environment a WAA = 1-s

Environment A WAA=1+s

Figure 3 Selection in continuous populations. Selection, s, in a continuous population may favor allele a on the left of the diagram, and allele A on the right. At equilibrium, the gene frequencies will form a sigmoid cline over the boundary between gene flow and selection (ax/s). The case of different environments is shown here; however, similar clines are formed in the case of intrinsic or frequency-dependent selection, for example, in contact zones between races differing in an underdominant chromosomal rearrangement, or in warning color pattern. From Mallet J (2001) Gene flow. In: Woiwood IP, Reynolds DR, and Thomas CD (eds.) Insect Movement: Mechanisms and Consequences, pp. 337-360. New York: CABI Publishing.

where w is the cline width, xe is the 'effective' selection coefficient, and K is a multiplier that depends on the type of selection. Strict application of this equation requires assumptions be met that might be rare in natural settings, including Gaussian dispersal, weak selection, and genetic equilibrium. The 'effective' selection coefficient acting on the clinal locus includes direct selection on the locus that displays clinal variation in addition to the cumulative levels of indirect selection on linked loci. The multiplier K varies from about 3 in the case of exogenous selection across an ecotone to 4 in the case of heterozygote disadvantage at the center of a cline. Frequency-dependent selection against rare genotypes can increase K to 8-12, because frequency-dependent selection is effectively weaker than heterozygote disadvantage. Because cline

Waa = 1-s width is proportional to the square root of K, different types of selection give cline widths that are similar to within about a factor of 2. In general, however, there are substantial deviations from the theoretical expectation when any type of selection is strong (s > 0.2).

Estimation of the clinal width is relatively straightforward. Traditionally, cline width has been defined as the geographic distance between populations that contain 10% or 20% and 80% or 90% of the parental gene frequencies, but in the theory of eqn [1], cline width is the inverse of the maximum slope of the cline. An estimate of width may assume that allele frequencies vary between 0.0 and 1.0 along the cline, or alternatively, if the populations are not fixed on either side, cline width w = Ap/slope, where Ap is the change in gene frequencies among parental populations at the ends of the cline and slope is the slope at the center of the cline.

Solar Power

Solar Power

Start Saving On Your Electricity Bills Using The Power of the Sun And Other Natural Resources!

Get My Free Ebook


Post a comment