Strong natural selection causes microscale allozyme variation in a marine snail.

Abstract

Natural selection is one of the most fundamental processes in biology. However, there is still a controversy over the importance of selection in microevolution of molecular traits. Despite the general lack ofdata most authors hold the view that selection on molecular characters may be important, but at lower rates than selection on most phenotypic traits. Here we present evidence that natural selection may contribute substantially to molecular variation on a scale of meters only. In populations of the marine snail Littorina saxatilis living on exposed rocky shores, steep microclines in allele frequencies between splash and surf zone groups are present in the enzyme aspartate aminotransferase (allozyme locus Aat; EC. 2.6.1.1). We followed one population over 7 years, including a period of strong natural perturbation. The surf zone part of the population dominated by the allele atwl' was suddenly eliminated by a bloom of a toxin-producing microflagellate. Downshore migration of splash zone snails with predominantlyAat120 alleles resulted in a drastic increase in surf zone frequency of Aat 20, from 0.4 to 0.8 over 2 years. Over the next four to six generations, however, the frequency of Aat120 returned to the original value. We estimated the coefficient of selection ofAat120 in the surfzone to be about 0.4. Earlier studies show similar or even sharperAat clines in other countries. Thus, we conclude that microclinal selection is an important evolutionary force in this system.

Gene Flow Versus Local Adaptation in the Northern Acorn Barnacle, Semibalanus Balanoides: Insights From Mitochondrial Dna Variation

“…Recent studies of S. balanoides by Schmidt and Rand (1999) and Schmidt et al (2000) and other marine invertebrates (McDonald 1991;Johannesson et al 1995) indicate that selection coefficients this high may in fact be common for enzyme polymorphisms. However, for Mpi in barnacles the spatial scale of this selection may be quite small (meters: Holm and Bourget 1994;Schmidt and Rand 1999), making the notion of local adaptation inappropriate for this mosaic system.…”

Regional structuring of genetic variation in short-lived rock pool populations of Branchipodopsis wolfi (Crustacea: Anostraca)

“…Kimura 1983;Futuyama 1986; but see e.g. Riddoch 1993;Johannesson et al 1995). It is very unlikely that and geographic distance at this site.…”

“…As in some cases (e.g. Riddoch 1993;Johannesson et al 1995) allozyme frequencies correlated to environmental variables, we have tested for an association between allele frequencies and all environmental variables measured. Associations between environmental variables and genetic characteristics (genetic diversity measures, allele frequencies) were assessed with principal component analysis run for a correlation matrix in a varimax rotation mode and with the Pearson correlation coefficient with Bonferroni adjusted probabilities (Systat 8, SPSS Inc.).…”

Are polymorphism and evolutionary rate of allozyme proteins limited by mutation or selection?

“…Furthermore, there are now many examples of evidence of selection acting at individual allozyme loci (e.g. Singh & Rhomberg, 1987;Karl & Avise, 1992;Berry & Kreitman, 1993;Johannesson et al, 1995;Pogson et al, 1995;Watt et al, 1996). All this evidence pointing to selection is perfectly compatible with variation in mutation rate being the dominant cause of variation in allozyme protein heterozygosity and genetic distance.…”