Coarsegrained Spatial Heterogeneity

Consider a landscape subdivided into spatial patches or habitats that induce different fitness responses from specific genotypes. We further assume that these genotypes experience only one of these patches in their lifetime (or at least, the selectively relevant portion of their lifetime for the unit of selection of interest). As an example, consider the northern acorn barnacle (Semibalanus balanoides) on the northeastern coast of the United States (Schmidt and Rand 2001). This, as well as many other marine species, has planktonic larval dispersal, resulting in high levels of gene flow and negligible population subdivision at the larval stage. The larvae then settle and enter a completely sessile stage for the remainder of their lives. The intertidal region that they settle in is a mosaic of habitats that greatly differ in environmental parameters that affect barnacle survivorship. A specific individual only experiences the habitat in which it settled, so this is coarse-grained, spatial heterogeneity. Schmidt and Rand (2001) performed genetic surveys on larvae and upon different aged adults found in four habitat types (Table 14.1). They estimated habitat-specific viabilities by measuring how genotype frequencies change with age within a particular habitat type. When genetic surveys were performed on mtDNA and upon an isozyme marker Gpi that codes for the enzyme glucose-6-phosphate isomerase, they could detect no heterogeneity either through space or age. These markers indicate the panmictic nature of this population and confirm its high level of gene flow and lack of population subdivision. However, a third genetic marker, the Mpi locus that codes for the enzyme mannose-6-phosphate isomerase, showed considerable heterogeneity. In particular, the age-specific surveys indicated a pulse of genotype-specific mortality that occurred over a two-week interval subsequent to metamorphosis from the larval to the adult form, leading to the estimated viabilities given in Table 14.1, although only the viabilities in the high intertidal zones deviated significantly from neutrality. This variation in viability associated with this locus arises in part from the differential ability to process mannose-6-phosphate through the glycolytic pathway. When mannose was supplemented in the diet and barnacles were exposed to temperature

Table 14.1. Habitat-Specific Viability Estimates for Mpi Genotypes in Northern Acorn Barnacle

Habitat SS SF FF

Table 14.1. Habitat-Specific Viability Estimates for Mpi Genotypes in Northern Acorn Barnacle

Habitat SS SF FF

Exposed substrate in high intertidal zone

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