2. Molecular Genetics and the Conservation of Diversity
23
There remains, however, disagreement over the criteria for recognising ESUs
(Nielsen and Powers 1995). I have argued that molecular signatures of substantial
historical isolation, such as reciprocal monophyly of mtDNA alleles (in animals)
should be the primary criterion (Moritz 1994; Moritz et al. 1995), whereas others
suggest that both molecular and phenotypic measures of divergence should be considered (e.g. Dizon et al. 1992; Vogler and DeSalle 1994; Waples 1995). The term
"Evolutionarily Significant Unit" itself is, perhaps, unfortunate as it suggests that
we can predict the evolutionary potential of individual popUlations from their genetic makeup. Other than for very specific selective factors (e.g., insecticide resistance), this is unlikely to be the case.
The view that both genetic and phenotypic criteria should be used for defining
ESUs confounds two different evolutionary processes - historical isolation and
adaptation (Fig. 1). Historical isolation, combined with genetic drift or divergent
selection will generate unique and irreplaceable combinations of genotypes, which
mayor may not be manifest as differences in phenotype (e.g., see below). This
process of isolation and divergence (but only rarely selection) can be studied readily
using molecular makers through analyses of phylogeography (Avise et al. 1987).
By contrast, the process of adaptation and divergent selection will generate differences in phenotype among populations, but because it can occur in the presence of
gene flow (Endler 1977) there need not be significant phylogeographic structure.
Importantly, recent experimental studies with natural popUlations have demonstrated that divergence of phenotypes through selection can occur rapidly and readaptive
divergence
phenotypic
divergence
L..-_ _ +-I~ phylogeographic
structure
historical isolation
Fig. 1. Illustration of two major components of genetic diversity within species. Adaptive
divergence, e.g. that along gradients of selection, is expected to result in phenotypic difference among popUlations and does not require complete suppression of gene flow. By contrast, historical isolation is expected to result in phylogeographic structure at neutral molecular markers, but does not necessarily result in phenotypic difference. Where both processes operate, the resulting phenotypically and phylogenetically distinct populations can be
recognised as distinct taxonomic entities (Avise and Ball 1990)
23
There remains, however, disagreement over the criteria for recognising ESUs
(Nielsen and Powers 1995). I have argued that molecular signatures of substantial
historical isolation, such as reciprocal monophyly of mtDNA alleles (in animals)
should be the primary criterion (Moritz 1994; Moritz et al. 1995), whereas others
suggest that both molecular and phenotypic measures of divergence should be considered (e.g. Dizon et al. 1992; Vogler and DeSalle 1994; Waples 1995). The term
"Evolutionarily Significant Unit" itself is, perhaps, unfortunate as it suggests that
we can predict the evolutionary potential of individual popUlations from their genetic makeup. Other than for very specific selective factors (e.g., insecticide resistance), this is unlikely to be the case.
The view that both genetic and phenotypic criteria should be used for defining
ESUs confounds two different evolutionary processes - historical isolation and
adaptation (Fig. 1). Historical isolation, combined with genetic drift or divergent
selection will generate unique and irreplaceable combinations of genotypes, which
mayor may not be manifest as differences in phenotype (e.g., see below). This
process of isolation and divergence (but only rarely selection) can be studied readily
using molecular makers through analyses of phylogeography (Avise et al. 1987).
By contrast, the process of adaptation and divergent selection will generate differences in phenotype among populations, but because it can occur in the presence of
gene flow (Endler 1977) there need not be significant phylogeographic structure.
Importantly, recent experimental studies with natural popUlations have demonstrated that divergence of phenotypes through selection can occur rapidly and readaptive
divergence
phenotypic
divergence
L..-_ _ +-I~ phylogeographic
structure
historical isolation
Fig. 1. Illustration of two major components of genetic diversity within species. Adaptive
divergence, e.g. that along gradients of selection, is expected to result in phenotypic difference among popUlations and does not require complete suppression of gene flow. By contrast, historical isolation is expected to result in phylogeographic structure at neutral molecular markers, but does not necessarily result in phenotypic difference. Where both processes operate, the resulting phenotypically and phylogenetically distinct populations can be
recognised as distinct taxonomic entities (Avise and Ball 1990)