I.17 Microevolution
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I.17 Microevolution Michael A. Bell OUTLINE 1. Evolution: Micro versus macro 2. ‘‘The ecological theater and the evolutionary play’’ 3. Microevolutionary mechanisms 4. Contemporary microevolution 5. The unintended consequences of human technology 6. Geographic variation 7. Phylogeography 8. Genomics and microevolution 9. Prospects Microevolution occurs within and among populations of a species and usually involves changes in the mean value or relative frequencies of alleles and phenotypes that are shared by most populations of the species. Divergence among populations of a species (i.e., conspecific populations ) is often associated with habitat differences, and such divergence often has important ecological consequences. Population genetics deals with evolution in terms of allele and genotype frequencies within populations, so it provides the theoretical foundation to study microevolution. Widespread species typically exhibit geographic variation, which has generally been thought to take thousands of generations to evolve. However, recent research on contemporary evolution suggests that geographic variation can evolve within a few generations after species colonize new habitats or experience environmental change. The high rate at which microevolution can occur is important because it means that pathogens, pests, and harvested natural populations can rapidly evolve traits that adversely affect people. DNA variation within and among conspecific populations can be studied as a product of microevolution, and it also provides powerful tools to tease apart the contributions of common ancestry and local adaptation to the evolution of geographic variation. Thus, previously intractable problems in microevolution and its applications to natural resource management can now be studied using the emerging technologies of molecular biology and genomics. GLOSSARY character displacement. This is the evolution of enhanced differences between species where they occur together as a result of selection against members of one or both species that use the same resources as members of the other species (i.e., ecological character displacement) or against individuals that tend to hybridize with members of the other species (i.e., reproductive character displacement). cline. A cline is a geographic gradient in the frequency or mean value of a phenotype or genotype. monophyletic group. This is a group of species that are more closely related to each other than any is to species outside the group. phenotypic plasticity. A change in an individual phenotype that does not alter its genetic constitution and is not inherited by its offspring. random walk. In population genetics, this is a change in allele frequencies from their initial values as a result of repeated episodes of genetic drift. taxon. A taxon (including higher taxon) is any named group (e.g., Vertebrata, Mammalia, Homo sapiens ) at any taxonomic rank (e.g., Kingdom, Class, Species); higher taxa are more inclusive. 1. EVOLUTION: MICRO VERSUS MACRO Biological evolution is change through time in the heritable properties of a lineage or monophyletic group (clade). Microevolution is generally confined to evolution within and among conspecific populations, and it occurs within relatively short time spans. In contrast, macroevolution involves changes in the number or characteristic properties (e.g., average body size) of the species of a clade. It depends on the variation among species generated by microevolution and unfolds over longer periods. Nevertheless, the definitions of microevolution and macroevolution have been controversial, and there is disagreement about their mechanistic relationships and even the value of the terms. The division between microevolution and macroevolution is usually placed at speciation because members of different species do not routinely interbreed, and the evolutionary fates of separate species are largely independent. Microevolution involves changes in the frequencies of alleles and genotypes and of interactions between different genes. These changes are manifested as recognizable changes in the mean values or frequencies of biochemical, physiological, behavioral, developmental, and morphological phenotypes . A separate set of macroevolutionary mechanisms influences the probability of speciation and extinction. Thus, properties of species that promote speciation or impede extinction will tend to increase in a monophyletic group over time. Both microevolution and macroevolution contribute to biodiversity, but microevolution affects individuals and changes the properties of populations, whereas macroevolution alters the relative frequencies of species with different properties. There are also practical reasons to distinguish microevolution and macroevolution. Microevolution can be studied using comparative, observational, or experimental methods to study individuals and populations over a few generations in the laboratory and field. Existing genetic properties and ecological conditions can be used to interpret microevolution. In contrast, macroevolutionary studies focus on differences among species. Careful species description, characterization of clades, and investigation of phylogenetic relationships among taxa are paramount in macroevolutionary research...