Variation and Evolution in Plants and Microorganisms Toward a New Synthesis 50 Years After Stebbins (2000) / Chapter Skim
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13 Gene Genealogies and Population Variation in Plants
13 Gene Genealogies and Population Variation in Plants
Pages 235-252
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SCHAAL AND KENNETH M OLSEN Early in the development of plant evolutionary biology, genetic drift, fluctuations in population size, and isolation were identified as critical processes that affect the course of evolution in plant species.
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Olsen nization of European tree species from refugia subsequent to Pleistocene glaciation, and such studies have been instructive in understanding the origin and domestication of the crop cassava. Currently, several technical limitations hinder the widespread application of a genealogical approach to plant evolutionary studies.
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However, inferring population structure solely from allele frequencies has its limitations. Allozyme alleles (or their DNA analogs, restriction fragment length polymorphisms, amplified fragment length polymorphisms, and microsatellites)
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1~. Coalescent theory provides a framework for studying the effects of populationlevel processes (e.g., population size fluctuations, selection, and gene flow)
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showing geographical distribution of alleles; allele genealogy (Middle) indicating true history of allelic divergence over time; unroofed minimum spanning tree (Bottom)
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At time to, genealogical relationships show paraphyly with respect to the two populations. At time t2, alleles show reciprocal monophyly and are congruent with the history of population divergence.
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In some cases, ancestral allelic variation may actually persist in populations after population divergence. These shared ancestral polymorphisms can easily be misinterpreted as evidence of interpopulation gene flow.
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Olsen In the above examples from Arabidopsis, population history has strongly affected patterns of genealogy and molecular evolution. Other loci within the Arabidopsis genome may reflect different evolutionary processes, in particular selection.
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defines phylogeography as "a field of study concerned with the principles and processes governing the geographic distribution of genealogical lineages, especially those within and among closely related species." Phylogeographic studies draw inferences about the history of population divergence based on associations between the geographical distribution of the alleles and their genealogical relationships. Because these studies are not based on equilibrium assumptions of gene flow and genetic drift, they have proved insightful in studying historical changes in patterns of gene flow, isolation, and secondary contact among divergent populations.
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(1997) have shown, via chloroplast DNA phylogenies, similar patterns in the structuring of variation among several different types of plants, including ferns, trees, and several members of the Saxifragaceae, suggesting that the present genetic structure of these species is strongly affected by their postglacial pattern of colonization.
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Populations of flabellifolia occur in mesic transitional forest patches in the ecotone between the lowland rainforest of the Amazon basin and the seasonally dry cerrado (savanna-scrub) found to the south and east on the Brazilian Shield plateau (Fig.
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5. Because the domestication of cassava is an extremely recent event evolutionarily speaking, one would not expect the divergence between cassava andJqabellifolia to be reflected in the G3pdh gene tree.
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Thus, the crop is most likely derived directly from flabellifolia, rather than from several hybridizing progenitor species as traditionally thought. In addition, we found that the cassava haplotypes occur inJqabellifolia populations along the southern border of the Amazon basin and not along the eastern border.
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and the analysis of DNA sequence data from two additional nuclear genes (K.M.O., unpublished data) will be useful in elucidating the historical processes that have led to the current phylogeographic structure in this study system.
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The larger effective population size results in increased coalescent times, which in turn, increases the likelihood of encountering ancestral polymorphisms. High-resolution nuclear markers such as random amplified polymorphic DNAs and amplified fragment length polymorphisms are historically unordered, and variants cannot be related easily in a genealogical manner.
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(1998) Population structure in the American oyster as inferred by nuclear gene genealogies.
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(1995) Separating population structure from population history: A cladistic analysis of the geographical distribution of mitochondrial DNA haplotypes in the tiger salamander, Ambystoma tigrinum.
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