Biomedical subjects

Cortland K Griswold

Publications and source records attributed to Cortland K Griswold.

3 recordsLinked to original sources

PubMed2003

The genetics of adaptation: the roles of pleiotropy, stabilizing selection and drift in shaping the distribution of bidirectional fixed mutational effects.

Pleiotropy allows for the deterministic fixation of bidirectional mutations: mutations with effects both in the direction of selection and opposite to selection for the same character. Mutations with deleterious effects on some characters can fix because of beneficial effects on other characters. This study analytically quantifies the expected frequency of mutations that fix with negative and positive effects on a character and the average size of a fixed effect on a character when a mutation pleiotropically affects from very few to many characters. The analysis allows for mutational distributions that vary in shape and provides a framework that would allow for varying the frequency at which mutations arise with deleterious and positive effects on characters. The results show that a large fraction of fixed mutations will have deleterious pleiotropic effects even when mutation affects as little as two characters and only directional selection is occurring, and, not surprisingly, as the degree of pleiotropy increases the frequency of fixed deleterious effects increases. As a point of comparison, we show how stabilizing selection and random genetic drift affect the bidirectional distribution of fixed mutational effects. The results are then applied to QTL studies that seek to find loci that contribute to phenotypic differences between populations or species. It is shown that QTL studies are biased against detecting chromosome regions that have deleterious pleiotropic effects on characters.

Acclimatization↗

PubMed2002

Time to the most recent common ancestor and divergence times of populations of common chaffinches (Fringilla coelebs) in Europe and North Africa: insights into Pleistocene refugia and current levels of migration.

We analyzed sequences from a 275-bp hypervariable region in the 5' end of the mitochondrial DNA control region in 190 common chaffinches (Fringilla coelebs) from 19 populations in Europe and North Africa, including new samples from Greece and Morocco. Coalescent techniques were applied to estimate the time to the most recent common ancestor (TMRCA) and divergence times of these populations. The first objective of this study was to infer the locations of refugia where chaffinches survived the last glacial episode, and this was achieved by estimating the TMRCA of populations in regions surrounding the Mediterranean that were unglaciated in the late Pleistocene. Although extant populations in Iberia, Corsica, Greece, and North Africa harbor haplotypes that are basal in a phylogenetic tree, this information alone cannot be used to infer that these localities served as refugia, because it is impossible to infer the ages of populations and their divergence times without also considering the population genetic processes of mutation, migration, and drift. Provided we assume the TMRCAs of populations are a reasonable estimate of a population's age, coalescent-based methods place resident populations in Iberia, Corsica, Greece, and North Africa during the time of the last glacial maximum, suggesting these regions served as refugia for the common chaffinch. The second objective was to determine when populations began diverging from each other and to use this as a baseline to estimate current levels of gene flow. Divergence time estimates suggest that European populations began diverging about 60,000 years before present. The relatively recent divergence of populations in North Africa, Italy, and Iberia may explain why classic migration estimates based on equilibrium assumptions are high for these populations. We compare these estimates with nonequilibrium-based estimates and show that the nonequilibrium estimates are consistently lower than the equilibrium estimates.

Africa, Northern↗

PubMedDate not supplied

Pleiotropic mutation, modularity and evolvability.

The relationship between pleiotropy and the rate of evolution of a phenotypic character (evolvability) in a population is explored using computer simulations. I present results that suggest the rate of evolution of a phenotypic character may not decline when that character is pleiotropically associated to an increasing number of other characters, provided that the characters are under pure directional selection such that they are far from their optima relative to the average magnitude of a mutation. These conditions may be relevant during adaptive radiations. Adding pleiotropic associations to a set of characters in which one is under directional selection and the other is under stabilizing selection increases the rate of adaptation of the character under directional selection provided that the new characters that come to be pleiotropically associated are under directional selection. Thus, increasing the number of pleiotropic associations under these conditions increases the rate of adaptation of a character.

Adaptation, Physiological↗