Biomedical subjects

B C Clarke

Publications and source records attributed to B C Clarke.

6 recordsLinked to original sources

PubMed1990

A model of quantitative traits under frequency-dependent balancing selection.

We describe a computer model that stimulates a combination of stabilizing and frequency-dependent selection acting on a quantitative character determined by several loci. The results correspond to many features of natural variations at both the phenotypic and genotypic levels. The model is robust, and its results are not strongly dependent either on the nature and shape of the function describing the stabilizing selection, or on the precise form of frequency dependence, except near the extrema. It suggests a mechanism for the maintenance of large amounts of variability, and shows a relation between population size and heterozygosity roughly corresponding to that found in nature. In this respect it is unlike the purely neutral model.

Alleles↗

PubMed1990

Mutational order: a major stochastic process in evolution.

Computer simulations in which selection acts on a quantitative character show that the randomness of mutations can contribute significantly to evolutionary divergence between populations. In different populations, different advantageous mutations occur, and are selected to fixation, so that the populations diverge even when they are initially identical, and are subject to identical selection. This stochastic process is distinct from random genetic drift. In some circumstances (large populations or strong selection, or both) mutational order can be greatly more important than random drift in bringing about divergence. It can generate a 'disconnection' between evolution at the phenotypic and genotypic levels, and can give rise to a rough 'molecular clock', albeit episodic, that is driven by selection. In the absence of selection, mutational order has little or no effect.

Biological Evolution↗

PubMed1989

The molecular-cytogenetic analysis of grasses and its application to studying relationships among species of the Triticeae.

An analysis of four species from the genus Secale, including the study of different accessions, has shown that the properties of DNA clones of monomer units from three repeated sequence loci, namely, Ter, Nor, and 5S DNA, proved to be representative of the entire loci from which they were isolated. This finding in Secale species, including the discovery of a new locus for 5S DNA on chromosome 5R, has been used to interpret information on the Ter, Nor, and 5S DNA loci from 15 species in the Triticeae complex. The evolutionary relationship among species suggested by the DNA sequence data has shown many consistencies with a number of other characters such as those used in classical systematics, as well as geographical distribution data and isozyme and chromosome-pairing studies. Apparent inconsistencies such as a close relationship between the R and P genomes at the Ter loci are interpreted in terms of amplification-deletion phenomena known to occur at repetitive sequence loci. In addition, this study included species endemic to Australia and thus provided a broad time span in which to consider some features of repeated sequence family evolution, such as the conservation of certain parts of 5S DNA spacer regions.

Base Sequence↗

PubMed1989

Phylogenetic relationships of Triticum tauschii, the D-genome donor to hexaploid wheat. 4. Variation and chromosomal location of 5S DNA.

The 5S DNA sequences in Triticum tauschii are organised in large clusters containing units that are primarily either 420 ("short") or 490 base pairs (bp) in length ("long"). The main cluster of short units was shown to be located on chromosome 1D in hexaploid wheat and is designated 5SDna-D1, while the cluster of long units was shown to be on chromosome 5D and is designated 5SDna-D2. The chromosomal locations in hexaploid wheat most likely correspond to those in T. tauschii and this could be shown directly for the 5SDna-D2 locus by using a T. tauschii 5D substitution in 'Chinese Spring' wheat. The sequence alignment of units derived from 5SDna-D1 and 5SDna-D2 revealed three apparent deletions in the noncoding spacer region, which were fixed in units from 5SDna-D1, and one deletion, which was fixed in units from 5SDna-D2. A minor size class, 400 bp long and closely related to the units from 5SDna-D1, was found in 2 of 415 accessions surveyed. A continuous range of quantitative changes in the number of 5S DNA units at the two loci was evident with up to a 10-fold relative abundance level of units being found in some accessions. Triticum tauschii var. typica was particularly noteworthy in that many accessions showed more units at 5SDna-D2 relative to 5SDna-D1. Partial thermal dissociation experiments with radioactive probes, synthesized from either the short or long 5S DNA units, hybridized to genomic DNA showed that the population of units at the respective loci were relatively homogeneous and clearly distinct from each other.(ABSTRACT TRUNCATED AT 250 WORDS)

Base Sequence↗

PubMed1988

Frequency-dependent selection, metrical characters and molecular evolution.

Computer models of selection acting on a quantitative character show that a combination of frequency-dependent and stabilizing selection can maintain many polymorphisms among the genes that determine the character. The models also show that the random order of mutations can give rise to selectively driven stochastic effects that are sometimes more important than random genetic drift. They suggest simple explanations for patterns of divergence between populations and species, and for apparent discrepancies between the rates of morphological and molecular evolution. They point towards a selective theory of 'molecular clocks'.

Animals↗

PubMed1979

The evolution of genetic diversity.

The existence within natural populations of large amounts of genetic variation in molecules and morphology presents an evolutionary problem. The 'neutralist' solution to this problem, that the variation is usually unimportant to the organism displaying it, has now lost much of its strength. Interpretations that assume widespread heterozygous advantage also face serious difficulties. A resolution is possible in terms of frequency-dependent selection by predators, parasites and competitors. The evidence for pervasive frequency-dependent selection is now very strong. It appears to follow naturally from the behaviour of predators, from the evolutionary lability of parasites, from the ecology of competition and, at the molecular level, from the phenomena of enzyme kinetics. Such selection can explain the maintenance not only of conventional polymorphism but also of continuous variation in both molecular and morphological characters. It can account for the occurrence of diversity within groups of haploid and self-fertilizing organisms, and for the evolution of differences between individuals in their systems of genetic control.

Alleles↗