School of Mathematics

Brian Charlesworth

Brian Charlesworth abstract

Brian Charlesworth, Institute of Evolutionary Biology, University of Edinburgh:

Natural Selection and the Genome

Natural selection is usually thought of as causing the evolution of propertiesof organisms like their structure, physiology and behaviour, which arecontrolled by the instructions encoded in their genomes. However, genomesthemselves are the product of evolution, and natural selection must play amajor role in shaping their organization. Detecting and quantifying this aspectof selection and its interactions with other evolutionary factors, such asmutation and random sampling effects due to finite population size, is a majorchallenge for biologists, especially as the intensity of selection on manygenomic features is probably so small as to be inaccessible to experimentalmeasurement.  It is increasingly recognised that patterns of DNA sequencevariation within populations offer considerable power to detect and estimateselective effects of the order of one in a million or less, if these patternsare compared with the predictions of models of the evolutionary processesinvolved. The utility of approaches based on such model-based analyses ofsequence data is illustrated with two examples from Drosophila populationgenetic studies. One concerns selection on codon usage bias, the non-random useof alternative codons for the same amino-acid, and the related problem of theGC content of sequences. The other concerns selection on the size of non-codingsequences, especially introns. The evidence for pervasive selection on theseand other features of genomes raises the old problem of  the ‘geneticload’; how can a population survive the action of selection at tens orhundreds of millions of sites throughout the genome? Two alternativeresolutions of this problem are presented.