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Effective Deme Sizes During Long-Term Evolution Estimated from Rates of Chromosomal Rearrangement

Russell Lande
Evolution
Vol. 33, No. 1, Part 1 (Mar., 1979), pp. 234-251
DOI: 10.2307/2407380
Stable URL: http://www.jstor.org/stable/2407380
Page Count: 18
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Effective Deme Sizes During Long-Term Evolution Estimated from Rates of Chromosomal Rearrangement
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Abstract

Evidence is presented that in most diploid organisms inversion and translocation heterozygosity and homozygosity rarely produce phenotypic effects, other than reducing heterozygote fertility. On this basis a model is constructed of a population subdivided into demes where spontaneous chromosomal rearrangements occur having heterozygote disadvantage. The initial fixation of such rearrangements in a deme can happen only by random genetic drift, because they are selected against when in the minority, but favored when in the majority. Interdeme migration of a few individuals per generation can prevent the local establishment of new rearrangements, suggesting that successful rearrangements generally arise in isolated populations. Once established in a deme, a negatively heterotic gene arrangement can spread in homozygous form through a subdivided population by random local extinction and colonization. For this process, the fixation rate in a species composed of many semi-isolated demes is approximately equal to the rate of establishment of new gene arrangements in a single one of its demes. Estimates of effective deme sizes during long-term evolution can thus be obtained from data on spontaneous occurrence rates, selection intensities against heterozygotes, and fixation rates of different kinds of chromosomal rearrangements in phylogeny. Spontaneous inversions, reciprocal translocations and Robertsonian fusions occur at rates between 10-4 and 10-3 per gamete per generation in a variety of animals. The usual absence of fixation of reciprocal translocations causing heterozygote semisterility and the common fixation, between congeneric species, of inversions and Robertsonian changes entailing several percent heterozygote disadvantage implies that during the evolution of many animal taxa effective deme sizes have been in the range of a few tens to a few hundreds of individuals. These results are rather insensitive to inaccuracies in the values of the spontaneous occurrence rates and fixation rates in phylogeny. The effective deme sizes estimated here are consistent with observations on the breeding structure of existing populations, and indicate considerable possibility for random genetic drift in macroevolution.

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