The Influence of Population Size on the Rate of Molecular Evolution in Raptorial and Non-Raptorial Birds

Abstract

Examining the relationship between population size and molecular evolutionary rate is a key issue in modern evolutionary research and remains an enduring debate. At its forefront are two opposing evolutionary theories. Ohta’s nearly neutral theory of evolution predicts an inverse relationship between population size and the rate of molecular evolution due to the increased action of genetic drift on selectively neutral and nearly neutral mutations in smaller populations. Conversely, positive selection theory predicts a directly proportionate relationship between population size and the rate of molecular evolution due to natural selection acting on selectively adaptive mutations which occur more abundantly in larger populations. Previous research relating population size to molecular evolutionary rates has not resolved this relationship due to the difficulty in selecting appropriate study species and in estimating their relative population sizes. In this study a novel population size estimation was used through the use of trophically determined population density variation, as a relevant proxy. Six pairs of bird species were compared and contrasted. Of each pair, one was a raptor species, that tend to have low population densities and thus smaller populations and one was a non-raptor species that conversely tend to have higher population densities and larger populations. This study used interspecific pair-wise comparisons of between 14 and 19 homologous nucleotide sequences per contrasted species-pair to determine the effect of population size on substitution rates. Non-raptor species displayed significantly faster rates of substitution across nearly every nucleotide sequence comparison made. These results support the positive selection theory of evolution and suggest natural selection acting on adaptive mutations as the regulatory agent of evolutionary change in these populations. This result shows population size and molecular evolutionary rates as having a directly proportionate relationship. The outcome of this study has significant implications for molecular research and methodology and for conservation biology. In particular, it reveals the likely vulnerability of raptors to reduced population sizes where the area of their habitat is diminished by human activity. As their rates of evolution are already low due to naturally lower population densities they may be more vulnerable to range loss and habitat disruption.