Molecular Biology and Evolution, Vol 16, 1575-1585, Copyright © 1999 by Society for Molecular Biology and Evolution
SE Stanley and RG Harrison
Patterns of molecular evolution in birds have long been considered
anomalous. Compared with other vertebrates, birds have reduced levels of
genetic divergence between groups of similar taxonomic ranks for a variety
of nuclear and mitochondrial markers. This observation led to the avian
constraint hypothesis, which identifies increased functional constraint on
avian proteins as the cause for the reduction in genetic divergence.
Subsequent investigations provided additional support for the avian
constraint hypothesis when rates of molecular evolution were found to be
slower in birds than in mammals in a variety of independent calibrations.
It is possible to test the avian constraint hypothesis as an explanation
for this avian slowdown by comparing DNA sequence data from protein-coding
regions in birds and homologous regions in mammals. The increased selective
constraints should lead to a reduction in the proportion of amino acid
replacement substitutions. To test for such a decrease, we calculated the
numbers of amino acid replacement substitutions per replacement site (dN)
and silent substitutions per silent site (dS) for the complete
mitochondrial cytochrome b gene using 38 avian and 43 mammalian comparisons
that were phylogenetically independent. We find that dN/dS is significantly
smaller in birds than in mammals. This difference cannot be explained by
differences in codon bias affecting dS values. We suggest that the avian
slowdown can be explained, at least in part, by a decreased tolerance for
amino acid substitutions in avian species relative to mammalian species.
ORIGINAL ARTICLE
Cytochrome b evolution in birds and mammals: an evaluation of the avian constraint hypothesis
Section of Ecology and Systematics, Cornell University, USA. scoob@amnh.org
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