Partitioning the variation in mammalian substitution rates.

We have used analysis of variance to partition the variation in synonymous and amino acid substitution rates between three effects (gene, lineage, and a gene-by-lineage interaction) in mammalian nuclear and mitochondrial genes. We find that gene effects are stronger for amino acid substitution rates than for synonymous substitution rates and that lineage effects are stronger for synonymous substitution rates than for amino acid substitution rates. Gene-by-lineage interactions, equivalent to overdispersion corrected for lineage effects, are found in amino acid substitutions but not in synonymous substitutions. The variance in the ratio of amino acid and synonymous substitution rates is dominated by gene effects, but there is also a significant gene-by-lineage interaction.