To Discover Genes That Have Undergone Accelerated Evolution in the Human

Multiple Choice

To discover genes that have undergone accelerated evolution in the human lineage, you compared the amino acid sequences of dozens of proteins from orthologous protein-coding genes in humans, chimpanzees, and mice. For each gene, you build an unrooted phylogenetic tree in which the branch lengths (a, b, or c) correspond to the number of amino acid substitutions in that branch, as depicted below. Primates and rodents diverged ~90 million years ago, and humans and chimpanzees diverged ~5.5 million years ago. For each individual gene shared by the three species, you therefore define the "normalized substitution rate" parameters h and k as h = (a/5.5) /[c/([2 × 90] - 5.5) ], and k = (b/5.5) /[c/([2 × 90] - 5.5) ]. Based on these definitions, which genes are more likely to be responsible for "uniquely human" traits?

A) Genes with very high h and k values
B) Genes with very low h and k values
C) Genes with very high h values but not very high k values
D) Genes with very high k values but not very high h values