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?

Question

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?

Quizplus · Accepted Answer

Genes with very high h and k values have shown an accelerated rate of change in both human and chimpanzee lineages compared to the mouse lineage. Even though such genes are interesting candidates to study general hominid evolution, the genes that have evolved uniquely in humans are probably those with high h but low k values. Note that in the normalized substitution rates, a and b are divided by 5.5 because each of them represents amino acid changes during 5.5 million years since the divergence of human and chimpanzee lineages. The parameter c represents the changes that occurred in the mouse lineage since its divergence from the primate lineage (90 million years ago), as well as changes that occurred in the primate lineage before human and chimpanzee lineages diverged (between 90 million and 5.5 million years ago). Therefore, c is normalized by dividing by the combined period (90 + 90 - 5.5).

To Discover Genes That Have Undergone Accelerated Evolution in the Human