Exam 15: Phylogenomics and the Molecular Basis of Adaptation

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By comparing gene trees and species trees,researchers can infer whether specific major duplication events occurred before or after particular species separated into different lineages.This allows researchers to identify the historical timing of gene family evolution and conclude whether the gene family is an ancient or recently evolved phenomenon.Rates of nonsynonymous versus synonymous substitution are used to test for positive selection to infer whether gene family expansion was favored by natural selection.

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First,the duplicated gene must rise to fixation (or high frequency)within a population due to drift,because most gene duplication events are initially selectively neutral.Second,one of the duplicate copies acquires a mutation that changes its function.Finally,the new,mutated allele rises to fixation (or high frequency)due to selection favoring the mutated allele with a new function.
[Graders: The final step does not require selection; in principle,the mutated allele could be selectively neutral and rise to fixation due to drift.]