Figure 8.12 Rates of nucleotide change in the NS gene that codes for "nonstructural" proteins based on 11 human influenza A virus samples isolated between 1933 and 1985. The number of years since isolation and DNA sequence divergence from an inferred common ancestor are positively correlated. The pattern of increasing substitutions as time since divergence increases is expected under the molecular clock hypothesis. The observed rate of substitution was approximately 1.9 x 10-3 substitutions per nucleotide site per year, a very high rate compared to most genes in eukaryotes. The line is a least-squares fit. Data from Buonagurio et al. (1986).

is, faster molecular evolution occurred when more sites were neutral and free to evolve by genetic drift. Slower molecular evolution occurred when a larger proportion of amino acid changes were eliminated by natural selection because they decreased or eliminated protein function. Thus, those sites that have not diverged in sequences compared among species may be constant due to selective constraint for function. Under this view, novel at the time, the portions of protein or DNA sequences that are invariant over time and shared among species indicate regions of functional importance. The neutral theory served as a key concept to explain why different loci might have molecular clocks that tick at different rates.

Dating events with the molecular clock

A useful application of the molecular clock is to date divergence events between species. For some organisms, the fossil record and geological context provides a means to date when species originated, went extinct, or exhibited evolutionary transitions.

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