Question

It has been hypothesized that the rate of molecular evolution exhibits a clock-like consistency, however, the clock appears to tick at different rates for different genes. As it happens, a clock based on amino acid substitutions appears to correspond to absolute time whereas a clock based on silent substitutions or substitutions in pseudogenes appears to "tick" in accordance with generation time (this is not a pattern we discussed in class so this question is asking you to stretch your thinking). Which specific evolutionary forces could account for the differences observed in the two molecular clocks?

Answer 1

The differences in the two molecular clocks can be explained by the influence of natural selection on amino acid substitutions and the lack of selective pressure on silent substitutions and substitutions in pseudogenes.

Step-by-step explanation:

The differences observed in the two molecular clocks can be attributed to specific evolutionary forces. The clock based on amino acid substitutions corresponds to absolute time because these substitutions have a direct impact on the function and structure of proteins. This means that amino acid substitutions are more likely to be subject to natural selection and the rate of evolution is determined by the selective pressures acting on the protein.

On the other hand, the clock based on silent substitutions or substitutions in pseudogenes is influenced by generation time rather than absolute time. Silent substitutions occur in DNA regions that do not code for proteins, so they are less likely to have a functional impact. Additionally, pseudogenes are non-functional copies of genes, so substitutions in these regions are not subject to selection pressures. As a result, these substitutions accumulate at a rate that is more closely tied to the generation time of a species.

Therefore, the differences observed in the two molecular clocks can be explained by the influence of natural selection on amino acid substitutions and the lack of selective pressure on silent substitutions and substitutions in pseudogenes.