The above (attached) represents the historical relationship between six taxa, A-E. The numbers along each branch represent the number of DNA substitutions at Gene X between each taxon at a particular sequence (each sequence is the same length) and the time-scale is on the right. All substitutions are unique (e.g., there is one difference between A and B, there are 32 differences between B and D; there are 33 differences between A and D).
a) What is the mean substitution rate per million years over this lineage?
b) What is the variance in substitution rate (per million years) over this lineage? Treat each branch with a number on it as an independent data point.
c) Do you think Gene X has been evolving neutrally over the entire history of this lineage? Justify your answer.