  In molecular systematics, evolutionary trees are reconstructed from sequences
at the tips under simple models of site substitution. A central question is how
much sequence data is required to reconstruct a tree accurately? The answer
depends on the lengths of the branches (edges) of the tree, with very short and
very long edges requiring long sequences for accurate tree inference,
particularly when these branch lengths are arranged in certain ways. For
four-taxon trees, the sequence length question was settled for the case of a
rapid speciation event in the distant past. Here, we generalize this result and
show that the same sequence length requirement holds even when the speciation
event is recent, provided that at least one of the four taxa is distantly
related to the others. However, this equivalence disappears if a molecular
clock applies, since the length of the long outgroup edge becomes largely
irrelevant in the estimation of the tree topology for a recent (but not a deep)
divergence. We also show how our results can be extended to models in which
substitution rates vary across sites, and to settings where more than four taxa
are involved.
