  In evolutionary biology, genetic sequences carry with them a trace of the
underlying tree that describes their evolution from a common ancestral
sequence. The question of how many sequence sites are required to recover this
evolutionary relationship accurately depends on the model of sequence
evolution, the substitution rate, divergence times and the method used to infer
phylogenetic history. A particularly challenging problem for phylogenetic
methods arises when a rapid divergence event occurred in the distant past. We
analyse an idealised form of this problem in which the terminal edges of a
symmetric four--taxon tree are some factor ($p$) times the length of the
interior edge. We determine an order $p^2$ lower bound on the growth rate for
the sequence length required to resolve the tree (independent of any particular
branch length). We also show that this rate of sequence length growth can be
achieved by existing methods (including the simple `maximum parsimony' method),
and compare these order $p^2$ bounds with an order $p$ growth rate for a model
that describes low-homoplasy evolution. In the final section, we provide a
generic bound on the sequence length requirement for a more general class of
Markov processes.
