Horizontal gene transfer (HGT) events in the species tree. The species tree is an ultrametric tree. Branch lengths in the species tree are the number of generations. We here focus on the HGT events that can change the topology of species A, B, and C. HGT events between species A and C (red arrow) or between species B and C (blue arrow) occurring below the divergence time (green line) of species A and B can change the topology of species A, B, and C. We assume that the transfer rate between species A and C (or between species B and C) is λ (per generation). Thus the probability distribution of the number of HGT events occurring between species A and C or species B and C below the divergence time L has a Poisson distribution with mean 2λL.
Liu et al. BMC Evolutionary Biology 2010 10:302 doi:10.1186/1471-2148-10-302