Gene trees and species trees are not the same

The relationship between species is usually represented as a bifurcating tr ee with the branching points representing speciation events. The ancestry o f genes taken from these species can also be represented as a tree, with th e branching points representing ancestral genes. The time back to the branc hing points, and even the branching order, can be different between the two trees. This possibility is widely recognized, but the discrepancies are of ten thought to be small. A different picture is emerging from new empirical evidence, particularly that based on multiple loci or on surveys with a wi de geographical scope. The discrepancies must be taken into account when es timating the timing of speciation events, especially the more recent branch es. On the positive side, the different timings at different loci provide i nformation about the ancestral populations.