One species contain many individuals who potentially mate with each other. If we consider a species as a unit of tree, speciation is mathematically identical with DNA replication to produce gene tree. Successive speciation will produce a tree structure, which is called species tree. However, a species tree can be inferred only through gene trees. If two adjacent speciation events are close enough and the population size is large enough, gene tree for a particular gene may be different from the species tree.