Speciation with gene flow could be common.

The likelihood of speciation in the face of homogenizing gene flow (i.e. without complete geographical isolation) is one of the most debated topics in evolutionary biology. Demonstrating this phenonemon is hampered by the difficulty of isolating the effects of time since population divergence vs. gene flow on levels of molecular genetic differentiation. For example, weak genetic differentiation between taxa could be due to recent divergence, gene flow, or a combination of these factors. Nonetheless, a number of convincing examples of speciation with gene flow have recently emerged, owing in part to the development of new analytical methods designed to estimate gene flow specifically. A recent example of speciation with gene flow in salamanders (Niemiller et al. 2008) further advances our understanding of this phenonemon, by showing that gene flow between cave and spring salamanders was ongoing during speciation, rather than having occurred after a long period of allopatric divergence. Future work on the ecological and genetic factors reducing gene flow will likely increase our understanding of the conditions that faciliate divergence in the face of gene flow.