It has long been assumed that geographical isolation, especially that caused by colonization ofislands, plays an important role in giving rise to new species.However, gene flow due to subsequentcontact, or in regions where closely related speciesoverlap, may counteract such interspecific divergence processes.These two types of gene flow may act upon the same species complex, and this, together with differences in effective population size, can make it more difficult to determine the evolutionary history of present-day species.