An American study has thrown up an interesting possibility of 'speciation reversal' in which the, well-documented speciation process (driven by geographical isolation) has apparently resulted in two separate lines recombining (https://www.theguardian.com/science/2018/mar/02/two-become-one-two-raven-lineages-merge-in-speciation-reversal). The California and Holarctic ravens were found in different parts of North America (and thought to have separated some 2 million years ago) but wide-spread hybridisation seems to be recombining them into a single species. Although there are not many reports of such a process, it seems eminently possible if geographical isolation is removed. Of course, the possibility exists that the two populations never really merited being classified as separate species.
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