More introgression:
There is a fish called the threespine stickleback.  (Speciation by Growing Apart NATURE vol. 510 no. 7505 June 12, 2014 page 510 reviewing Genetics of Ecological Divergence During Speciation M. E. Arnegard et al. doi:10.1038/nature 13301 ) There have been two species.  One feeds on lake bottoms and the other at the surface.  When they hybridize they have a problem, since half way in between is a poor food source or at least their genes don’t provide the success of the pure lines.

As I said before, there’s something to be learned here.  Just saying that they never were really species doesn’t do it.  They clearly were on the way.  I wonder if anybody has checked the situation out for hybrid breakdown – you know – cross two lines, things go well for a few generations and then the line goes infertile.  There aren’t many examples of that in animals, and I’m pretty sure that it’s important.  Speciation, and in particular early speciation, lies at the very heart of selection for outbreeding depression. 

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