Here’s the problem with the biological species concept, which states that two populations represent different species if they are reproductively isolated (unable to interbreed). There are all sorts of organisms that can interbreed and have viable hybrid offspring but are considered separate species.
Usually, this kind of interbreeding happens within a genus, between two very closely related species that have recently diverged, but it does turn our concrete idea of individual species a bit fuzzy. I’ve even complained about television shows that have different animal families interbreeding.
But a recent study published in the American Naturalist shows a deep hybridization between two distinct fern genera, Cystopteris and Gymnocarpium. Fortunately for me and my righteous soap box, the two fern species that hybridized are in the same family, the Cystoperidaceae (though until very recently they were in different families). The hybrid is also sterile, though it can reproduce vegetatively.
The authors of the study suggest that this kind of deep hybridization in the ferns maybe one reason why there aren’t more species representing this relatively ancient division of plants…they’re very slow to reach reproductive isolation (Rothfels et al 2015). This hybridization is compared to a human-lemur or elephant-manatee mating, both of which would be impossible because animals reach reproductive isolation much more quickly than ferns.
Not only does this deep hybridization occur, but the hybrid is pretty common and the lead author on the manuscript says, “It seems to be quite happy.” Well, that’s all we really can ask for isn’t it? Happy ferns, surrounded by fronds.
NB: I recently blogged about a lizard hybrid that was also bringing our definition of species into question. Read more about that here.
AND you can read more about the different species concepts here.