We usually think of speciation as a bifurcating process — a single lineage splitting into two. The relationships of those species can often be determined using DNA sequences. But we know that there are exceptions, like horizontal gene transfer in bacteria. And hybrid speciation in plants. These exceptions often interfere with our ability to reconstruct the evolutionary relationships of those species.
Hybrid speciation occurs when two species produce hybrids that are both fit and capable of becoming reproductively isolated from the two parental species. The new species will often exploit a niche unique from the parent species as well. This differs from a hybrid zone, which often exists between closely related species in close proximity. Hybrid zones tend to have an excess of F1 hybrids, whereas hybrid species are in Hardy-Weinberg equilibrium and are genetically and ecologically unique from the two parental species.
The phenomenon of hybrid speciation was thought to be limited to plants (for one of the most rigorous sets of analyses see the work of Loren Rieseberg on sunflowers). Three recent studies, however, are causing us the rethink the role of hybrid speciation in animals. Before I get any further I must draw a distinction between homoploid hybrid speciation and polyploid hybrid speciation. Polyploid hybrid speciation occurs when a hybridization event produces polyploid progeny which are reproductively isolated from the parental species due to differences in chromosome number. This is fairly common in plants and frogs. Homoploid hybrid speciation involves no changes in chromosome number, so we must invoke other mechanisms to explain the subsequent reproductive isolation between the hybrids and their parental species.
My buddy Dietmar showed that a hybrid species can become reproductively isolated from its parental species if the hybrids find a new host. He performed this analysis in Rhagoletis which are well known for speciating by host shift. The hybrid species oviposits on the fruit of a plant that was recently introduced into North America. It exhibits prezygotic behavioral isolation from the parental species, and each species has high affinity for its host plant of preference. Dietmar published his work in July of 2005.
When another paper showing evidence of homoploid hybrid speciation, this time in Heliconius butterflies, came out earlier this year (June, 2006), I was surprised by all the attention it received. Not because the research wasn’t solid, but because people acted as if this was the first time someone had shown evidence for homoploid hybrid speciation in animals. There was nary a reference to Dietmar’s work on Rhagoletis. Carl Zimmer said that the previous “evidence from animals has been suggestive”, and that this was a new mechanism of speciation that hadn’t been previously considered. The new work was unique not because of the mechanism the authors proposed, but because they were able to recreate the hybrid species in the lab. That’s the impressive aspect of the paper.
But how unique is it to recreate a hybrid species in the lab? For animals, I’d say it’s pretty special. But it’s nothing new when one takes a look at non-animal research. Loren Rieseberg (the guy I mentioned above who works on hybrid speciation in sunflowers) and colleagues have done it in Helianthus.
And that brings us to the most recent paper on hybrid speciation. This one looks at butterflies from the genus Lycaeides. The hybrid species shows adaptations to an alpine environment in the Sierra Nevadas. Once again, we see an important requirement for hybrid speciation: a new niche for the hybrid population to exploit. If the hybrids specialize on a new host or in a unique environment, back-hybrids to the parental population will be at a selective disadvantage due to an intermediate phenotype.
How about other requirements? I’d argue that chromosomal rearrangements are important. Not polyploidization, which results in immediate reproductive isolation, but the types of rearrangements that have been observed in sunflowers and Rhagoletis. These rearrangements don’t cause reproductive isolation themselves, but they prevent the flow of genes between speciating populations by suppressing recombination (see this model). This allows the populations (both the parental and hybrid species) to accumulate genetic difference that can lead to host preference, environmental adaptation, and mating behavior in the face of some gene flow.