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.
Hybridism is not that rare in animals. It has been observed in many reptiles, a few mammals, and lots of birds. I don't know about insects. Many species are in fact chromosomally polymorphic, and hybrids often add to this, e.g., in Sorex araneus in Scandanavia.
Of all the mechanisms of speciation, hyrbridism is the oldest proposed - Aristotle adduces it, Linnaeus thought it possible, and arguably Mendel thought it was the driver of evolution, and was trying to demonstrate this.
John, you got any references from the twentieth or twenty-first centuries?
you know about spock, right? how about 23rd century bitches!
I only had a short look at the following paper yesterday and I don't have online access in the momment. Still, I guess it will fit to this thread:
Disotell TR.: 'Chumanzee' evolution: the urge to diverge and merge. Genome Biol. 2006 Nov 24;7(11):240 [Epub ahead of print]
ABSTRACT: A recent analysis of the human and chimpanzee genomes compared with portions of other primate genomes suggests that the divergence of the human and chimpanzee lineages beginning around 6 million years ago was not a simple clean split.
Sparc, the human-chimp hybridization stuff is far more speculative than these papers. These papers show highly conclusive evidence that these species arose via a hybridization event between two other extant species. The paper you mention (which is a review of some other papers) suggests that the the ancestors of humans and chimps hybridized as the two lineages diverged. That result is far more speculative.
I think this "magic bullet" approach to speciation is too coarse an approach to speciation. As if there are a group of processes that are different ways to make species such as hybridization, the founder effect, or even that allopatric and sympatric speciation are qualitatively distinct.
I'm sure that there have been pretty good cases of "pure" hybrid speciation documented, e.g. Rieseberg's work, some yeasts etc., but I think hybridization is important in a lot more subtle ways. Dobzhansky's group in the 1950s and 1960s did a lot of cool stuff with the founder effect and saw that a founder event alone couldn't cause divergent evolution, couldn't cause a peak shift. You had to have the right variation in the large parent population from which you drew your founders. What was the right kind of variation? Many geographic races of flies all hybridized up into one giant ball of linkage. Similarly, more recent work has also shown this to be the case, without citing Dobzhansky, interestingly. For example, Rundle et al. 1998 write that previous experiments that purport to show increases in reproductive isolation due to founder effects "are subject to a number of criticisms, including the use of hybrid populations recently collected from the wild".
While some people (cough, Coyne and, cough, cough, Orr) like to play up Rundle et al.'s result as bad for founder effect speciation, I think it just shows that founder events can be very powerful given the right genetic variation, and that hybridization can create the right kind of variation. I guess part of my point is that hybridization might be important in other more subtle ways in addition to those you outline above.
But at the same time I want to maintain that it's not always clear which process we should say actually caused a speciation event. Variation caused by hybridization or the founder event? In the butterflies you mention, is it adaptation to a novel environment where back-hybrids experience a decrease in fitness or is it the hybridization event that caused speciation? How should we untangle these?
Matt, regardless of the ultimate "cause" of the speciation (or reproductive isolation), I think these studies are of interest. While they do not show why the species are reproductively isolation, they do show that a unique and isolated gene pool can arise via the hybridization of two other fairly isolated gene pools.
It would be neat to figure out what aspect of hybridization allows for speciation to occur. Whether it's the hybrization event that primes speciation or something else.