Science and sensibility

Remember last month when all the heavy weights of the science blogging world were talking bout Heliconius heurippa - the Andean butterfly that appears to have arisen by hybridisation between two other Heliconius butterflies? Yeah, if I wanted to be big deal blogger that's probably when I should have put my two cents in. Of course being a big deal is hardly my motivation so I have no shame in arriving late to this particular party.

The basic story is certainly available from all the usual outlets, in summary Jesus Mavarez and his colleagues have used molecular genetics and lab crosses to show that a rather pretty tropical butterfly has arisen from interspecific hybridisation between two closely related species. Most commentators have understandably highlighted how this story contrasts with the usual lineage splitting modes of speciation but there is more to this story than that. One of the things I'm really interested in is studying the processes and events that have occurred during the evolution of different groups to generate their current day distributions. So, rather than rehash what has been so excellently explained elsewhere lets see what this research has to say about the events that have generated the unique distribution this group of butterflies display

The hybrid species H. heurippa surrounding by examplesof the parent species that generated it

Scientists have been talking about Heliconius butterflies for well over 100 years. They are interesting because individual species are highly polymorphic across their range with different geographic 'races' each bearing a different wing pattern. This polymorphism is important because the wing patterns are warning signals to potential predators that these butterflies are not palatable. These isolated populations do occasionally interbreed but the progeny of these crosses bare kaleidoscopic wing patterns that give no clear signal to predators which, unaware of their prey's chemical defences, will make hybrids into a disappointing lunch.

Interesting biogeographical distributions like the one that the Heliconius present us generate interesting hypotheses to test. What sort of evolutionary processes could generate disjunct populations bearing distinct colour polymorphisms in tropical America? Most people would argue this is another example of the ubiquitous and lasting effects the last ice age generated. They would argue a formally continuous population of each species was broken into subpopulations as glaciers generated 'forest islands' separated by uninhabitable ice. During this isolation warning signals drifted apart in different populations so that when the ice finally receded hybrids generated by interbreeding between the formally separate populations were met with fitness costs (ie getting eaten). The cost of being eaten generates a selective pressure to maintain the most common wing pattern (as a clear signal to predators) in each region so the geographical races each maintain their unique colour patterns.

A model for the generaton of disjunct populations following glaciation

The glacial hypothesis is certainly a reasonable suggestion, molecular evidence from New Zealand invertebrates (and in fact flightless birds) seems to tell a similar story and the effects of the ice age in the northern hemisphere are very well documented. Still, there are other interpretations that could be made by the present day Heliconius distributions. Some argue that the distribution could equally well be described by an ongoing series of 'peripatric' speciation events. In this hypothesis there will occasionally be dips in the predator driven selective pressure to have a common wing pattern (perhaps a new predator invades an area but doesn't immediately learn the signal). Following such a dip hybrids won't be so strongly selected against and a polymorphic population may arise. Of course eventually the selection pressure will return, predators will steer clear of the most common of the wing patterns which will be driven to fixation at the cost of all the other patterns. The end result here is something very much like we see in nature, populations broken into geographic races, each bearing a different wing pattern that is recognised by the local predators.

A model for the generation of disjunct populations as a result of peripatric events

So how are we to choose between these two hypotheses, neither of which is obviously silly? The answer is, as always, by getting more evidence. The one piece of evidence that the glacial hypothesis needs (and that some may have overlooked) is something to say the Mesoamerican forest was fragmented during the last ice age. The best way to study past distributions of forest is 'stratigraphical palynology' which amounts to looking for pollen grains in different layers of soil or lake beds. The most recent palynological evidence suggests that Mesoamerican forests actually survived the ice age intact. If story holds up then the glacial hypothesis will need to be discarded or radically reworked.

On the other hand Mavarez et. al's revelation that H. heurippa is a hybrid species provides support for the peripatric hypothesis. The peripatric model requires that occasionally selection pressures can be loosened to the extent that wing patterns other than the most common one can prosper and that once one of these patterns is finally fixed (by the return of selective pressure for the most common wing type) a mechanism exists to isolate the population from its parents. Mavarez et. al tell us that a Venezuelan Heliconius population they have studied contains 8% hybrids as proof hybridisation happens and then go on to conclusively prove H. heurippa is the result of hybridisation. So what stops these hybrids from falling back into their parent lineages? One of the most exciting (and rather under reported) results of the paper was that the wing patterns generated by hybridisation can themselves be isolating mechanisms. H. heurippa males studied for this paper showed marked preference for their own wing pattern when it came to choosing mates. It is possible that this preference comes as a package with the wing pattern (a paper published this year suggests wing colour and mate preference are the result of two linked genes or pleiotropic effects of one gene in one of H. heurippa's parents) or that mate preference evolves once selection for the most common wing type builds up and canny males select their mates in an attempt to ensure their offspring also have the common wing pattern.

The evidence that hybrid phenotypes which are usually so strongly selected against can occasionally forge viable new populations not only helps understand the details of how new species arise but also shines light on how variations within species may arise and be maintained

Some more reading

Aydin Örstan on the same stuff

Mavarez et. al on the hybrid origin of H. heurippa

Palynological evidence that andean forests weren't fragmented in the last ice age