14 March 2013

King of the rock! Founder populations on islands

One of the downsides of your children moving to the teenage phase is that you tend to miss out on the latest movies for kids. Many of these movies that I watched with younger versions of my sons have become personal favourites. Recently, I happened to see what I consider to be the best of the Disney films of the modern era - Mulan. I like the story, I like Eddie Murphy as the dragon, I like the songs and I especially like that Mulan isn't a princess! I came across it on the telly last week and enjoyed catching up with the characters after a few years. One scene that always made me laugh was when the soldiers decide to go skinnydipping while Mulan is bathing (for those of you that haven't had the pleasure, Mulan pretends to be a man so that she can join the Emperor's army to save China as well her father's honour). Particularly amusing is when the soldiers play king of the rock with one of them sitting on a rock in midstream and knocking allcomers back into the water. I recall playing a similar game in my youth. In the game the king has an advantage because the challenger is coming out of the water whereas the king is on solid footing on the rock with gravity on their side. This time while watching the movie the scene also reminded me of a paper that has just been published by my colleagues Rob Cruickshank, Hannah Buckley and me in the journal Trends in Ecology and Evolution. What?

All of us are interested in how populations form in new habitats. Rob and I tend to be more interested in the long term evolutionary patterns whereas Hannah is more interested in the ecological processes. Over the last few years we have learned how to speak to each other so that the evolutionary biologists can understand the ecologists and vice versa. There are several areas in which this seems to help. One of the puzzles of looking at populations that have established in new habitats, such as islands, recently glaciated terrain, volcanic areas and so on, is that, despite a constant 'rain' of colonisers carrying different genes, most populations have a reduced genetic diversity. There are several ideas as to why this is the case. Recently, Jon Waters from University of Otago and his colleagues speculated that the 'founder (first established population) takes all'. There may be several explanations for why this happens but Jon and his team favoured what is termed 'space pre-emption'. In other words the first established population is king of the rock and newly arriving individuals with different genetic haplotypes will find it physically difficult to find the space they need to survive and establish and then to push the king off his rock. So the order of arrival of different populations is crucially important, a population arriving second to an island A will struggle to establish whereas it will find it easy if it is first to island B.


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We followed on from Jon's paper by suggesting other reasons by which this phenomenon could play out. First, certain populations may have a competitive advantage (a useful behaviour or physical trait) that allows that group to be successful. An individual is more likely to become king of the rock if they are strong, large or agile. Second, niche specialisation to the particular area could help the established population. The more that you have played king of the rock in one area, the more you learn about best places to stand to anchor yourself to the rock. Third, incoming populations may be genetically incompatible with the established population which will affect their chance of mating. Those that turn up at the swimming hole just to relax will not end up as the king of the rock. Fourth, there may not have been enough time for new populations to have arrived on the island. You may be king of the rock simply because there are no or few challengers. There are no doubt other possibilities to explain this phenomenon and the most important thing is that all of these factors could be at work.

What is most important about this debate is that we are dealing with a biological process that falls a little between two major types of study. Ecological studies tend to be interested in what is happening over a few generations. Evolutionary studies tend to be interested in what is happening over thousands of generations. As such, historically ecologists and evolutionary biologists have not worked together that closely and even have different languages for similar concepts. Processes like species establishing on islands take place somewhere between these ranges in the tens to hundreds of generations. As most founding events take place over the intermediate range of generations our understanding has often been slowed by misunderstandings between the different types of research. The upside of the Waters paper and our response is that scientists from ecology and evolution backgrounds are finding ways to communicate and understand one another. Now that I think about it, that's one of the messages of Mulan as well.

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