Ben Haller<\/p><\/div>\n

Nexus: What was the broad question you were trying to answer in this work?
\n<\/b>Ben Haller: We are interested in the origins of biodiversity.\u00a0 The world contains quite a large number of different species.\u00a0 There are about 400,000 different known species of beetles alone.\u00a0 And beyond beetles, of course, the world is full of different kinds of plants and animals and fungi and bacteria and so forth.<\/p>\n

Perhaps the central question of evolutionary biology is what led to all this biodiversity.\u00a0 Although Darwin supplied the biggest piece of the puzzle with his theory of evolution by natural selection, there is still much left that we don’t understand about the origin of species.<\/p>\n

More specifically, our question was how variation in the environment might generate biodiversity.\u00a0 Landscapes vary in temperature, in elevation, in rainfall, and in all sorts of other environmental variables.\u00a0 Environments vary biotically, too; some forests are dominated by conifers, while other forests are dominated by deciduous trees, for example, and that creates very different environments for all of the birds and mammals and insects and so forth that live in those forests.\u00a0 We know that this environmental variation promotes biodiversity; organisms in different environments will evolve to “fit” the environment they are in, and this can lead to the development of new species.<\/p>\n

Previous theoretical models have simulated this process.\u00a0 Simulated organisms would be placed into a simulated environment that had some sort of variation in it, and the simulated organisms would evolve divergently and become different species.\u00a0 But these models only looked at extremely simple, artificial environments.\u00a0 We wanted to look at more realistic environments, with more random variation in environmental conditions, to see whether a sort of patchy quilt of different environments across a landscape had a different effect on biodiversity than the simple forms of variation used in earlier models.<\/p>\n

In your paper you found something called a \u201crefugium effect.\u201d What do you mean by that?
\n<\/b>We found that complex environmental variation promotes the creation of new biodiversity, through a phenomenon that we have called the “refugium effect”. A refugium is a place of refuge.<\/p>\n

Imagine the sort of simple environment that has been studied before, with a smooth, continuous gradient from environment type A to environment type B.\u00a0 Maybe a gradient in elevation, for example; our model does not make any assumption about what aspect of the environment is varying, but we can think of it as elevation.\u00a0 If a species is adapted to a particular elevation in this environment, it can be hard for that species to move around in the landscape.\u00a0 It lives at its optimal elevation, and if it ventures outside of that zone, it encounters conditions that it is not well-adapted to, and it has trouble establishing new populations and colonizing the landscape.\u00a0 As a result, that species might never diversify into other species adapted to other elevations.<\/p>\n

Now imagine a landscape with a more complex topographic profile, with high points and low points scattered somewhat randomly across the landscape.\u00a0 That’s the sort of environmental variation that we modeled in this study.\u00a0 A species adapted to a particular elevation can find spots that have that elevation in many places in the landscape.\u00a0 These are refugia.\u00a0 The species can immediately disperse into those refugia and establish new colonies.\u00a0 Those colonies will be largely isolated from each other, however, due to their geographic separation, so they can follow independent evolutionary trajectories.\u00a0 And importantly, the refuge provided by a refugium is limited, because the refugium is surrounded by different habitat; land at a different elevation, here.\u00a0 A population might scrape out a living in a small refugium, but many of its offspring are going to end up in that surrounding environment, and so there will be a strong pressure to become better-adapted to that surrounding environment.\u00a0 While the refugium provides an initial foothold, in other words, the pressure is ultimately to leave the refugium and become adapted to the surrounding conditions.\u00a0 And when that happens, a new ecotype, or perhaps a new species, has been created.<\/p>\n

You created a model of an environment in order to investigate how complexity can affect biodiversity. What kind of environment does your model represent and what kinds of organisms live there?
\n<\/b>Well, it’s really fairly abstract.\u00a0 While we\u2019re trying to bring our model closer to the real world, I don’t want to give the impression that we are simulating anything like real environments and real organisms.\u00a0 In our model, the environment varies in just a single characteristic, and the pattern of variation, while relatively complex, is still much simpler than the variation in the real world.\u00a0 Similarly, the organisms in our model vary in just a single trait, corresponding to the single axis of variation in the environment.<\/p>\n

At present our model is also of asexual organisms that disperse at birth; this is similar to many plants, for example, but is less applicable to most animals.\u00a0 The model is never going to be a sort of hyper-realistic model of real-world environments and organisms; that would not actually be desirable, as it would then be just as impossible to analyze and understand as the real world is!<\/p>\n

The power of theoretical work is that you can distill a real-world question down into a very simple, abstract form and get a very simple, abstract answer, uncluttered by all of the complications that exist in the real world.\u00a0 At the same time, though, the specific things I’ve just mentioned are things we’d like to work on next: looking at landscapes and organisms that vary in more ways than one, and looking at sexual organisms with perhaps more animal-like movement behaviors.
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Besides the refugium effect, were there other new findings from the study?
\n<\/b>We also showed that environmental variation is a bit like the story of Goldilocks and the Three Bears; for promoting biodiversity, you can have too little variation (which doesn’t promote diversification), or too much variation (which makes it too difficult to diversify), or the variation can be “just right”.\u00a0 That had been shown before for very simple environments, but we generalized that result to a broader class of landscapes and variation, which is an important result.<\/p>\n

And finally, we showed that the spatial scale of environmental variation is important to diversification.\u00a0 Imagine a landscape with very fine-grained variation, and a species with a very long dispersal distance.\u00a0 The species will not be able to adapt to fit different local conditions, because it doesn’t “fit into” any one local patch in the environment.\u00a0 Now imagine a landscape with broad-scale variation, continent-scale variation perhaps, and a species with a much shorter dispersal distance.\u00a0 Now whole populations of the species will “fit into” different local patches with different local conditions, and so those populations will diverge and become different ecotypes or different species.<\/p>\n