Survival Of The Different

The biosphere is even more ridiculously diverse than it appears through the lens of natural history documentaries. Amazingly, scientists have been able to estimate the numbers of different species that are alive today and, amazingly, there are tens of millions, most of which have yet to be named, far less described. Even at local scales diversity can be bewilderingly high – for example, in a couple of acres of rainforest you can find hundreds of species of trees and thousands of species of insects. And importantly, these species are very different in just about any way we think of to measure them. The plants have different leaves, different branching patterns and flower at different times. The animals look very different, eat different things, inhabit different places, move in different ways.

Why is there not just one best tree species? One best beetle and one best bird? Isn't this how evolution works, causing species to adapt toward the environment that they find themselves in? Why doesn't natural selection leave only the best adapted species? Indeed, natural selection as commonly understood should lead diversity to collapse. The physical environment implies a single, best way to live. Whichever individuals are closest to this optimum are the fittest, and get to survive.

But that is obviously not what happens. Ecologists have mulled over this paradox of diversity for over a century, and although there is still a lot to learn, they are mostly agreed that for diversity to be maintained in the long term there needs to be some form of difference advantage. All else being equal, it is advantageous to be different from everyone else. If you're a tree, and the other trees are evergreen, you're better off deciduous. If you're a bird, and the other birds eat big seeds, you're better off if you eat small seeds. Whatever kind of thing you are, if everyone else likes it cold, you're better off if you like it hot. If everyone else is red, you are better off blue.

Without such an explicit advantage of being different, ecologists think that ecosystems would indeed become dominated by one, or a very small number, of best adapted species, a la traditional natural selection. But, if there is a 'niche axis' – defined by diet, temperature tolerance, colour, or pretty much anything else – the community can support the long-term coexistence of a number of species spread along that axis. If you have many such axes combined, you can get very high diversity, like you see in rainforests. My colleague Lindsay Turnbull has described niches as the 'dark matter of ecology'. The only way for physicists to explain why galaxies don't fly apart is to invoke dark matter. The only way for ecologists to understand why diversity doesn't collapse is to invoke niches.

Why is 'survival of the different' not common parlance like 'survival of the fittest'? My view is that the traditional view of natural selection is easier to understand, because it is based on a simplistic view of the natural world. In this view, the world is just physical – the terrain, climate, and so on. From the point of view of one individual the physical world is mostly fixed, and this in turn implies a single best, fixed, optimum way to live. Natural selection should take evolution toward that optimum, then stay there. Survival of the fittest.

What this view misses is that, to most animals and plants, the world is mostly biological, consisting of a constantly changing mixture of other animals and plants, which themselves are living complex lives, and also evolving. This teeming froth of ecology, which is much harder to think about, is what gives rise to difference advantage. You're better off eating small seeds because there are small seeds about, and nobody else eats them – at least for now. And why are there small seeds about? Because, in part, some plants have evolved small seeds to evade animals that eat big seeds. Until the animals catch up. You're better off if you like it cold, because when a cold snap occurs, your warm-adapted competitors won't grow so well and this will free up resources for you. Until some other species cottons on to the cold adapted strategy, in which case it might suddenly be better to become hot adapted. And so it has gone on, for billions of years.

A particular form of ecology that ecologists think leads to a large amount of diversity everywhere is attack by enemies. Over evolutionary time your predators and diseases home in on anything that is regular about you and the others in your species, from your movement patterns to your biochemistry. Therefore, being different from the others, in any way, helps you escape your enemies. You're better off blue, because your main predator has learned to search for red. You're better off deciduous, because most of the local pests are specialized to attack evergreens.

Of course, all else is isn't equal. Yes, ecology tends to confer an advantage to being different, but it also confers a disadvantage when you move too far away from what you do best. There's no point in evolving to eat seeds that are smaller than any seeds that exist. And the physical environment does matter: there is no point in evolving to tolerate temperatures that are much hotter or colder than ever occur where you live. The type and amount of diversity in a community, therefore, results from a tension between processes that favour difference, and those that favour conformity. As a result of this tension, each community becomes adapted to its physical environment, but nonetheless maintains a diversity of species within in it. Most rainforest trees are evergreen, but there are deciduous trees here and there. Most trees in lowland England are deciduous, but we still have our native Yew and Box.

Is there a useful societal and economic analogy here? All else being equal, are companies, individuals, or even countries, better off if they are different? Does difference advantage work in tension against forces that favour conformity in these cases, as happens in nature? If so, and if we think that diversity is a good thing, then we could ask ourselves what we can do to create or amplify difference advantages in different situations – especially if, as I discuss in a separate essay, diversity is beneficial in promoting productivity and stability.

© 2017 Drew Purves All Rights Reserved

@drewpurves