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Darwin Now pod 5: Darwinian demons

Updated Wednesday 4th November 2009

If only the fittest survive in nature, you might expect the world to be taken over by a small number of rampant species. So why, instead of dull monotony, are we faced with infinite variety?

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Copyright British Council

Transcript

Music: Variations on a Theme by Giuliani

Rissa de la Paz:

Welcome to Darwin Now. We’ll be tackling questions that lie at the heart of life’s seemingly infinite variety – how can such a diversity of species exist? And more urgently than ever, how might this richness be preserved? Jonathan Silvertown, Professor of Ecology at the Open University in the UK, argues that any question about the rise of diversity or its demise, is at the most basic level, an evolutionary one.

 

Jonathan Silvertown:

An evolutionary approach enhances our understanding of biodiversity in a number of ways. I mean firstly you have to recognise that species are the product of evolution, and so you can’t really understand biodiversity unless you understand evolution. But also one can think of evolution as a play and ecology as a theatre, the ecological theatre and the evolutionary play. Anything that happens in evolution is happening in the context provided by ecology. In other words, it’s the relationships between animals, plants, microbes and their physical environment which produce the selection pressures which result in evolution.

The theatre, the ecology, provides the context in which evolution happens and Darwin understood this very clearly, and you can see in The Origin of Species how he was thinking about the way in which the context of a species, what it eats, what eats it, its environment influence how it’s adapted, it’s adapted to its environment and also to other organisms. And one way of putting that is to talk about the evolutionary play in the ecological theatre.

 

Rissa de la Paz:

The beauty of the evolutionary play is that it works to a remarkably simple script. All it needs are three basic ingredients to unfold.

 

Jonathan Silvertown:

It starts off with the observation that not all organisms belonging to a species are the same. I mean look at any roomful of people, it’s perfectly obvious that we differ. Many of those differences are heritable, can be passed onto our children, so you’ve got variation, you’ve got inheritance, and then finally some of that variation would be more or less useful in adapting an organism to its environment, in other words in making it easier to find a mate or easier to survive or find food, but something would ultimately influence the transmission of that individual’s genes.

So if you put those three things together – variation, inheritance and natural selection – you automatically get change, and that change is evolution.

 

Rissa de la Paz:

So far so good but if the evolutionary spotlight favours the individual that survives best and leaves the most offspring, surely only one type of character could take the leading role.

 

Jonathan Silvertown:

If you take a simplistic view of what Darwin was proposing then there’s really a very simple rule as to which kind of individual will succeed over and replace all others, and it’s what you might call a Darwinian demon. It’s an organism that reproduces very soon after it itself was born, it produces a huge number of offspring, in theory an infinite number of offspring, it doesn’t waste time swapping its genes in sex, it just reproduces asexually, and ideally it would live for ever. If that were possible, if an organism like that could exist it would surely beat all-comers, and that one might call a Darwinian demon.

 

Rissa de la Paz:

Are there actually examples of Darwinian demons in nature? Cast your eye around any English garden and you’d be surprised.

 

Jonathan Silvertown:

There are many examples of things that are going in that direction, and you can see them succeeding stupendously as a result. Aphids are demonic, they reproduce so quickly that when the females are born, and they’re born without sex during the summer when they’re reproducing at their fastest rate, the females are born and they already have their own offspring inside.

 

If you want an example from a plant, dandelions do pretty well. They do live a long time, or they can do, and they also reproduce asexually, they can produce huge numbers of seeds. Of course, you know, they’re not Darwinian demons but they have some of the characteristics of Darwinian demons.

 

Rissa de la Paz:

What’s perhaps surprising is that every species is potentially a Darwinian demon because all populations have the potential to double and so increase geometrically – two producing four, producing eight, sixteen and so on.

 

Jonathan Silvertown:

Now this has to be inherent in all species or else no species would get started because all new species start off in the minority, they start off as something new and rather rare, and they can never become common, common enough to survive unless they reproduce successfully. So that’s inherent in Darwin’s theory.

But then the question is what stops it happening, why don’t those successful species take over completely? So what the whole concept of the Darwinian demon does, it points out that diversity is not the only necessary consequence of evolution, you have to explain how you get diversity out of evolution, it isn’t obvious, if you like, at first sight.

 

Rissa de la Paz:

So there’s the paradox: if Darwinian demons are the logical consequence of evolution by natural selection, the result would be dull monotony, not marvellous variety.

Jonathan Silvertown:

Darwin was well aware of this difficulty, and he writes in his autobiography that he remembered the place on the road when he was driving in his carriage when the solution occurred to him. In modern parlance, what Darwin realised was that when plants and animals and different organisms adapt they occupy different niches in what he called the economy of nature. Well the economy of nature is what we call ecology, so there are different ecological niches.

 

Rissa de la Paz:

A famous example of ecological niches in the animal world involves the finches of the Galapagos Islands. Different species of finch vary in the size and shape of their beaks, depending on the sort of food they eat. So for instance, one species might have a stout beak for crushing hard seeds, another, a probing beak for picking insects from vegetation. Beaks also vary in size according to the type of seed a bird eats. So the beak of the finch is a signature of how each species makes its living on the islands – its ecological niche. What about niches in the plant world?

 

Jonathan Silvertown:

It’s clear that there has to be some way in which plants obtain a living in different ways. If they didn’t there would then be just one plant that did better than all others, and instead of having 400,000 species of flowering plants we’d have one or two. Perhaps one per continent or something, which would be quite the reverse of biodiversity.

But if you actually look at what plants require and how they obtain it, you don’t see the picture that you see in finches where each species has a clear difference. You see a lot of obvious similarities because all plants require light, they all require water, they all require basic plant nutrients, nitrogen, phosphorus, potassium and other minerals in smaller quantities. They all seem to obtain these things in much the same way. All green plants have green leaves with which they capture light and photosynthesise, they capture carbon dioxide from the atmosphere through little apertures on the leaf called stomates, and their characteristics are really fairly similar.

Rissa de la Paz:

So how is it possible for plants to carve out a vast array of different niches when they at first sight seem to obtain their resources in similar ways? How might you go about studying how plant niches evolve?

 

Jonathan Silvertown:

If you really want to get at the evolution of niches, what one needs to do is to go somewhere where you can study plant niches in situ in the kind of environment where they evolved. There are places, and there’s one particular place where you can look at species today which evolved, as far as we can tell, in that very place with each other, and that place is the Cape of South Africa. The beauty of the Cape is its huge floristic diversity. There are some 9,000 species, half of them evolved from just 33 ancestors, something like 70% of the species in the Cape are endemic, that is to say they’re found nowhere else, and there are families in the Cape which occur nowhere else. In fact, there are even nature reserves with two or three families present in them that occur nowhere else.

 

Rissa de la Paz:

With David Gowing and other colleagues, Silvertown has been studying a group of plants rather similar to rushes called the restios. There are about 350 species in the Cape, flourishing in a type of heathland vegetation called fynbos.

 

Jonathan Silvertown:

They’re all quite closely related to each other, and you can get in a single site a dozen, even two dozen living together, and what we’re looking at is how they manage this, whether we can actually find out what the different niches of restios within a particular site are. And what we’ve discovered is that the species separate out on soil moisture gradients, so that you could describe the niches of different restios in terms of their ‘preferences’, and I put the word preference in inverted commas, their ‘preferences’ for areas of different soil moisture.

 

Rissa de la Paz:

It looks as though subtle variations in soil moisture content can favour different species of restio. So you can get a patchwork of species living together on one site simply because they vary in small but distinct ways in how much water they need. It’s remarkable how such subtle differences can lead to such dramatic diversity. Could it be unique to this type of vegetation?

 

Jonathan Silvertown:

It was something of a surprise that we found the restios separated out on soil moisture gradients as clearly as they did. We thought they might because we’d already discovered a phenomenon rather like this in meadows in England. However, there is a huge difference between meadow communities in Britain and the communities where restios live in South Africa, which is a kind of heathland called fynbos. So in meadows in England, these are flooded regularly, every year in winter the meadows are flooded and then they dry out in the summer, in fynbos there is some flooding but the key environmental factor in fynbos is not water but fire, and every 15 to 50 years depending on chance to some extent these fynbos communities, these heathland communities completely burn to the ground.

This is the natural process that’s been going on for millions of years and the plants are adapted to it. In fact they even require smoke and fire to trigger germination of their seeds in many cases, so then the fynbos community comes back, it recovers in a phoenix-like fashion from buried roots that then sprout and seeds that then germinate.

So in a situation like that, where fire drives the whole system, it’s kind of surprising to discover just how important water is in enabling species to co-exist with each other by having different niches.

 

Rissa de la Paz:

And yet the contrast between South African heathland and English meadows is itself a strong pointer that niches based on soil moisture content may be more widespread than you might think.

 

Jonathan Silvertown:

If you look at all the differences between fynbos in South Africa and meadows in England, they are so great that to find that the plants behave in a similar way does suggest there’s something very fundamental about the processes, they’re probably physiological processes that are causing plants to behave in this way.

There’s not a single species in common between fynbos and meadows. Many of the families are completely different, so we’re sampling, if we compare the two communities, one in the southern hemisphere, one in the northern hemisphere, one driven by flood, the other one driven by drought, one woody, the other one mainly herbaceous, we’re dealing with species that are sampled from very different parts of what you might call the green part of the tree of life.

And if you then discover that these species sampled in very different places within the evolutionary tree are actually behaving the same way, there’s every reason to suppose that all the species between them if you like on the tree, all the other species which we haven’t sampled yet, are probably behaving in a similar way too, and that then makes this result about as general as you can get.

 

Rissa de la Paz:

Of course, adapting to the availability of water isn’t the only way in which plants create separate niches in their environment. Light is another example: different species in a forest can live at different heights in the canopy. Plants also vary in the way they disperse their seeds and this might allow them to occupy different niches. What’s crucial is that by specialising in some way, plants are demonstrating one aspect of why ecology and evolution are so intimately linked. It’s what’s known as a trade-off.

 

Jonathan Silvertown:

What a trade-off is is essentially, it’s a very simple idea, it’s the idea that you can’t have your cake and eat it too, that every adaptation obviously has some advantages but it also has some inherent disadvantages as well.

Let’s take the example of a tree that’s tolerant of shade. You can be tolerant of shade but you can’t grow fast if you’re going to grow in a shady place, so there’s a trade-off between shade tolerance and rate of growth. This can produce different niches because there’ll be plants that are intolerant of shade but can grow very fast when they have light, and there’ll be at the other extreme the reverse, plants that are tolerant of shade but necessarily have to grow slowly.

For just about everything you can think of there are always swings and roundabouts, there are advantages and disadvantages, and adaptation sort of finds a way through the compromises that have to be made between these opposing forces. And that’s how adaptation evolves.

 

Rissa de la Paz:

So how does the concept of a trade-off help us to understand diversity?

 

Jonathan Silvertown:

Diversity is a result of the compromise that the species make when faced with these trade-offs. Essentially what trade-offs do is they force specialisation. You can’t be a jack of all trades because of trade-offs. Consequently, choosing one particular compromise between properties that trade-off, between physiological demands, if you like, results in specialisation. So a plant can specialise as being a slow growing, shade tolerant species or as a shade intolerant, fast growing species or something in between, but it can’t occupy all of those niches, given that there are other specialists around competing with it.

 

Rissa de la Paz:

So if no single plant species can occupy all niches, what does this mean for Darwinian demons? Can they actually exist?

 

Jonathan Silvertown:

They can try, they can do very well in one particular avenue producing lots and lots of offspring, but there’s a cost to that, and the cost of producing lots of offspring is often to grow rather poorly and to have a shortened life. So it isn’t possible in reality to live for ever and have lots of offspring, you tend to find that, for example, in trees that those species that produce the most seeds are actually very short lived, things like birches. Those trees that live a lot longer don’t start to reproduce until they’re quite old, oaks for example. And while they will produce a lot of seeds in the end, they have to save up resources, they have to postpone reproduction for a long time in order to do that.

So there are many trade-offs of that kind that force specialisation. The thing about the Darwinian demon is, if you like, that it’s a generalist and it’s good at everything, and that turns out just to be not possible.

 

Rissa de la Paz:

And yet, there are plants today which have many of the characteristics of Darwinian demons and it seems that there are certain conditions that actually favour their survival. Take species that are introduced from one environment to another and end up invading their new habitats with astonishing success.

 

Jonathan Silvertown:

What’s curious from a Darwinian point of view is that very often these species are doing better than where they evolved and they’re doing better in their new habitat than the species that evolved in that habitat. So at first glance this is very much anti-Darwinian because you have species that are doing better where they are not adapted because they didn’t evolve there than they did back home, and they’re also doing better than the natives.

An example in many parts of the world might be Japanese knotweed, which is a plant that reproduces asexually, it’s actually fragments of the plant can grow, and they grow at an incredibly fast rate, they spread enormously, and it turns out that this plant, all populations of it pretty much throughout the world outside Japan where it comes from all belong to a single clone, they’re all one genetic individual. For a while this plant is showing all the signs of being a Darwinian demon, it is spreading in a demonic way.

Incidentally, it’s very expensive to eradicate from derelict land, and on the site of the London Olympics for 2012 they’re having to spend something like £50,000 a hectare just to remove Japanese knotweed. And if you don’t remove it the roots will come up through the tarmac and destroy the building sites. So it’s very important to remove it, and it’s a very good example of that sort of demonic behaviour.

Why does something behave like that? Well we don’t really know, but I think that the likeliest answer is that in its native environment a plant like Japanese knotweed is attacked by insects and other herbivores that have evolved with it, so it’s got a whole fauna of natural enemies. Also fungi and things like that keep it in check and then it’s evolved with. When plants travel, and it’s, the same is true to some extent of animals as well, they tend to leave behind their natural enemies. This is certainly true of anything that spreads by seeds because the enemies, you know, caterpillars and all that sort of thing will be left behind.

So I think this tells you why some of these plants are such a nuisance when they travel but not when they’re at home, and also of course suggests a solution to the problem, which is to introduce the natural enemies to the areas where these plants have been, become established in order to try and reintroduce a check on their populations.

 

Rissa de la Paz:

So to preserve biodiversity, the demonic tendencies of any one species need to be kept in check, allowing a variety of species to co-exist. There are practical ways of doing this, depending on the habitat.

 

Jonathan Silvertown:

One way this needs to be done is to control the nutrient inputs to plant habitats, because when nutrients go up this tends to favour those few species that are best able to turn those nutrients into offspring, and this then out-competes the other species so you lose species.

Another way is through grazing. Many species’ rich grasslands, for example, those you find in limestone habitats, depend upon grazing to keep in check the grasses which will in other circumstances totally overtake the habitat. So to a large extent, managing these kinds of plant habitats for diversity is basically an exercise in controlling the demonic tendencies of species.

 

Rissa de la Paz:

There are factors today which can influence the spread of demonic species and cause mayhem in the ecological theatre. Climate change can disrupt communities of plants and animals living in different areas of the globe and encourage the invasion of new species into those areas. Advances in plant breeding, including genetic modification, may introduce new demonic species into habitats. We need to be aware of the ecological impact of these developments if we’re to safeguard diversity and ensure that the lights don’t go out prematurely on our planet’s evolutionary play.

Music: Variations on a Theme by Giuliani

Rissa de la Paz:

This podcast was produced as a collaboration between the British Council and the Open University.

 

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