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Breaking Science: Darwin, evolution & natural selection...

Updated Sunday 22nd March 2009

How Darwin's theories became accepted, palaeontology provides evidence of natural selection, and dinosaurs – an accident in selection.

In the final programme of the series, the Naked Scientists bring you a Darwin bicentenary special from the Open University’s lecture on one of the drivers of evolution: natural selection. Discover how Darwin succeeded in gaining acceptance for his radical theories; how the evidence for natural selection in palaeontology has since been discovered; and find out why dinosaurs were an accident in selection.

Dinosaur model in Crystal Palace, London Creative commons image Icon Alex J White under CC-BY-NC licence under Creative-Commons license
A dinosaur in Crystal Palace park, London. [Photo posed by model]

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Chris Smith: Coming up, it’s the Darwin bicentenary. So this week, we attended the Open University’s annual lecture on one of his drivers of evolution, and that’s natural selection.

Jim Moore: The powerful drive that overcame the social stigma of being an evolutionist was Darwin’s radical belief in the unity of all life. Darwin takes it a step further and unites everyone and says that it’s our arrogance to believe that we’re not related to animals; it’s the arrogance of the slaveholder lording it over his slaves whom he likes to regard as another species.

Chris Smith: And Jim Moore will be telling us how Darwin did succeed in gaining acceptance for, what were then, very radical theories. Also, how the evidence for natural selection in palaeontology, which Darwin struggled to find at the time, has since been discovered.

Peter Skelton: In his day, they knew it only went back to the Jurassic, but then reptilian forms, more primitive reptilian forms, they went back into the Carboniferous, and then there were amphibians lower down than that, and fish lower down than that, so you could see that the fossil record did provide the big picture, but what it didn’t provide was, in his day, the evidence for microevolution. But, today, we have plenty of examples because we know where to prospect for such examples.

Chris Smith: And Peter Skelton will be digging through the layers of the small changes in fossils for us very soon. Plus, we’ll be speaking to Mike Benton about an accident in selection: dinosaurs.

Mike Benton: They took over certain roles on land, particularly they were plant eaters, and they were good at that, but they just did that for 25 million years, they didn’t do anything else.

Chris Smith: So how did they come to rule the Earth? Hello, I’m Chris Smith, and this is Breaking Science which is produced in association with the Open University.

Darwin is commonly held up as the father of the theory of evolution, but why? Was it his beard that gave him credibility? To find out, Ben Valsler spoke to Darwin biographer Jim Moore, and he began by asking him what drove Darwin to formulate the concept of evolution in the face of what was initially really quite profound religious and political hostility to his ideas.

Jim Moore: Darwin was driven by different things at different times, just like all of us. He was complex; he changed; he became more conservative generally speaking as he got older. But if you mean what drove Darwin to become an evolutionist, one has to say it has to be something as powerful as the forces that were ranged against evolutionists.

When Darwin is less than 30 years old, he comes back from travelling around the world, most of it was on land, not at sea but he gets home, and within weeks, probably a few months, he’s become an evolutionist. Why does he do that? It’s a bad career move. And in our new book, Darwin’s Sacred Cause, Adrian Desmond and I say that that powerful drive that overcame the social stigma of being an evolutionist was Darwin’s radical belief in the unity of all life. That common descent unites every species, the human race as well as all races of animals and plants, and that leads him to a powerful image that was part of the ideological foundations of the antislavery movement. The notion of a family tree of humanity for traditional Christians rooted in Adam and Eve as the father and mother of humankind. Darwin takes it a step further and unites everyone and says that it’s our arrogance to believe that we’re not related to animals; it’s the arrogance of the slaveholder lording over his slaves whom he likes to regard as another species.

Ben Valsler: This may well have been the driving force but still it was a long time before he published. It was a long time before these ideas actually made it out there. Was there a tempering force as well that made him look for all the right evidence and made him make sure he could prove this before he would publish?

Jim Moore: Darwin kept his thoughts to himself to begin with. He was in the process of becoming involved in the Royal Society as secretary of the Geological Society of London. He was welcomed to the inner sanctum of elite natural history. His sponsors were Cambridge clergymen, professors. He had a grant from the Whig Chancellor of the Exchequer, a huge amount of money to publish his Beagle research. He was a young man on the make. He was pushing all the right buttons, he was going all the right places, and yet he carried this terrible secret in his private notebooks. He needed a theory, and he began calling his speculations ‘my theory.’

That was his project, ‘my theory.’ And towards the end of 1838 he works out what we now call natural selection. By 1839, when he’s getting married and having children, he’s developed that, and he knows within three years, he leaves London, he takes shelter in the countryside, he knows he’s onto something really big. It’s going to change the course of the history of science if he can convince people.

Now at that stage you don’t go public. You take every precaution that’s necessary to convince people beforehand that what you carry with you is true. It’s not disreputable; it’s the answer to the mystery of the diversity of life on earth.

So he commits himself for the next 17 years, that’s sort of 20 years in all since he devises natural selection, to answering in advance every conceivable objection that the heavyweights of science in his day could bring against what he’s doing, and that leads him into huge research projects. And finally he gets around to putting pen to paper and he plans a huge book, maybe a half million words in three volumes which no one would read, and in the middle of all of that, you know, he gets outed by this guy named Wallace, everyone knows this story, Darwin has to condense his work into something which he entitles ‘On the Origin of Species’.

Ben Valsler: Do you think the pressure of having these other younger researchers formulating very, very similar theories based on very similar principles, Wallace was looking at series of islands much like Darwin had, do you think this forced him to make some concessions in his work?

Jim Moore: Darwin was not aware that Wallace was working on a theory, until the paper arrived in June 1858. Darwin felt safe in his non-competitive ecological niche as a theoriser of evolution. He knew that all the other theorisers were discredited or spoke ill of. He wasn’t like them. He wasn’t telling anybody what he was like. He still believed he had an inside track on natural selection.

Now, what did he do with that theory once he knew that Wallace was onto the same thing? He believed Wallace was onto the same thing. Darwin read the paper in haste; we can all see now that they are not talking about the same thing in the same way – Wallace rejects the selection analogy for example. Absolutely basic analogy with domestic animal breeding, Wallace absolutely rejects it, always rejects it. So there’s a fundamental difference between Darwin and Wallace to begin with.

I can’t see that Darwin gives up anything. I’d have to think about it for a while before I gave you a technical answer, but it seems to be that what Wallace says and does over the next 10 to 15 years makes Darwin more attached to what he always thought. Wallace did push him hard, and Darwin said once, “It terrifies me to disagree with you,” and that was public hyperbolae, but this unprepossessing sort of guy, who left school when he was 13, he didn’t go to Cambridge. I mean he would have, Wallace would have joined the Open University and he’d have got a fine PhD, had there been an Open University in 1840.

This was an incredibly bright and underused talent, you know. And Darwin knew that. You know, they were socially chalk and cheese, and yet this guy was dorking him, and Darwin took preventative measures, hedging about his theories to make sure, obvious example is sexual selection, Darwin is so goaded by Wallace, because Wallace doesn’t believe that male competition and female choice causes sexual dimorphism in nature. Darwin expands his work on sexual selection so two thirds of his book, on ‘The Descent of Man’, and Selection in Relation to Sex is the rest of the title, two thirds of that book is about birds and bees and pigeons and furry mammals before he ever gets to humans. Typical Darwin, he has to do the whole panoply of nature to prove that sexual selection is right and (brackets) Wallace is wrong.

Ben Valsler: And finally, what was it about Darwin that means that he stands out now? There were other people researching similar things that may not have hit exactly the same theory, but Darwin really was the man that stands out as being the father of evolution.

Jim Moore: Evolution needs a father, as Steve Jones would say. Newton is pictured by Blake’s geometer outside the British library on Euston Road - unfairly perhaps. You think of Einstein. You think of Einstein as a brain, you know. You might think of Freud as being something really slippery. But Darwin’s a grandpa! He has a beard. He has a big family. He’s wealthy. He lives in the country. He’s contented. He cut the image of what it was like to be a gentleman of science in his day, and he still does. Darwin is cuddly. Apart from the fact that this old man is not reliable with children because he teaches them falsehoods, some people say, this old gent is like anybody’s grandpa. You could really warm to the guy.

Now I’ve studied Darwin for many, many, many years, and I’m not particularly enamoured of him. The more I’ve got to know him, I suppose the more I’ve got to know anybody, the less I’ve been enamoured of him.

Chris Smith: So never get to know your heroes too well, it seems, is the moral of that story. Thank you very much to the Open University science historian and Darwin biographer, Jim Moore. And incidentally you can find out more about his biographical work on the Open University’s website, which is at open2.net/breakingscience.

In just a moment, how picking up snails could help with the study of evolution, but first to palaeontology and how a group of fossils called rudists are giving us insights into long-term natural selection.

Rudists were a species of shellfish that first appeared about 160 million years ago in what’s now termed the Upper Jurassic. But very often an issue with using the fossil record to follow the evolution of a species like this is that you only get to see occasional snapshots of how the changes occurred; in other words, there are lots of gaps, as Peter Skelton explained to Diana O’Carroll.

Peter Skelton: There are two problems: there’s the problem of a vertical incompleteness; there is also the problem of geographical incompleteness. If you are thinking about how does speciation take place, a lot of examples of speciation involve geographically separated populations, either gradually drifting apart from one another, or some small population that’s been separated rather rapidly changing, as for example picture-wing flies in Hawaii, which all evolved probably from one immigrant form that came from California, they’ve evolved very rapidly in isolation from the parent populations.

So if you’re going to capture that whole story in the fossil record, you not only have to have extraordinarily complete fossil records in each place but in all the different places so that you can catch the geographical dimensions, as well as the vertical dimensions, and that’s very, very difficult. The best example where we can do that is in oceanic oozes because the ocean floor is a huge basin where all this plankton, you know, shelly material from the plankton can come raining down, and there, with some of these microfossils, the foraminifera, coccoliths, things like that, this microscopic fossils. We do actually have very good examples of change, both on the geographical scale and the timescale.

Diana O’Carroll: Darwin was looking for geological samples to back up his theory, but what happened when he went to find them?

Peter Skelton: In his day, they weren’t there, that's the simple answer. There were one or two people had talked about changes in gastropods in lake deposits and what have you, but basically Darwin went out hoping to find examples of gradual evolutionary change, written all over the book of sedimentary deposition, and he couldn’t find any anywhere. So he rather turned away from geology. As I say, he had the right answer: incompleteness.

He gave up, pretty much, on palaeontology for microevolutionary change. He saw that palaeontology provided the evidence for the tree of life. He could see that, for example, if you go back through the fossil record, the fossil record of mammals only, in his day, they knew it only went back to the Jurassic, but then reptilian forms, more primitive reptilian forms, they went back into the Carboniferous, and then there were amphibians lower down than that, and fish lower down than that. So he could see that the fossil record did provide the big picture, but what it didn’t provide was, in his day, the evidence for microevolution. But, today, we have plenty of examples because we know where to prospect for such examples.

Diana O’Carroll: You’ve spoken about the macro or large scale side of natural selection, but how are you exploring it and what are rudists?

Peter Skelton: I’m looking at the circumstances in which major changes can take place. With my particular group of rudists, for example, the ancestral forms look like a rather bloated cockle with, you know how a cockle, each half of a cockle has that little beak on top, and that’s where the two valves join and it allows the shell to open, well the earliest rudists look like, you know, a cockle to which somebody has taken a bicycle pump and sort of puffed it up and made it sort of fat and globose, and they sat on the sea floor living pretty blameless lives.

By the end of their evolutionary career - and I should say they became extinct at the end of the Cretaceous when most of the dinosaurs became extinct. They had evolved forms which look like ice cream cones, with incredibly complex lids, with lots of little pores on, where water was drawn in through the pores and along the canals which were inside the lid. The mantle margin of the animal would feed from water that was strained through; this thing that looks absolutely nothing like a bivalve at all.

So what I’m interested in is why these things underwent such extraordinary changes. In what circumstances did they have reason to change in that way, and of course, in my case because I’m looking at something that’s extinct, I can really only look at the circumstantial reasons, the environmental reasons, but it’s changes in ecological context which cause these big changes. There would have been important genetic changes associated with this, but since they’re extinct I don’t know anything about their genes.

Diana O’Carroll: What kind of ecological changes have you come up with to explain this?

Peter Skelton: Right, okay. Part of it is size related because the early forms lived implanted in the sediment and they achieved stabilisation just by the passive accumulation of sediment around them, and they could grow up from the sedimentary substrate. But as they became bigger, it became possible for them to start adopting a new strategy, which is growing like a big spread out snowshoe, like a calcareous croissant, living on the sediment surface, okay. With that sort of morphology they were well defended from not being washed away, which would have happened if they had been smaller.

This allowed them to start occupying environments that the smaller forms couldn’t occupy. They occupy environments where fast currents tended to sweep the sediment along, anything growing in the sediment couldn’t rely on sedimentary accumulation, so they could exploit new environments, and this is what we find that as the rudists diversified to these different forms they were beginning to invade new environments, new niches and they became incredibly diverse and very, very successful. But if you happened to be in the way of an asteroid zapping the earth, as happened at the end of the Cretaceous, no matter how well adapted you are, it’s curtains.

Diana O’Carroll: So, although they’re actually extinct, is there anything about today that is a little bit like a rudist that’s occupying that same niche?

Peter Skelton: Not really, the closest we come is an analogy for the forms that live implanted in the sediment, are things called pen shells. If you go, for example, to the Great Pearl Bank in the Arabian Gulf, you’ll find of course the pearl oysters there, but these pen shells - they look like big hams, they’re called pinna - that live implanted in the sediment, and they too grow out of the sediment and feed from the overlying water, but that’s a pretty approximate analogue.

Chris Smith: So no rudists on our beaches these days, just nudists. That was Peter Skelton, who’s a palaeobiologist with the Open University talking to Diana O’Carroll.

Now from shells that can change with their environment to a whole set of extinct animals that just happened to exploit an ecosystem, not because they were better adapted but because they were in the right place at the right time. But what was this group of animals? Here’s Mike Benton.

Mike Benton: We’re very interested in the origin of dinosaurs and more widely in big origins through the history of life. If you go back through geological time you see that different groups have come and gone, and famously, of course, dinosaurs, in the popular terms, ruled the Earth for 150 million years, and then they died out and the mammals came on the scene. So there’s a big question about how do these big transitions happen. We wanted to try and look at that and try to tease it apart and just see is there any way we can try to get to the basis of this big debate.

Chris Smith: And how have you done that?

Mike Benton: So what we did, we decided we would look, in great detail, at the exact timing of when the very first dinosaurs appeared and then follow step by step, if you like, turning the pages of geological time, going through the first 20 or 30 million years of dinosaurs on the Earth, and what we found surprised us. We discovered of course that dinosaurs did not appear overnight, sort of big bang suddenly they’re everywhere, nor did they appear in a progressive way where they sort of step by step replaced precursor groups. We wanted to look at the range of body types or variability in body forms, and this is termed, by biologists, the amount of morphospace that species occupy.

So, for example, birds occupy a fairly limited amount of morphospace because they’ve all got wings, they all fly, they all stand on their two hind legs, whereas reptiles today occupy quite a wide range of morphospace. There’s an enormous difference in the body shape of a turtle and a crocodile and a lizard and a snake, let’s say.

Chris Smith: And how does that give you a clue as to the order in which these things arose and what drove the appearance of different types of dinosaurs?

Mike Benton: Looking at the range of body types tells you something about the number of niches, the number of kinds of ecological things that any group is doing, whether it’s living or fossil. And what we found was that the dinosaurs did appear and they took over certain roles on land, particularly they were plant eaters, and they were good at that, but they just did that for 25 million years, they didn’t do anything else. Whereas the group that they were competing with, which are collectively called the crurotarsans, a bit of a mouthful, we can just call that crocodile ancestors more broadly, they maintained a great diversity of forms, and they were plant eaters and flesh eaters and included the big carnivores of their day.

And during this critical 25-million-year period, the dinosaurs did their thing but they did not supplant these crurotarsans, and then it took a second extinction event, about 200 million years ago, when the majority of these crocodile ancestors finally bit the dust, due to some other cause, some major change in climate it seems, and then the dinosaurs expanded their range of activity, their range of adaptation.

Chris Smith: Do you know why they were so slow off the blocks in the first place, to get into this niche of eating things other than plants, because that presumably is what was holding them back?

Mike Benton: Yes, and I think what this illustrates is that dinosaurs were good at lots of things, and they had lots of advantages, clearly they ruled the Earth for 150 million years, but they were not overwhelmingly better than the crurotarsans, and they failed to supplant them. There is no evidence then that there was active competition going on. And these crocodile ancestors were clearly good as flesh eaters and fish eaters and various kinds of plant eaters. They were as good as the dinosaurs, and it took some other event, some chance catastrophe that wiped them out.

But what it does is shows us, allows us to anatomise or dissect, in a lot more detail than people had done up to now, exactly what was going on, but also to begin to look at the processes, and one thing we can say for sure is that the dinosaurs were extremely successful in their day, but they were not overwhelmingly better or more progressive than the animals they eventually supplanted.

Chris Smith: Mike Benton, from the University of Bristol, with a menu for natural selection.

You’re listening to Breaking Science with me, Chris Smith, and back to the OU lecture now, where Ben met Jonathan Silverton who says that snails are the key to observing natural selection as it’s happening. With more about the MegaLab Initiative to study snails as it’s known, here’s Ben Valsler.

Ben Valsler: Also as part of this year, again there’s a large Darwin theme throughout everything, I understand that the OU are running something called Evolution MegaLab, could you tell me a bit more about this?

Jonathan Silverton: Yes, well the Evolution MegaLab is my favourite project actually. It is the largest survey of genetic variation in any species other than humans that has been undertaken. We’re doing it as a public citizen science project which means that anybody listening to this broadcast can participate. And what we’re looking at is variation in the banded snail, which is found pretty much anywhere in Europe where snails occur.

And if you go to evolutionmegalab.org you will find full instructions about what to do. It’s a suitable thing for children, grownups, whoever, and you can help in a real scientific experiment to see whether there’s been evolutionary change in these snails, in relation to climate change and in relation to changes in the abundance of thrushes which eat banded snails.

Ben Valsler: Most people, when they find a snail in their garden, will in fact just throw it over the fence, or perhaps stamp on it, because of course they eat our plants, they eat our vegetables if we have a vegetable patch. Are we asking people not to do that just for a little while so that we can get some decent data?

Jonathan Silverton: Well there are snails and snails, and I have a garden and I don’t much like the big garden snail which is the brown thing about the size of two thumbs. We’re not studying those. We’re studying smaller ones, to find out exactly what, I do urge you to go to evolutionmegalab.org because then you can find pictures and everything you need to know, but basically these are about the size of a fingernail on your little finger. They’re beautifully coloured. You get them different colours, some have stripes, bands of varying numbers; they can have one, they can have five. Some have no bands but they’re still banded snails.

And it’s this variation which makes them so interesting. We understand the genetics that lies behind this variation and so that’s how it’s a wonderful organism for looking at evolution. These snails actually don’t eat your lettuces, they tend to graze - the smaller kind I’m talking about - tend to graze on algae and dead matter, dead leaves, things like that. So actually don’t stamp on them, put them back when you’ve finished looking at them.

Ben Valsler: As a professor of ecology you understand better than most the fact that one species, such as the banded snail, might actually be an indicator of a greater health of an environment, is that the sort of thing we’re looking at with Evolution MegaLab?

Jonathan Silverton: Not exactly, I mean it’s true that, you know, the species are all kind of connected up. I mean everything eats something else unless it’s a plant, in which case it is eaten probably and certainly has interactions with fungi and all sorts of other organisms. So, in that sort of sense, we’re all connected. But the idea of Evolution MegaLab is to see how the environment, and how it’s changed over the last 40 years, may have changed the genetic constitution of the population genetics of the banded snails. We don’t know, because it’s an open question, it’s a scientific question, we don’t know what the answer is and we would like people to help us find out.

Chris Smith: So watch where you step, it’s an important experiment. Jonathan Silverton there, who’s Professor of Ecology at the Open University, he was speaking to Ben Valsler.

That’s it for this week’s Breaking Science and in fact for the series, but we will be back with more science one way or another very soon.

In the meantime you can follow up on any of the items included in this, or one of our previous programmes, on the Open University’s website, that’s at open2.net/breakingscience. Alternatively you can follow the links to get there from the BBC Radio Five Live Up All Night web pages.

The production this week was by Diana O’Carroll from thenakedscientist.com and I’m Chris Smith. Thank you for listening and goodbye!

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Background

These are the sources used by the Naked Scientists in making the show:

Interviews

Jim Moore, Open University

Peter Skelton, Open University

Mike Benton, Bristol University

Jonathan Silvertown, Open University

'Superiority, Competition, and Opportunism in the Evolutionary Radiation of Dinosaurs' by Stephen L. Brusatte, Michael J. Benton, Marcello Ruta, Graeme T. Lloyd in Science, pub 11.09.08

 

 

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