Evolutionary biology 101

It has been said that evolution by natural selection is the best idea that anyone has ever had. But what actually is it? How does it work? And how has our view of evolution changed from Darwin to DNA sequencing? Steve Jones, from university College London, answers these questions in a crash course introduction to the word of evolutionary biology...

Steve - Natural selection is overwhelmingly something simple: inherited differences in the ability to reproduce. It’s a two-part exam: in order to reproduce, you need to stay alive, so there’s natural selection on survival, and you also need to find a mate and have offspring, so there is what Darwin called ‘sexual selection’. If you have genes that make it more likely that you will survive, become an adult, find a mate and have offspring then those genes will become more common over the generations, and that is natural selection. I call it a factory for making almost impossible things. It trudges forwards, it doesn’t take leaps. It takes very very slow, steady, directionless steps, and gets to unexpected and almost impossible places at the end. For example, people often say, “What use is half an eye?” The answer is, “a lot more use than no eye at all, or 49% of an eye”.

Will - And what was Darwin’s role in the history of evolution?

Steve - First of all, the idea of evolution is much, much older than Darwin. Darwin, for example, refers quite often to a guy named William Jones, who was a merchant and a linguist. And what Jones did was to realize that the languages of Europe were related to each other and were also related to a number of Indian languages. And he deduced that they had actually all descended from a common ancestor, and as they descended they changed – in other words they’d evolved. But the idea of natural selection – although you can pick up earlier notions which hint at it – the only person who really thought it through was Darwin himself. So that’s Darwin’s central role in evolution. Secondly, he brought together an astonishing collection of at first sight unrelated topics and facts into what he called “one long argument”, in The Origin of Species. And at the end of it, if you approach The Origin with an open mind, it’s almost impossible to deny that his arguments hold water. So Darwin really was the inventor of the science of evolution.

Will - It’d often claimed that another British naturalist, Alfred Russell Wallace, co-discovered natural selection?

Steve - I think in some senses he did, although he didn’t put it quite as clearly as Darwin. Darwin planned to spend most of his life writing a book that was going to be called ‘Natural Selection’. And fortunately, Wallace – who was himself a very considerable figure – came up with the same idea, although with far less evidence to support it.  And Wallace wrote to Darwin, and bounced Darwin into writing The Origin of Species within a year. And The Origin as a result is a much better book than the big Natural Selection book would ever have been, it would have been unreadable. So I think Wallace’s role was important but secondary.

Will - Were there any differences between what Wallace thought about evolution and what Darwin thought?

Steve - Wallace in his later years saw some sort of progress in evolution. But Darwin saw no progress, he wrote, “never see higher or lower”. And Wallace was convinced that somehow human evolution had an extra added onto it, perhaps by God, which led to spirituality and consciousness and things like that. And Darwin would have nothing of that; he really thought it was nonsensical and that it wasn’t scientific, and he rather fell out with Wallace about it. So that was the main difference between them.

Will - How would you say that the field has moved on since then – how was genetics put in the context of evolutionary biology?

Steve - Genetics is the keystone of the Darwinian arch – it locks the whole system into place. Darwin kind of believed in the mixing of bloods as the pattern of inheritance – that male and female bloods mixed and made the next generation. And Darwin was written to by a Scottish engineer named Fleeming Jenkin. And Jenkin pointed out to him that if you have red paint and yellow paint, and you mix them together, you get purple paint. Now let’s imagine that red is very advantageous – but in the next generation red has disappeared and you’ll never get it back because you’ve got purple. So how can natural selection work? But what Mendel showed was that inheritance wasn’t based on fluids, it was based on particles called genes. And they came together and separated apart without mixing in the sense that paint mixes, so you could get red back in a later generation, even though the animal might not look red for a generation or two. You can’t do evolutionary biology without doing genetics.

Will - And how has the human genome project extended our understanding of evolution?

Steve - I think it’s made us realise that the physicist’s view of biology is wrong. It was physics that uncovered the double helix – the structure of DNA. And physics, at least in those days, and in some ways still is, a very beautiful subject; it makes the complicated simple. And what happened with the double helix was that it made something that seemed completely baffling – biological replication – simple with this extraordinarily elegant molecule. But what the human genome project has done is to remind us how evolution works. It doesn’t work like physics: it’s not a great force of simplification, it’s a force of complication. Evolution does not plan ahead; it’s a tinkerer, it’s not an engineer. It simply waits for what turns up; it’s a series of successful mistakes. It builds on the ruins of what has failed. And if you look at the genome, that’s exactly what you see: a series of successful mistakes, it’s filled with failures, filled with ruins, filled with bafflement, and a small part of it seems to do the job.

Will - One of the central features of evolutionary biology is the idea of the relatedness of all living things on Earth.

Steve - Yes, I think that is central. What is really quite remarkable is that the genetic code, with a few details, applies to everything, which suggests that we all do descend from one unique ancestor 3.5 billion years ago. The mere existence of that code is a statement about the astonishing power of Darwin’s idea of common descent. And everything that we’ve done in biology since then has affirmed the truth of that statement. What genetics has done to some extent is to get over the problem that natural selection eliminates history. It would make sense to a non-evolutionist to look at a bird and a bat and say that these things are related because they’ve both got wings. But if you look at the DNA of birds and bats then it’s immediately clear that actually bats are much closer to other mammals in their DNA than they are to birds, and flighted birds are much closer to ostriches than they are to bats. So genetics clarifies the past – not entirely, by any means – but it makes the past much clearer than it would otherwise be.

Will - And what’s the significance of that in telling us about our own evolution?

Steve - Well if you just take Homo sapiens as a mammal, and you stick us on the mammalian evolutionary tree based on bones and fossils, then we fit exactly where you’d expect, which is close to chimpanzees, not quite as close to gorillas, less close to orang-utans, and the DNA simply confirms that. And that itself is pretty damned impressive, but it’s done much more than that. If you compare humans and chimps, we differ only in about 5% of our DNA. So if you take the point that the amount of physical change hasn’t been that great, then it makes you realize that most things that make us human are not actually involved in physical change, they’re involved in mental adjustments to the Universe. The most striking one is that humans cannot live on raw food alone. If you, or anybody else, were just to eat raw food, you would die of starvation in a few months. And that’s because we no longer have the physical ability to chew, to process and to digest raw food. Why? Because we have invented an external stomach known as a frying pan or sauce pan, which predigests the food for us. Now that’s something that no other animal does; it has turned us into the cultured primate. And I think that’s the major thing that the DNA sequence has told us: in DNA terms we’re a boring, ordinary primate. But in every other way we have stepped beyond that.

Will - And you’ve written about how humans might have stopped evolving, at least in some parts of the world?

Steve - Well, what I’m basically saying is that natural selection in humans, in the developed world, and at least for the time being, has greatly slowed down. If you look at the figures all over the developed world, the variance in completed family size has collapsed, particularly when it comes to males. It used to be that some males – rich males – had hundreds of children, while other males had none because they couldn’t find a female. And all those differences were fuel for natural selection. In Shakespeare’s time, two English babies out of every three died before they were twenty one. And in Darwin’s time, one English baby in two died before they were twenty one. And natural selection is built on dead babies. Now we’ve got 99% of all English infants after the first three months surviving to be twenty one, so the natural selection machine has run out of fuel. And that’s why I say that natural selection has slowed down and stopped. 

Will - Steve Jones, Professor of Genetics at University College London.