Today, no area of biology remains unaffected by the discovery of evolution. The evidence that today's species evolved from different ones, and that this process still continues, is overwhelming. Very little modern biology would make sense without the over-arching framework of evolution. If Darwin were reincarnated today, he would recognise many of his ideas, perhaps slightly reformulated, in the conventional scientific wisdom. The big principle of natural selection would be one of them. But he would also observe debate, perhaps even controversy, about this fundamental pillar of his thinking. Not whether natural selection happens, not whether it drives much of evolution; but whether it is the only driving force.
He would also find many new layers of detail filling some of the gaps in his theories. The most important and far-reaching of these is DNA, the magic molecule that carries genetic `information', the physical form of heredity. Darwin was sure that organisms could pass on their characteristics to their offspring, but he had no idea how this process was implemented, and what physical form it took. Today we are so familiar with the role of genes, and their chemical structure, that any discussion of evolution is likely to focus mainly on DNA chemistry. The role of natural selection, indeed the role of organisms, has been downgraded: the molecule has triumphed.
We want to convince you that it won't stay that way.
Evolution by natural selection, the great advance that Darwin and Wallace brought to public attention, is nowadays considered to be `obvious' by scientists of most persuasions and by most nonspecialists outside the US Bible Belt. This consensus has arisen partly because of a general perception that biology is `easy', it isn't a real, hard-to-understand science like chemistry or physics, and most people think that they know enough about it by a kind of osmosis from the general folk information. This assumption showed up amusingly at the Cheltenham Science Festival in 2001, when the Astronomer Royal Sir Martin Rees and two other eminent astronomers gave talks on `Life Out There'.
The talks were sensible and interesting, but they made no contact with real modem biology. They were based on the kind of biology that is currently taught in schools, most of which is about thirty years out of date. Like almost everything in school science, because it takes at least that long for ideas to `trickle down' from the research frontiers to the classroom. Most `modem mathematics' is at least 150 years old, so thirty-year-old biology is pretty good. But it's not what you should base your thinking on when discussing cutting-edge science.
Jack, in the audience, asked: `What would you think of three biologists discussing the physics of the black hole at the centre of the galaxy?' The audience applauded, seeing the point, but it took a couple of minutes for the scientists on the platform to understand the symmetry. They were then as contrite as they could be without losing their dignity.
This kind of thing happens a lot, because we are all so familiar with evolution that we think we understand it. We devote the rest of this section to a reasonable account of what the average person thinks about evolution. It goes like this.
Once upon a time there was a little warm pond full of chemicals, and they messed about a bit and came up with an amoeba. The amoeba's progeny multiplied (because it was a good amoeba) and some of them had more babies (something funny here ... ) and some had fewer, and some of them invented sex and had a much better time after that. Because biological copying wasn't very good in those days, all of their progeny were different from each other, carrying various copying mistakes called mutations.
Nearly all mutations were bad, on the principle that putting a bullet randomly through a piece of complex machinery is unlikely to improve its performance, but a few were good. Animals with good mutations had many more babies, and those had the good mutation too, so they thrived and bred. Their progeny carried the good mutation into the future. However, many more bad mutations accumulated, so natural selection killed those off. Luckily, another new mutation appeared, which made a new character for a new species (better eyes, or swimming fins, .or scales), which was altogether better and took over.
These later species were fishes, and one of them came out on land, growing legs and lungs to do so. From these first amphibians arose the reptiles, especially the dinosaurs (while the unadventurous fishes were presumably just messing about in the sea for millions of years, waiting to be fish and chips). There were some small, obscure mammals, who survived by coming out at night and eating dinosaur eggs. When the dinosaurs died, the mammals took over the planet, and some evolved into monkeys, then apes, then Stone Age people.
Then evolution stopped, with amoebas in ponds content to remain amoebas and not wanting to be fishes, fishes not wanting to be dinosaurs but just living their little fishy lives, the dinosaurs wiped out by a meteorite. The monkeys and apes, having seen what it was like to be at the peak of evolution, are now just slowly dying out - except in zoos, where they are kept to show us what our progenitors used to be like. Humans now occupy the top branch of the tree of life: since we are perfect, there's nowhere for evolution to go any more, which is why it has stopped.
If pressed for more detail, we dredge up various things we've learned, mostly from newspapers, about things called genes. Genes are made from a molecule called DNA, which takes the form of a double helix and contains a kind of code. The code specifies how to make that kind of organism, so human DNA contains the information needed to make a human, whereas cat DNA contains the information for a cat, and so on. Because the DNA helix is double, it can be split apart, and the separate parts can easily be copied, which is how living creatures reproduce. DNA is the molecule of life, and without it, life would not exist. Mutations are mistakes in the DNA copying process - typos in the messages of life.
Your genes specify everything about you - whether you'll be homosexual or heterosexual, what kinds of diseases you will be susceptible to, how long you will live ... even what make of car you will prefer. Now that science has sequenced the human genome, the DNA sequence for a person, we know all of the information required to make a human, so we know everything there is to know about how human beings work.
Some of us will be able to add that most DNA isn't in the form of genes, but is just `junk' left over from some distant part of our evolutionary history. The junk gets a free ride on the reproductive roller-coaster, and it survives because it is `selfish' and doesn't care what happens to anything except itself.
Here ends the folk view of evolution. We've parodied it a little, but not by as much as you might hope. The first part is a lie-to-children about natural selection; the second part is uncomfortably close to `neo-Darwinism', which for most of the past 50 years has been the accepted intellectual heir to The Origin. Darwin told us what happens in evolution; neo-Darwinism tells us how it happens, and how it happens is DNA.
There's no question that DNA is central to life on Earth. But virtually every month, new discoveries are being made that profoundly change our view of evolution, genetics, and the growth and diversification of living creatures. This is a vast topic, and the best we can do here is to show you a few significant discoveries and explain why they are significant.
Just as physics replaced Newton by Einstein, there has been a major revolution in the basic tenets of biology, so we now have a different, more universal view of what drives evolution. The `folk' evolutionary viewpoint: `I've got this new mutation. I have become a new kind of creature. Is it going to do me any good?' is not the way modern biologists think.
There are many things wrong with our folk-evolution story. In fact we've deliberately constructed it so that every single detail is wrong. However, it's not very different from many accounts in popular science books and television programmes. It assumes that primitive animals alive today are our ancestors, when they are our cousins. It assumes that we `came from' apes, when of course the ape-like ancestor of man is the same creature as the man-like ancestor of modem apes. More seriously, it assumes that mutations in the genetic material, the changes that natural selection has to work on - indeed, to select among - are checked out as soon as they appear, and labelled `bad' (the organism dies, or at least fails to breed) or `good' (the animal contributes its progeny to the future).