22 March, 2009

Sunday Sensational Science

Evolution as Mountaineering

I haven't forgotten Neil DeGrasse Tyson, I promise. We'll get back to him. Right now, though, I'm on a Richard Dawkins spree.

There are two books anyone who's confused about evolution should read: The Blind Watchmaker and Climbing Mount Improbable. The poster below sums up the result rather well:

Most of us probably have moments when we wonder just how the hell we made it from bacteria to enormously complex multi-celled entities. Evolution is elegantly simple - almost unbelievably simple. I think that's why people have such a hard time with it. We're used to complex things being created by other complex things. It's hard to imagine simplicity giving rise to mind-boggling complexity.

Well, until you've read these two books, it might be.

In Climbing Mount Improbable, Dawkins presents an excellent metaphor. Seen from one side, the mountain seems impassible. There seems to be no way up to the summit. But go round the back, and you find a nice, easy slope that you can amble along. It'll take a long time, but evolution has bags of it. Three billion years, in fact.

And no one had to make any leaps.

Two things stand out in my mind from these books, because they helped me understand just how we climbed the mountain.

We've probably all heard of the sneer that something random like mutation has the same chance of assembling a living creature that a hurricane has of assembling a Boeing 747 while blowing through a junkyard. Dawkins takes off on this theme in The Blind Watchmaker, and introduces the metaphor of the Stretched DC8:
Stretched DC8 macromutations are mutations that, although they may be large in the magnitude of their effects, turn out not to be in terms of their complexity. The Stretched DC8 is an airliner that was made by modifying an earlier airliner, the DC8. It is like a DC8, but with an elongated fuselage. It was an improvement at least from one point of view, in that it could carry more passengers than the original DC8. The stretching is a large increase in length, and in that sense is analogous to a macromutation. More interestingly, the increase in length is, at first sight, a complex one. To elongate the fuselage of an airliner, it is not enough just to insert an extra length of cabin tube. You also have to elongate countless ducts, cables, air tubes and electric wires. You have to put in lots more seats, ashtrays, reading lights, 12-channel music selection and fresh-air nozzles. At first sight there seems to be much more complexity in a Stretched DC8 than there is in an ordinary DC8, but is there really? The answer is no, at least to the extent that the 'new' things in the stretched plane are just 'more of the same.'
Dawkins follows with snakes as his example. Creating a longer snake is a lot like creating a longer DC8 - you just have to duplicate a segment. Big change on the surface, dead easy once you understand what's going on. And since genes are more a recipe than a blueprint, it's not surprising that they might sometimes cook up a body with a little more than what the recipe called for. Three eggs rather than two. It might make for a rather sticky cake - or it might be delightfully rich, in which case three eggs will become incorporated into the recipe. Natural selection works with mutations like this all the time, selecting the good and discarding the bad. And after a long time, and many more random changes to the recipe, you'll end up with something completely different. Trace the recipe back through its various incarnations, though, and you'll have no trouble seeing how things got from cake to, say, fondue. None of those changes, even the big ones, are any more complicated than duplicating a bit of an airliner to make a longer one.

But what about random mutation? People get hung up on that word, "random." In Climbing Mount Improbable, Dawkins gets them unhooked:
Even mutations are, as a matter of fact, non-random in various senses, although these senses aren't relevant to our discussion because they don't contribute constructively to the improbable perfection of organisms. For example, mutations have well-understood physical causes, and to this extent they are non-random. The reason X-ray machine operators take a step back before pressing the trigger, or wear lead aprons, is that X-rays cause mutations. Mutations are also more likely to occur in some genes than in others. There are 'hot spots' on chromosomes where mutation rates are markedly higher than the average. This is another kind of non-randomness. Mutations can be reversed ('back mutations'). For most genes, mutation in one direction is equally probable. For some, mutation in one direction is more frequent than back mutation in the reverse direction. This gives rise to so-called 'mutation pressure' - a tendency to evolve in a particular direction regardless of natural selection. This is yet another sense in which mutation can be described as non-random. Notice that mutation pressure does not systematically drive in the direction of improvement. Nor do X-rays.
So the "random" part of evolution isn't quite so random as it seems. We don't have to sit around waiting for a stray mutation to come out of nowhere. All sorts of things cause mutations. Some mutations are more likely than others. And there's plenty of them available to provide variations for natural selection to choose from. Most of them will be discarded or corrected. A few lucky mutations will be allowed to stay around. And evolution happens.

There's plenty more where that comes from. Both books together take care of lingering confusions - unless, of course, you're one of the willfully confused.

Once you've read both books, the climb up Mount Improbable seems like no more than a casual summer stroll.


Chris said...

Creationists are always trying to use the word "random" to describe processes which are usually anything but. Thus, if an advantage is conferred by one mutation out of a potential ten million, creationists want you to believe that the "odds" of gaining the advantage are one in ten million - which is not only absurd but against common sense.

Efrique said...

My own paperback copy of The Blind Watchmaker from the 80s is now yellowed and falling apart. I can't bear to throw it out or replace it.

george.w said...

I read Climbing Mount Improbable for the first time last year, so it's time to give Blind Watchmaker another go. They are awesome books, and I'd recommend them to any creationist who doesn't want to sound like an idiot discussing evolution.