This is a familiar creationist argument, stated in this case by a non-creationist:

Consider the replacement processes needed in order to change each of the resident genes at L loci in a more primitive genome into those of a more favorable, or advanced, gene. Suppose that at each such gene locus, the argument runs, the proportion of gene types (alleles) at that gene locus that are more favored than the primitive type is K^−1. The probability that at all L loci a more favored gene type is obtained in one round of evolutionary “trials” is K^−L, a vanishingly small amount. When trials are carried out sequentially over time, an exponentially large number of trials (of order K^L) would be needed in order to carry out the complete transformation, and from this some have concluded that the evolution-by- mutation paradigm doesn’t work because of lack of time.

Basically, what creationists argue is that the evolution of new genes is linear and sequential — there is no history, no selection, it works entirely by random replacement of the whole shebang, hoping that in one dazzling bit of luck that the entire sequence clicks into the right sequence, and then it all works. If that were the way the process occurred, then they’d be right, and evolution would be absurdly improbable and would take an untenable length of time.

Another way to think of it is a bizarre version of the hangman guessing game, where one person thinks of a word, and the second person has to guess what it is. In the normal version of the game, the second person guesses letters one by one, and they’re placed in the appropriate spot. In the creationist version, you only get to guess a whole sequence of letters in each round, and you are only told if you are right or wrong, not which letters are in the correct position in the word. Not only does it become a really boring game, but it also becomes extremely unlikely that anyone can solve it in a reasonable amount of time.

Evolution does not work like that. It works in parallel, changing and testing each variant simultaneously in many individuals, and then selection for the most favorable subset of changes latches them in place, making the matching letters more likely to be fixed. Or, as the paper by Wilf and Ewens explains,

But a more appropriate model is the following. After guessing each of the letters, we are told which (if any) of the guessed letters are correct, and then those letters are retained. The second round of guessing is applied only for the incorrect letters that remain after this first round, and so forth. This procedure mimics the “in parallel” evolutionary process. The question concerns the statistics of the number of rounds needed to guess all of the letters of the word successfully.

That’s the question. If purely random changes would require a ridiculous length of time to match a target, proportional to K^L, how long would it take if we actually use more reasonable, biologically relevant model? Wilf and Ewens state the model in mathematical terms and derive a theoretical answer, and you won’t be surprised that it’s significantly shorter; you might be impressed at how much shorter the operation would take.

Instead of a time proportional to K^L, it will take a time proportional to K log L.

That’s very much shorter! To put some representative numbers on it, imagine a protein that is 300 amino acids long, made up of 20 possible amino acids, and I’m going to ask you to guess the sequence. Under the creationist model, you wouldn’t even want to play the game — it would take you on the order of 20³⁰⁰ trials to hit that one specific arrangement of amino acids. On the other hand, if you took a wild guess, writing down a random 300 amino acids, and I then told you which amino acids in which position were correct, you’d be able to progressively work out the exact sequence in only 20 log 300 trials, or around 50 guesses.

Notice that this is not a concrete estimate of the time it would take for something to evolve! It’s a grossly simplified version of the story: the example overstates the power of selection (amino acids won’t be locked in, but will only be less likely to change), and overstates the required accuracy of matching to a target (there would be more tolerance for variation), and the whole idea of meeting a specific target is not necessarily a good model. As a guide to short-circuiting the invalid assumptions of creationists, though, it’s handy to have a simple mathematical formula to remove that naive combinatorial model from the table.

Wilf HS, Ewens WJ (2010) There’s plenty of time for evolution. arXiv:1010.5178v1.

Researchers in Maryland have discovered an interesting quirk: lung smooth muscle expresses on its surfaces a protein that is the same as the bitter taste receptor. This could be useful, since they also discovered that activating that receptor with bitter substances causes the muscle to relax, opening up airways, and could represent a new way to treat asthma. That’s a fine discovery.

But man, it really tells us something about human psychology. I’m getting all this mail right now, and just about all of it asks the same question: Why do lungs have taste receptors? What is the purpose of sensing taste with the lungs? Even the investigators speculate this way:

Most plant-based poisons are bitter, so the researchers thought the purpose of the lung’s taste receptors was similar to those in the tongue — to warn against poisons. “I initially thought the bitter-taste receptors in the lungs would prompt a ‘fight or flight’ response to a noxious inhalant, causing chest tightness and coughing so you would leave the toxic environment, but that’s not what we found,” says Dr. Liggett.

Weird. I guess the teleological impulse really is etched deep into most people’s minds. I’m going to suggest that everyone just relax, let go, and embrace a simpler assumption.

There is no purpose.

That should be our default assumption. Gene regulatory networks are complicated, with expression of all kinds of genes coupled to other genes, so my first thought was that this was a simple biological accident, and totally unsurprising. I’ve looked at enough developmental gene expression papers to know that genes get switched on and off in all kinds of complicated patterns that have nothing to do with proximal function and everything to do with the network of connections between them; sometimes if gene A is active, the only ‘purpose’ is because A is coregulated by factor X which also switches on gene B, and B is the next step in a physiological or developmental program that is adaptive for the organism.

Another way to think of it: the handle on your teapot is wobbling loose, so you bring the home toolbox into the kitchen to tighten it up with your screwdriver. Your toolbox also contains wrenches and a hammer, but we don’t speculate that the reason you brought the hammer is that you need it right then to fix the teapot. The purpose of bringing the hammer is that it’s in the same handy toolbox as your screwdriver, which is not really a purpose at all.

Now the way evolution works is that this purposeless variation may fortuitously find a purpose — a gene in the T2R family of G-protein coupled receptors is uselessly misexpressed in the lungs, but a clever doctor finds a way to take advantage of it to treat asthma, or you may spot a vagrant mouse skittering across your kitchen counter, and suddenly the hammer becomes a useful implement of pest control — but the root of that innovation isn’t purpose, but purposelessness and serendipity.

There’s another reason to be unimpressed with the purpose of the expression of this gene in the lungs. Many of you may already be familiar with another quirk of the bitter receptor — its expression is variable in people. A common observation to make in genetics labs is the existence of non-tasters, tasters, and super-tasters to a substance called phenylthiocarbamide, or PTC. The mechanism of that is variability in this same kind of receptor gene now found to be expressed in lung tissue. Shouldn’t we be used to the random element of the expression of this gene by now?

Right now on Discovery…it’s First Life with David Attenborough, which is supposed to be about the origin of life on Earth. There’s a pretty severe metazoan bias, unfortunately, so it’s really about the origin of animals, but still it’s cool stuff about the Cambrian.

Williams was one of the giants of 20th century evolutionary biology, and he died on Wednesday. Michael Ruse offers a brief summary of his career, while Edge has personal testimonials from people who knew him. I never met him, to my regret, but knew his work, which was enormously influential.

If you’re more bored than I am (and I’m not bored at all — I’m neck deep in work), try out these evolution games on the web. Any good? Useful for the kiddies? I don’t know. Somebody else review them for me.