John Wilkins has replied to Larry Moran on the role of “chance” in evolution (incidentally, Moran replies to Wilkins on the same topic, but a different post by Wilkins). Here’s what Larry wrote:

Nobody denies the power of natural selection and nobody claims that natural selection is random or accidental. However, the idea that everything is due to natural selection is the peculiar belief of a relatively small number of people, of whom Richard Dawkins is the most outspoken.

A great deal of evolution is the result of chance or accident, as is a great deal of the rest of the universe. It’s perfectly okay to say, as a first approximation, that lots of evolution is random or accidental. This is a far closer approximation to the truth than saying it’s the all the result of design by natural selection.

This is a favorite topic of Moran’s (see his webpage for more). Wilkins takes a closer look at Moran’s point, focusing especially on some philosophical issues, but the argument is lacking due to a sloppy treatment of the statistics and a failure to adequately define “chance” from a statistical perspective.

First, a quibble on probability distributions. Wilkins writes:

There are a number of meanings to “chance” in this case. One of them is that without a perturbing cause, ensembles of events will tend to form a Poisson distribution – the “bell curve” of beginner’s statistics.

I like where he was going with a definition of “chance” — random draws from a probability distribution — but the “‘bell curve’ of beginner’s statistics” is the normal distribution. Given enough trials, a Poisson distribution will converge on a normal distribution (the central limit theorem). But a Poisson random variable specifically refers to how many events will occur in a certain window of time. For example, one could model the number of cars driving past your house every hour using a Poisson distribution. The inverse of this is the exponential distribution, which deals with the issue of waiting time — the expected length of time between the events (how long do we expect to wait between cars driving past your house). But I digress.

Throughout his entire argument, Wilkins never gives a true definition of “chance” (neither does Moran, for that matter, but his blog entry was much shorter). And, while Wilkins does hint that he means random draws from a probability distribution, he also suggests something else:

Yes, molecular changes can be “random” in the sense that they are external to the theory of evolution. They are not external to the theory of chemistry. In the domain of subatomic physics, the randomness of radioactive decay or gamma radiation has to do more with statistical properties than actual chance.

“Statistical properties” mean, to me, random draws from a probability distribution. So, what is chance if chance is not a stochastic process? Oftentimes, people conflate randomness with a uniform distribution — equal probabilities of all possible outcomes. But when we model a random process, we assume some distribution that approximates the randomness of the natural event we’d like to simulate. In evolutionary biology, this is often done with the binomial distribution — either allele A_{1} or allele A_{2} get passed on to a child, either a locus obtains a mutation or it does not, either two alleles coalesce at generation t-1 or they do not.

The examples mentioned above are the chance aspects of evolution. Evolution, in a nutshell, results from differential inheritance of alleles. The expected frequency of a neutral allele in the next generation is simply its frequency in the previous generation. But, in finite populations, there exists some variance around that mean. Processes that can be modeled as random draws from probability distributions — mutation and drift, for example — can lead to differential inheritance of alleles. Moran is arguing that these types of processes trump natural selection in terms of importance throughout evolution history. Many of the arguments Wilkins makes are irrelevant to that point, instead focusing on the theistic implications of “chance” (which I won’t touch with a ten foot cross).

But the big question remains: is Moran correct? Is evolution due mostly to the stochastic processes described above, or does natural selection (non-random draws from a probability distribution) drive most changes? And that’s what a lot of people are studying. Of course, it depends on which aspects of evolution one is most interested in. If one is studying evolution at the DNA sequence level, then, yes, evolution is mostly due to the mutations that accumulate over a given amount of time (which can be modeled as a Poisson distribution). If, however, one is interested in the evolution of the amino acid sequences encoded by protein coding genes, then we begin to see many more examples of both selective constraint (stasis due to natural selection) and adaptive evolution (natural selection driving change). Finally, the morphology of organisms probably contains the greatest evidence for natural selection. A cetacean‘s streamlined morphology, a bird’s wing, and every type of eye out there were all shaped by natural selection.

Moran’s peeve — and one that I agree with — is that people tend to interpret everything in an adaptationist light. This is especially problematic when it comes to molecular evolution. Much of what people explain using natural selection may merely be a spandrel.