Wednesday, 4 February 2015

What happens when an expert argues within his area of competence

Anyone who has spent some time wading through special creationist propaganda will have encountered the appeal to authority, where the author will tout his irrelevant credentials before proceeding to peddle yet another special creationist error. Too often, laypeople think that being an expert in one area of science somehow grants the scientists insight into other scientific areas. That's false. Outside of their area of expertise, a scientist's opinion is no more authoritative than any other educated layperson.

Special creationists are dimly aware of this, and have attempted to counter this by attacking defenders of mainstream science who cite scientific articles by evolutionary biologists for appealing to authority. What the special creationists ignore is that it is entirely relevant to appeal to relevant authority. If you were arguing with a germ theory denialists, it is entirely relevant to cite the relevant microbiology literature as their professional opinion is directly relevant to the subject.

Evolutionary geneticist Jerry Coyne provides two examples of what happens when a professional argues within his area of competence, and when he argues outside of it. The examples should serve to show special creationists the difference between an appeal to relevant authority, and a fallacious appeal to authority.

Earlier this month, Coyne responded to a podcast by theologian / philosopher William Lane Craig on the historical Adam which attempted to rebut criticisms of monogenism - the belief that the entire human race descended exclusively from two people living a few thousand years ago. Craig's professional sphere of competence - theology / philosophy - is far removed from population genetics, which immediately raises the question of the competence of Craig to offer a dissenting opinion. As Coyne points out, Craig's attempts rebuttal falls far short:
Craig gets around the genetic data by saying that the population-size estimates by geneticists are based on mathematical models, and “It could well be the case that these mathematical models are simply incorrect.” Well, maybe, but they use conservative assumptions, and there are two different models giving pretty much the same results. If the models are wrong, let Craig present some cogent criticisms and, perhaps, make his own model, or have a Christian geneticist do it. In the interim, on one side we have two sets of decent scientific estimates of historical population size, and on the other we have Craig’s bluster. I’ll go with the science. 
Craig does level one criticism of the models: they assume a constant mutation rate in humans. That’s not a bad assumption, actually, for we have no reason to think that the rate of errors in DNA replication changed drastically in the last ten thousand years. But Craig says that the mutation rate could have been much higher in the past than we see now. That would then give us a misleadingly high population sizes if we use the lower present mutation rates. If they were much higher in the past, then maybe there could have been just two people in H. sapiens, and the huge mutation rates in their immediate descendants would give us the genetic diversity present today.
There are two points against this. First, human mutation rates are not estimated by direct observation, but from population-genetic estimates, with some estimates based on data from many generations. So if mutation rates were higher in the past, much of that would already have been accounted for. 
Second, if mutation rates did change over time, you’d expect them to be higher not in ancient times, but recently, since now we’re exposed to all kinds of mutagens (like chemicals and X-rays) that we didn’t have in the past. 
Craig’s desperate invocation of the nonuniformity of mutation rates reminds me of those theologians who, seeing a contradiction between their beliefs in a young Earth and the fact that we can see light from stars millions of light-years away, invoke either a non-uniformity of the speed of light (“it was higher in the past”) or God’s creation of light in transit from the stars along with the stars (after all, what good would stars be to humans unless we could see their light as soon as God made them?). This isn’t science, but apologetics—an attempt to save an a priori emotional commitment. 
Craig gloats about the fact that the “Y-chromosome Adam” (the single male from which all our Y chromosomes come) and the “mitochondrial Eve” (the single female from which all our mitochondria descend) lived about the same time, in contrast to what I said in my post. So they could have been Adam and Eve! Indeed, a few years ago estimates based on a limited number of Y chromosomes showed that these ancestors did live at non-overlapping times. But more recent analyses show that there could have been some overlap.
This, however, hardly supports the idea that the genomes of all modern humans came from a couple who lived at the same time and mated with each other. There are huge error bars around these times. So, for example, the Y chromosome Adam could have lived any time between 120,000 and 160,000 years ago, while “mitochondrial Eve” could have lived any time between 99,000 and 148,000 years ago. While there’s overlap, there’s also 32,000 years when they don’t overlap. This is not good support for the claim that the two individuals lived at exactly at the same time. 
But there’s a bigger problem. As I note in the Albatross, “although all the Y chromosomes of modern humans descend from this one individual, the rest of our genome descends from a multitude of different ancestors who lived at various times ranging from 100,000 to about 4 million years ago. Our genome testifies to literally hundreds of ‘Adams and Eves’ who lived at different times—a result of the fact that different parts of our DNA were inherited differently based on the vagaries of reproduction and the random division of genes at when sperm and eggs are formed.” It’s not just mitochondrial and Y-chromosomal DNA we have to consider, but the entire human genome. And that shows clearly that parts of the genome go way back before the DNA on the Y and on the mitochondrion. Indeed, parts of our genome originated even before our divergence from the ancestors of chimps! We have some variable genes, for example, with variants that are more closely related to gene forms in chimps than to other genes in humans. That shows that the variability was hanging around in our common ancestor, and that the variability has persisted over more than five million years. 
But none of this so-called “contemporary” DNA data refutes the data showing that the human species was never as small as two individuals. It’s a separate issue.
Near the end of the podcast, Craig gives his own take, and that’s a literal view of Genesis: Adam and Eve were real people and the ancestors of all humanity. As he says,”I’m inclined to stick to the literal Adam and Eve until I’m actually forced by the evidence to abandon that view, and I’m far from that point.”
Craig's view however is not that of an informed population geneticist, but that of a philosopher theologian with no professional competence in the subject, and as Coyne points out, it is one that is flatly refuted by the evidence. On this point, evolutionary geneticist and Evangelical Christian Dennis Venema is worth quoting:
Studies based on SNP/LD approaches have now estimated ancestral population dynamics for various human groups over time in more detail than is pos- sible with mutation-based estimates. African groups have a higher effective population size (~7,000) than do non-African groups (~3,000) over the last 200,000 years. This approach, though based on methods and assumptions independent of previous work, nonetheless continues to support the conclusion that humans, as a species, are descended from an ancestral population of at least several thousand individuals. More importantly, the scalability of this approach reveals that there was no significant change in human population size at the time modern humans appeared in the fossil record (~200,000 years ago), or at the time of significant cultural and religious development at ~50,000 years ago.

Taken individually and collectively, population genomics studies strongly suggest that our lineage has not experienced an extreme population bottle- neck in the last nine million years or more (and thus not in any hominid, nor even an australopithecine species), and that any bottlenecks our lineage did experience were a reduction only to a population of several thousand breeding individuals. As such, the hypothesis that humans are genetically derived from a single ancestral pair in the recent past has no support from a genomics perspective, and, indeed, is counter to a large body of evidence. (Emphasis mine)
If Craig really believes that the evidence is not sufficient to force him to abandon monogenism, then it is painfully clear that he has failed to understand the evidence, which as even Evangelical Christian biologists such as Venema point out rule out universal human descent from two people living several thousand years ago.


Craig, as a philosopher / theologian is arguing well outside his area of competence, and advancing a minority opinion to boot, when he attempts to rebut monogenism. He has the burden of proof, and has failed to meet it. Any special creationist who appeals to Craig as a 'respected apologist' to justify citing his podcast as evidence for monogenism is committing the logical fallacy of appealing to authority. Conversely, citing Coyne is a case of appealing to appropriate authority. The difference is one that special creationists desperately need to grasp.