Branko Kozulic responds to Professor Moran, Part II
|April 13, 2014||Posted by vjtorley under Intelligent Design|
I would like to thank Professor Moran for kindly addressing the issues raised in my previous post, “Branko Kozulic responds to Professor Moran”. The answers did help me clarify some points that I was struggling with, in my thinking. In this reply, I am speaking in my own name only. (Note by vjtorley: I have made a few minor corrections to Dr. Kozulic’s English, but the views expressed below are his, not mine. However, since he does not have posting privileges on Uncommon Descent, I have agreed to put up this post at his request, as I’m a firm believer in free speech.)
At the outset, I would like to express my conviction that in matters relating to science, Professor Moran and I probably agree on most things, and not just on findings based strictly on experimental data. For example, I would strongly support his effort to give organic chemistry a prominent role in the teaching of biochemistry. In matters of philosophy, we would probably disagree on some issues; however, this is not at all uncommon, among educated people. But let’s talk about science.
The idea of 100 mutations being fixed in the human population in each generation over a period of 185,000 generations, or 5,000,000 years, has always appeared intuitively unrealistic to me, possibly because I am primarily a practical scientist. My first question was: How many mutations are fixed now, in my generation? I am pretty sure that the answer is zero. Fixation means, we agree, that all individuals in the human population acquire the same mutation in a generation, or, equivalently, that the other allele is completely lost. If so, then the question logically follows: When did this change from 100 to zero fixed mutations per generation take place? Alternatively, is there perhaps something wrong with my initial assumptions?
After wandering around, a solution entered my mind while reading the following sentence in Professor Moran’s reply: “In my calculation, the values of Ne are irrelevant, so any value will work equally well, as long as you realize that you are starting with an ancestral population containing existing variation.” Here, the key phrase is “containing existing variation.” That is the essential assumption. What it means in reality is this: throughout the whole period leading up to the fixation – i.e. in the case we are considering, a population of 10,000 individuals over 40,000 (=4N) generations (which, when multiplied by 20 years/generation, equals 800,000 years) – a group of individuals may split off from the population, but no newcomers are allowed to enter it – that is, the population must remain closed in terms of mating. Here, a newcomer is defined as any individual from a group that had lived separately, say on an adjacent island or across a mountain, for some generations. Whenever a newcomer joins the population, the fixation process is broken, because migrants bring with them new variation that did not exist initially. Migration is a powerful force acting against genetic divergence among sub-populations (Hartl & Clark, Principles of Population Genetics, pp. 295-309).
Herein lies the weak point in Professor Moran’s calculations. Based on our knowledge of human history and behavior, we know that groups of people grow in numbers, split in two and merge, repeatedly. This commonly known fact has to be ruled out, in order to allow for the fixation of 100 mutations per generation. And now we can see that the value of Ne will have a major impact: a large Ne makes it more likely that a newcomer will join the population, and also that a group will split away from it and then re-enter it (in part) later on.
In view of the above, I believe I am entitled to continue rejecting the feasibility of 22,000,000 mutations being fixed in the human population over a period of 5,000,000 years, or 100 mutations each generation. Here it is appropriate to mention a reservation relating to the exact number of fixed differences between the human and chimp genomes that has been expressed by Professor Felsenstein: the actual number might be lower than 22,000,000.
There are two remaining issues, i.e. the pattern of fixation and orphans/singletons found in the sequenced genomes, which have yet to be addressed by Professor Moran. But regardless of whether or not he addresses them in the future, I would like to make it clear that my involvement in the public discussion ends with this post.