Wednesday, April 16, 2008

Random Genetic Drift

The anti-evolution film Expelled: No Intelligence Allowed is being released on Friday and the US National Centre for Science Education site Expelled Exposed has gone live. The site is a point-by-point examination and de-bunking of the claims made in the film. The site hit Digg and is currently working out the high traffic issues, but some of the comments at Digg were interesting.

I am not an evolutionary biologist, but the comments seemed off. Most of the pro-science commenters there equate evolution with Darwinism - that is, speciation by mutation and natural selection. Natural selection is one evolutionary mechanism - indeed, the one people are most familiar with - but it is not the only one. Larry Moran at Sandwalk kicked off a bit of a discussion of this when he asked his readers to define evolution, and many different ideas were kicked around. Larry also has a nice explanation of one of those mechanisms, random genetic drift, in a piece at From the article:
"If a population is finite in size (as all populations are) and if a given pair of parents have only a small number of offspring, then even in the absence of all selective forces, the frequency of a gene will not be exactly reproduced in the next generation because of sampling error. If in a population of 1000 individuals the frequency of "a" is 0.5 in one generation, then it may by chance be 0.493 or 0.505 in the next generation because of the chance production of a few more or less progeny of each genotype. In the second generation, there is another sampling error based on the new gene frequency, so the frequency of "a" may go from 0.505 to 0.510 or back to 0.498. This process of random fluctuation continues generation after generation, with no force pushing the frequency back to its initial state because the population has no "genetic memory" of its state many generations ago. Each generation is an independent event. The final result of this random change in allele frequency is that the population eventually drifts to p=1 or p=0. After this point, no further change is possible; the population has become homozygous. A different population, isolated from the first, also undergoes this random genetic drift, but it may become homozygous for allele "A", whereas the first population has become homozygous for allele "a". As time goes on, isolated populations diverge from each other, each losing heterozygosity. The variation originally present within populations now appears as variation between populations." (Suzuki, D.T., Griffiths, A.J.F., Miller, J.H. and Lewontin, R.C. in An Introduction to Genetic Analysis 4th ed. W.H. Freeman 1989 p.704)
(A quote of a quote? How meta.) I think some of the numbers were miscopied from Suzuki et al. so I've changed them here, but it isn't the actual numbers that are important so much as the concept. Read the full article, it's quite informative and covers a variety of examples of genetic drift.


Larry Moran said...

Thanks for the link.

Here's the 2006 version: Random Genetic Drift.