Random genetic drift is a process that alters allele frequencies within a population. The change is due to "random" events. It differs from natural selection where the change is due to selection for alleles that confer selective advantage on the reproductive success of an individual. Here's one description,
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 0.501 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.When you look at the number of alleles that have become fixed in a population, it seems clear that random genetic drift is the dominant mechanism of evolution. However, it does not cause adaptation and many biologists think that adaptation is the only important mechanism of evolution.
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)
It seems that philosophers have recently become interested in drift. As far as I can see, most philosophers who write about evolution are completely ignorant of random genetic drift. It's refreshing to see that there's an entry on drift in the Stanford Encyclopedia of Philosophy [Genetic Drift].
The article is very philosophical and difficult to read. Here's an excerpt.
As will be discussed further below, much of the twentieth century was marked by debates among biologists about the relative importance of drift and selection in evolution. Were those debates at least in part the result of conceptual unclarity? Millstein (2002) argues that we need not accept this inadvertent consequence of Beatty’s argument, and that selection can, in fact, be distinguished from drift. In order to do this, three extensions should be made to Beatty’s account. First, similar to Hodge (1987), Millstein suggests that a proper distinction between drift and selection relies on causation, specifically, that drift processes are indiscriminate sampling processes in which any heritable physical differences between entities (organisms, gametes, etc.) are causally irrelevant to differences in reproductive success, whereas natural selection processes are discriminate sampling processes in which any heritable physical differences between entities (organisms, gametes, etc.) are causally relevant to differences in reproductive success. These more precise characterizations of “indiscriminate sampling” and “discriminate sampling” are intended to replace the metaphorical “sampling” talk, retaining the term “sampling” as a useful shorthand only. Second, we should be careful to distinguish the process of drift from the outcomes that drift produces, and the process of selection from the outcomes that selection produces. (Of course, the importance of distinguishing process from outcome is not a novel insight; what is novel here is its application to the problem of distinguishing drift from selection. The distinction has sometimes been rendered as “process vs. product” rather than “process vs. outcome” in the philosophical literature, but given the teleological and other misleading connotations of “product”, the term “outcome” is preferable and “product” should be avoided). Third, we should characterize drift and selection as processes rather than outcomes (as in the first of the three points). If we do these three things, then drift and selection are conceptually distinct and the problem Beatty raises is dissolved; discriminate sampling processes where unlikely outcomes obtain are still selection processes. On this view, it is further acknowledged that it is possible for drift and selection to produce the same outcomes, which helps explain the persistence of biologists’ debates over the relative importance of drift and selection without making them seem trivial (see Millstein 2002 for additional discussion of Beatty’s arguments).After reading this lengthy article, it's not clear to me what philosophers can contribute to our understanding of random genetic drift.