Gene Flow and Genetic Drift

Beyond natural selection itself, population genetics recognises two further, quite different processes that can each independently change the relative frequencies of different alleles within a population's gene pool from one generation to the next: gene flow and genetic drift. Understanding both of these processes is essential to a complete picture of how evolution actually proceeds within real populations, because natural selection, although historically the most emphasised of the evolutionary mechanisms, is very often not the only mechanism actually at work within any given real population at any given point in time.

Gene flow, also frequently called gene migration, occurs whenever individual organisms physically move out of one population of a species and into another, geographically separate population of the very same species, and then go on to interbreed successfully with members of this new, previously separate population, in the process carrying their own personal set of alleles into that new population's shared gene pool. This physical movement and subsequent interbreeding of migrating individuals has a direct, measurable effect on the allele frequencies of BOTH of the two populations involved in the exchange: the original population that the migrating individuals have left behind necessarily loses whatever particular alleles those specific departing individuals happened to be carrying, while the new population that the migrants have joined correspondingly gains those same alleles, which may previously have been comparatively rare, or entirely absent altogether, within that particular recipient population's own gene pool before the migration event took place. When gene flow between two geographically separate populations of the same species continues to occur steadily, generation after generation, over a sufficiently long span of evolutionary time, its cumulative long-term effect is to make the allele frequencies of the two exchanging populations progressively more SIMILAR to one another, since each population is continually receiving a fresh, regular supply of whatever alleles happen to be common in the other — in this specific sense, ongoing gene flow acts as a homogenising force that actively counteracts, and can even entirely prevent, any tendency of two populations to gradually become genetically distinct from one another through the independent, unconnected action of local natural selection or genetic drift occurring separately within each of the two populations.

Genetic drift is a fundamentally different kind of process from gene flow, and, just as importantly, a fundamentally different kind of process from natural selection itself: it refers to random, entirely chance-driven changes in a population's allele frequencies occurring from one generation to the very next, arising simply because only a limited, necessarily finite sample of one parent generation's full complement of alleles happens, purely as a matter of chance sampling, to actually be passed on and survive successfully into the following generation. Genetic drift, unlike natural selection, has absolutely nothing whatsoever to do with which particular alleles happen to be more advantageous, more 'fit', or better adapted to the prevailing local environmental conditions — an allele can become more common, or, equally, can become rarer or even disappear entirely from a population's gene pool, purely through the operation of genetic drift, entirely independently of whether that particular allele happens to be helpful, harmful, or entirely neutral in its effect on the organism carrying it. Genetic drift has, by a very wide margin, its greatest and most disruptive potential effect specifically within SMALL populations: in any small population, the play of pure chance in determining exactly which particular individuals happen to survive and successfully reproduce in any given generation can shift a population's overall allele frequencies quite dramatically, sometimes within the span of just a very few generations; in a sufficiently LARGE population, by clear contrast, such purely chance-driven fluctuations in who survives and reproduces tend, on average, to largely cancel each other out across the very large number of individuals involved, and so have comparatively little measurable overall effect on the population's aggregate allele frequencies. …