When a small number of individuals from an original population immigrate to a certain place, the new population founded by those people might have much less genetic diversity compared with those of the original population (Fig. 1). This is generally the case, because a few founder people cannot have sufficient genetic variation to reflect that of the mother population. Such a drastic decrease of genetic diversity within a population is known as the founder effect. This effect most often occurs when a segment of a larger population becomes isolated as the result of colonization of a geographically isolated area, a catastrophic event, artificial isolation, and so on. These founding populations contain only a fraction of the genetic diversity of the population from which they originated.
Figure 1. Founder effect.
When founders are individuals sampled randomly from the mother population, the founder effect has the same influence as genetic drift, which is the stochastic change of gene frequencies due to random mating. The effect of genetic drift becomes stronger when the population size is smaller (1); therefore, the founder effect is similar to the effect of genetic drift when the size of the initial population in a given geographical area suddenly becomes extremely small. In particular, the drastic reduction in size of a population has the same effect, which is now called the "bottleneck effect" (Figs. 2 and 3). Strictly speaking, the founder effect can be considered to be a particular form of the bottleneck effect. The difference between the founder effect and the bottleneck effect is that the former occurs in a different place, whereas the latter takes place in the same location.
