A temporary, frequently very drastic reduction in the size of the population, followed by a return in the number of individuals in the population or species to the original size, is apparently important in evolution. This process and the evolutionary and genetic processes that accompany it are called the bottle-neck effect. The bottle-neck effect occurs, for example, when the size of the population is radically reduced by a certain biotic or abiotic factor whose action is only temporary. The same effect occurs when a new location is colonized by a small group of individuals of a certain species, in the extreme case one fertilized or even parthenogenetic female. Such an event can lead to accelerated evolution through accelerated anagenesis, as the gene pool of the founding population can differ drastically from the gene pool of the initial population (see XXVI.5).The direction of evolution is determined not only by the selection pressures, to which the population is exposed, but also by the structure of the gene pool of the given population. As the gene pools of the original and founding populations differ, the course of their evolution can also differ. This founder effect (Mayr 1963)has a number of genetic consequences and can even lead to the formation of a new species. In addition, as was discussed in Section VII.3, selection pressure does not act as strongly in small populations as in large populations, so that mutants that would be rapidly eliminated by natural selection can survive here.The bottle-neck effectthus allows evolution to overcome shallow valleys in the adaptive landscape (Wright 1931; Wright 1982).
At first glance it might seem that the bottle-neck effect would lead to the same decrease in polymorphism as a reduction in the size of the population. However, theoretical models of this phenomenon indicate that this need not be true, that the bottle-neck effect frequently does not lead to a substantial reduction in polymorphism. This is a result of the fact that the reduction in the size of the population is followed by its re-expansion, during which no alleles basically become fixed. The population expands into free ecological space and is thus not exposed to almost any intraspecies competition, so that the bearers of all the alleles transfer their genes to their progeny. If an allele “survives” the process of reduction of the population, in the following period of exponential increase it will apparently not be eliminated, even in the period when the number of individuals in the expanding population is still very small compared with the size of the original population. It is, however, apparent that a temporary reduction in the size of the population always leads to the loss of most rare alleles, i.e. alleles that were present in such low frequency in the original population that they disappeared from the gene pool at the instant when the size of the population was reduced. In this case, however, these are mostly neutral or almost neutral mutations that are maintained in the population by mutation pressure, i.e. as a consequence of constant formation of the same alleles through mutations. Thus passage through the bottle neck affects primarily type 1 polymorphism that is not of such fundamental importance in evolutionary and ecological processes as type 2 polymorphism (see VIII.3).
Study of the genetic polymorphism of a certain population yields information on whether it passed through a bottle-neck in the past (Emerson, Paradis, & Thebaud 2001). If a certain DNA segment is sequenced for a greater number of representatives of the relevant population and the mutual similarity of all pairs of sequences is compared, three fundamentally different results can be obtained. If the population was stable for a prolonged time and was relatively large, it contains a large amount of polymorphism, where some pairs of alleles differ in many positions, while other pairs differ just in few positions. A histogram of the number of mutations in which a pair of sequences differs is very irregular – it does not have one clear maximum. If the population was limited in size for a long period of time, the overall polymorphism, both in the numbers of alleles and in the numbers of differences between the alleles, is substantially smaller but the histogram of positions in which the individual alleles differ from one another is also irregular. If the population passed through a bottle neck and later substantially increased in size, the amount of polymorphism is also large, while the histogram of the number of different positions has a regular bell shape (Vonhaeseler, Sajantila, & Paabo 1996)(Fig. V.6).