Frozen Evolution. Or, that’s not the way it is, Mr. Darwin. A Farewell to Selfish Gene.

A new species can be most readily formed by gradual evolution outside of direct contact with the parent species, i.e. by allopatric speciation (Fig. XXI.3). If, for example, a geographically isolated population is formed, which branched off from the population of the original species, and this population is reproductively isolated from the parent population for a sufficiently long time, genetic changes can gradually accumulate in its gene pool that finally lead to phenotype and subsequently also ecological differentiation of the two populations. If the two populations come into contact before they are sufficiently differentiated, the two species can again merge into a single species. Otherwise, the two species can exist sympatrically next to one another (if there was differentiation of their niches) or one of the species can force the other species out of the location or even globally.
If an originally uniform population is divided by a barrier (mountain range, river, for aquatic organisms a strip of land) into two comparably large populations and these populations differentiate in time both genetically and phenotypically, then this is called vicariant speciation or dichopatric speciation. On the other hand, if only a very small population splits off from the parent population and then gradually develops into a new species, this is termed peripatric speciation (Mayr 1999) (se Fig. XXI.3). Empirical data and the results of theoretical analyses, for example comparison of the differences in the sizes of the ranges of occurrence of young sister species, indicate that peripatric speciations are apparently more common than vicariant speciations (Barraclough & Nee 2001); however, sometimes quite the opposite is stated {8917}. In addition, it seems that these types of peripatric speciations more frequently lead to the evolution of species that have different phenotypes than the parent species. For example, mutually related species and geographical races of kingfishers of the genus Tanysiptera, which occur on tiny islands in the vicinity of New Guinea and which probably originated by peripatric speciation, differ in their phenotype substantially more than the related species and races occurring on New Guinea (Mayr 1963).
The more frequent occurrence of peripatric speciations can have a quite prosaic cause. The splitting off of small populations, e.g. by introduction outside of the original range of occurrence or splitting off of tiny subpopulations on the fluctuating edge of a range of occurrence can occur far more often than division of the original range by a newly formed barrier. In most cases, these new subpopulations disappear or merge with the main population after some time. However, a certain percentage of them can lead to the formation of a new species.
There can be at least two reasons for greater phenotype differentiation of species formed by peripatric speciation. Populations at the very edge of the range of occurrence and even more so populations formed by introduction outside of this range mostly find themselves in different natural conditions than those in which most of the populations of their species live. Thus there are also different selection pressures acting on them and, as a consequence, their genotype also substantially changes with time. However, if the original range is divided into two parts of approximately the same size, the natural conditions in the two parts will probably be rather similar. Thus, the sister species will be differentiated more by the action of evolutionary drives and genetic drift than by the action of different selection pressures. Thus, the differences between the species will very frequently tend to be selectionally neutral and need not substantially affect the phenotype.
The second reason for the greater differences in species formed by peripatric speciation can lie in the founder effect and following transition of a species from the evolutionary frozen to a plastic state. The existence of the founder effect was derived in the middle of the last century by Ernst Mayr (Mayr 1963). He stated that species cohesion exists in sexually reproducing species because their gene pool represents an integrated whole – an adaptive gene complex, in which approximately constant representation of the individual alleles is spontaneously maintained through a form of genetic homeostasis. If a new allele appears in the gene pool, either by penetration from an external gene flow or formed directly on site through mutation, it will not be capable of functioning as well in the context of the other alleles present in the gene pool of the population as the original alleles, which have been repeatedly tested in all the possible combinations. Thus, it will be eliminated in the population in time. Massive penetration of foreign alleles, e.g. as a consequence of merging of two originally separate populations, can even lead to a drastic reduction in the average fitness of the members of the population. This will be caused both by the fact that migrants coming from different conditions and their progeny will have phenotypes that are poorly adapted to local conditions and also by the fact that crosses that have emerged, bearing untested combinations of local and foreign alleles, can have reduced viability and fertility – alleles derived from distant populations will not be sufficiently mutually compatible. If an originally uniform population divides into two daughter populations, each of them will bear approximately the same gene pool, in which the frequencies of the individual alleles will remain mutually interconnected and the overall composition of the gene pool will thus remain stabilized. Thus, the gene pools will have only limited ability to evolve. The action of strong selection pressures can force the frequencies of the individual alleles to deviate somewhat form the original values; however, the greater this deviation, the greater will be the resistance of the gene pool of the population to the particular selection pressure. Following reduction of the selection pressure, the frequencies of the alleles will return to the original value (see the genetic homeostasis effect, IV.9.2). In contrast, a small population formed by splitting off from the large population will take only a small part of the overall genetic polymorphism with it and, subsequently, they will lose most of the remaining polymorphism through drift. Even if the population rapidly increases in size in the new environment without competitors, a great many alleles will be completely missing in it or will occur randomly, as a consequence of the founder effect, with very unusual frequency. This can completely disturb the homeostatic stabilization of the composition of the gene pool and the population will respond much more readily and willingly (and irreversibly) to selection pressures. In addition, in a population with drastically reduced polymorphism, new alleles, for example formed by mutations, will always find themselves (i.e. in each newly born individual) in the context of an almost identical set of alleles. Thus, their selection coefficients will not change from one generation to the next, i.e. will not, for example, oscillate between positive and negative values. Thus, the selection of new alleles can be far more effective in a non-polymorphic population than in a polymorphic population (Flegr 1998, Flegr 2010) and the newly formed daughter species will be able to differ substantially from the parent species.