Genetic revolution

Eldredge and Gould originally suggested that the punctualist character of evolution is a result of a genetic revolution that occurs through the founder effect. This was based on the peripatric speciation model, created by Ernst Mayr (Mayr 1963). Mayr emphasized that a large population located in the central part of the geographic area of the particular species has only limited potential for the formation of anagenetic changes. As a result of migration, alleles constantly flow in from the other parts of the geographic area of the species and these alleles prevent the formation of optimal adaptations to local conditions. Thus, only those alleles that are capable of withstanding the competition from alleles coming in from the rest of the population survive in the long run, i.e. alleles that are capable of providing their carriers with reasonable fitness in combination with the widest possible spectrum of the alleles of other genes and not, for example, alleles that provided their carriers with ideal adaptation to local conditions and possible changes in these conditions. In addition, normalizing natural selection is much more effective in large populations compared with small populations. If it was necessary to overcome a valley in the adaptive landscape for a change in the phenotype to become possible, i.e. if the transition form between the old and new forms exhibited lower fitness, the emergence of new forms in the population would be impossible. The genetical composition of the gene pool of large populations located in central parts of the area of occurrence thus remains unchanged over long periods of time and reacts very slowly to any changes in the environment. In contrast, much more favorable conditions for anagenetic evolution occur during peripatric speciation in small populations, which are constantly formed and disappear at the edges of the area of occurrence of the given species. The populations are smaller so that their members can more easily overcome valleys in the adaptive landscape through genetic drift. The populations are also geographically and thus also genetically isolated from the other populations of the given species so that foreign alleles do not enter their gene pool and the composition of the gene pool can better adapt to the momentary local natural conditions. It is an important property of small populations that the composition of their gene pool can differ very substantially from that of the parent population thanks to the founder effect. The particular population was most probably established by only a very few, frequently mutually related individuals, so that their genetic composition could differ at random from that of the parent population. As a result of genetic drift acting in small populations, other alleles can disappear from the gene pool. A genetic revolution can occur in the population as a result of the altered composition of the gene pool. The selection values of the individual alleles are mostly affected by their own frequency and the frequencies of the other alleles in the same locus or in other loci. If the frequency of an allele increases through the effect of selection pressure or randomly (by genetic drift), its selection value changes and the relevant selection pressure ensures that, in time, its frequency will return back to the equilibrium value. This genetic homeostasis (see IV.9.2) is capable of maintaining approximately constant frequency of the individual alleles in the gene pool of the population even when the natural conditions and thus also the external selection pressures acting on the population change. The most important component of the external environment of an allele consists in the frequencies of the other alleles, because these frequencies determine which alleles a certain allele will most often encounter in future zygotes and thus what its selection value will be. Where, as a consequence of the founder effect and as a result of subsequent genetic drift, a great many alleles disappear from the gene pool of the population and the frequencies of other alleles change drastically, a number of the remaining alleles escape from mutual bonds and thus also the phenotype of organisms in the population can begin to change through natural selection. Thus, in contrast to large, genetically interconnected populations, small populations can evolve and can lead to the formation of a new species with a different phenotype. The appearance of a new species in the paleontological record at a certain location thus reflects, not the formation of a new species at the particular location, but rather the invasion of a species that was formed by peripatric speciation at some other location. The population in which the new species evolved was small and the anagenesis of its members was relatively rapid because of genetic revolution, so that it is not very probable that its members would be preserved in the paleontological record.
            In later works, Gould abandoned the model assuming the participation of genetic revolution (Gould 2002). He stated that his main reason for abandoning this model was the fact that, at the present time, geneticists tend to think that selection occurs more effectively in large populations than in small populations. The fact that the existence of genetic revolution was not confirmed experimentally also probably played a certain role. Gould also concluded that the participation of this mechanism is not necessary for explaining the punctuated character of evolution and that the mechanism of peripatric speciation alone explains it sufficiently. In his opinion, it is not necessary for acceleration of anagenetic processes in small populations, as a period of the order of 10,000 years is adequate for the accumulation of a sufficient number of evolutionary changes even at the normal rate of evolution. The fact that the population is located close to the edge of the geographic area of the species, where different natural conditions are most probably present, that it is genetically isolated from the rest of the species, so that it can adapt to these conditions and that, because of its smaller size, it can overcome any valleys in the adaptive landscape through genetic drift apparently provide sufficient explanation for the faster anagenesis of these populations. As soon as a reproduction barrier is formed between species, the members of the new species can invade the geographic area of the old species without the newer species becoming “dissolved” in the more numerous population of the old species.
            Personally, I am of the opinion that Gould abandoned the genetic revolution model prematurely. The present-day skeptical opinion of this mechanism on the part of a large fraction of geneticists is probably a result of the methodical difficulties associated with its experimental verification rather than the existence of data that would be contrary to it. The fact that selection is more effective in large populations than in small populations is undoubtedly true; however it is in not way contrary to the statement that selection is more effective in genetically homogeneous populations than in genetically polymorphic populations. The polymorphism of the population is the critical parameter here. A small population can very rapidly grow into a large population, while the original degree of polymorphism is renewed in the population much more slowly. Thus, for a very long time after splitting off, the population can remain in a completely ideal state from the viewpoint of selection, i.e. in the state of a very numerous, genetically uniform and thus evolutionarily plastic population. Only after a longer time is polymorphism accumulated in the population, as a consequence of which the heritability of the individual phenotype traits and the heritability of the overall fitness are reduced (see IV.9.2). As a consequence, the particular species becomes evolutionarily frozen and, for the rest of its existence, more or less passively awaits a change in the external conditions that will lead to its extinction or to new peripatric speciation which will renew its evolutionary plasticity (Flegr 1998), {14400, 15429}. I am of the opinion that, in the field of genetics, the research related to the mechanisms of genetic homeostasis and conversion of nonadditive heritability to additive heritability in small or genetically homogeneous populations will lead to great support for this model of evolution. In the field of paleontology, the model of punctualist evolution encompassing the genetic revolution mechanism could substantially support or confirm current indications that suggest that the punctualist model better describes the evolution of sexually reproducing species, while the gradualist model better describes the evolution of species whose members reproduce asexually.

Was this information useful for you?
You voted 'yes'.
The classical Darwinian theory of evolution can explain the evolution of adaptive traits only in asexual organisms. The frozen plasticity theory is much more general: It can also explain the origin and evolution of adaptive traits in both asexual and sexual organisms Read more