IX.5.4 Different mechanisms can maintain genetic polymorphism within one species and give rise to inter-specific genetic diversity

The relative importance of mutation pressure on the one hand and selection on the other hand for maintenance of intraspecies polymorphism and for mutual divergence of individual species are two quite independent aspects.Comparison of intraspecies polymorphism with interspecies divergence, i.e. the occurrence of different nucleotides in the same position in the genes of closely related species, has shown that there is a very strong positive correlation between polymorphism and divergence.However, in case of interspecies divergence, there is mostly, but not always, a relative excess of nonsynonymous mutations (Fay, Wyckoff, & Wu 2002; Ohta 1996).However, it is not entirely clear whether this surplus is formed as a consequence of positive selection of evolutionarily advantageous mutations in the formation of a new species (Kreitman 1996)(as thought by selectionists) or through the effect of a reduction in the effectiveness of negative selection, i.e. selection eliminating negative and slightly negative mutations, in the period prior to a reduction in the size of the population accompanying a speciation event (Ohta 1996)(as thought by neutralists).In any case, the results of molecular taxonomic studies demonstrate that the mechanisms responsible for maintenance of intraspecies molecular polymorphism and the mechanisms responsible for generating interspecies diversity are different.In my opinion, at the present time, the available information favors the alternative that, for point mutations, in determining the degree of polymorphism at the intraspecies level, negative selection and genetic drift are of fundamental importance; in contract, positive selection, most probably frequency-dependent selection,is important for maintenance of alozyme polymorphism.At the DNA level, fixation of neutral and especially slightly negative mutations plays an important role in the divergence of species, apparently because speciation is generally accompanied by a reduction in the size of the population  together with the associated reduction in the efficiency of all types of selection.Understandably, from the standpoint of the anagenesis of species, the fixation of positive mutations through the action of positive selection is a more important phenomenon; this occurs most readily in genetically not very polymorphic but quite numerous populations in the period following the origin of a new species (see XXVI.5.3).However, these mutations will tend to be quite rare and will probably not be distinguishable against the background of other types of sequence changes, many of which will be fixed together with positive mutations by genetic draft.

Was this information useful for you?
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
Draft translation from: Evoluční biologie, 2. vydání (Evolutionary biology, 2nd edition), J. Flegr, Academia Prague 2009. The translation was not done by biologist, therefore any suggestion concerning proper scientific terminology and language usage are highly welcomed. You can send your comments to flegratcesnet [dot] cz. Thank you.