V.6 The theory of neutral evolution is of key importance for the method of studying one of the two components of biological evolution, i.e. cladogenesis.
When it was realized that the effectiveness of natural selection can be very low in small populations, suggestions began to arise as to the degree to which this basic pillar of Darwinism can play a role in biological evolution.Some authors concluded that the effective size of real populations is so small that natural selection cannot basically play any role here.With reference to the relationships mostly derived by Kimura, they argued that most traits were fixed during evolution by genetic drift, so that biological evolution basically did not occur as Darwinist evolution, but rather as neutral evolution.
This conclusion was rejected by most evolutionary biologists, including M. Kimura.Primarily, it was found that mutations with selection coefficients so large that they cannot act as selectionally neutral even in a relatively small population occur in nature with a non-negligible frequency .At the present time, it has even been found that the populations of a great many species are apparently so large that even a prototype of neutral mutation, i.e. a synonymous mutation, can act as a selectionally significant mutation in at least some phases of evolution (Akashi 1995; Berg 1996).Primarily, however, a great many authors have repeatedly emphasized that the nature of evolution is not so much determined by how many mutations are fixed by a particular mechanism, but rather primarily through which mutations become fixed.The number of mutations fixed by selection can be incomparably fewer than the number of mutations fixed by genetic drift or genetic draft.However, because primarily those rare mutations that fundamentally affect the phenotype of organisms will be preferentially fixed by selection, it will be selection that plays a major role in determining the nature of biological evolution.
Regardless of the basic evolutionary importance of mutations affected by selection, it must be borne in mind that there is an extremely large class of mutations that act as effectively neutral in populations of normal size.These are mostly synonymous point mutations and also mutations in those parts of the DNA that do not code a functional protein or RNA and do not even participate in the regulation of biological processes.Simultaneously, it is probable that most other mutations that are not synonymous and that can thus be manifested in the structure of coded proteins or in the regulation of their synthesis have such a small effect on the overall fitness of the organism that they act as neutral mutations in populations of normal size.When molecular biologists study a sequence of nucleic acids or proteins, the vast majority of the differences between the individual sequences, from a practical standpoint almost all that they encounter, tend to fall into the category of effectively neutral mutations.
In Chapter IX (DNA Sequence Evolution), it will be shown that the study of neutral mutations is of substantial importance for the evolutionary biologist when he attempts to reconstruct the progress of cladogenesis of a certain taxon.Determination of the molecular traits that are shared by the individual species permits determination, with well-quantifiable probability, of the order in which the individual species branched off from their common developmental base.The number of fixed mutations in the gene pools of the individual species also permits dating of the instances of splitting off of the individual phylogenetic branches.
However, these neutral mutations are insignificant and play no role in the formation of adaptive structures or in increasing the complexity of organisms, i.e. in anagenesis.Natural selectionplays an exclusive and irreplaceable role in the anagenesis of organisms.