XI.3 Extra copies of genes usually originate from gene or genome duplication

The number of copies of a particular gene can increase in the genome through two different mechanisms.For example, genome polyploidization can occur as a consequence of errors in the cell division or as a consequence of fusion of two cells.Simultaneous multiplication occurs in this case, most frequently duplication of all the genes.This polyploidization occurs quite frequently and repeatedly in all groups of organisms during evolution, so that some taxa contain species that mutually differ in the number of chromosomal sets.Genes in extra chromosomal sets are freely available for molecular evolution, can more or less freely mutate and can thus act as a substrate for the formation of new genes.Comparison of the content of genes in the framework of whole genomes of higher taxa indicates that polyploidization was probably also important in macroevolutionary processes (Wolfe 2001).

            The second mechanism consists in duplication of an individual gene itself or part thereof, usually as a consequence of nonreciprocal recombination between two chromosomes.This leads to deletion on one chromosome and duplication of a certain DNA section on the other one.The chromosome with deletion in the section of a vitally important gene is eliminated from the population immediately in haploid organisms and gradually in diploid organisms, while the chromosome with duplication can lead to the formation of a new gene under suitable selection pressure.

            An important and frequently discussed aspect remains the probability with which a duplicated gene changes in time to a new gene and the probability with which, on the other hand, it will be inactivated by mutation and converted into a nonfunctional pseudogene.In accordance with empirical observations, theoretical models indicate that the probabilities of both alternatives equal approximately 50% (Clark 1994; Hughes 1994; Walsh 1995; Wagner 1998).Mutations that change a functional gene into a nonfunctional pseudogene occur incomparably more frequently than mutations that form a new gene, which is useful for the organism, from an extra gene.On the other hand, the probability of fixation of a pseudogene in the population (by genetic drift or draft) is much less than the probability of fixation of positive mutations by natural selection.

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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.