Species cohesion in asexually reproducing organisms
The basic mechanism of species cohesion, sexual reproduction, cannot function in asexually reproducing organism; however, simultaneously, a different process that is capable of ensuring species cohesion can exist here, i.e. genetic draft (Gillespie 2000; Gillespie 2001). Genetic draft, which is also known under the older name hitchhiking, consists in elimination of genetic (and thus also phenotype) polymorphism from the gene pool together with an allele that is subject to selection (see IX.4.4.1). The effects of this process are very limited amongst sexually reproducing organisms. They are related only to elimination of polymorphism in genes in close genetic linkage with a gene that is the actual object of selection. The effect of draft is manifested most strongly here in the sections of the genome in which genetic recombination does not occur, for example, in the non-recombining parts of the sex chromosomes. It is assumed that genetic draft is responsible for the absence of intraspecific polymorphism, which is typical for these genome sections. The importance of genetic draft is much greater in asexually reproducing species, as all the genes of the individual are in absolute genetic linkage. Thus, if a single advantageous mutation appears in the population at a certain moment, that has a sufficiently large selection coefficient and sufficient luck to become fixed, the alleles of all the genes that occurred in the genome of the mutant are also fixed. All the other alleles occurring in the population at the given moment are, on the other hand, eliminated (Fig. XX.4). If the fixation of the new alleles is so fast that there is no time for the formation of new genetic variability in the progeny of the particular mutant, the entire species can become genetically and, to a considerable degree, also phenotypically uniform. Over time, new mutations will accumulate in the gene pool of the species and genotype and phenotype polymorphism will be renewed. However, over shorter or longer intervals, new, advantageous mutations will repeatedly “sweep out” this polymorphism and thus renew the gene and phenotype uniformity of the species.
Continuous elimination of negative mutations will understandably contribute to the maintenance of the uniformity of members of an asexual species. As soon as negative mutations appear in the genome of an individual and become an object of gradual elimination through the action of negative (purifying) selection, all the copies of alleles occurring in the genome of the individual are also fated to disappear. Of course, this disappearance occurs only for copies occurring in the particular individual, so that the final effect on the overall polymorphism of the species or population will be less than for positive mutations. However, because negative mutations occur far more often than positive mutations, their overall importance for maintenance of species cohesion can actually be greater than the effect of positive mutations.
- more or less1606
- not at all2391