regions (within that repeating unit) Noncoding regions evolve very rapidly Coding regions evolve very slowly Restriction on homogeneity Population size Requirement of doses Requirements The horizontal transfer of mutations among the family members (homogenization) The spread of mutations in the population (fixation) Hypotheses To Explain Concerted Evolution High sequence similarity between paralogs may be maintained by homologous recombination events that lead to gene conversion, effectively copying some sequence from one and overwriting the homologous region in the other. Another possible hypothesis that has yet to be disproved is that rapid waves of gene duplication are responsible for the apparently "concerted" homogeneity of tandem and unlinked repeats seen in concerted evolution. Rapid amplification of a gene, usually assisted by recombination events in IS elements, in bacteria, or in other repetitive genetic elements (ERV, LINE, SINE, etc.), for example, in eukaryotes. Unchecked transposition events of these transposable elements are thought to be associated with increases in the copy number of the gene. In sexually reproducing organisms unequal crossing over during meiosis may be responsible for amplification due to misalignment of repeated sequences. Redistribution of genes requires transposition, probably assisted by the same repetitive genetic elements as in 1). Homogenization of alleles by gene conversion may also play a role in sexually reproducing organisms. Some genes may be more prone to gene conversion than others, thus reinforcing the unity of the genes within a gene family of a species. Evolution and speciation Findings of concerted evolution, particularly in ribosomal DNA genes, led the Cambridge molecular geneticist Gabriel Dover to his controversial proposal of molecular drive, which in his view was an evolutionary principle distinct both from natural selection and from genetic drift. Closely related species or even populations may differ in their nucleolus organizing regions (NORs), which are genomic regions that contain many copies of ribosomal RNA genes in eukaryotes, typically found within or adjacent to highly repetitive parts of the genome such as centromeres or telomeres in mammals such as the house mouse Mus musculus or insects such as the grasshopper Podisma pedestris. Future Research In future studies, it may want to be looked at whether if repeats are relocated to different parts of the genome in related species what exactly would happen to the ancestral copies. Another aspect that could be studied includes tandem arrays of new ribosomal genes when they are born and how they become lost over time. The