Although the members of many gene familes, such as the globin gene family, have diversified in form and function, the members of many other gene families do not evolve independently of one another. For instance, almost all organisms have many copies (up to thousands) of the ribosomal RNA genes, and within any one species the multiple copies of rRNA genes are very similar, both structurally and functionally. In other words, within a given species, the multiple copies of these rRNA genes evolve in concert with one another, a phenomenon called concerted evolution. In this animation, we examine the phenomenon of concerted evolution and two mechanisms by which it may occur.
Gene duplication is an important way for species to create novel proteins. Duplicate copies are free to mutate and acquire unique functions, while a copy of the original gene can continue performing its original function. Most duplicate genes evolve independently from each other. One example is the globin gene family.
In some cases, however, all copies of duplicate genes are identical, or nearly so. For instance, almost all organisms have many copies (up to thousands) of the ribosomal RNA genes. Ribosomes are made up of large and small subunits. A small portion of each consists of proteins, but the bulk of the subunits consist of ribosomal RNA (rRNA), shown in green. Having many copies of the rRNA genes that can be transcribed into rRNA ensures that organisms can rapidly produce many ribosomes and thereby maintain a high rate of protein synthesis.
What about the rRNA genes in different species? rRNA genes do evolve, as they are slightly different from one species to the next. However, within any one species, the multiple copies of rRNA genes are very similar. This similarity makes sense, because ideally every ribosome in a species should synthesize proteins in the same way.
Concerted evolution is the phenomenon in which the genes in a gene family evolve in concert with each other. Two different mechanisms appear to be responsible for concerted evolution, the first being unequal crossing over. This example shows a gene family in which one gene has just acquired a mutation. During meiosis in a diploid species, homologous chromosomes align and recombine by crossing over. However, highly repeated genes are easy to misalign. After recombination, one chromosome may acquire additional copies of the mutated version, while the other one loses copies.
In another generation, this could happen again, increasing the prevalence of the mutated version in a particular chromosome.
The process continues for many generations. Over time, a novel substitution will either become fixed or lost entirely from the repeat. In either case, all the copies of the repeat will remain very similar to one another.
The second mechanism that produces concerted evolution is biased gene conversion. This mechanism can be much faster than unequal crossing over, and has been shown to be the primary mechanism for concerted evolution of rRNA genes. DNA strands break often, and are repaired by the DNA repair systems of cells.
A copy of the rRNA gene on another chromosome may be used to repair the damaged copy, and the sequence used as a template can replace the original sequence. In many cases, this repair system appears to be biased in favor of using particular sequences as templates for repair, and thus the favored sequence rapidly spreads across all copies of the gene. In this way, changes may appear in a single copy and then rapidly spread to all the other copies. Note that in concerted evolution, mutations do still occur, but once they arise in one copy, they either spread rapidly across all the copies or are lost from the genome completely.
Two different mechanisms appear to be responsible for concerted evolution, a phenomenon in which all of the copies in a gene family remain similar and appear to be evolving in concert with each other.
The first mechanism is unequal crossing over. When homologous chromosome pairs align and recombine by crossing over during meiosis, it is easy for highly repeated genes to become displaced in alignment. The end result is that one chromosome will gain extra copies of the repeated gene and the other chromosome will have fewer copies. Thus, over time, a novel substitution will either become fixed or lost entirely from the repeat. In either case, all the copies of the repeat will remain very similar to one another.
The second mechanism is biased gene conversion. This mechanism can be much faster than unequal crossing over, and has been shown to be the primary mechanism for concerted evolution of rRNA genes. In gene conversion, if damage occurs to one of the genes in a repeat, a copy of the gene on another chromosome may be used to repair the damaged copy, and the sequence that is used as a template can thereby replace the original sequence. In this way, changes may appear in a single copy and then rapidly spread to all the other copies.
Regardless of the mechanism responsible, the net result of concerted evolution is that the copies of a highly repeated gene do not evolve independently of one another. Mutations still occur, but once they arise in one copy, they either spread rapidly across all the copies or are lost from the genome completely. This process allows the products of each copy to remain similar through time in both sequence and function.