Animation 21.1: Concerted Evolution

INTRODUCTION

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.

CONCLUSION

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.

Textbook Reference: Key Concept 21.3 Changes in the Size and Arrangement of Genomes Can Result in New Features