Concerted evolution is a process that may explain the observation that paralogous genes within one species

Species

In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

 are more closely related to each other than to members of the same gene family in another species, even though the gene duplication

Gene duplication

Gene duplication is any duplication of a region of DNA that contains a gene; it may occur as an error in homologous recombination, a retrotransposition event, or duplication of an entire chromosome.The second copy of the gene is often free from selective pressure — that is, mutations of it have no...

 event preceded the speciation

Speciation

Speciation is the evolutionary process by which new biological species arise. The biologist Orator F. Cook seems to have been the first to coin the term 'speciation' for the splitting of lineages or 'cladogenesis,' as opposed to 'anagenesis' or 'phyletic evolution' occurring within lineages...

 event. The high sequence similarity between paralogs is maintained homologous recombination

Homologous recombination

Homologous recombination is a type of genetic recombination in which nucleotide sequences are exchanged between two similar or identical molecules of DNA. It is most widely used by cells to accurately repair harmful breaks that occur on both strands of DNA, known as double-strand breaks...

 events that lead to gene conversion

Gene conversion

Gene conversion is an event in DNA genetic recombination, which occurs at high frequencies during meiotic division but which also occurs in somatic cells. It is a process by which DNA sequence information is transferred from one DNA helix to another DNA helix, whose sequence is altered.It is one...

, effectively copying some sequence from one and overwriting the homologous region in the other.

Example

An example can be seen in bacteria: Escherichia coli

Escherichia coli

Escherichia coli is a Gram-negative, rod-shaped bacterium that is commonly found in the lower intestine of warm-blooded organisms . Most E. coli strains are harmless, but some serotypes can cause serious food poisoning in humans, and are occasionally responsible for product recalls...

 has seven operons

Operon

In genetics, an operon is a functioning unit of genomic DNA containing a cluster of genes under the control of a single regulatory signal or promoter. The genes are transcribed together into an mRNA strand and either translated together in the cytoplasm, or undergo trans-splicing to create...

 encoding various Ribosomal RNA

Ribosomal RNA

Ribosomal ribonucleic acid is the RNA component of the ribosome, the enzyme that is the site of protein synthesis in all living cells. Ribosomal RNA provides a mechanism for decoding mRNA into amino acids and interacts with tRNAs during translation by providing peptidyl transferase activity...

. For each of these genes, rDNA

Ribosomal DNA

Ribosomal DNA codes for ribosomal RNA. The ribosome is an intracellular macromolecule that produces proteins or polypeptide chains. The ribosome itself consists of a composite of proteins and RNA. As shown in the figure, rDNA consists of a tandem repeat of a unit segment, an operon, composed of...

 sequences are essentially identical among all of the seven operons (sequence divergence of only 0.195%). In a closely related species, Haemophilus influenzae

Haemophilus influenzae

Haemophilus influenzae, formerly called Pfeiffer's bacillus or Bacillus influenzae, Gram-negative, rod-shaped bacterium first described in 1892 by Richard Pfeiffer during an influenza pandemic. A member of the Pasteurellaceae family, it is generally aerobic, but can grow as a facultative anaerobe. H...

 its six ribosomal RNA operons are entirely identical. When the 2 species are compared together however, the sequence divergence of the 16S rRNA gene between them is 5.90% .

Requirements

1. Amplification of a gene, usually assisted by recombination events in IS elements (in bacteria) or in similar high copy number repetitive genetic elements (ERV

Endogenous retrovirus

Endogenous retroviruses are sequences in the genome thought to be derived from ancient viral infections of germ cells in humans, mammals and other vertebrates; as such their proviruses are passed on to the next generation and now remain in the genome....

, LINE, SINE

Siné

Maurice Sinet , known as Siné, is a French cartoonist.As a young man he studied drawing and graphic arts, while earning a living as a cabaret singer. After his military service he started publishing his drawings and also worked as a photo-retoucher for porn magazines. His first published drawing...

, etc). This increases the copy number of the gene. In sexual reproducing organisms unequal crossing over during meiosis may be responsible for amplification due to misalignment of repeated sequences.

2. Redistribution of genes requires transposition, probably assisted by the same repetitive genetic elements as in 1).

3. Homogenization of alleles by gene conversion

Gene conversion

Gene conversion is an event in DNA genetic recombination, which occurs at high frequencies during meiotic division but which also occurs in somatic cells. It is a process by which DNA sequence information is transferred from one DNA helix to another DNA helix, whose sequence is altered.It is one...

may play a role in sexual reproducing organisms.

Some genes have a bias and are more prone to gene conversion than others, thus reinforcing the unity of the genes within a gene family of a species.