A duplicated gene has three possible fates:
- Remain as two redundant copies. The copies are in danger of being
lost, since selection is weaker on redundant genes, but concerted
evolution (see below) can help.
- Decay into a pseudogene and eventually become lost.
- Evolve a new function, becoming a separate gene.
HLA apparently has examples of all three processes.
Duplicate genes can interact and thereby
become more similar, in two different ways. Gene conversion can
change one copy to be identical to another. Unequal crossing-over
can create and destroy whole copies, and if this happens repeatedly
the family will tend to look very uniform. This is called concerted
because the whole gene family evolves in concert (together) rather
than as individuals.
Possible evolutionary advantages of concerted evolution:
- Multiple copies remain functional rather than decaying.
- A favorable mutation can be spread to all copies.
- Concerted genes remain in usable form longer, and are therefore
available to evolve new functions.
- Additional copies are easy to generate (by unequal crossing over)
so the family can increase in number quickly if this is selectively
advantageous.
- Hybrid copies produced by crossing-over or conversion may have
useful new properties.
Possible evolutionary disadvantages of concerted evolution:
- Useful new functions may be wiped out by convergence. (Example:
color vision difficulties in humans.)
- A harmful mutation may spread by copying before it can be
eliminated by selection.
- Crossing over can increase or reduce the family size too much.
- If the family members are not tandem (lined up in a row),
crossing-over between them can lead to inviable chromosomes.
These come in several kinds.
Multiple copies of a gene in a row. These are unusual
because they tend to evolve as a group, not as individuals.
The number of copies can easily increase or decrease due to unequal
crossing-over, and gene conversion can easily cause copies to become identical.
Examples: ribosomal RNA genes, immunoglobulin V-chain genes.
Families of this kind are sometimes involved in high-demand operations;
having a thousand identical copies lets you put out gene product faster.
Multiple copies in random locations in
the genome (interspersed with other genes).
These tend to evolve mainly as independent genes.
They can also show partial concerted evolution, especially if they are
close together. Usually there is more gene conversion than unequal
crossing-over, since crossing-over would lead to chromosome
rearrangements.
A gene superfamily is a group of distantly related genes, often sharing
similarities across only part of their sequences. The immunoglobulin superfamily is
an example. These are normally interspersed families, because concerted
evolution interferes with this kind of differentiation in tandem repeats.
Transposons produce an unusual sort of multi-gene
family because they duplicate so frequently that selection for duplicating
ability, on the gene level, can compete with selection on the organism
level. Humans have huge families of transposons making up a significant
percentage of the whole genome. Rather than unequal crossing-over
or gene conversion, these families evolve mainly by transposition to
new locations, but the other processes can happen too.
- How could one gene copy escape from a tandem repeat and develop
its own function?
- Since there is selection on individual transposons within the
genome, favoring transposons that transpose more often, why doesn't
the whole genome fill up with transposons?
- If you were trying to improve the human color-vision system, what
could you do? Concerted evolution seems to be purely a nuisance here.