Evolutionary change by natural selection (or genetic drift) requires variation in DNA. Where does this variation come from?

Point mutation

Single bases in DNA can be altered by chemical processes, triggered by, for example, ionising radiation or chemical mutagens. Sometimes DNA-copying mechanisms may insert the wrong base.

Insertion/deletion

Sometimes, chunks of DNA are added or lost during DNA copying. This can lead to loss or duplication of genes (or control regions).

Recently, it has been discovered that the human genome shows surprisingly high levels of variation caused by insertion or deletion. This so-called copy number variation affects around 12 per cent of the human genome.

Transposable elements

Some parts of the genome - transposable elements - can leap about from place to place in the genome. Potentially, transposable elements may play an important role in evolution by moving genes or changing how they are controlled.

Transposable elements come in many shapes and sizes. They have been likened to genetic 'parasites' or free-loaders, providing nothing to the cell but getting copied from generation to generation. Some look like viruses that have integrated into the genome and no longer make infectious virus particles. Some cut themselves out of the genome and move to another location; others make copies that spread through the genome.

Transposable elements make up nearly half the human genome, but they remain poorly understood. Are they purely selfish? Or do they provide advantages by supplying a new source of genetic variation?

Gene conversion

Because of complementary base pairing, strands of DNA tend to stick together like Velcro. Sometimes, genes or other sections of DNA in different parts of a chromosome (or on a different chromosome) become intermingled if they have the same or similar base sequences. When this mess is sorted out by DNA repair, it can result in the 'wrong' DNA being inserted. So, instead of two similar genes, a chromosome may end up with two copies of the same gene – a process known as gene conversion.

Gene transfer

By a variety of processes, whole genes can sometimes be moved between genomes. Some viruses have a habit of snatching genes from their hosts, and bacteria are good at swapping genes among themselves. This is one reason that antibiotic resistance can spread so quickly.

Horizontal gene transfer, as it is known, can also affect eukaryotic cells, but appears to be less common. Some genes seem to have moved from the mitochondrial genome to the nucleus. When the human genome was first sequenced, it was thought that some genes might have been transferred from bacteria, but this now seems less likely.

Translocations

Sometimes, bits of chromosome are moved around the genome, or stay in the same place but their orientation is flipped. Translocations can disrupt genes or bring together genes and control regions.

Chromosomes from different species are often very dissimilar, but it is possible to see strings of genes that have stuck together through evolutionary history.

Genome duplication

One of the most important evolutionary changes is the duplication of an entire genome. Initially, this leads to polyploidy - where an organism has extra copies of all its chromosomes. Polyploidy is relatively common in plants (banana and apple are triploid; cotton, potato, cabbage and peanut are tetraploid; strawberries, pansies and sugar cane are octaploid). It occurs in some animals (e.g. flatworms) but is very rare in mammals.

p>In the past, whole genome duplications are thought to have occurred several times in the lineages leading to modern organisms. Over time, genes are lost and shuffled around, obscuring the original event.