Evolutionary change by natural selection (or genetic
drift) requires variation in DNA. Where does this variation come
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.
Sometimes, chunks of DNA are added or lost during DNA copying.
This can lead to loss or duplication of genes (or control
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.
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
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.
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
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
Chromosomes from different species are often very dissimilar,
but it is possible to see strings of genes that have stuck together
through evolutionary history.
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
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