Evolutionary change by natural selection (or genetic drift) requires variation in DNA. Where does this variation come from?
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 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 a significant proportion 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 part 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 freeloaders, 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?
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.
Through a variety of processes, whole genes can sometimes be moved between genomes. Some viruses have a habit of snatching genes from their hosts – the algae-infecting virus EhV has acquired the genes encoding an entire metabolic pathway from its host – and bacteria are good at swapping genes among themselves.This is one reason why antibiotic resistance can spread so quickly.
Horizontal gene transfer, as it is known, can also affect eukaryotic cells, but this seems 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 finding was challenged. However, researchers have identified transfer of bacterial genes into the genomes of cells in the body, particularly cancer cells.
Sometimes, bits of chromosome are moved around the genome, or they 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 – Hox genes being one example.
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: bananas and apples are triploid; cotton, potatoes, cabbages and peanuts are tetraploid; and strawberries, pansies and sugar cane are octaploid. It occurs in some animals (eg flatworms) but is very rare in mammals.
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.
The additional genes created by genome evolution may have provided the raw material for extensive gene diversification, enabling newly duplicated genes to take on new functions. This could have played an important part in the evolution of the vertebrate nervous system.Lead image:
Alper Cugun/Flickr CC BY
- Copy Number Variation and Human Disease (2008)
- Wikipedia: Transposable element
- Initial sequencing and analysis of the human genome (2001)
- Wikipedia: Gene conversion
- Evolution of viruses by acquisition of cellular RNA or DNA nucleotide sequences and genes: an introduction (2000)
- Horizontal gene transfer of an entire metabolic pathway between a eukaryotic alga and its DNA virus (2009)
- News: Researchers challenge recent claim that humans acquired 223 bacterial genes during evolution
- News: Bacterial DNA in human genomes
- Two rounds of whole genome duplication in the ancestral vertebrate (2005)
- Synaptic scaffold evolution generated components of vertebrate cognitive complexity (2013)
- Evolution of GluN2A/B cytoplasmic domains diversified vertebrate synaptic plasticity and behaviour (2013)
Questions for discussion
- Is mutation bad for an individual and good for a species?