Asked by: Fiona Roberts, Brighton
Evolution is propelled by two forces: random mutations in the genes of living organisms, plus natural selection of organisms whose mutations make them better able to survive long enough to reproduce and thus pass on the same genes to their offspring. These mutations can be anything from a single ‘point mutation’ – where one of the four ‘bases’ making up DNA changes into another – to wholesale loss or duplication of entire sequences of genes. Point mutations are often caused by mutagenic chemicals like cadmium, while larger changes can be due to simple errors made by the genetic systems of cells. When they occur, around 70 per cent of mutations are harmful to organisms, and many are weeded out because they do not promote survival. Only around one-third of them are either harmless or actively beneficial.