Evolution is caused by mutations in the DNA of organisms. Mutations change the properties of proteins, which are machines that carry out the functions necessary for life. Mutations have either a positive or negative effect on the survival and propagation of organisms and are submitted to natural selection. Evolution can be mimicked in the laboratory by directed or random mutagenesis of the DNA that encodes for a protein. In directed mutation one specifically selected building block of the protein is changed into another, whereas in random mutagenesis the changes are made randomly. One example for random mutagenesis is DNA family shuffling, where the DNA of different organisms encoding for the same protein is cut into small pieces and reassembled into DNA that contains parts originated from different organisms.