Adaptive mutation

Adaptive mutation, also called directed mutation or directed mutagenesis is a controversial evolutionary theory. It posits that mutations, or genetic changes, are much less random and more purposeful than traditional evolution, implying that organisms can respond to environmental stresses by directing mutations to certain genes or areas of the genome. There have been a wide variety of experiments trying to support (or disprove) the idea of adaptive mutation, at least in microorganisms. The most widely accepted theory of evolution states that organisms are modified by natural selection where changes caused by mutations improve their chance of reproductive success. Adaptive mutation states that rather than mutations and evolution being random, they are in response to specific stresses. In other words, the mutations that occur are more beneficial and specific to the given stress, instead of random and not a response to anything in particular. The term stress refers to any change in the environment, such as temperature, nutrients, population size, etc. Tests with microorganisms have found that for adaptive mutation, more of the mutations observed after a given stress were more effective at dealing with the stress than chance alone would suggest is possible. This theory of adaptive mutation was first brought to academic attention in the 1980s by John Cairns. Adaptive mutation is a controversial claim leading to a series of experiments designed to test the idea. Three major experiments are the SOS response, responses to starvation in Escherichia coli, and testing for revertants of a tryptophan auxotroph in Saccharomyces cerevisiae (yeast). The E. coli strain FC40 has a high rate of mutation, and so is useful for studies, such as for adaptive mutation. Due to a frameshift mutation, a change in the sequence that causes the DNA to code for something different, FC40 is unable to process lactose. When placed in a lactose-rich medium, it has been found that 20% of the cells mutated from Lac- (could not process lactose) to Lac+, meaning they could now utilize the lactose in their environment.