AUXOTROPHIC A mutant that cannot synthesize a required nutrient, such as an amino acid.
BOTTLENECK A severe reduction in population size that causes the loss of genetic variation. The role of random drift is increased, whereas the power of selection is reduced, by bottlenecks.
CHAPERONES A class of protein that binds to other proteins and thereby promotes their proper folding during synthesis or following damage.
CHEMOSTAT A device that allows the continuous growth of a bacterial population on a growth-rate-limiting resource. The resource flows into the chemostat at a constant rate; depleted medium and cells are washed out at the same rate. The population grows and consumes the resource until the bacteria reach an equilibrium density at which their growth rate equals the flow rate through the vessel.
DIGITAL ORGANISMS A type of computer-based artificial life that can be used to investigate certain scientific questions. The genomes of digital organisms are computer programs and, like computer viruses, are able to self-replicate. Digital organisms can also mutate and evolve spontaneously, whereas computer viruses are deliberately modified by hackers.
EPISTASIS Any non-additive interaction between two or more mutations at different loci, such that their combined effect on a phenotype deviates from the sum of their individual effects.
FITNESS The average reproductive success of a genotype in a particular environment. Often expressed relative to another genotype, such as the ancestor in evolution experiments.
GENETIC LOAD The loss of fitness that is caused by producing offspring that carry deleterious mutations, and the resulting decrease in the rate of population growth.
HITCHHIKING The process by which a neutral, or even deleterious, mutation increases in frequency owing to its physical linkage with a beneficial mutation elsewhere in the genome.
ISOGENIC Genotypes that have been engineered to be identical, with the exception of one or more mutations of interest.
PLEIOTROPY The side-effect of a mutation that affects a primary trait or function on a secondary trait or function.
PORIN A protein channel across the outer membrane of a Gram-negative bacterium that allows the diffusion of molecules into the periplasm, which is located between the outer and inner membranes.
RANDOM DRIFT The change in frequency of genotypes in a population that is caused by chance differences in survival and reproduction, as opposed to consistent differences in their fitness.
REPLICATE POPULATIONS Two or more populations that started from the same ancestral genotype and were propagated under identical conditions as part of an evolution experiment. By having replicates in each of several environments, it is possible to distinguish statistically between systematic responses of the populations to a particular environmental feature (for example, temperature) and other responses that might reflect the chance effects of mutation and drift.
SEQUENCE SPACE The universe of all possible sequences or genotypes. For example, even a small viral genome of 1,000 nucleotides has 3,000 one-step neighbours, nearly 9,000,000 two-step neighbours, and more than 10600 variants at all possible distances of the same genome length.
SERIAL TRANSFER A culture regime in which some proportion of a population is periodically diluted into fresh medium, in which the population grows until it exhausts the limiting resource and then waits until the next transfer cycle. Selection favours rapid exponential growth as well as the ability to respond quickly following transfer into fresh medium.
STATIONARY PHASE The period in a serial-transfer regime after the limiting resource has been depleted, such that population growth ceases. A population can be kept in this phase indefinitely by never transferring it to fresh medium, and it eventually declines owing to starvation.
