by mutations. This process happens by reproduction. In a particular generation the 'best fit' survive with higher probability, passing their genes to their offspring. The sign of the change in this probability defines mutations to be beneficial, neutral or harmful to organisms. Measurement An organism's mutation rates can be measured by a number of techniques. One way to measure the mutation rate is by the fluctuation test, also known as the Luria–Delbrück experiment. This experiment demonstrated that bacteria mutations occur in the absence of selection instead of the presence of selection. This is very important to mutation rates because it proves experimentally mutations can occur without selection being a component—in fact, mutation and selection are completely distinct evolutionary forces. Different DNA sequences can have different propensities to mutation (see below) and may not occur randomly. The most commonly measured class of mutations are substitutions, because they are relatively easy to measure with standard analyses of DNA sequence data. However substitutions have a substantially different rate of mutation (10−8 to 10−9 per generation for most cellular organisms) than other classes of mutation, which are frequently much higher (~10−3 per generation for satellite DNA expansion/contraction). Substitution rates Many sites in an organism's genome may admit mutations with small fitness effects. These sites are typically called neutral sites. Theoretically mutations under no selection become fixed between organisms at precisely the mutation rate. Fixed synonymous mutations, i.e. synonymous substitutions, are changes to the sequence of a gene that do not change the protein produced by that gene. They are often used as estimates of that mutation rate, despite the fact that some synonymous mutations have fitness effects. As an example, mutation rates have been directly inferred from the whole genome sequences of experimentally evolved replicate lines of Escherichia coli B. Mutation accumulation lines A particularly labor-intensive way of characterizing the mutation rate is the mutation accumulation line. Mutation accumulation lines have been used to characterize mutation rates with the Bateman-Mukai Method and direct sequencing of well-studied experimental organisms ranging from intestinal bacteria (E. coli), roundworms (C. elegans), yeast (S. cerevisiae), fruit flies (D. melanogaster), and small ephemeral plants (A. thaliana). Variation in mutation rates Mutation rates differ between species and even between different regions of the genome of a single species. These different rates of nucleotide substitution are measured in substitutions (fixed mutations) per base pair per generation. For example, mutations in intergenic, or