Mutation--The Engine of Evolution: Studying Mutation and Its Role in the Evolution of Bacteria

Abstract

Mutation is the engine of evolution in that it generates the genetic variation on which the evolutionary process depends. To understand the evolutionary process we must therefore characterize the rates and patterns of mutation. Starting with the seminal Luria and Delbruck fluctuation experiments in 1943, studies utilizing a variety of approaches have revealed much about mutation rates and patterns and about how these may vary between different bacterial strains and species along the chromosome and between different growth conditions. This work provides a critical overview of the results and conclusions drawn from these studies, of the debate surrounding some of these conclusions, and of the challenges faced when studying mutation and its role in bacterial evolution.

Figure 1.

Different types of mutations (represented by differently colored arrows) occur at different frequencies (represented by arrow thickness). Selection acts as a sieve and allows only a subset of these mutations to persist and become the differences we see between genomes. Such differences are referred to as substitutions. Various types of mutations have different fitness effect distributions, and will be differently affected by selection. (A) Under normal levels of selection, selection will introduce its own biases into patterns of variation. Thus, biases in the patterns of observable substitutions between genomes are likely not to reflect mutational biases. (B) When selection is extremely relaxed, it is expected to affect patterns of variation to a much lesser extent, because it will affect only mutations with very high-fitness effects. Under such conditions, observed substitutions between genomes approximate a random sample of the mutations that have occurred. Because of this, when selection is relaxed, biases in the patterns of substitutions observed between genomes will better approximate mutational biases.