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species is on the order of the mutation rate or lower (Levin, 1981, 1988; Levin and Lenski, 1983; see also Hartl and Dykhuizen, 1984). The low rates of recombination in bacterial populations have at least four important (and interesting) ramifications for at least the local (short-term) population genetics of adaptive evolution in bacterial populations.

Sequential evolution

Within each bacterial lineage, adaptive evolution will proceed by the sequential accumulation of favorable mutations, rather than by recombinational generation of gene combinations; in this respect, bacterial evolution will be similar to that depicted in the top portion of Muller's famous diagram of evolution in asexual and sexual populations (Crow and Kimura, 1965). The evolution of a better genotype ABC from its less fit ancestor abc will proceed in stages, one gene at a time, with the order and rate of evolution depending primarily on the fitness of the intermediates (aBc, aBC, etc.). If, individually and collectively, the intermediates have a selective advantage over the abc ancestor, this evolution can proceed quite rapidly (Evans, 1986). If, on the other hand, the intermediates are not favored, it may take a great deal of time before the best genotype, ABC, is assembled. Moreover, the best genotype will most likely arise by mutation from an intermediate form, rather than through the recombinational merger of different intermediate forms; for a more extensive and formal (mathematical) consideration of the process of mutation and selection in bacteria, see Gerrish and Lenski (1998).

Compensatory evolution

In bacteria, adaptation to the effects of deleterious genes that have become fixed (because of their being favored in another environment or for other reasons) is likely to be through amelioration of those effects by the ascent of compensatory mutations at other loci, rather than by the evolution of more fit variants at the deleterious loci (Bjorkman et al., 1998; Schrag and Perrot, 1996). In sexual eukaryotes as well as bacteria, these compensatory mutations are likely to be the first to arise when, as is probable, there are more ways to improve fitness of the organism than mutation (reversion) at the deleterious locus. If recombination is common, however, these compensatory mutations and the genes whose deleterious effect they ameliorate will rapidly become separated, especially if these loci are not closely linked. Because of the low rate of recombination in bacterial populations and the fact that, when recombination does occur, only a small fraction of the genome is replaced, the deleterious and

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