Microbial Evolution


Microbial evolution is the study of the patterns and processes that affect the dynamics of microbial diversity over time.

Keywords: adaptation; species; recombination; mutation; selection

Figure 1.

Average fitness improvement for 12 Escherichia coli populations during selection. Fitness is calculated as the ratio of doublings of the evolving populations to their unevolved ancestor in direct competition (Lenski and Travisano, ). The rate of improvement declined significantly over 1000 generations (P < 0.01). Curve is the best fit of a hyperbolic model [y = xo + ax/(b + x)] to data from Travisano .

Figure 2.

Contrasting evolutionary responses to selection after 1000 generations of selection. Escherichia coli populations selected in a glucose‐supplemented environment (horizontal hatching) improved in fitness in the selected environment (b) with little genetic variance for fitness in that environment (a). In a novel environment supplemented with a different nutrient (maltose, slanted hatching), the glucose‐selected populations show no improvement in fitness but substantial variation. In contrast, maltose‐selected populations improved in fitness in both selected (maltose) and novel (glucose) environments, with little genetic variance for fitness in either. Values are means ± 95% confidence intervals. Data from Travisano .

Figure 3.

(a) Pseudomonas fluorescens rapidly diversifies, as measured by colony morphology, in complex nutrient medium under spatially heterogeneous conditions (blue circles/dotted line), but does not diversify when only a single carbon source is supplied (red circles, solid line). (b) Spatially heterogeneous conditions were obtained by cultivating the bacteria in tubes without shaking, so that an oxygen gradient forms as the bacterial population grows. Diversity (H) calculated using the Shannon–Weaver index. Lines are the best fit to a linear model [y = b0 + b1x]. However, nutrient complexity has only moderate effect on maximum population density. Curves are the best fit to a hyperbolic model. Data from Travisano and Rainey .

Figure 4.

Variation in chromosome length among natural isolates of Escherichia coli. Nineteen isolates were grouped into five phylogenetic groups (A, B1, B2, D, E) based upon electrophoretic variation at 20 loci. Statistically significant variation in chromosome size was observed among the five phylogenetic groups (P < 0.05). Values are means ± range. Data from Bergthorsson and Ochman .



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Further Reading

Baumberg S, Young JPW, Wellington EMH and Saunders JR (eds) (1995) Population Genetics of Bacteria. Cambridge: Cambridge University Press.

Chadwick DJ and Goode J (eds) (1997) Antibiotic resistance: origins, evolution, selection and spread. Ciba Foundation Symposium 207.

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How to Cite close
Travisano, Michael(May 2005) Microbial Evolution. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1038/npg.els.0001746]