Experimental Evolution

Evolution in the Laboratory

Above. Different strains of E. coli can be monitored by labeling the strains with a neutral genetic marker, which causes them to appear red or white when plated on an indicator agar. Markers allows us to track the success of different strains as they compete against one another in the laboratory. Strains can also be frozen and thawed out again, so we can do side-by-side comparisons of any given strain to its direct ancestors, allowing us to understand the changes that have taken place as they evolve in under laboratory conditions.

Microorganisms are ideal for watching evolution in action: they are easy to grow and culture, and they reach astoundingly large populations sizes.  They can also double as often as every few minutes.  For all of these reasons, they can evolve rapidly, allowing us to track evolutionary change in response to different laboratory conditions.  Microbes are also genetically tractable, so we can identify exactly what genetic changes took place and isolate their effects apart from other changes in the genome in order to understand how and why these changes improved survival and reproduction.  Laboratory evolution experiments allow us to to investigate basic evolutionary principles in a highly controlled setting.

As part of my graduate research, I identified some of the different mutations that arose as laboratory populations of bacteria adapted to novel environments, examining the extent to which selection to improve in one set of conditions generated loss of fitness in other, unselected environments. This tendency for improvements under one set of selective conditions to reduce fitness in another results from pleiotropy (a single gene affecting multiple traits). Genetic architecture thus influences evolution, and evolutionary studies can in turn be used to probe the genetic architecture of traits important for fitness. We are continuing to use experimental evolution as a tool to gain insight into the genes and pathways important for evolution.

Relevant Publications

Ostrowski, EA., Woods, R., and R.E. Lenski.  2008.  Genetic basis of parallel and divergent phenotypic responses in evolving populations of Escherichia coli.  Proceedings of the Royal Society of London B 275:277-284.

Ostrowski, E.A., Rozen, D.E., and R.E. Lenski.  2005.  Pleiotropic effects of beneficial mutations in Escherichia coli.  Evolution 59:2343-2352.