What is adaptation by natural selection? Perspectives of an experimental microbiologist

Abstract

Ever since Darwin, the role of natural selection in shaping the morphological, physiological, and behavioral adaptations of animals and plants across generations has been central to understanding life and its diversity. New discoveries have shown with increasing precision how genetic, molecular, and biochemical processes produce and express those organismal features during an individual's lifetime. When it comes to microorganisms, however, understanding the role of natural selection in producing adaptive solutions has historically been, and sometimes continues to be, contentious. This tension is curious because microbes enable one to observe the power of adaptation by natural selection with exceptional rigor and clarity, as exemplified by the burgeoning field of experimental microbial evolution. I trace the development of this field, describe an experiment with Escherichia coli that has been running for almost 30 years, and highlight other experiments in which natural selection has led to interesting dynamics and adaptive changes in microbial populations.

Fig 1. Incubator used in early experiment on adaptation by natural selection.

Image from Dallinger (1887), now in the public domain (http://commons.wikimedia.org/w/index.php?curid=10531922).

Fig 2. Experimental populations of E. coli, centered on the population that evolved the ability to use citrate in the LTEE.

Photo by Brian Baer and Neerja Hajela, Michigan State University (http://commons.wikimedia.org/w/index.php?curid=4277502).

Fig 3. Clusters of “snowflake” yeast that evolved a multicellular life history.

Confocal micrographs showing many clusters (left) and one at higher magnification (right). Colors show depth in z-axis. Unpublished images by Shane Jacobeen, Will Ratcliff, and Peter Yunker, Georgia Institute of Technology.