Experimental Evolution

by

Mark A. Phillips, Michael R. Rose

• LAST REVIEWED: 24 May 2018
• LAST MODIFIED: 24 May 2018
• DOI: 10.1093/obo/9780199941728-0107

Introduction

The field of evolutionary biology is primarily focused on the study of the evolutionary processes that are responsible for the diversity of life on Earth. Experimental evolution (EE) explores the microevolutionary dynamics underlying these processes by studying populations across multiple generations under controlled conditions. By studying these populations as they evolve in response to deliberately imposed conditions, scientists are able to directly study evolution in real time. Most EE studies are concerned with characterizing the phenotypic and genetic changes underlying adaptation. Historically, the field has been chiefly concerned with characterizing phenotypic evolution, particularly life history and physiology. But with the advent of next-generation sequencing, EE has emerged as a powerful tool for parsing the genetic foundations of adaptation. EE research can also be divided into two other main categories: studies that feature asexual populations, and studies that feature sexual populations. Studies of asexual microbes feature large population sizes and sometimes thousands of generations. Asexual adaptation appears to be limited primarily by the occurrence of beneficial de novo mutations. In studies featuring sexual populations, population sizes tend to be much smaller, and the number of generations is usually far fewer. Sexual adaptation is thought to be fueled primarily by standing genetic variation, as opposed to the emergence of beneficial mutations. By studying adaptation in both sexual and asexual systems, EE allows insights into adaptation across a variety of evolutionary scenarios. While the term experimental evolution is sometimes applied to experiments undertaken in nature, or “the wild,” this annotated bibliography will be limited exclusively to laboratory EE.

Pioneering Experiments on Evolutionary Dynamics

In the earliest well-documented EE study, Dallinger 1887, unicellular organisms were cultured in an incubator for several years, and the temperature was gradually increased from 60℉ to 158℉. Dallinger showed that while early cultures were unable to survive at 158℉, those present at the end of the experiment were seemingly unaffected by such high temperatures. These results ultimately led him to conclude he had found evidence for Darwinian adaptation. Another set of pioneering experiments were those performed by Georges Teissier and Philippe L’Héritier in the mid-1900s using Drosophila. As described by Gayon 1998, the duo created the first population cages and maintained their populations under conditions that promoted extreme competition among larvae. By studying the response to selection in this experiment, they showed that balanced polymorphisms could exist in constant environments, and that genetic background could affect the selective value of a genotype due to frequency-dependent selection. Both of these ideas existed only in theory at that time. In a similar vein, McDonald and Ayala 1974 used populations of D. pseudoobscura to test the idea that environmental heterogeneity can foster the maintenance of genetic variation in natural populations. McDonald and Ayala did this by placing their experimental populations in treatment groups subjected to different levels of environmental heterogeneity for a little over a dozen generations. They found a positive correlation between levels of environmental variation and genetic variation. In addition to the studies just mentioned, Wright 1977 compiled results from a number of other pioneering EE studies (see chapters 7–9); these experiments involved insects and non-insect species, tests of frequency dependence, toxin resistance, and body size, to name just a few of the characters studied.

• Dallinger, W. 1887. The president’s address. Journal of the Royal Microscopical Society 10:184–199.

  Dallinger cultured populations of a unicellular organisms in an incubator over a seven-year period while gradually increasing the temperature from 60℉ to 158℉. Early cultures showed signs of reduced survival at 73℉ and were not able to survive at 158℉. His experiment eventually resulted in cultures able to survive at 158℉ but unable to grow at 60℉.

• Gayon, J. 1998. Darwinism’s struggle for survival. Cambridge, UK: Cambridge Univ. Press.

  In chapter 10, Gayon describes Drosophila selection experiments performed by Georges Teissier and Philippe L’Héritier starting in 1932. The two maintained populations in the lab with high levels of competition due to limited nutrient resources, and then characterized the response to selection. Their experiments led to discoveries about the maintenance of balanced polymorphisms and the effect of genetic background on the selective value of specific genotypes.

• McDonald, J. F., and F. J. Ayala. 1974. Genetic response to environmental heterogeneity. Nature 250:572–574.

  DOI: 10.1038/250572a0

  The authors performed a selection experiment using D. pseudoobscura to investigate how environmental heterogeneity impacts genetic variation. They maintained eighteen populations independently in cages featuring different levels of environmental heterogeneity based on manipulating food, yeast, temperature, and illumination. Levels of genetic variation were then measured and compared between populations after twelve to fifteen generations. They determined there was a positive correlation between levels of environmental variation and genetic variation.

• Wright, S. 1977. Evolution and the genetics of populations. Vol. 3, Experimental results and evolutionary deductions. Chicago: Univ. of Chicago Press.

  Chapters 7 through 9 summarize and highlight a number of pioneering studies in EE. Chapters 7 and 8 deal with experiments featuring artificial selection: chapter 7 focuses on non-insect systems, while chapter 8 focuses exclusively on experiments with insects. Chapter 9 deals with experiments that involve natural selection in the lab.