Proc Nati Acad Sci USA Vol 91 pp 6808 6814 July 1994 Colloquium Paper This paper was presented at a colloquium entitled Tempo and Mode in Evolution organized by Walter M Fitch and Francisco J Ayala held January 27 29 1994 by the National Academy of Sciences in Irvine CA Dynamics of adaptation and diversification A 10 000 generation experiment with bacterial populations experimental evolution macroevolutlon natural selection cell size Escherichia cola RICHARD E LENSKI AND MICHAEL TRAvISANO Center for Microbial Ecology Michigan State University East Lansing MI 48824 1325 profound problems of inference inherent in purely observational approaches to studying evolutionary dynamics while also highlighting the power of our particular experimental system Imagine then that you have discovered a well preserved and clearly stratified fossil bed that provides a record of evolution extending thousands of generations for the particular organism that you study You could measure the size and shape of the organisms that were preserved and perhaps deduce the rate of change in these traits But even from a near perfect record you would have great difficulty inferring the evolutionary processes selection drift mutation recombination and migration affecting these morphological traits It might be difficult even to exclude the hypothesis that any phenotypic trends reflect nonheritable changes caused by the direct effects of a changing environment on the ABSTRACT We followed evolutionary change in 12 populations of Escherichia coli propagated for 10 000 generations in identical environments Both morphology cell size and fitness measured in competition with the ancestor evolved rapidly for the first 2000 generations or so after the populations were introduced into the experimental environment but both were nearly static for the last 5000 generations Although evolving in identical environments the replicate populations diverged significantly from one another in both morphology and mean fitness The divergence in mean fitness was sustained and implies that the populations have approached different fitness peaks of unequal height in the adaptive landscape Although the experimental time scale and environment were microevolutionary in scope our experiments were designed to address questions concerning the origin as well as the fate of genetic and phenotypic novelties the repeatability of adaptation the diversification of lineages and thus the causes and consequences of the uniqueness of evolutionary history In fact we observed several hallmarks of macroevolutionary dynamics including periods of rapid evolution and stasis altered functional relationships between traits and concordance of anagenetic and cladogenetic trends Our results support a Wrightian interpretation in which chance events mutation and drit play an important role in adaptive evolution as do the complex genetic interactions that underlie the structure of organisms organism Fifty years after the publication of Simpson s Tempo and Mode in Evolution 1 evolutionary biologists are still fascinated by and struggling to understand the dynamics of adaptation and diversification especially for those traits that affect the reproductive success of individual organisms How quickly do populations change in these traits and are their rates of change constant or variable How rapidly do populations diverge from one another in these traits and are rates of adaptation and diversification tightly or loosely coupled How repeatable is evolution and how sensitive are evolutionary outcomes to a population s initial genetic state What are the relative roles of chance phylogeny and adaptation in evolution How do the answers to these questions depend on the genetic system of an organism and on the traits examined We have embarked on an experimental program to investigate these questions We believe that experiments complement historical and comparative studies and when appropriately designed may forge an important link between micro and macroevolutionary studies Before describing our experiments however we present an imaginary framework for such research This imaginary framework illustrates the But imagine that you could infer that the environment had not changed for thousands of generations so that any phenotypic trends must have resulted from underlying genetic changes Moreover you could be sure that there was no influx of genotypes from other populations and that the population was initially homogeneous so that all of the genetic variation in the fossil population must have arisen in situ You could then confidently assess the tempo and mode of morphological evolution Now imagine that you found many fossil beds all in identical environments and having the same initial genetic state You could evaluate the repeatability of evolution by examining the parallelism or divergence of the populations from one another Any repeatability or lack thereof would also bear on the success of specific hypotheses that sought to address the adaptive significance of particular phenotypic trends And the fantasy continues Imagine that you could resurrect these organisms not merely bits of fossil DNA but the entire living organisms and reconstruct their environment exactly as it was during the thousands of generations preserved in the fossil bed You could measure not only the organism s morphology but also its functional capacities and genetic composition You could even place derived and ancestral forms in competition to determine their relative fitness in the fossil environment You could assess which phenotypes promoted ecological success and you could evaluate the similarity of the adaptive solutions achieved by the replicate populations thereby disentangling the roles of chance and necessity 2 in evolutionary dynamics Still more opportunities exist in this fantastic world You could travel back in time and manipulate populations by The publication costs of this article were defrayed in part by page charge payment This article must therefore be hereby marked advertisement in accordance with 18 U S C 1734 solely to indicate this fact Present address Microbial Evolution Project The Institute of Physical and Chemical Research RIKEN Hirosawa 2 1 Wakoshi Saitama ken 351 01 Japan 6808 Colloquium Paper Lenski and Travisano altering their evolutionary history or their environment and then return to the present to examine the effect of these conditions on the dynamics of adaptation and
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