1996
Kibota T T; Lynch M
Estimate of the genomic mutation rate deleterious to overall fitness in E. coli Journal Article
Nature, vol. 381, no. 6584, pp. 694–696, 1996.
Abstract | Links | BibTeX | Altmetric | Tags: Descendant Experiments, Fitness Trajectories, Mutation Rates
@article{kibota1996,
title = {Estimate of the genomic mutation rate deleterious to overall fitness in \textit{E. coli}},
author = {Travis T. Kibota and Michael Lynch},
doi = {10.1038/381694a0},
year = {1996},
date = {1996-01-01},
urldate = {1996-01-01},
journal = {Nature},
volume = {381},
number = {6584},
pages = {694--696},
abstract = {Mutations are a double-edged sword: they are the ultimate source of genetic variation upon which evolution depends, yet most mutations affecting fitness (viability and reproductive success) appear to be harmful. Deleterious mutations of small effect can escape natural selection, and should accumulate in small population. Reduced fitness from deleterious-mutation accumulation may be important in the evolution of sex, mate choice, and diploid life-cycles, and in the extinction of small populations. Few empirical data exist, however. Minimum estimates of the genomic deleterious-mutation rate for viability in \textit{Drosophila melanogaster} are surprisingly high, leading to the conjecture that the rate for total fitness could exceed 1.0 mutation per individual per generation. Here we use \textit{Escherichia coli} to provide an estimate of the genomic deleterious-mutation rate for total fitness in a microbe. We estimate that the per-microbe rate of deleterious mutations is in excess of 0.0002.},
keywords = {Descendant Experiments, Fitness Trajectories, Mutation Rates},
pubstate = {published},
tppubtype = {article}
}
Mutations are a double-edged sword: they are the ultimate source of genetic variation upon which evolution depends, yet most mutations affecting fitness (viability and reproductive success) appear to be harmful. Deleterious mutations of small effect can escape natural selection, and should accumulate in small population. Reduced fitness from deleterious-mutation accumulation may be important in the evolution of sex, mate choice, and diploid life-cycles, and in the extinction of small populations. Few empirical data exist, however. Minimum estimates of the genomic deleterious-mutation rate for viability in Drosophila melanogaster are surprisingly high, leading to the conjecture that the rate for total fitness could exceed 1.0 mutation per individual per generation. Here we use Escherichia coli to provide an estimate of the genomic deleterious-mutation rate for total fitness in a microbe. We estimate that the per-microbe rate of deleterious mutations is in excess of 0.0002.
1995
Travisano M; Mongold J A; Bennett A F; Lenski R E
Experimental Tests of the Roles of Adaptation, Chance, and History Journal Article
Science, vol. 267, no. January, 1995.
Abstract | Links | BibTeX | Altmetric | Tags: Cell Morphology, Correlated Responses, Descendant Experiments, Fitness Trajectories, Historical Contingency, Methods and Miscellaneous, Parallelism and Divergence
@article{Travisano1995,
title = {Experimental Tests of the Roles of Adaptation, Chance, and History},
author = {Michael Travisano and Judith A. Mongold and Albert F. Bennett and Richard E. Lenski},
url = {https://www.science.org/lookup/doi/10.1126/science.7809610},
doi = {https://doi.org/10.1126/science.7809610},
year = {1995},
date = {1995-01-01},
urldate = {1995-01-01},
journal = {Science},
volume = {267},
number = {January},
abstract = {The contributions of adaptation, chance, and history to the evolution of fitness and cell size were measured in two separate experiments using bacteria. In both experiments, populations propagated in identical environments achieved similar fitnesses, regardless of prior history or subsequent chance events. In contrast, the evolution of cell size, a trait weakly correlated with fitness, was more strongly influenced by history and chance.},
keywords = {Cell Morphology, Correlated Responses, Descendant Experiments, Fitness Trajectories, Historical Contingency, Methods and Miscellaneous, Parallelism and Divergence},
pubstate = {published},
tppubtype = {article}
}
The contributions of adaptation, chance, and history to the evolution of fitness and cell size were measured in two separate experiments using bacteria. In both experiments, populations propagated in identical environments achieved similar fitnesses, regardless of prior history or subsequent chance events. In contrast, the evolution of cell size, a trait weakly correlated with fitness, was more strongly influenced by history and chance.
Johnson P A; Lenski R E; Hoppensteadt F C
Theoretical Analysis of Divergence in Mean Fitness between Initially Identical Populations Journal Article
Proceedings of the Royal Society, London, pp. 125–130, 1995.
Abstract | Links | BibTeX | Altmetric | Tags: Fitness Trajectories, Parallelism and Divergence, Theory and Simulations
@article{Johnson1995,
title = {Theoretical Analysis of Divergence in Mean Fitness between Initially Identical Populations},
author = {Paul A. Johnson and Richard E. Lenski and Frank C. Hoppensteadt},
url = {https://royalsocietypublishing.org/doi/10.1098/rspb.1995.0019},
doi = {10.1098/rspb.1995.0019},
year = {1995},
date = {1995-01-01},
urldate = {1995-01-01},
journal = {Proceedings of the Royal Society, London},
pages = {125--130},
abstract = {Initially identical populations in identical environments may subsequently diverge from one another not only via the effects of genetic drift on neutral alleles, but also by selection on beneficial alleles that arise stochastically by mutation. In the simple case of one locus with two alleles in a haploid organism, a full range of combinations of population sizes, selection pressures, mutation rates and fixation probabilities reveals two qualitatively distinct dynamics of divergence among such initially identical populations. We define a non-dimensional parameter \textit{k} that describes conditions for the occurrence of these different dynamics. One dynamic (\textit{k} > 1) occurs when beneficial mutations are sufficiently common that substitutions within the populations are essentially simultaneous; the other dynamic (\textit{k} < 1) occurs when beneficial mutations are so rare that substitutions are likely to occur as isolated events. If there are more than two alleles, or multiple loci, divergence among the populations can be sustained indefinitely if \textit{k} < 1. The parameter \textit{k} pertains to the nature of biological evolution and its tendency to be gradual or punctuated.},
keywords = {Fitness Trajectories, Parallelism and Divergence, Theory and Simulations},
pubstate = {published},
tppubtype = {article}
}
Initially identical populations in identical environments may subsequently diverge from one another not only via the effects of genetic drift on neutral alleles, but also by selection on beneficial alleles that arise stochastically by mutation. In the simple case of one locus with two alleles in a haploid organism, a full range of combinations of population sizes, selection pressures, mutation rates and fixation probabilities reveals two qualitatively distinct dynamics of divergence among such initially identical populations. We define a non-dimensional parameter k that describes conditions for the occurrence of these different dynamics. One dynamic (k > 1) occurs when beneficial mutations are sufficiently common that substitutions within the populations are essentially simultaneous; the other dynamic (k < 1) occurs when beneficial mutations are so rare that substitutions are likely to occur as isolated events. If there are more than two alleles, or multiple loci, divergence among the populations can be sustained indefinitely if k < 1. The parameter k pertains to the nature of biological evolution and its tendency to be gradual or punctuated.
Travisano M; Vasi F K; Lenski R E
Long-Term Experimental Evolution in Escherichia coli. III. Variation Among Replicate Populations in Correlated Responses to Novel Environments Journal Article
Evolution, vol. 49, no. 1, pp. 189–200, 1995.
Abstract | Links | BibTeX | Altmetric | Tags: Correlated Responses, Genotypes and Phenotypes, Parallelism and Divergence
@article{Travisano1995b,
title = {Long-Term Experimental Evolution in \textit{Escherichia coli}. III. Variation Among Replicate Populations in Correlated Responses to Novel Environments},
author = {Michael Travisano and Farida K. Vasi and Richard E. Lenski},
url = {https://onlinelibrary.wiley.com/doi/10.1111/j.1558-5646.1995.tb05970.x},
doi = {https://doi.org/10.1111/j.1558-5646.1995.tb05970.x},
year = {1995},
date = {1995-01-01},
urldate = {1995-01-01},
journal = {Evolution},
volume = {49},
number = {1},
pages = {189--200},
abstract = {Twelve populations of \textit{Escherichia coli} were founded from a single clone and propagated for 2000 gen- erations in identical glucose-limited environments. During this time, the mean fitnesses of the evolving populations relative to their common ancestor improved greatly, but their fitnesses relative to one another diverged only slightly. Although the populations showed similar fitness increases, they may have done so by different underlying adaptations, or they may have diverged in other respects by random genetic drift. Therefore, we examined the relative fitnesses of independently derived genotypes in two other sugars, maltose and lactose, to determine whether they were ho- mogeneous or heterogeneous in these environments. The genetic variation among the derived lines in fitness on maltose and lactose was more than 100-times greater than their variation in fitness on glucose. Moreover, the glucose-adapted genotypes, on average, showed significant adaptation to lactose, but not to maltose. That pathways for use of maltose and glucose are virtually identical in \textit{E. coli}, except for their distinct mechanisms of uptake, suggests that the derived genotypes have adapted primarily by improved glucose transport. From consideration of the number of generations of divergence, the mutation rate in \textit{E. coli}, and the proportion of its genome required for growth on maltose (but not glucose), we hypothesize that pleiotropy involving the selected alleles, rather than random genetic drift of alleles at other loci, was the major cause of the variation among the derived genotypes in fitness on these other sugars.},
keywords = {Correlated Responses, Genotypes and Phenotypes, Parallelism and Divergence},
pubstate = {published},
tppubtype = {article}
}
Twelve populations of Escherichia coli were founded from a single clone and propagated for 2000 gen- erations in identical glucose-limited environments. During this time, the mean fitnesses of the evolving populations relative to their common ancestor improved greatly, but their fitnesses relative to one another diverged only slightly. Although the populations showed similar fitness increases, they may have done so by different underlying adaptations, or they may have diverged in other respects by random genetic drift. Therefore, we examined the relative fitnesses of independently derived genotypes in two other sugars, maltose and lactose, to determine whether they were ho- mogeneous or heterogeneous in these environments. The genetic variation among the derived lines in fitness on maltose and lactose was more than 100-times greater than their variation in fitness on glucose. Moreover, the glucose-adapted genotypes, on average, showed significant adaptation to lactose, but not to maltose. That pathways for use of maltose and glucose are virtually identical in E. coli, except for their distinct mechanisms of uptake, suggests that the derived genotypes have adapted primarily by improved glucose transport. From consideration of the number of generations of divergence, the mutation rate in E. coli, and the proportion of its genome required for growth on maltose (but not glucose), we hypothesize that pleiotropy involving the selected alleles, rather than random genetic drift of alleles at other loci, was the major cause of the variation among the derived genotypes in fitness on these other sugars.
1994
Vasi F K; Travisano M; Lenski R E
Long-Term Experimental Evolution in Escherichia coli. II. Changes in Life-History Traits During Adaptation to a Seasonal Environment Journal Article
The American Naturalist, vol. 144, no. 3, pp. 432–456, 1994, ISSN: 0003-0147.
Abstract | Links | BibTeX | Altmetric | Tags: Demography and Ecology, Parallelism and Divergence
@article{Vasi1994,
title = {Long-Term Experimental Evolution in \textit{Escherichia coli}. II. Changes in Life-History Traits During Adaptation to a Seasonal Environment},
author = {Farida K. Vasi and Michael Travisano and Richard E. Lenski},
url = {https://www.journals.uchicago.edu/doi/10.1086/285685},
doi = {10.1086/285685},
issn = {0003-0147},
year = {1994},
date = {1994-09-01},
urldate = {1994-09-01},
journal = {The American Naturalist},
volume = {144},
number = {3},
pages = {432--456},
abstract = {Twelve populations of the bacterium \textit{Escherichia coli} were propagated for 2,000 generations in a seasonal environment, which consisted of alternating periods of feast and famine. The mean fitness of the derived genotypes increased by ∼35% relative to their common ancestor, based on competition experiments in the same environment. The bacteria could have adapted, in principle, by decreasing their lag prior to growth upon transfer to fresh medium (L), increasing their maximum growth rate (V_{m}), reducing the concentration of resource required to support growth at half the maximum rate (K_{s}), and reducing their death rate after the limiting resource was exhausted (D). We estimated these parameters for the ancestor and then calculated the opportunity for selection on each parameter. The inferred selection gradients for V_{m} and L were much steeper than for K_{s} and D. The derived genotypes showed significant improvement in V_{m} and L but not in K_{s} or D. Also, the numerical yield in pure culture of the derived genotypes was significantly lower than the yield of the common ancestor, but the average cell size was much larger. The independently derived genotypes are somewhat more variable in these life-history traits than in their relative fitnesses, which indicates that they acquired different genetic adaptations to the seasonal environment. Nonetheless, the evolutionary changes in life-history traits exhibit substantial parallelism among the replicate populations.},
keywords = {Demography and Ecology, Parallelism and Divergence},
pubstate = {published},
tppubtype = {article}
}
Twelve populations of the bacterium Escherichia coli were propagated for 2,000 generations in a seasonal environment, which consisted of alternating periods of feast and famine. The mean fitness of the derived genotypes increased by ∼35% relative to their common ancestor, based on competition experiments in the same environment. The bacteria could have adapted, in principle, by decreasing their lag prior to growth upon transfer to fresh medium (L), increasing their maximum growth rate (Vm), reducing the concentration of resource required to support growth at half the maximum rate (Ks), and reducing their death rate after the limiting resource was exhausted (D). We estimated these parameters for the ancestor and then calculated the opportunity for selection on each parameter. The inferred selection gradients for Vm and L were much steeper than for Ks and D. The derived genotypes showed significant improvement in Vm and L but not in Ks or D. Also, the numerical yield in pure culture of the derived genotypes was significantly lower than the yield of the common ancestor, but the average cell size was much larger. The independently derived genotypes are somewhat more variable in these life-history traits than in their relative fitnesses, which indicates that they acquired different genetic adaptations to the seasonal environment. Nonetheless, the evolutionary changes in life-history traits exhibit substantial parallelism among the replicate populations.
Leroi A M; Lenski R E; Bennett A F
Evolutionary Adaptation to Temperature. III. Adaptation of Escherichia coli to a Temporally Varying Environment Journal Article
Evolution, vol. 48, no. 4, pp. 1222, 1994, ISSN: 00143820.
Abstract | Links | BibTeX | Altmetric | Tags: Descendant Experiments
@article{Leroi1994b,
title = {Evolutionary Adaptation to Temperature. III. Adaptation of \textit{Escherichia coli} to a Temporally Varying Environment},
author = {Armand M. Leroi and Richard E. Lenski and Albert F. Bennett},
url = {https://www.jstor.org/stable/2410380?origin=crossref},
doi = {10.2307/2410380},
issn = {00143820},
year = {1994},
date = {1994-08-01},
urldate = {1994-08-01},
journal = {Evolution},
volume = {48},
number = {4},
pages = {1222},
abstract = {Six lines of the bacterium \textit{Escherichia coli} were propagated for 2,000 generations in a temporally varying environment. The imposed environmental regime consisted of alternating days at 32°C and 42°C, with rapid transitions between them. These derived lines are competitively superior to their ancestor in this variable temperature regime. We also measured changes in the fitness of these lines, relative to their common ancestor, in both the constant (32°C and 42°C) and transition (from 32°C to 42°C and from 42°C to 32°C) components of this temporally varying environment, to determine whether the bacteria had adapted to the particular constant temperatures or the transitions between them, or both. The experimentally evolved lines had significantly improved fitness in each of the constant environmental components (32°C and 42°C). However, the experimental lines had not improved in making the sudden temperature transitions that were a potentially important aspect of the temporally variable environment. In fact, fitness in making at least one of the transitions (between 32°C and 42°C) unexpectedly decreased. This reduced adaptation to the abrupt transitions between these temperatures is probably a pleiotropic effect of mutations that were responsible for the increased fitness at the component temperatures. Among the six experimental lines, significant heterogeneity occurred in their adaptation to the constant and transition components of the variable environment.},
keywords = {Descendant Experiments},
pubstate = {published},
tppubtype = {article}
}
Six lines of the bacterium Escherichia coli were propagated for 2,000 generations in a temporally varying environment. The imposed environmental regime consisted of alternating days at 32°C and 42°C, with rapid transitions between them. These derived lines are competitively superior to their ancestor in this variable temperature regime. We also measured changes in the fitness of these lines, relative to their common ancestor, in both the constant (32°C and 42°C) and transition (from 32°C to 42°C and from 42°C to 32°C) components of this temporally varying environment, to determine whether the bacteria had adapted to the particular constant temperatures or the transitions between them, or both. The experimentally evolved lines had significantly improved fitness in each of the constant environmental components (32°C and 42°C). However, the experimental lines had not improved in making the sudden temperature transitions that were a potentially important aspect of the temporally variable environment. In fact, fitness in making at least one of the transitions (between 32°C and 42°C) unexpectedly decreased. This reduced adaptation to the abrupt transitions between these temperatures is probably a pleiotropic effect of mutations that were responsible for the increased fitness at the component temperatures. Among the six experimental lines, significant heterogeneity occurred in their adaptation to the constant and transition components of the variable environment.
Lenski R E; Travisano M
Dynamics of adaptation and diversification: a 10,000-generation experiment with bacterial populations. Journal Article
Proceedings of the National Academy of Sciences of the United States of America, vol. 91, no. 15, pp. 6808–6814, 1994, ISSN: 0027-8424.
Abstract | Links | BibTeX | Altmetric | Tags: Cell Morphology, Fitness Trajectories, Parallelism and Divergence
@article{Lenski1994,
title = {Dynamics of adaptation and diversification: a 10,000-generation experiment with bacterial populations.},
author = {Richard E. Lenski and Michael Travisano},
url = {http://www.pnas.org/cgi/doi/10.1073/pnas.91.15.6808},
doi = {10.1073/pnas.91.15.6808},
issn = {0027-8424},
year = {1994},
date = {1994-07-01},
urldate = {1994-07-01},
journal = {Proceedings of the National Academy of Sciences of the United States of America},
volume = {91},
number = {15},
pages = {6808--6814},
abstract = {We followed evolutionary change in 12 populations of \textit{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 drift) play an important role in adaptive evolution, as do the complex genetic interactions that underlie the structure of organisms.},
keywords = {Cell Morphology, Fitness Trajectories, Parallelism and Divergence},
pubstate = {published},
tppubtype = {article}
}
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 drift) play an important role in adaptive evolution, as do the complex genetic interactions that underlie the structure of organisms.
Leroi A M; Bennett A F; Lenski R E
Temperature acclimation and competitive fitness: an experimental test of the beneficial acclimation assumption. Journal Article
Proceedings of the National Academy of Sciences of the United States of America, vol. 91, no. 5, pp. 1917–1921, 1994, ISSN: 0027-8424.
Abstract | Links | BibTeX | Altmetric | Tags: Descendant Experiments
@article{Leroi1994,
title = {Temperature acclimation and competitive fitness: an experimental test of the beneficial acclimation assumption.},
author = {Armand M. Leroi and Albert F. Bennett and Richard E. Lenski},
url = {http://www.pnas.org/cgi/doi/10.1073/pnas.91.5.1917},
doi = {10.1073/pnas.91.5.1917},
issn = {0027-8424},
year = {1994},
date = {1994-03-01},
urldate = {1994-03-01},
journal = {Proceedings of the National Academy of Sciences of the United States of America},
volume = {91},
number = {5},
pages = {1917--1921},
abstract = {Phenotypic acclimation is generally assumed to confer an advantage in the environment that stimulates the response. To test this beneficial acclimation assumption explicitly, we investigated the consequences of temperature acclimation for the fitness of \textit{Escherichia coli} at two temperatures, 32°C and 41.5°C. Both temperatures permit growth and long-term persistence of the genotypes in serial culture. We found that prior acclimation to 32°C, relative to acclimation to 41.5°C, enhanced fitness at 32°C, consistent with the assumption. But prior acclimation to 41.5°C actually reduced fitness at 41.5°C, relative to acclimation to 32°C. Hence, the assumption that acclimation always confers an advantage is demonstrated to be false. Acclimation to 41.5°C did, however, improve survival at 50°C, a lethal temperature. This protective response has been shown to be associated with the induction of stress proteins. The reduced competitive fitness caused by acclimation at 41.5°C may reflect a physiological burden associated with expression of stress proteins when they are not needed to prevent lethal damage. Whatever the cause, acclimation to the higher temperature decreased competitive fitness at that temperature.},
keywords = {Descendant Experiments},
pubstate = {published},
tppubtype = {article}
}
Phenotypic acclimation is generally assumed to confer an advantage in the environment that stimulates the response. To test this beneficial acclimation assumption explicitly, we investigated the consequences of temperature acclimation for the fitness of Escherichia coli at two temperatures, 32°C and 41.5°C. Both temperatures permit growth and long-term persistence of the genotypes in serial culture. We found that prior acclimation to 32°C, relative to acclimation to 41.5°C, enhanced fitness at 32°C, consistent with the assumption. But prior acclimation to 41.5°C actually reduced fitness at 41.5°C, relative to acclimation to 32°C. Hence, the assumption that acclimation always confers an advantage is demonstrated to be false. Acclimation to 41.5°C did, however, improve survival at 50°C, a lethal temperature. This protective response has been shown to be associated with the induction of stress proteins. The reduced competitive fitness caused by acclimation at 41.5°C may reflect a physiological burden associated with expression of stress proteins when they are not needed to prevent lethal damage. Whatever the cause, acclimation to the higher temperature decreased competitive fitness at that temperature.
1993
Lenski R E; Bennett A F
Evolutionary Response of Escherichia coli to Thermal Stress Journal Article
The American Naturalist, vol. 142, pp. S47–S64, 1993, ISSN: 0003-0147.
Abstract | Links | BibTeX | Altmetric | Tags: Descendant Experiments
@article{Lenski1993,
title = {Evolutionary Response of \textit{Escherichia coli} to Thermal Stress},
author = {Richard E. Lenski and Albert F. Bennett},
url = {https://www.journals.uchicago.edu/doi/10.1086/285522},
doi = {10.1086/285522},
issn = {0003-0147},
year = {1993},
date = {1993-07-01},
urldate = {1993-07-01},
journal = {The American Naturalist},
volume = {142},
pages = {S47--S64},
abstract = {We used a clone of the bacterium \textit{Escherichia coli} previously adapted to 37°C to found replicate populations propagated at constant 32°C, constant 37°C, constant 42°C, and a daily alternation between 32° and 42°C. Several criteria indicate that 42° was stressful for the ancestor, while 32° and 37°C were not. For example, 42°C was within 1°C of the limit for extinction, and yield was substantially reduced at this temperature. Adaptation was assayed by competing derived genotypes against their common ancestor at various temperatures. Bacteria adapted much more rapidly to 42°C than to either lower temperature. Also, bacteria propagated in the alternating environment exhibited greater adaptation to 42°C than to 32°C. Adaptation was temperature-specific in all groups, but adaptation to 42°C entailed little loss of fitness at lower temperatures. Nor did adaptation to 42°C much extend the upper limit for population persistence, although we isolated more thermotolerant mutants by imposing hard selection. Thus, whereas the stressful 42°C environment consistently led to more rapid adaptive evolution than did nonstressful regimes, superstressful temperatures caused either extremely rapid adaptive evolution or extinction. Although defining stress in general terms is difficult, one can evaluate specific criteria and test evolutionary hypotheses using appropriate experimental systems.},
keywords = {Descendant Experiments},
pubstate = {published},
tppubtype = {article}
}
We used a clone of the bacterium Escherichia coli previously adapted to 37°C to found replicate populations propagated at constant 32°C, constant 37°C, constant 42°C, and a daily alternation between 32° and 42°C. Several criteria indicate that 42° was stressful for the ancestor, while 32° and 37°C were not. For example, 42°C was within 1°C of the limit for extinction, and yield was substantially reduced at this temperature. Adaptation was assayed by competing derived genotypes against their common ancestor at various temperatures. Bacteria adapted much more rapidly to 42°C than to either lower temperature. Also, bacteria propagated in the alternating environment exhibited greater adaptation to 42°C than to 32°C. Adaptation was temperature-specific in all groups, but adaptation to 42°C entailed little loss of fitness at lower temperatures. Nor did adaptation to 42°C much extend the upper limit for population persistence, although we isolated more thermotolerant mutants by imposing hard selection. Thus, whereas the stressful 42°C environment consistently led to more rapid adaptive evolution than did nonstressful regimes, superstressful temperatures caused either extremely rapid adaptive evolution or extinction. Although defining stress in general terms is difficult, one can evaluate specific criteria and test evolutionary hypotheses using appropriate experimental systems.
Bennett A F; Lenski R E
Evolutionary Adaptation to Temperature II. Thermal Niches of Experimental Lines of Escherichia coli Journal Article
Evolution, vol. 47, no. 1, pp. 1–12, 1993, ISSN: 0014-3820.
Abstract | Links | BibTeX | Altmetric | Tags: Descendant Experiments
@article{Bennett1993,
title = {Evolutionary Adaptation to Temperature II. Thermal Niches of Experimental Lines of \textit{Escherichia coli}},
author = {Albert F. Bennett and Richard E. Lenski},
url = {https://onlinelibrary.wiley.com/doi/10.1111/j.1558-5646.1993.tb01194.x},
doi = {10.1111/j.1558-5646.1993.tb01194.x},
issn = {0014-3820},
year = {1993},
date = {1993-02-01},
urldate = {1993-02-01},
journal = {Evolution},
volume = {47},
number = {1},
pages = {1--12},
abstract = {Groups of replicated lines of the bacterium \textit{Escherichia coli} were propagated for 2,000 generations at constant 32, 37, or 42°C, or in an environment that alternated between 32 and 42°C. Here, the authors examine the performance of each group across a temperature range of 12-44°C measuring the temperatures over which each line can maintain itself in serial dilution culture (the thermal niche). Thermal niche was not affected by selection history: average lower and upper limits remained about 19 and 42°C for all groups. No significant differences among groups were observed in rate of extinction at more extreme temperatures. Increases in mean fitness were temperature specific, with the largest increase for each group occurring near its selected temperature. Thus, the temperature at which mean fitness relative to the ancestor was greatest (the thermal optimum) diverged by 10°C for the groups selected at constant 32°C versus constant 42°C. Tradeoffs in relative fitness (decrements relative to the ancestor elsewhere within the thermal niche) did not necessarily accompany fitness improvements.},
keywords = {Descendant Experiments},
pubstate = {published},
tppubtype = {article}
}
Groups of replicated lines of the bacterium Escherichia coli were propagated for 2,000 generations at constant 32, 37, or 42°C, or in an environment that alternated between 32 and 42°C. Here, the authors examine the performance of each group across a temperature range of 12-44°C measuring the temperatures over which each line can maintain itself in serial dilution culture (the thermal niche). Thermal niche was not affected by selection history: average lower and upper limits remained about 19 and 42°C for all groups. No significant differences among groups were observed in rate of extinction at more extreme temperatures. Increases in mean fitness were temperature specific, with the largest increase for each group occurring near its selected temperature. Thus, the temperature at which mean fitness relative to the ancestor was greatest (the thermal optimum) diverged by 10°C for the groups selected at constant 32°C versus constant 42°C. Tradeoffs in relative fitness (decrements relative to the ancestor elsewhere within the thermal niche) did not necessarily accompany fitness improvements.
1992
Bennett A F; Lenski R E; Mittler J E
Evolutionary Adaptation to Temperature. I. Fitness Responses of Escherichia coli to Changes in its Thermal Environment Journal Article
Evolution, vol. 46, no. 1, pp. 16-30, 1992, ISSN: 0014-3820.
Abstract | Links | BibTeX | Altmetric | Tags: Descendant Experiments
@article{nokey,
title = {Evolutionary Adaptation to Temperature. I. Fitness Responses of \textit{Escherichia coli} to Changes in its Thermal Environment},
author = {Albert F. Bennett and Richard E. Lenski and John E. Mittler},
url = {https://onlinelibrary.wiley.com/doi/10.1111/j.1558-5646.1992.tb01981.x},
doi = {10.1111/j.1558-5646.1992.tb01981.x},
issn = {0014-3820},
year = {1992},
date = {1992-02-01},
urldate = {1992-02-01},
journal = {Evolution},
volume = {46},
number = {1},
pages = {16-30},
abstract = {Replicate lines of \textit{Escherichia coli} were propagated for 2,000 generations in four different thermal regimes: constant 32, 37, or 42°C (thermal specialists), or a daily alternation between 32 and 42°C (32/42°C: thermal generalists). The ancestor had previously been propagated at 37°C for 2,000 generations. All experimental groups showed improved relative fitness in their own thermal environment (direct response of fitness), but rates of fitness improvement varied greatly among temperature groups. The 42°C group responded most rapidly and extensively, followed by the 32 and 32/42°C groups, whose fitness improvements were indistinguishable. The 37°C group, which experienced the ancestral temperature, had the slowest and least extensive fitness improvement.},
keywords = {Descendant Experiments},
pubstate = {published},
tppubtype = {article}
}
Replicate lines of Escherichia coli were propagated for 2,000 generations in four different thermal regimes: constant 32, 37, or 42°C (thermal specialists), or a daily alternation between 32 and 42°C (32/42°C: thermal generalists). The ancestor had previously been propagated at 37°C for 2,000 generations. All experimental groups showed improved relative fitness in their own thermal environment (direct response of fitness), but rates of fitness improvement varied greatly among temperature groups. The 42°C group responded most rapidly and extensively, followed by the 32 and 32/42°C groups, whose fitness improvements were indistinguishable. The 37°C group, which experienced the ancestral temperature, had the slowest and least extensive fitness improvement.
1991
Lenski R E; Rose M R; Simpson S C; Tadler S C
Long-term experimental evolution in Escherichia coli. I. Adaptation and divergence during 2,000 generations Journal Article
Am. Nat., vol. 138, no. 6, pp. 1315–1341, 1991.
Abstract | Links | BibTeX | Altmetric | Tags: Fitness Trajectories, Parallelism and Divergence
@article{Lenski1991,
title = {Long-term experimental evolution in \textit{Escherichia coli}. I. Adaptation and divergence during 2,000 generations},
author = {Richard E. Lenski and M R. Rose and S C. Simpson and S C. Tadler},
url = {https://www.jstor.org/stable/2462549},
doi = {https://doi.org/10.1086/285289},
year = {1991},
date = {1991-01-01},
urldate = {1991-01-01},
journal = {Am. Nat.},
volume = {138},
number = {6},
pages = {1315--1341},
publisher = {JSTOR},
abstract = {We assess the degree to which adaptation to a uniform environment among independently evolving asexual populations is associated with increasing divergence of those populations. In addition, we are concerned with the pattern of adaptation itself, particularly whether the rate of increase in mean fitness tends to decline with the number of generations of selection in a constant environment. The correspondence between the rate of increase in mean fitness and the within-population genetic variance of fitness, as expected from Fisher's fundamental theorem, is also addressed. Twelve \textit{Escherichia coli} populations were founded from a single clonal ancestor and allowed to evolve for 2,000 generations. Mean fitness increased by about 37%. However, the rate of increase in mean fitness was slower in later generations. There was no statistically significant within-population genetic variance of fitness, but there was significant between-population variance. Although the estimated genetic variation in fitness within populations was not statistically significant, it was consistent in magnitude with theoretical expectations. Similarly, the variance of mean fitness between populations was consistent with a model that incorporated stochastic variation in the timing and order of substitutions at a finite number of nonepistatic loci, coupled with substitutional delays and interference between substitutions arising from clonality. These results, taken as a whole, are consistent with theoretical expectations that do not invoke divergence due to multiple fitness peaks in a Wrightian evolutionary landscape.},
keywords = {Fitness Trajectories, Parallelism and Divergence},
pubstate = {published},
tppubtype = {article}
}
We assess the degree to which adaptation to a uniform environment among independently evolving asexual populations is associated with increasing divergence of those populations. In addition, we are concerned with the pattern of adaptation itself, particularly whether the rate of increase in mean fitness tends to decline with the number of generations of selection in a constant environment. The correspondence between the rate of increase in mean fitness and the within-population genetic variance of fitness, as expected from Fisher's fundamental theorem, is also addressed. Twelve Escherichia coli populations were founded from a single clonal ancestor and allowed to evolve for 2,000 generations. Mean fitness increased by about 37%. However, the rate of increase in mean fitness was slower in later generations. There was no statistically significant within-population genetic variance of fitness, but there was significant between-population variance. Although the estimated genetic variation in fitness within populations was not statistically significant, it was consistent in magnitude with theoretical expectations. Similarly, the variance of mean fitness between populations was consistent with a model that incorporated stochastic variation in the timing and order of substitutions at a finite number of nonepistatic loci, coupled with substitutional delays and interference between substitutions arising from clonality. These results, taken as a whole, are consistent with theoretical expectations that do not invoke divergence due to multiple fitness peaks in a Wrightian evolutionary landscape.
