1997
Bennett A F; Lenski R E
Evolutionary adaptation to temperature. VI. Phenotypic acclimation and its evolution in Escherichia coli Journal Article
Evolution, 51 (1), pp. 36–44, 1997, ISSN: 0014-3820.
Abstract | Links | BibTeX | Altmetric | Tags: Descendant Experiments
@article{Bennett1997,
title = {Evolutionary adaptation to temperature. VI. Phenotypic acclimation and its evolution in \textit{Escherichia coli}},
author = {Albert F. Bennett and Richard E. Lenski},
url = {https://onlinelibrary.wiley.com/doi/10.1111/j.1558-5646.1997.tb02386.x},
doi = {10.1111/j.1558-5646.1997.tb02386.x},
issn = {0014-3820},
year = {1997},
date = {1997-02-01},
urldate = {1997-02-01},
journal = {Evolution},
volume = {51},
number = {1},
pages = {36--44},
abstract = {Acclimation refers to reversible, nongenetic changes in phenotype that are induced by specific environmental conditions. Acclimation is generally assumed to improve function in the environment that induces it (the beneficial acclimation hypothesis). In this study, we experimentally tested this assumption by measuring relative fitness of the bacterium \textit{Escherichia coli} acclimated to different thermal environments. The beneficial acclimation hypothesis predicts that bacteria acclimated to the temperature of competition should have greater fitness than do bacteria acclimated to any other temperature. The benefit predicted by the hypothesis was found in only seven of 12 comparisons; in the other comparisons, either no statistically demonstrable benefit was observed or a detrimental effect of acclimation was demonstrated. For example, in a lineage evolutionarily adapted to 37°C, bacteria acclimated to 37°C have a higher fitness at 32°C than do bacteria acclimated to 32°C, a result exactly contrary to prediction; acclimation to 27°C or 40°C prior to competition at those temperatures confers no benefit over 37°C acclimated forms. Consequently, the beneficial acclimation hypothesis must be rejected as a general prediction of the inevitable result of phenotypic adjustments associated with new environments. However, the hypothesis is supported in many instances when the acclimation and competition temperatures coincide with the historical temperature at which the bacterial populations have evolved. For example, when the evolutionary temperature of the population was 37°C, bacteria acclimated to 37°C had superior fitness at 37°C to those acclimated to 32°C; similarly, bacteria evolutionarily adapted to 32°C had a higher fitness during competition at 32°C than they did when acclimated to 37°C. The more surprising results are that when the bacteria are acclimated to their historical evolutionary temperature, they are frequently competitively superior even at other temperatures. For example, bacteria that have evolved at either 20°C or 32°C and are acclimated to their respective evolutionary temperatures have a greater fitness at 37°C than when they are acclimated to 37°C. Thus, acclimation to evolutionary temperature may, as a correlated consequence, enhance performance not only in the evolutionary environment, but also in a variety of other thermal environments.},
keywords = {Descendant Experiments},
pubstate = {published},
tppubtype = {article}
}
Acclimation refers to reversible, nongenetic changes in phenotype that are induced by specific environmental conditions. Acclimation is generally assumed to improve function in the environment that induces it (the beneficial acclimation hypothesis). In this study, we experimentally tested this assumption by measuring relative fitness of the bacterium Escherichia coli acclimated to different thermal environments. The beneficial acclimation hypothesis predicts that bacteria acclimated to the temperature of competition should have greater fitness than do bacteria acclimated to any other temperature. The benefit predicted by the hypothesis was found in only seven of 12 comparisons; in the other comparisons, either no statistically demonstrable benefit was observed or a detrimental effect of acclimation was demonstrated. For example, in a lineage evolutionarily adapted to 37°C, bacteria acclimated to 37°C have a higher fitness at 32°C than do bacteria acclimated to 32°C, a result exactly contrary to prediction; acclimation to 27°C or 40°C prior to competition at those temperatures confers no benefit over 37°C acclimated forms. Consequently, the beneficial acclimation hypothesis must be rejected as a general prediction of the inevitable result of phenotypic adjustments associated with new environments. However, the hypothesis is supported in many instances when the acclimation and competition temperatures coincide with the historical temperature at which the bacterial populations have evolved. For example, when the evolutionary temperature of the population was 37°C, bacteria acclimated to 37°C had superior fitness at 37°C to those acclimated to 32°C; similarly, bacteria evolutionarily adapted to 32°C had a higher fitness during competition at 32°C than they did when acclimated to 37°C. The more surprising results are that when the bacteria are acclimated to their historical evolutionary temperature, they are frequently competitively superior even at other temperatures. For example, bacteria that have evolved at either 20°C or 32°C and are acclimated to their respective evolutionary temperatures have a greater fitness at 37°C than when they are acclimated to 37°C. Thus, acclimation to evolutionary temperature may, as a correlated consequence, enhance performance not only in the evolutionary environment, but also in a variety of other thermal environments.
Bennett A F; Lenski R E
Phenotypic and evolutionary adaptation of a model bacterial system to stressful thermal environments Book Chapter
Bijlsma, R.; Loeschcke, V. (Ed.): Environmental Stress, Adaptation and Evolution, pp. 135–154, Birkhäuser Basel, Basel, 1997, ISBN: 978-3-0348-8882-0.
Abstract | Links | BibTeX | Altmetric | Tags: Descendant Experiments
@inbook{Bennett1997b,
title = {Phenotypic and evolutionary adaptation of a model bacterial system to stressful thermal environments},
author = {Albert F. Bennett and Richard E. Lenski},
editor = {R. Bijlsma and V. Loeschcke},
url = {https://doi.org/10.1007/978-3-0348-8882-0_8
http://link.springer.com/10.1007/978-3-0348-8882-0_8},
doi = {10.1007/978-3-0348-8882-0_8},
isbn = {978-3-0348-8882-0},
year = {1997},
date = {1997-01-01},
urldate = {1997-01-01},
booktitle = {Environmental Stress, Adaptation and Evolution},
pages = {135--154},
publisher = {Birkhäuser Basel},
address = {Basel},
abstract = {We studied both phenotypic and evolutionary adaptation to various thermal environments using the bacterium \textit{Escherichia coli} as an experimental model system. We determined that 42°C was stressful to a bacterial clone adapted to 37°C, based on reductions in both absolute and competitive fitness, as well as induction of a heat stress response. This clone was also used to found replicated populations that were propagated for thousands of generations under several different thermal regimes, including 42°C. Evolutionary adaptation of the populations to 42°C resulted in an increase in both absolute and relative fitness at that temperature, measured respectively as an increase in the number of descendants (and their biovolume) and in competitive ability relative to the ancestral clone. The replicated experimental lineages achieved their evolutionary improvement by several distinct pathways, which produced differential preadaptation to a non-stressful nutrient environment. Adaptation to this stressful temperature entailed neither a change in the ancestral thermal niche nor any pronounced tradeoffs in fitness within the thermal niche, contrary to \textit{a priori} predictions. This study system has several important advantages for evaluating hypotheses concerning the effects of stress on phenotypic and evolutionary adaptation, including the ability to obtain lineages that have evolved in controlled and defined environments, to make direct measurements of fitness and to quantify the degree of stress imposed by different environments.},
keywords = {Descendant Experiments},
pubstate = {published},
tppubtype = {inbook}
}
We studied both phenotypic and evolutionary adaptation to various thermal environments using the bacterium Escherichia coli as an experimental model system. We determined that 42°C was stressful to a bacterial clone adapted to 37°C, based on reductions in both absolute and competitive fitness, as well as induction of a heat stress response. This clone was also used to found replicated populations that were propagated for thousands of generations under several different thermal regimes, including 42°C. Evolutionary adaptation of the populations to 42°C resulted in an increase in both absolute and relative fitness at that temperature, measured respectively as an increase in the number of descendants (and their biovolume) and in competitive ability relative to the ancestral clone. The replicated experimental lineages achieved their evolutionary improvement by several distinct pathways, which produced differential preadaptation to a non-stressful nutrient environment. Adaptation to this stressful temperature entailed neither a change in the ancestral thermal niche nor any pronounced tradeoffs in fitness within the thermal niche, contrary to a priori predictions. This study system has several important advantages for evaluating hypotheses concerning the effects of stress on phenotypic and evolutionary adaptation, including the ability to obtain lineages that have evolved in controlled and defined environments, to make direct measurements of fitness and to quantify the degree of stress imposed by different environments.
Souza V; Turner P E; Lenski R E
Long-term experimental evolution in Escherichia coli. V. Effects of recombination with immigrant genotypes on the rate of bacterial evolution Journal Article
Journal of Evolutionary Biology, 10 (5), pp. 743, 1997, ISSN: 1010061X.
Abstract | Links | BibTeX | Altmetric | Tags: Descendant Experiments
@article{Souza1997,
title = {Long-term experimental evolution in \textit{Escherichia coli}. V. Effects of recombination with immigrant genotypes on the rate of bacterial evolution},
author = {Valeria Souza and Paul E. Turner and Richard E. Lenski},
url = {https://turnerlab.yale.edu/sites/default/files/souza_etal_1997.pdf
https://onlinelibrary.wiley.com/doi/abs/10.1046/j.1420-9101.1997.10050743.x},
doi = {https://doi.org/10.1046/j.1420-9101.1997.10050743.x},
issn = {1010061X},
year = {1997},
date = {1997-01-01},
urldate = {1997-01-01},
journal = {Journal of Evolutionary Biology},
volume = {10},
number = {5},
pages = {743},
abstract = {This study builds upon an earlier experiment that examined the dynamics of mean fitness in evolving populations of \textit{Escherichia coli} in which mutations were the sole source of genetic variation. During thousands of generations in a constant environment, the rate of improvement in mean fitness of these asexual populations slowed considerably from an initially rapid pace. In this study, we sought to determine whether sexual recombination with novel genotypes would reaccelerate the rate of adaption in these populations. To that end, treatment populations were propagated for an additional 1000 generations in the same environment as their ancestors, but they were periodically allowed to mate with an immigrant pool of genetically distinct Hfr (high frequency recombination) donors. These donors could transfer genes to the resident populations by conjugation, but the donors themselves could not grow in the experimental environment. Control populations were propagated under identical conditions, but in the absence of sexual recombination with the donors. All twelve control populations retained the ancestral alleles at every locus that was scored. In contrast, the sexual recombination treatment yielded dramatic increases in genetic variation. Thus, there was a profound effect of recombination on the rate of genetic change. However, the increased genetic variation in the treatment populations had no significant effect on the rate of adaptive evolution, as measured by changes in mean fitness relative to a common competitor. We then considered three hypotheses that might reconcile these two outcomes: recombination pressure, hitchhiking of recombinant genotypes in association with beneficial mutations, and complex selection dynamics whereby certain genotypes may have a selective advantage only within a particular milieu of competitors. The estimated recombination rate was too low to explain the observed rate of genetic change, either alone or in combination with hitchhiking effects. However, we documented complex ecological interactions among some recombinant genotypes, suggesting that our method for estimating fitness relative to a common competitor might have underestimated the rate of adaptive evolution in the treatment populations.},
keywords = {Descendant Experiments},
pubstate = {published},
tppubtype = {article}
}
This study builds upon an earlier experiment that examined the dynamics of mean fitness in evolving populations of Escherichia coli in which mutations were the sole source of genetic variation. During thousands of generations in a constant environment, the rate of improvement in mean fitness of these asexual populations slowed considerably from an initially rapid pace. In this study, we sought to determine whether sexual recombination with novel genotypes would reaccelerate the rate of adaption in these populations. To that end, treatment populations were propagated for an additional 1000 generations in the same environment as their ancestors, but they were periodically allowed to mate with an immigrant pool of genetically distinct Hfr (high frequency recombination) donors. These donors could transfer genes to the resident populations by conjugation, but the donors themselves could not grow in the experimental environment. Control populations were propagated under identical conditions, but in the absence of sexual recombination with the donors. All twelve control populations retained the ancestral alleles at every locus that was scored. In contrast, the sexual recombination treatment yielded dramatic increases in genetic variation. Thus, there was a profound effect of recombination on the rate of genetic change. However, the increased genetic variation in the treatment populations had no significant effect on the rate of adaptive evolution, as measured by changes in mean fitness relative to a common competitor. We then considered three hypotheses that might reconcile these two outcomes: recombination pressure, hitchhiking of recombinant genotypes in association with beneficial mutations, and complex selection dynamics whereby certain genotypes may have a selective advantage only within a particular milieu of competitors. The estimated recombination rate was too low to explain the observed rate of genetic change, either alone or in combination with hitchhiking effects. However, we documented complex ecological interactions among some recombinant genotypes, suggesting that our method for estimating fitness relative to a common competitor might have underestimated the rate of adaptive evolution in the treatment populations.
1996
Turner P E; Souza V; Lenski R E
Tests of Ecological Mechanisms Promoting the Stable Coexistence of Two Bacterial Genotypes Journal Article
Ecology, 77 (7), pp. 2119–2129, 1996, ISSN: 00129658.
Abstract | Links | BibTeX | Altmetric | Tags: Descendant Experiments
@article{Turner1996,
title = {Tests of Ecological Mechanisms Promoting the Stable Coexistence of Two Bacterial Genotypes},
author = {Paul E. Turner and Valeria Souza and Richard E. Lenski},
url = {http://doi.wiley.com/10.2307/2265706},
doi = {10.2307/2265706},
issn = {00129658},
year = {1996},
date = {1996-10-01},
urldate = {1996-10-01},
journal = {Ecology},
volume = {77},
number = {7},
pages = {2119--2129},
abstract = {A series of competition experiments with two genotypes of \textit{Escherichia coli} showed that each genotype was favored when it was the minority, allowing their coexistence in a stable polymorphism. In these experiments, glucose was the sole source of carbon provided, and its concentration was limiting to population density. Thus, the stable polymorphism did not conform to a simple model of competitive exclusion. In similar experiments also with glucose as the sole resource, we considered two hypotheses that might explain the observed coexistence: (1) a strictly demographic trade—off, such that one genotype is competitively superior when glucose is abundant whereas the other genotype is the better competitor for sparse glucose; and (2) a cross—feeding interaction, whereby the superior competitor for glucose excretes a metabolite that acts as a second resource for which the other genotype is the better competitor. Although there was a demographic tradeoff, the advantage to the superior competitor at high glucose concentrations was too large (given the initial concentration of glucose used in these experiments) to allow the second genotype to invade when rare at the observed rate. Therefore, the second genotype must have had some other advantage that allowed it to readily invade a population of the superior competitor for glucose. Indeed, the second genotype could increase in abundance after glucose was depleted, but only in the presence of the superior competitor for glucose, thus implicating a cross—feeding interaction. These results confirmed earlier studies showing that populations of \textit{E. coli} can maintain ecologically relevant genetic diversity even in a simple environment.},
keywords = {Descendant Experiments},
pubstate = {published},
tppubtype = {article}
}
A series of competition experiments with two genotypes of Escherichia coli showed that each genotype was favored when it was the minority, allowing their coexistence in a stable polymorphism. In these experiments, glucose was the sole source of carbon provided, and its concentration was limiting to population density. Thus, the stable polymorphism did not conform to a simple model of competitive exclusion. In similar experiments also with glucose as the sole resource, we considered two hypotheses that might explain the observed coexistence: (1) a strictly demographic trade—off, such that one genotype is competitively superior when glucose is abundant whereas the other genotype is the better competitor for sparse glucose; and (2) a cross—feeding interaction, whereby the superior competitor for glucose excretes a metabolite that acts as a second resource for which the other genotype is the better competitor. Although there was a demographic tradeoff, the advantage to the superior competitor at high glucose concentrations was too large (given the initial concentration of glucose used in these experiments) to allow the second genotype to invade when rare at the observed rate. Therefore, the second genotype must have had some other advantage that allowed it to readily invade a population of the superior competitor for glucose. Indeed, the second genotype could increase in abundance after glucose was depleted, but only in the presence of the superior competitor for glucose, thus implicating a cross—feeding interaction. These results confirmed earlier studies showing that populations of E. coli can maintain ecologically relevant genetic diversity even in a simple environment.
Bennett A F; Lenski R E
Evolutionary Adaptation to Temperature. V. Adaptive Mechanisms and Correlated Responses in Experimental Lines of Escherichia coli Journal Article
Evolution, 50 (2), pp. 493–503, 1996, ISSN: 0014-3820.
Abstract | Links | BibTeX | Altmetric | Tags: Descendant Experiments
@article{Bennett1996,
title = {Evolutionary Adaptation to Temperature. V. Adaptive Mechanisms and Correlated Responses in Experimental Lines of \textit{Escherichia coli}},
author = {Albert F. Bennett and Richard E. Lenski},
url = {https://onlinelibrary.wiley.com/doi/10.1111/j.1558-5646.1996.tb03862.x},
doi = {10.1111/j.1558-5646.1996.tb03862.x},
issn = {0014-3820},
year = {1996},
date = {1996-04-01},
urldate = {1996-04-01},
journal = {Evolution},
volume = {50},
number = {2},
pages = {493--503},
abstract = {We previously demonstrated temperature-specific genetic adaptation in experimental lines of \textit{Escherichia coli}. Six initially identical populations were propagated for 2000 generations under each of five regimes: constant 20°C, 32°C, 37°C, and 42°C, and a daily switch between 32°C and 42°C. Glucose was the sole carbon source in all cases. Here, we examine the physiological bases of adaptation to determine whether the same mechanisms evolved among the replicate lines within each thermal regime and across different regimes. Specifically, we investigate whether changes in glucose transport may account for the temperature-specific adaptation. We compared each line's direct response of fitness to glucose with its correlated response to maltose; glucose and maltose enter the cell by different pathways, but their catabolism is identical. Except for lines maintained at the ancestral temperature (37°C), almost all derived lines had significantly different fitnesses (relative to their common ancestor) in glucose and maltose, supporting the hypothesis that adaptation involved changes in glucose transport. An alternative explanation, that maltose transport decayed by genetic drift, appears unlikely for reasons that are discussed. Although most lines showed evidence of temperature-specific adaptation in glucose transport, several different mechanisms may underlie these improvements, as indicated by heterogeneity in correlated responses (across temperatures and substrates) among replicate lines adapted to the same regime. This heterogeneity provides a latent pool of genetic variation for responding to environmental change.},
keywords = {Descendant Experiments},
pubstate = {published},
tppubtype = {article}
}
We previously demonstrated temperature-specific genetic adaptation in experimental lines of Escherichia coli. Six initially identical populations were propagated for 2000 generations under each of five regimes: constant 20°C, 32°C, 37°C, and 42°C, and a daily switch between 32°C and 42°C. Glucose was the sole carbon source in all cases. Here, we examine the physiological bases of adaptation to determine whether the same mechanisms evolved among the replicate lines within each thermal regime and across different regimes. Specifically, we investigate whether changes in glucose transport may account for the temperature-specific adaptation. We compared each line's direct response of fitness to glucose with its correlated response to maltose; glucose and maltose enter the cell by different pathways, but their catabolism is identical. Except for lines maintained at the ancestral temperature (37°C), almost all derived lines had significantly different fitnesses (relative to their common ancestor) in glucose and maltose, supporting the hypothesis that adaptation involved changes in glucose transport. An alternative explanation, that maltose transport decayed by genetic drift, appears unlikely for reasons that are discussed. Although most lines showed evidence of temperature-specific adaptation in glucose transport, several different mechanisms may underlie these improvements, as indicated by heterogeneity in correlated responses (across temperatures and substrates) among replicate lines adapted to the same regime. This heterogeneity provides a latent pool of genetic variation for responding to environmental change.
Mongold J A; Bennett A F; Lenski R E
Evolutionary Adaptation to Temperature. VI. Phenotypic Acclimation and Its Evolution in Escherichia coli Journal Article
Evolution, 50 (1), pp. 35–43, 1996, ISSN: 0014-3820.
Abstract | Links | BibTeX | Altmetric | Tags: Descendant Experiments
@article{Mongold1996b,
title = {Evolutionary Adaptation to Temperature. VI. Phenotypic Acclimation and Its Evolution in \textit{Escherichia coli}},
author = {Judith A. Mongold and Albert F. Bennett and Richard E. Lenski},
url = {https://onlinelibrary.wiley.com/doi/10.1111/j.1558-5646.1996.tb04470.x},
doi = {10.1111/j.1558-5646.1996.tb04470.x},
issn = {0014-3820},
year = {1996},
date = {1996-02-01},
urldate = {1996-02-01},
journal = {Evolution},
volume = {50},
number = {1},
pages = {35--43},
abstract = {Following an environmental change, the course of a population's adaptive evolution may be influenced by environmental factors, such as the degree of marginality of the new environment relative to the organism's potential range, and by genetic factors, including constraints that may have arisen during its past history. Experimental populations of bacteria were used to address these issues in the context of evolutionary adaptation to the thermal environment. Six replicate lines of \textit{Escherichia coli} (20°C group), founded from a common ancestor, were propagated for 2000 generations at 20°C, a novel temperature that is very near the lower thermal limit at which it can maintain a stable population size in a daily serial transfer (100-fold dilution) regime. Four additional groups (32/20, 37/20, 42/20, and 32-42/20°C groups) of six lines, each with 2000 generation selection histories at different temperatures (32, 37, 42, and daily alternation of 32 and 42°C), were moved to the same 20°C environment and propagated in parallel to ascertain whether selection histories influence the adaptive response in this novel environment. Adaptation was measured by improvement in fitness relative to the common ancestor in direct competition experiments conducted at 20°C. All five groups showed improvement in relative fitness in this environment; the mean fitness of the 20°C group after 2000 generations increased by about 8%. Selection history had no discernible effect on the rate or final magnitude of the fitness responses of the four groups with different histories after 2000 generations. The correlated fitness responses of the 20°C group were measured across the entire thermal niche. There were significant tradeoffs in fitness at higher temperatures; for example, at 40°C the average fitness of the 20°C group was reduced by almost 20% relative to the common ancestor. We also observed a downward shift of 1-2°C in both the upper and lower thermal niche limits for the 20°C selected group. These observations are contrasted with previous observations of a markedly greater rate of adaptation to growth near the upper thermal limit (42°C) and a lack of trade-off in fitness at lower temperatures for lines adapted to that high temperature. The evolutionary implications of this asymmetry are discussed.},
keywords = {Descendant Experiments},
pubstate = {published},
tppubtype = {article}
}
Following an environmental change, the course of a population's adaptive evolution may be influenced by environmental factors, such as the degree of marginality of the new environment relative to the organism's potential range, and by genetic factors, including constraints that may have arisen during its past history. Experimental populations of bacteria were used to address these issues in the context of evolutionary adaptation to the thermal environment. Six replicate lines of Escherichia coli (20°C group), founded from a common ancestor, were propagated for 2000 generations at 20°C, a novel temperature that is very near the lower thermal limit at which it can maintain a stable population size in a daily serial transfer (100-fold dilution) regime. Four additional groups (32/20, 37/20, 42/20, and 32-42/20°C groups) of six lines, each with 2000 generation selection histories at different temperatures (32, 37, 42, and daily alternation of 32 and 42°C), were moved to the same 20°C environment and propagated in parallel to ascertain whether selection histories influence the adaptive response in this novel environment. Adaptation was measured by improvement in fitness relative to the common ancestor in direct competition experiments conducted at 20°C. All five groups showed improvement in relative fitness in this environment; the mean fitness of the 20°C group after 2000 generations increased by about 8%. Selection history had no discernible effect on the rate or final magnitude of the fitness responses of the four groups with different histories after 2000 generations. The correlated fitness responses of the 20°C group were measured across the entire thermal niche. There were significant tradeoffs in fitness at higher temperatures; for example, at 40°C the average fitness of the 20°C group was reduced by almost 20% relative to the common ancestor. We also observed a downward shift of 1-2°C in both the upper and lower thermal niche limits for the 20°C selected group. These observations are contrasted with previous observations of a markedly greater rate of adaptation to growth near the upper thermal limit (42°C) and a lack of trade-off in fitness at lower temperatures for lines adapted to that high temperature. The evolutionary implications of this asymmetry are discussed.
Kibota T T; Lynch M
Estimate of the genomic mutation rate deleterious to overall fitness in E. coli Journal Article
Nature, 381 (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.
Mongold J A; Bennett A F; Lenski R E
Experimental investigations of evolutionary adaptation to temperature. Incollection
Bennet, Albert F.; Johnston, I. A. (Ed.): Animals and Temperature: Phenotypic and Evolutionary Adaptation, pp. 239 – 264, Cambridge University Press, Cambridge, 1996, ISBN: 9780511721854.
Abstract | Links | BibTeX | Altmetric | Tags: Descendant Experiments
@incollection{Mongold1996c,
title = {Experimental investigations of evolutionary adaptation to temperature.},
author = {Judith A. Mongold and Albert F. Bennett and Richard E. Lenski},
editor = {Albert F. Bennet and I. A. Johnston},
url = {https://www.cambridge.org/core/books/animals-and-temperature/EAC172DF2783C04EDEA19E84D40E0A6A
https://www.cambridge.org/core/books/abs/animals-and-temperature/experimental-investigations-of-evolutionary-adaptation-to-temperature/E2D8AD8E2D50A525012CD0A6A22A507E},
doi = {https://doi.org/10.1017/CBO9780511721854},
isbn = {9780511721854},
year = {1996},
date = {1996-01-01},
urldate = {1996-01-01},
booktitle = {Animals and Temperature: Phenotypic and Evolutionary Adaptation},
pages = {239 -- 264},
publisher = {Cambridge University Press},
address = {Cambridge},
abstract = {Organismal biologists employ two principal methods in their investigation of the natural world: comparison and experiment. The latter is most familiar in the context of laboratory investigations of functional mechanisms. Experiment is the classic application of the scientific method, including such elements as rigorous and replicated design, controlled manipulation of a single variable of interest and the incorporation of a control group into the study. While experimental science has been crucial to our understanding of how organisms work, to date it has had relatively less application in studying how those organisms came to be the way they are, i.e. in studies of the evolution of organismal characters. In such evolutionary studies, comparative investigations have been by far the dominant methodological tradition.
In the study of evolutionary adaptation to temperature, for example, virtually all our knowledge is derived from comparative studies of different populations, species, or other taxa inhabiting different thermal environments (for reviews, see Precht \textit{et al.}, 1973; Prosser, 1973; Hochachka & Somero, 1984; Cossins & Bowler, 1987). The comparative approach involves the measurement of a character and its correlation with environmental temperature. If the character (e.g. a rate process) is thermally dependent, then it is measured either over a similar range of temperatures or at a single temperature common to the different groups examined. The pattern of character on environmental temperature is then analysed and interpreted, most frequently in an adaptive context (Prosser, 1986; Cossins & Bowler, 1987; Bennett, 1996), and compared with the pattern found for other biological systems inhabiting similar thermal environments.},
keywords = {Descendant Experiments},
pubstate = {published},
tppubtype = {incollection}
}
Organismal biologists employ two principal methods in their investigation of the natural world: comparison and experiment. The latter is most familiar in the context of laboratory investigations of functional mechanisms. Experiment is the classic application of the scientific method, including such elements as rigorous and replicated design, controlled manipulation of a single variable of interest and the incorporation of a control group into the study. While experimental science has been crucial to our understanding of how organisms work, to date it has had relatively less application in studying how those organisms came to be the way they are, i.e. in studies of the evolution of organismal characters. In such evolutionary studies, comparative investigations have been by far the dominant methodological tradition.
In the study of evolutionary adaptation to temperature, for example, virtually all our knowledge is derived from comparative studies of different populations, species, or other taxa inhabiting different thermal environments (for reviews, see Precht et al., 1973; Prosser, 1973; Hochachka & Somero, 1984; Cossins & Bowler, 1987). The comparative approach involves the measurement of a character and its correlation with environmental temperature. If the character (e.g. a rate process) is thermally dependent, then it is measured either over a similar range of temperatures or at a single temperature common to the different groups examined. The pattern of character on environmental temperature is then analysed and interpreted, most frequently in an adaptive context (Prosser, 1986; Cossins & Bowler, 1987; Bennett, 1996), and compared with the pattern found for other biological systems inhabiting similar thermal environments.
In the study of evolutionary adaptation to temperature, for example, virtually all our knowledge is derived from comparative studies of different populations, species, or other taxa inhabiting different thermal environments (for reviews, see Precht et al., 1973; Prosser, 1973; Hochachka & Somero, 1984; Cossins & Bowler, 1987). The comparative approach involves the measurement of a character and its correlation with environmental temperature. If the character (e.g. a rate process) is thermally dependent, then it is measured either over a similar range of temperatures or at a single temperature common to the different groups examined. The pattern of character on environmental temperature is then analysed and interpreted, most frequently in an adaptive context (Prosser, 1986; Cossins & Bowler, 1987; Bennett, 1996), and compared with the pattern found for other biological systems inhabiting similar thermal environments.
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, 267 (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.
1994
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, 48 (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.
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, 91 (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, 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, 47 (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, 46 (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.