Publications

@article{Coyne1996,

title = {Mechanisms of Punctuated Evolution},

author = {Jerry A. Coyne and B. Charlesworth and Santiago F. Elena and Vaughn S. Cooper and Richard E. Lenski},

url = {https://www.sciencemag.org/lookup/doi/10.1126/science.274.5293.1748},

doi = {10.1126/science.274.5293.1748},

issn = {0036-8075},

year  = {1996},

date = {1996-12-01},

urldate = {1996-12-01},

journal = {Science},

volume = {274},

number = {5293},

pages = {1748--1750},

abstract = {In their report, "Punctuated evolution caused by selection of rare beneficial mutations," Santiago F. Elena \textit{et al.} describe (1) punctuated changes in a morphological character (cell size) in a population of \textit{Escherichia coli} kept in batch culture with daily transfer for 3000 generations. They state that their observations might explain some aspects of punctuated evolutionary change, a phenomenon often said to be in consistent with classical neo-Darwinism (2). The controversy about punctuated equilibrium involves two questions: (i) How ubiquitous is the pattern of "jerky" evolution in nature? and (ii) Do novel evolutionary processes cause this pattern? Elena \textit{et al.} answer the second question by proposing that stasis in the fossil record arises (as it does in their bacteria) from an absence of genetic variation, while rapid changes in the record reflect the spread of new beneficial mutations.},

keywords = {},

pubstate = {published},

tppubtype = {article}

}

@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}

}

@article{Mongold1996,

title = {Experimental rejection of a nonadaptive explanation for increased cell size in \textit{Escherichia coli}},

author = {Judith A. Mongold and Richard E. Lenski},

url = {https://journals.asm.org/doi/10.1128/jb.178.17.5333-5334.1996},

doi = {10.1128/jb.178.17.5333-5334.1996},

issn = {0021-9193},

year  = {1996},

date = {1996-09-01},

urldate = {1996-09-01},

journal = {Journal of Bacteriology},

volume = {178},

number = {17},

pages = {5333--5334},

abstract = {Populations of \textit{Escherichia coli} that have been serially propagated for thousands of generations in glucose minimal medium show heritable increases in both cell size and growth rate. We sought to test the hypothesis that the increased cell size of the derived genotypes could be explained solely by their faster growth. The regression of cell size on growth rate differed significantly between populations having ancestral and derived genotypes, with the latter producing larger cells over almost the entire range of growth rates. Thus, the physiological coupling between cell size and growth rate has been evolutionarily altered.},

keywords = {Cell Morphology},

pubstate = {published},

tppubtype = {article}

}

@article{Elena1996,

title = {Punctuated Evolution Caused by Selection of Rare Beneficial Mutations},

author = {Santiago F. Elena and Vaughn S. Cooper and Richard E. Lenski},

url = {https://www.sciencemag.org/lookup/doi/10.1126/science.272.5269.1802},

doi = {10.1126/science.272.5269.1802},

issn = {0036-8075},

year  = {1996},

date = {1996-06-01},

urldate = {1996-06-01},

journal = {Science},

volume = {272},

number = {5269},

pages = {1802--1804},

abstract = {For more than two decades there has been intense debate over the hypothesis that most morphological evolution occurs during relatively brief episodes of rapid change that punctuate much longer periods of stasis. A clear and unambiguous case of punctuated evolution is presented for cell size in a population of \textit{Escherichia coli} evolving for 3000 generations in a constant environment. The punctuation is caused by natural selection as rare, beneficial mutations sweep successively through the population. This experiment shows that the most elementary processes in population genetics can give rise to punctuated evolutionary dynamics.},

keywords = {Cell Morphology},

pubstate = {published},

tppubtype = {article}

}

@article{Travisano1996,

title = {Long-Term Experimental Evolution in \textit{Escherichia coli}. IV. Targets of Selection and the Specificity of Adaptation},

author = {Michael Travisano and Richard E. Lenski},

url = {https://academic.oup.com/genetics/article/143/1/15/6016887},

doi = {10.1093/genetics/143.1.15},

issn = {1943-2631},

year  = {1996},

date = {1996-05-01},

urldate = {1996-05-01},

journal = {Genetics},

volume = {143},

number = {1},

pages = {15--26},

abstract = {This study investigates the physiological manifestation of adaptive evolutionary change in 12 replicate populations of \textit{Escherichia coli} that were propagated for 2000 generations in a glucose-limited environment. Representative genotypes from each population were assayed for fitness relative to their common ancestor in the experimental glucose environment and in 11 novel single-nutrient environments. After 2000 generations, the 12 derived genotypes had diverged into at least six distinct phenotypic classes. The nutrients were classified into four groups based upon their uptake physiology. All 12 derived genotypes improved in fitness by similar amounts in the glucose environment, and this pattern of parallel fitness gains was also seen in those novel environments where the limiting nutrient shared uptake mechanisms with glucose. Fitness showed little or no consistent improvement, but much greater genetic variation, in novel environments where the limiting nutrient differed from glucose in its uptake mechanisms. This pattern of fitness variation in the novel nutrient environments suggests that the independently derived genotypes adapted to the glucose environment by similar, but not identical, changes in the physiological mechanisms for moving glucose across both the inner and outer membranes.},

keywords = {Correlated Responses, Genotypes and Phenotypes, Parallelism and Divergence},

pubstate = {published},

tppubtype = {article}

}

@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}

}

@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}

}

@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}

}

@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}

}

@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}

}

@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}

}

@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}

}

@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},

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}

}

@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},

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}

}

@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}

}

@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}

}

@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}

}

@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}

}