EVOLUTIONARY ADAPTATION TO TEMPERATURE. I. FITNESS RESPONSES OF ESCHERICHIA COLI TO CHANGES IN ITS THERMAL ENVIRONMENT

We used bacteria to study experimentally the process of genetic adaptation to environmental temperature. Replicate lines of Escherichia coli, founded from a common ancestor, were propagated for 2,000 generations in 4 different thermal regimes as 4 experimental groups: 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. Adaptation of the groups to temperature was measured by improvement in fitness relative to the ancestor, as estimated by competition experiments. All four experimental groups showed improved relative fitness in their own thermal environment (direct response of fitness). However, 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. The correlated fitness responses of each group, again relative to the common ancestor, were measured over the entire experimental range of temperatures. No necessary tradeoff between direct and correlated responses of fitness was apparent: for example, the improved fitness of the 42°C group at 42°C was not accompanied by a loss of fitness at 37°C or 32°C. However, the direct fitness responses were usually greater than the correlated responses, judged both by comparing direct and correlated responses of a single group at different temperatures and by comparing direct and correlated responses of different groups at a single temperature. These comparisons indicate that the observed adaptation was, in fact, largely temperature specific. Also, the fitness responses of the generalist group across a range of temperatures were less variable than those of the thermal specialist groups considered as whole.     

EVOLUTIONARY ADAPTATION TO TEMPERATURE. IV. ADAPTATION OF ESCHERICHIA COLI AT A NICHE BOUNDARY

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.     

EVOLUTIONARY ADAPTATION TO TEMPERATURE II. THERMAL NICHES OF EXPERIMENTAL LINES OF ESCHERICHIA COLI

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, we 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. In addition, no significant differences among groups were observed in rate of extinction at more extreme temperatures. Within the thermal niche, we measured the mean fitness of the evolved groups relative to their common ancestor. 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 about 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, although tradeoffs were observed for a few of the lines. We conclude that adaptation in this system was quite temperature specific, but substantial divergence among groups in thermal optima had little effect on the limits of their thermal niches and did not necessarily involve tradeoffs in fitness at other temperatures.     

EVOLUTIONARY ADAPTATION TO TEMPERATURE. V. ADAPTIVE MECHANISMS AND CORRELATED RESPONSES IN EXPERIMENTAL LINES OF ESCHERICHIA COLI

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.     

EVOLUTIONARY ADAPTATION TO TEMPERATURE. IX. PREADAPTATION TO NOVEL STRESSFUL ENVIRONMENTS OF ESCHERICHIA COLI ADAPTED TO HIGH TEMPERATURE

Abstract Stressful environments may be considered as those that reduce fitness, sometimes due in part to the increased metabolic expenditure required to sustain life. Direct adaptation to a stressor is expected to increase fitness and reduce maintenance metabolism, with the latter leading to increased biomass production. In this study, we test the general hypothesis that such adaptation to one stressor can preadapt organisms to novel stressful environments. Six lines of Escherichia coli propagated for 2000 generations at 41–42°C (42 group), a stressful temperature, were compared to six control lines propagated for 2000 generations at 37°C (37 group) and to the common ancestor of both groups. We assayed biovolume yield (a measure of growth efficiency) and competitive fitness in the 42 group's selective high temperature environment as well as five novel stressful environments–acid, alkali, ethanol, high osmolarity and peroxide. As previously reported, at high temperature the 42 group had both higher yield and fitness than the 37 group and ancestor. In the novel environments, the 42 group generally produced yields higher than the 37 group (and marginally higher than the ancestor), but we found no differences in competitive fitness among the 37 and 42 groups and the ancestor. We also found that the performance of lines within groups was not correlated across stressful environments for either yield or relative fitness. Because previous adaptation to one stressor did not improve our measure of Darwinian fitness in novel stressful environments, we conclude that the 42 group shows no useful preadaptation, or cross‐tolerance, to these types of environments.     

EVOLUTIONARY ADAPTATION TO TEMPERATURE. VI. PHENOTYPIC ACCLIMATION AND ITS EVOLUTION IN ESCHERICHIA COLI

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.     

EVOLUTIONARY ADAPTATION TO TEMPERATURE. VII. EXTENSION OF THE UPPER THERMAL LIMIT OF ESCHERICHIA COLI

What factors influence the ability of populations to adapt to extreme environments that lie outside their current tolerance limits? We investigated this question by exposing experimental populations of the bacterium Escherichia coli to lethally high temperatures. We asked: (1) whether we could obtain thermotolerant mutants with an extended upper thermal limit by this selective screen; (2) whether the propensity to obtain thermotolerant mutants depended on the prior selective history of the progenitor genotypes; and (3) how the fitness properties of these mutants compared to those of their progenitors within the ancestral thermal niche. Specifically, we subjected 15 independent populations founded from each of six progenitors to 44°C; all of the progenitors had upper thermal limits between about 40°C and 42°C. Two of the progenitors were from populations that had previously adapted to 32°C, two were from populations adapted to 37°C, and two were from populations adapted to 41–42°C. All 90 populations were screened for mutants that could survive and grow at 44°C. We obtained three thermotolerant mutants, all derived from progenitors previously adapted to 41–42°C. In an earlier study, we serendipitously found one other thermotolerant mutant derived from a population that had previously adapted to 32°C. Thus, prior selection at an elevated but nonlethal temperature may predispose organisms to evolve more extreme thermotolerance, but this is not an absolute requirement. It is evidently possible to obtain mutants that tolerate more extreme temperatures, so why did they not become prevalent during prior selection at 41–42°C, near the upper limit of the thermal niche? To address this question, we measured the fitness of the thermotolerant mutants at high temperatures just within the ancestral niche. None of the four thermotolerant mutants had an advantage relative to their progenitor even very near the upper limit of the thermal niche; in fact, all of the mutants showed a noticeable loss of fitness around 41°C. Thus, the genetic adaptations that improve competitive fitness at high but nonlethal temperatures are distinct from those that permit tolerance of otherwise lethal temperatures.     

An experimental evolutionary study on adaptation to temporally fluctuating pH in Escherichia coli

In this study, we use the bacterium Escherichia coli to examine evolutionary responses to environmental acidity fluctuating temporally among pH 5.3, 6.3, 7.0, and 7.8 (5,000-15 nM [H(+)]). Two experimental protocols of temporal variation were used. One group (six replicate lines) of populations evolved for 2,000 generations during exposure to a cycled regime fluctuating daily between pH 5.3 and 7.8. The other group (also in six replicate lines) evolved during exposure for 2,000 generations to a randomly shifting regime fluctuating stochastically each day among pH 5.3, 6.3, 7.0, and 7.8. Adaptation to these fluctuating acidity regimes was measured as a change in fitness relative to the common ancestor by direct competition experiments in both constant and fluctuating pH regimes. For comparisons with constant pH evolution, a group evolved at a constant pH of 5.3 and another group evolved at pH 7.8 were also tested. This study initiated the first long-term laboratory natural selection experiment on adaptation to variable acidity and addressed key questions concerning patterns of adaptation (trade-offs, specialists, generalists, plasticity, transitions, and acclimation) in temporally fluctuating environments.     

Prior evolution in stochastic versus constant temperatures affects RNA virus evolvability at a thermal extreme

It is unclear how historical adaptation versus maladaptation in a prior environment affects population evolvability in a novel habitat. Prior work showed that vesicular stomatitis virus (VSV) populations evolved at constant 37°C improved in cellular infection at both 29°C and 37°C; in contrast, those evolved under random changing temperatures between 29°C and 37°C failed to improve. Here, we tested whether prior evolution affected the rate of adaptation at the thermal‐niche edge: 40°C. After 40 virus generations in the new environment, we observed that populations historically evolved at random temperatures showed greater adaptability. Deep sequencing revealed that most of the newly evolved mutations were de novo. Also, two novel evolved mutations in the VSV glycoprotein and replicase genes tended to co‐occur in the populations previously evolved at constant 37°C, whereas this parallelism was not seen in populations with prior random temperature evolution. These results suggest that prior adaptation under constant versus random temperatures constrained the mutation landscape that could improve fitness in the novel 40°C environment, perhaps owing to differing epistatic effects of new mutations entering genetic architectures that earlier diverged. We concluded that RNA viruses maladapted to their previous environment could “leapfrog” over counterparts of higher fitness, to achieve faster adaptability in a novel environment.     

ADAPTATION OF PHOTOSYNTHESIS IN LAMINARIA SACCHARINA (PHAEOPHYTA) TO CHANGES IN GROWTH TEMPERATURE1

The effect of growth temperature on photosynthetic metabolism was studied in the kelp Laminaria saccharina (L.) Lamour. Photosynthesis was subject to phenotypic adaptation, with almost constant photosynthetic rates being achieved at growth temperatures between 0 and 20° C. This response involved: (1) an inverse relationship between growth temperature and photosynthetic capacity, (2) a reduction in the Q₁₀ value for photosynthesis of L. saccharina grown at 0 and 5° C compared with 10, 15 and 20° C grown sporophytes, and (3) an acquired tolerance of photosynthesis to temperatures between 15–25° C (which inhibited photosynthesis in 0 and 5° C grown L. saccharina) in sporophytes grown at 10, 15 and 20° C. The physiological basis of these adaptations is discussed in terms of observed changes in activities and kinetics of the Calvin cycle enzyme ribulose-1, 5-bisphosphate carboxylase (oxygenase) and efficiency of light harvesting-electron transport systems.     

TEMPORAL VARIATION FAVORS THE EVOLUTION OF GENERALISTS IN EXPERIMENTAL POPULATIONS OF DROSOPHILA MELANOGASTER

In variable environments, selection should favor generalists that maintain fitness across a range of conditions. However, costs of adaptation may generate fitness trade‐offs and lead to some compromise between specialization and generalization that maximizes fitness. Here, we evaluate the evolution of specialization and generalization in 20 populations of Drosophila melanogaster experimentally evolved in constant and variable thermal environments for 3 years. We developed genotypes from each population at two temperatures after which we measured fecundity across eight temperatures. We predicted that constant environments would select for thermal specialists and that variable environments would select for thermal generalists. Contrary to our predictions, specialists and generalists did not evolve in constant and spatially variable environments, respectively. However, temporal variation produced a type of generalist that has rarely been considered by theoretical models of developmental plasticity. Specifically, genotypes from the temporally variable selective environment were more fecund across all temperatures than were genotypes from other environments. These patterns suggest certain allelic effects and should inspire new directions for modeling adaptation to fluctuating environments.     

RAPID LABORATORY EVOLUTION OF ADULT LIFE-HISTORY TRAITS IN DROSOPHILA MELANOGASTER IN RESPONSE TO TEMPERATURE

Three replicate lines of Drosophila melanogaster were cultured at each of two temperatures (16.5°C and 25°C) in population cages for 4 yr. The lifespans of both sexes and the fecundity and fertility of the females were then measured at both experimental temperatures. The characters showed evidence of adaptation; flies of both sexes from each selection regime showed higher longevity, and females showed higher fecundity and fertility, than flies from the other selection regime when they were tested at the experimental temperature at which they had evolved. Calculation of intrinsic rates of increase under different assumptions about the rate of population increase showed that the difference between the lines from the two selection regimes became less the higher the rate of population increase, because the lines were more similar in early adulthood than they were later. Despite the increased adaptation of the low-temperature lines to the low temperature, like the high temperature lines they produced progeny at a higher rate at the higher temperature. The lines may have independently evolved adaptations to their respective thermal regimes during the experiment, or there may have been a trade-off between adaptation to the two temperatures, or mutation pressure may have lowered adaptation to the temperature that the flies no longer encountered.     

Temperature-related divergence in experimental populations of Drosophila melanogaster. II. Correlation between fitness and body dimensions

From a laboratory stock of Drosophila melanogaster (Oregon), reared for more than 20 years at 18° C, a new population was derived and maintained at 28° C for 8 years. The chromosomal and cytoplasmic contribution to genetic divergence between the two populations was estimated. Six body traits and reproductive fitness were taken into account. The third chromosome is responsible for the adaptive difference for temperature between the two lines. Temperature-selected genes which control body size are located on the second and third chromosomes, although the contribution of each chromosome depends on the environment in which the flies develop. The correlation between the chromosomal and cytoplasmic contributions to different traits and fitness, changes with temperature. At 28° C the correlation between fitness and each body trait is proportional to the response to selection exhibited by each of them, but this is not true at 18° C. Body size has, therefore, an adaptive significance in relation to temperature, which is expressed only in the environment where selection occurs. Cytoplasmic genes affect almost all characters to an extent similar to that of chromosomal genes. Inter-chromosomal and nucleo-cytoplasmic interactions are present and also change with temperature. In general, genes selected in a given environment produce greater phenotypic changes in that environment than in another. The population that experienced both temperatures is fitter in both environments, suggesting that the capacity to adapt to warm temperatures depends on genes other than those which are involved in the adaptation to cold.     

Male‐limited secondary sexual trait interacts with environment in determining female fitness

Selection for secondary sexual trait (SST) elaboration may increase intralocus sexual conflict over the optimal values of traits expressed from shared genomes. This conflict can reduce female fitness, and the resulting gender load can be exacerbated by environmental stress, with consequences for a population's ability to adapt to novel environments. However, how the evolution of SSTs interacts with environment in determining female fitness is not well understood. Here, we investigated this question using replicate lines of bulb mites selected for increased or decreased prevalence of a male SST—thickened legs used as weapons. The fitness of females from these lines was measured at a temperature to which the mites were adapted (24°C), as well as at two novel temperatures: 18°C and 28°C. We found the prevalence of the SST interacted with temperature in determining female fecundity. At 28°C, females from populations with high SST prevalence were less fecund than females from populations in which the SST was rare, but the reverse was true at 18°C. Thus, a novel environment does not universally depress female fitness more in populations with a high degree of sexually selected dimorphism. We discuss possible consequences of the interaction we detected for adaptation to novel environments.     

Indirect selection of thermal tolerance during experimental evolution of Drosophila melanogaster

Natural selection alters the distribution of a trait in a population and indirectly alters the distribution of genetically correlated traits. Long‐standing models of thermal adaptation assume that trade‐offs exist between fitness at different temperatures; however, experimental evolution often fails to reveal such trade‐offs. Here, we show that adaptation to benign temperatures in experimental populations of Drosophila melanogaster resulted in correlated responses at the boundaries of the thermal niche. Specifically, adaptation to fluctuating temperatures (16–25°C) decreased tolerance of extreme heat. Surprisingly, flies adapted to a constant temperature of 25°C had greater cold tolerance than did flies adapted to other thermal conditions, including a constant temperature of 16°C. As our populations were never exposed to extreme temperatures during selection, divergence of thermal tolerance likely reflects indirect selection of standing genetic variation via linkage or pleiotropy. We found no relationship between heat and cold tolerances in these populations. Our results show that the thermal niche evolves by direct and indirect selection, in ways that are more complicated than assumed by theoretical models.     

Experimental Evolution and Its Role in Evolutionary Physiology

Four general approaches to the study of evolutionary physiology—phylogenetically-basedcomparisons, genetic analyses and manipulations, phenotypicplasticity and manipulation, and selection studies—areoutlined and discussed. We provide an example of the latter,the application of laboratory selection experiments to the studyof a general issue in environmental adaptation, differencesin adaptive patterns of generalists and specialists. A cloneof the bacterium Escherichia coli that had evolved in a constantenvironment of 37°C was replicated into 6 populations andallowed to reproduce for 2,000 generations in a variable thermalenvironment alternating between 32 and 42°C. As predictedby theory, fitness and efficiency of resource use increasedin this new environment, as did stress resistance. Contraryto predictions, however, fitness and efficiency in the constantancestral environment of 37°C did not decrease, nor didthermal niche breadth or phenotypic plasticity increase. Selectionexperiments can thus provide a valuable approach to testinghypotheses and assumptions about the evolution of functionalcharacters.     

Influence of soil temperature on niacin and thiamine in honey mesquite

Summary The influence of soil temperature was examined on niacin and thiamine concentration in honey mesquite (Prosopis glandulosa var.glandulosa) seedlings. The seedlings were grown in soil temperature regimes of 21, 27, and 32°C in a controlled environment growth room. Nodulation randomly occurred on the roots of the seedlings, necessitating separate analysis according to the occurrence of nodulation. Roots of nodulated seedlings from the 21°C soil temperature regime contained greater quantities of niacin and thiamine compared to root samples from seedlings grown in either 27 or 32°C regimes. Niacin concentration of non-nodulated seedlings was highest in samples from seedlings grown in the 27°C soil temperature regime and lowest in samples from seedlings grown in the 21°C regime. Thiamine concentration was the greatest from non-nodulated seedlings grown in the 27°C soil temperature regime, while the thiamine concentration of non-nodulated samples from the 32°C regime was the least. Optimal soil temperature for honey mesquite root growth appears to be about 27°C. At sub-optimal soil temperatures niacin might have limited ‘growth’ while at supra-optimal soil temperatures, thiamine might be a limiting factor. College of Agricultural Sciences Contribution No. T-9-164.     

Adaptation to temperature stress by Vibrio fischeri facilitates this microbe's symbiosis with the Hawaiian bobtail squid (Euprymna scolopes)

For microorganisms cycling between free‐living and host‐associated stages, where reproduction occurs in both of these lifestyles, an interesting inquiry is whether adaptation to stress during the free‐living stage can impact microbial fitness in the host. To address this topic, the mutualism between the Hawaiian bobtail squid (Euprymna scolopes) and the marine bioluminescent bacterium Vibrio fischeri was utilized. Using microbial experimental evolution, V. fischeri was selected to low (8°C), high (34°C), and fluctuating temperature stress (8°C/34°C) for 2000 generations. The temperatures 8°C and 34°C were the lower and upper growth limits, respectively. V. fischeri was also selected to benign temperatures (21°C and 28°C) for 2000 generations, which served as controls. V. fischeri demonstrated significant adaptation to low, high, and fluctuating temperature stress. V. fischeri did not display significant adaptation to the benign temperatures. Adaptation to stressful temperatures facilitated V. fischeri’s ability to colonize the squid host relative to the ancestral lines. Bioluminescence levels also increased. Evolution to benign temperatures did not manifest these results. In summary, microbial adaptation to stress during the free‐living stage can promote coevolution between hosts and microorganisms.     

ROLE OF SEX AND MIGRATION IN ADAPTATION TO SINK ENVIRONMENTS

Understanding the effects of sex and migration on adaptation to novel environments remains a key problem in evolutionary biology. Using a single‐cell alga Chlamydomonas reinhardtii, we investigated how sex and migration affected rates of evolutionary rescue in a sink environment, and subsequent changes in fitness following evolutionary rescue. We show that sex and migration affect both the rate of evolutionary rescue and subsequent adaptation. However, their combined effects change as the populations adapt to a sink habitat. Both sex and migration independently increased rates of evolutionary rescue, but the effect of sex on subsequent fitness improvements, following initial rescue, changed with migration, as sex was beneficial in the absence of migration but constraining adaptation when combined with migration. These results suggest that sex and migration are beneficial during the initial stages of adaptation, but can become detrimental as the population adapts to its environment.     

Increased susceptibility to repeated freeze-thaw cycles in Escherichia coli following long-term evolution in a benign environment

Background  

In order to study the dynamics of evolutionary change, 12 populations of E. coli B were serially propagated for 20,000 generations in minimal glucose medium at constant 37°C. Correlated changes in various other traits have been previously associated with the improvement in competitive fitness in the selective environment. This study examines whether these evolved lines changed in their ability to tolerate the stresses of prolonged freezing and repeated freeze-thaw cycles during adaptation to a benign environment.