Optimal switching to diapause in relation to the onset of winter

A model is presented that specifies optimal switching times for the induction of diapause relative to the onset of winter in a deterministic environment. Fitness is defined as an individual's contribution to the overwintering population in diapause at the time of the first hard frost. The fitness of a nondiapausing female is determined by the switching times of her offspring. If age-specific fecundity and survivorship do not change significantly from generation to generation, the optimal switching time precedes the onset of winter by a constant amount of time equal to the age of first reproduction plus the time to produce one offspring plus the difference in ages between the sensitive age and the diapause age. This result is independent of the time at which the original female began to reproduce. However, if fecundity or survivorship decreases toward the end of the season, the optimal switching time depends upon the time when she began to reproduce and should be more conservative by an amount of time that can be computed. Possible tests of the model are proposed.     

Timing of diapause in relation to temporally variable catastrophes

This paper analyses the response of the distribution of diapause switching times in an arthropod population with respect to variation in a catastrophe, which kills nondiapause individuals. For concreteness, the catastrophe will be taken to the onset of winter. The relationship between an individual's switching time and the decision whether to diapause is defined as follows: If she passes through the end of the sensitive period, during which the diapause decision is made, before her switching time, she will complete development and reproduce rather than diapause. If she passes through the sensitive period on or after her switching time, she will diapause. The model follows the evolution of the distribution of switching times for a population over a sequence of years. Random variation in the end of the season is created by sampling from a normal distribution of times at the end of the growing season. The model is for a haploid population in which the distribution of switching times that a female passes to her offspring is normally distributed having her switching time as its mean. This approximates a sexual population with strong positive assortative mating and heritability near 1. This mode of inheritance permits a rapid response to yearly changes in the end of the season as a contrast to earlier models, which incorporated a slow response. Patterns of temporal change in the median switching time are analyzed. The influences of three parameters are considered: the mean and standard deviation of the end of the season, and standard deviation of the offspring distribution. The main conclusion differs from the results of earlier models in that the end of the season must be extremely variable before the median of the distribution of switching times displays conservative behavior, i. e., before it becomes significantly earlier than the median expected when the end of the season is constant and equal to the mean of the normal distribution for the end of the season used in the simulation. Previous models predicted a conservative response even in moderately variable environments.     

Bet-Hedging Diapause Strategies in Stochastic Environments

In many insect species, adult emergence spreads over several years because of the existence of prolonged diapause in certain individuals. From stochastic models, we show that diversified bet-hedging strategies (mixed strategies with emergence after 1 or 2 yr) are more fit than simple diapause strategy (emergence after 1 yr) or fixed prolonged diapause strategy (emergence after 2 yr) in isolated chestnut weevil populations. This conclusion applies to a large range of survival rates in prolonged diapause and is insensitive to initial conditions, magnitude of temporal autocorrelation, distribution of demographic parameters, and quoted values of population size limitation. However, the shape of the fitness distribution as a function of prolonged diapause frequency changes greatly in the absence of population size limitation. Whatever the survival rate during prolonged diapause, we find that there is no genotypic advantage to extending diapause for all chestnut weevil larvae to more than 1 yr. Our models predict selection of bet-hedging strategies over a large range of prolonged diapause frequencies. This result is consistent with the existence of several mixed strategies in a population. Emergences after 3 yr are not crucial for selection or for the dynamics of mixed strategies in the chestnut weevil.     

Genetic variance for diapause expression and associated life histories in Drosophila melanogaster

The dipteran Drosophila melanogaster can express a form of reproductive quiescence or diapause when exposed to low temperature and shortened photoperiod. Among natural populations in the eastern United States, the frequency of lines that express reproductive diapause in the laboratory varies substantially and predictably with latitudinal origin. The goals of the present study were twofold: (1) to examine the impact of genetic variance for diapause expression on multiple traits associated with organismal fitness; and (2) to evaluate the potential for fitness trade-offs between diapause and nondiapause phenotypes that may result in the observed cline. Even prior to diapause entry or expression, inbred lines that express and do not express reproductive diapause in laboratory assays were constitutively distinct for life span, age-specific mortality rates, fecundity profiles, resistance to cold and starvation stress, lipid content, development time, and egg-to-adult viability. Furthermore, estimates of genetic correlations based on line means revealed significant differentiation for genetic variance/covariance matrices between diapause and nondiapause lines. The data indicate the potential for life-history trade-offs associated with variation for the diapause phenotype. The observed cline in diapause incidence in the eastern United States may be generated by these tradeoffs and the associated spatial and/or temporal variation in relative fitness of these two phenotypes in natural populations.     

Fecundity of the European pine sawfly Neodiprion sertifer (Hymenoptera, Diprionidae) related to cyclic outbreaks: Invariance effects

At the end of ontogenesis, individual variability of the fecundity of females of Neodiprion sertifer constitutes 30–120 eggs (with rare fluctuations from 16 to 140 eggs). Average fecundity as an adaptive index in a certain biocenosis, however, depends on many factors, including the structural organization of the sawfly population. The presence of a long diapause in some individuals results in the cleavage of the offspring into groups with different dates of reactivation. At the same time, mainly heavy eonymphs, giving rise to large females, stay in the diapause. Their participation in the reproduction together with females, raised in other years under different conditions, equalize the dimensional structure of the breeding fraction of the population. During population waves, revealed during 35 years of my continuous observations, fecundity fluctuated closely to the average value with deviations constituting 19–25%. It hardly depends on the population density of the phyllophagan. The intraspecific structuredness of the population in relation to the size of females and its transformation by the moment of their emergence promotes the evolutionary stability of the degree of fecundity of the sawfly.     

FITNESS CONSEQUENCES OF MATING SYSTEM,SEED WEIGHT,AND EMERGENCE DATE IN A WINTER ANNUAL,COLLINSIA VERNA

Selfed and outcrossed progeny of 60 maternal parents were produced to investigate the joint and individual effects of mating system, seed weight, and emergence date on the expression of characters related to fitness and adult fecundity. A series of analyses of variance investigated these effects through time and indicated that 1) mating system explained 56% of the variance in seed weight, 2) seed weight explained 51% and mating system explained 38% of the variance in emergence date, and 3) mating system explained 71% and seed weight explained 15% of the variance in fecundity. Outcrossed-seed means differed significantly from selfed-seed means for all traits measured. On average, outcrossed seeds were larger, germinated earlier, had higher percentage emergence, and produced plants that were more fecund than selfed seeds. The coefficient of inbreeding depression increased through time in this study, from 0.05 for seed weight to 0.23 for fecundity. Seed weight and emergence date were positively correlated, both phenotypically and genetically, for both mating systems. Genetically, this indicates that genes that increase the value of seed weight also increase the value of emergence date and vice versa. Phenotypically, the positive correlation indicates that larger seeds germinate later. Outcrossed seeds were significantly larger but germinated earlier than selfed seeds, suggesting that mating system has an overriding effect in influencing fitness. In light of the selection on emergence date quantified in a previous study, seed weight, emergence date, and mating system may be functioning as a cluster of characters on which selection acts jointly in this species.     

Environmental heterogeneity and the maintenance of genetic variation for reproductive diapause in Drosophila melanogaster

Drosophila melanogaster has colonized temperate habitats on multiple continents over a historical time period, and many traits vary predictably with latitude. Despite considerable attention paid to clinal variation in Drosophila, the mechanisms generating such patterns in nature remain largely unidentified. In D. melanogaster, the expression of reproductive diapause can be induced by exposure to low temperatures and shortened photoperiods. Both diapause expression itself and the underlying genetic variance for diapause expression have widespread impacts on organismal fitness, and diapause incidence exhibits a 60% cline in frequency in the eastern United States. The major aim of this study was to evaluate whether the relative fitness of diapause and nondiapause genotypes varies predictably with environment. In experimental population cages in the laboratory, the frequency of genotypes that express diapause increased over time when flies were exposed to environmental stress, whereas the frequency of nondiapause genotypes increased when flies were cultured under benign control conditions. Other fitness traits correlated with the genetic variance for diapause expression (longevity, mortality rates, stress resistance, lipid content, preadult viability, fecundity profiles, and development time) also diverged between experimental treatments. Similarly, sampling of isofemale lines from natural populations revealed that the frequency of diapause incidence cycled over time in seasonal habitats: diapause expression was at high frequency following the winter season and subsequently declined throughout the summer months. In contrast, diapause expression was low and temporally homogeneous in isofemale line collections from human-associated urban habitats. These data suggest that genetic variation underlying the diapause-nondiapause dichotomy may be actively maintained by selection pressures that vary spatially and temporally in natural populations.     

Does artificial selection on diapause incidence cause correlational changes in other life‐history traits? Case study in a spider mite population

Theoretical studies suggest that the timing of entering hibernation by arthropods has large effects on long-term fitness, incurring strong selection pressure on diapause attributes every year. On the other hand, diapause attributes are often genetically correlated with other important life-history traits such as fecundity or development time. To understand the evolutionary process of life cycle formation, there is a need to investigate not only diapause attributes themselves but also their genetic association with other life-history traits. The Kanzawa spider mite, Tetranychus kanzawai Kishida (Acari: Tetranychidae), is a small herbivore that lives on the undersurface of host plant leaves. This mite has been investigated for the mode of inheritance of diapause attributes, but scarcely for genetic correlations with other life-history traits. Here, I investigated whether diapause proneness, measured as the proportion of diapausing females under short-day conditions, is genetically correlated with fecundity or development time under long-day conditions using artificial selection experiments. Diapause incidence responded to the selection for both increasing and decreasing directions, suggesting that high genetic variance in diapause proneness is maintained in the study population. However, the change in proportion of diapausing females during the selection period was not associated with responses in fecundity or development time. These results suggest that diapause proneness and other life-history traits have different genetic backgrounds, and thus diapause proneness may freely evolve without being constrained by changes in other life-history traits.     

Timing in the life histories of insects

This paper presents a heuristic model illustrating some major problems in analyzing seasonal life histories of multigeneration insects. The concept of the critical interval is introduced and defined as the age classes that survive at the end of a period of population growth. These conclusions follow from the results: The optimal age for occupying a habitat depends upon the duration of the habitat as well as the life history of the insect. Two positions of the initial age distribution may give local maxima for fitness. The critical interval should often include the youngest age classes to maximize fitness while the optimal position of the initial age distribution may be at a much older age. In this case, conflicts arise between the positions of the critical interval at the end of one growing period and the initial age distribution at the start of the next. The length of the critical interval that maximizes fitness in a particular environment may be relatively small in which case mortality at the end of a growing period may be high and timing would appear to be poor even though fitness is maximized. In this model, optimum generation lengths exist which are not the shortest attainable. Finally, the length of time that a habitat remains suitable influences all of the above results and must be taken into account in analyzing the adaptedness of life history traits.     

LIFE-CYCLE COMPONENTS OF SELECTION IN ERIGERON ANNUUS: I. PHENOTYPIC SELECTION

The magnitude and direction of phenotypic selection on emergence date and seedling size in Erigeron annuus was measured to determine the heterogeneity of selection among sites and the proportion of fitness variance explained by seedling size and emergence date. Three disturbance treatments (open, annual vegetation, perennial vegetation) were imposed to test the hypothesis of stronger selection on seedlings in competitive environments. Selection was most heterogeneous early in the life cycle, with significant spatial heterogeneity in the magnitude of selection on a local scale. The disturbance treatments affected only fecundity selection gradients and selection was strongest in open plots. Significant variation in the sign of selection differentials on emergence date was observed for establishment and fall viability selection episodes; at later stages selection varied in magnitude but not direction. Seedlings in the earliest cohort experienced high mortality during establishment, but increased size and fecundity later in the life cycle. Both stabilizing and disruptive selection on emergence date were observed during establishment, but in general selection was purely directional. At Stony Brook most selection on emergence date operated indirectly through seedling size, whereas at the Weld Preserve direct selection was stronger. There were persistent effects of both seedling emergence date and rosette diameter on adult fitness components, and October rosette diameter explained 18% of the total phenotypic variance in fecundity. Overall, viability fitness components were much more important than fecundity selection. Winter survivorship was the single most important episode of selection.     

THE GENETIC BASIS OF LIFE-HISTORY CHARACTERS IN A POLYCHAETE EXHIBITING PLANKTOTROPHY AND LECITHOTROPHY

The polychaete Streblospio benedicti is unusual in that several field populations consist of individuals that exhibit either planktotrophic or lecithotrophic larval development. Planktotrophy in this species involves production of many small ova that develop into feeding larvae with a two- to three-week planktonic period. Lecithotrophy involves production of fewer, larger ova that develop into nonfeeding larvae that are brooded longer and have a brief planktonic stage. Reciprocal matings were performed to investigate genetic variance components and the correlation structure of life-history traits associated with planktotrophy and lecithotrophy. Our objective was to better understand persistence of this developmental dichotomy in Streblospio benedicti, and among marine invertebrates in general. Substantial additive genetic variation (75–98% of total) was detected for the following characters at first reproduction: female length; position of the first gametogenic setiger and first brood pouch; ovum diameter; three traits related to fecundity (numbers of ova per ovary, larvae per brood pouch, and larvae per brood); median larval planktonic period and the presence of larval swimming setae; but not for total number of brood pouches; larval length; larval feeding; and larval survivorship. Based on the unusual geographic distribution of development modes in this species, we hypothesize that the developmental traits have evolved in allopatry and have only recently come into contact in North Carolina. The high additive contribution to variance observed for many traits may be inflated due to (a) nonrandom breeding in nature, and (b) examination of only one component of an age-structured population at one time. Nuclear interaction variance and maternal variance accounted for 84% of the total variation in larval survivorship. This observation supports other empirical studies and theoretical predictions that nonadditive components of variance will increase in importance in individual traits that are most closely tied to fitness. A network of life-history trait correlations was observed that defines distinct planktotrophic and lecithotrophic trait complexes. Negative genetic correlations were present between fecundity and egg size, between fecundity and position of the first gametes, and between larval survivorship and median planktonic period. Positive genetic correlations were detected between fecundity and female size at first reproduction and between planktonic period and the presence of swimming setae. Intergenerational product-moment correlations were negative for larval length and fecundity, planktonic period and egg size, female size and larval survivorship, and fecundity and larval survivorship. If the genetic correlation structure observed in the laboratory persists in the field, it may constrain responses of individual characters to directional selection, and indirectly perpetuate the dichotomies associated with planktotrophy and lecithotrophy.     

Timing of diapause termination in relation to variation in winter climate

In temperate insects, winters are typically endured by entering diapause, which comprises a deep resting stage. Correct timing of diapause termination is vital for synchronization of emergence with conspecifics and for mobilizing resources when conditions for growth and reproduction become favourable. Although critical to survival, the intrinsic and extrinsic drivers of diapause termination timing are poorly understood. In the present study, we investigate diapause development under a range of durations (10–24 weeks) spent at different temperatures (?2 to 10 °C) in the pupal diapausing butterfly Pieris napi Linnaeus (Lepidoptera:Pieridae). We determine: (i) the maximum cold temperature for diapause development; (ii) if pupae in diapause count cold days or cold sums; and (iii) whether diapause termination is distinct or gradual. The results indicate large and idiosyncratic effects of high and low nonlethal temperatures on diapause development in P. napi. Although all temperatures tested lead to diapause termination, a thermal optimum between 2 and 4 °C is observed. Lower temperatures lead to decreased eclosion propensity, whereas higher temperatures slow down development and increase emergence desynchronization. These data suggest that, rather than a simple cold‐summing process with a distinct diapause termination point, there are trade‐offs between time and temperature at the low and high end of the thermal range, resulting in a nonlinear thermal landscape showing a ridge of increasing eclosion propensity at moderate temperatures. The present study suggests that the effects of temperature on diapause development should be included in projections on post‐winter phenology models of insects, including pest species.     

LIFE-CYCLE COMPONENTS OF SELECTION IN ERIGERON ANNUUS: II. GENETIC VARIATION

Genetic variation for seedling and adult fitness components was measured under natural conditions to determine the relative importance of the seedling stage for lifetime fitness in Erigeron annuus. Variation in lifetime reproductive success can result from both the persistent effects of genetic variation expressed among seedlings and from variation in adult fitness components. Analysis of covariance was used to separate the stage specific from the cumulative effects of genetic variance expressed earlier in the life cycle. E. annuus produces seeds through apomixis, which allowed measurement of the fitness of replicate genotypes from germination through the entire life cycle. There were significant differences among genotypes for date of emergence, seedling size, survivorship and fecundity, but heritabilities were low, indicating slow response to selection. For all characters, environmental components of variance were one to two orders of magnitude larger than genetic variance components, resulting in broad sense heritabilities less than 0.1. For seedling size and fecundity, all of the genetic variance was in the form of genotype-environment interactions, often with large negative genetic correlations across environments. In contrast, genotypes differed in mean survivorship through one year, but there were no genotype-environment interactions for viability. Genetic differences in viability were primarily expressed as differences in overwinter survivorship. Genotype × environment interactions among sites and blocks were generated early in the life cycle while the genotype × environment interactions in response to competitive environment (open, annual cover, perennial cover) first appeared in adult fecundity. Genetic variation in lifetime fitness was not significant, despite a fourfold difference in mean fitness among genotypes.     

The fitness effects of delayed switching to sex in a facultatively asexual insect

Facultative reproductive strategies that incorporate both sexual and parthenogenetic reproduction should be optimal, yet are rarely observed in animals. Resolving this paradox requires an understanding of the economics of facultative asexuality. Recent work suggests that switching from parthenogenesis to sex can be costly and that females can resist mating to avoid switching. However, it remains unclear whether these costs and resistance behaviors are dependent on female age. We addressed these questions in the Cyclone Larry stick insect, Sipyloidea larryi, by pairing females with males (or with females as a control) in early life prior to the start of parthenogenetic reproduction, or in mid‐ or late life after a period of parthenogenetic oviposition. Young females were receptive to mating even though mating in early life caused reduced fecundity. Female resistance to mating increased with age, but reproductive switching in mid‐ or late life did not negatively affect female survival or offspring performance. Overall, mating enhanced female fitness because fertilized eggs had higher hatching success and resulted in more adult offspring than parthenogenetic eggs. However, female fecundity and offspring viability were also enhanced in females paired with other females, suggesting a socially mediated maternal effect. Our results provide little evidence that switching from parthenogenesis to sex at any age is costly for S. larryi females. However, age‐dependent effects of switching on some fitness components and female resistance behaviors suggest the possibility of context‐dependent effects that may only be apparent in natural populations.     

松毛虫赤眼蜂滞育诱导及解除条件研究

【目的】以柞蚕Antheraea pernyi卵为繁殖寄主,对松毛虫赤眼蜂Trichogramma dendrolim滞育诱导及解除条件进行研究,以解决赤眼蜂工厂化生产和大面积应用中面临的的中、长期储存问题。【方法】通过观测不同发育阶段(寄生柞蚕卵在26℃培养40、96和144 h)、滞育诱导温度(10、13和16℃)和诱导时间对松毛虫赤眼蜂滞育的影响,确定松毛虫赤眼蜂滞育诱导条件;通过观测滞育诱导温度和滞育后的贮藏温度对滞育解除的影响,确定松毛虫赤眼蜂滞育解除条件。【结果】在松毛虫赤眼蜂的不同发育阶段对其进行持续的低温刺激均能使其导入滞育,但以小幼阶段(26℃培养40 h)开始效果最佳,寄生卵在26℃培养40 h后,转入10℃和13℃下连续诱导31 d,滞育率可达100%和99.12%。滞育诱导温度和滞育后的贮藏温度对松毛虫赤眼蜂解除滞育所需时间和解除滞育后的羽化出蜂率有较大影响,10℃诱导滞育后置于1℃冷藏的赤眼蜂解除滞育所需时间最短,解除滞育后的羽化出蜂率和单卵出蜂数更高,更耐储存。此条件下冷藏约30 d开始打破滞育,在正常发育下温度下羽化出蜂,60 d羽化出蜂率达到95.24%,冷藏4个月后羽化出蜂率仍在60%以上,单卵出蜂数高于50头。【结论】松毛虫赤眼蜂最佳滞育诱导条件为26℃培养40 h后,转入10℃连续低温诱导31 d;最佳滞育解除条件为1℃低温储存,但储存期不能超过4个月。     

Variability in diapause duration in the chestnut weevil: mixed ESS,genetic polymorphism or bet‐hedging?

Within-generation variability in diapause duration can be viewed either as a mixed Evolutionary Stable Strategy (ESS), a genetic polymorphism of pure strategies, or as bet-hedging. Diapause variability expressed by a single genotype that maximizes mean geometric fitness at the cost of mean arithmetic fitness is a bet-hedging strategy. Bet-hedging differs from mixed ESS and stable genetic polymorphism of pure strategies because in these latter the expected pay-offs for all phenotypes are equal. In insects, individuals with a prolonged diapause (long cycle) lose at least one reproductive opportunity and suffer lower survival before reproduction than those with a short diapause (short cycle). If long-cycle individuals compensate this cost by better adult performance, the compensation leads to a trade-off which could result in mixed ESS or genetic polymorphism of pure strategies since the overall fitness of the two morphs may be similar. In this paper, we show that in the chestnut weevil Curculio elephas adult performance, measured as sex ratio, longevity, weight, and realized fecundity of females, are similar in individuals emerged after one and two years. Long-cycle morphs emerge slightly before short-cycle ones but this eventual advantage for fertility probably does not compensate higher larval mortality and missed reproductive opportunity in long-cycle phenotypes. Therefore, the cost associated with prolonged diapause cannot be completely compensated for by a better adult performance. From these results, and previous data, we conclude that variability in diapause duration cycle is better explained as bet-hedging than mixed ESS or genetic polymorphism of pure strategies.     

Effects of inbreeding on variation in diapause duration and early fecundity in the Kanzawa spider mite

In arthropods, both diapause duration and ability to produce eggs in early adult life (early fecundity) are important life‐history traits for successful settling in a new habitat. In herbivorous spider mites (Acari: Tetranychidae), inbreeding frequently occurs because new colonies are established by only one or a few females. In the present study, we investigated the impact of inbreeding on the phenotypic variance in diapause duration and early fecundity in the Kanzawa spider mite, Tetranychus kanzawai Kishida. Diapause duration was compared between the inbreeding treatment (strongly inbred strains) and the control (strains immediately taken from the stock culture) under winter‐mimicking laboratory conditions. The variance in diapause duration was smaller in the inbreeding treatment than in the control, though the magnitude of decrease of variance was less than expected. On the other hand, early fecundity did not show any reduction of variance. The present results revealed that inbreeding reduces phenotypic variation, as expected in theory.     

Laboratory relationships between adult lifetime reproductive success and fitness surrogates in a Drosophila littoralis population

The difficulties in measuring total fitness of individuals necessitate the use of fitness surrogates in ecological and evolutionary studies. These surrogates can be different components of fitness (e.g. survival or fecundity), or proxies more uncertainly related to fitness (e.g. body size or growth rate). Ideally, fitness would be measured over the lifetime of individuals; however, more convenient short-time measures are often used. Adult lifetime reproductive success (adult LRS) is closely related to the total fitness of individuals, but it is difficult to measure and rarely included in fitness estimation in experimental studies. We explored phenotypic correlations between female adult LRS and various commonly used fitness components and proxies in a recently founded laboratory population of Drosophila littoralis. Noting that survival is usually higher in laboratory conditions than in nature, we also calculated adjusted adult LRS measures that give more weight to early reproduction. The lifetime measures of fecundity, longevity, and offspring viability were all relatively highly correlated with adult LRS. However, correlations with short-time measures of fecundity and offspring production varied greatly depending on the time of measurement, and the optimal time for measurement was different for unadjusted compared to adjusted adult LRS measures. Correlations between size measures and adult LRS varied from weak to modest, leg size and female weight having the highest correlations. Our results stress the importance of well-founded choice of fitness surrogates in empirical research.     

The diapause syndrome in the seed-eating caterpillar of Coleophora alticolella

Adult emergence in Coleophora alticolella held at 15 °C was accelerated by exposure to L18:D6 in autumn and midwinter. The effect decreased during winter and exposure of individuals, held at low temperature over winter, to L18:D6 or L6:D18 at 15 °C at the end of March resulted in the same mean emergence date. Long daylength experienced at 5 °C did not promote emergence nor did exposure to low temperature during winter. The number of adults emerging increased with the length of time cultures were held on short day but was always below 50% of the larvae. When larvae were exposed to L18:D6 and L6:D18 at 15 °C at the end of March, on long day 61% adults emerged and 39% remained in diapause, whereas on short day, 25% became adult and 75% remained diapausing larvae. The possibility of cohort splitting, with some individuals undergoing prolonged diapause, is discussed.     

Quantifying the Role of Population Subdivision in Evolution on Rugged Fitness Landscapes

Natural selection drives populations towards higher fitness, but crossing fitness valleys or plateaus may facilitate progress up a rugged fitness landscape involving epistasis. We investigate quantitatively the effect of subdividing an asexual population on the time it takes to cross a fitness valley or plateau. We focus on a generic and minimal model that includes only population subdivision into equivalent demes connected by global migration, and does not require significant size changes of the demes, environmental heterogeneity or specific geographic structure. We determine the optimal speedup of valley or plateau crossing that can be gained by subdivision, if the process is driven by the deme that crosses fastest. We show that isolated demes have to be in the sequential fixation regime for subdivision to significantly accelerate crossing. Using Markov chain theory, we obtain analytical expressions for the conditions under which optimal speedup is achieved: valley or plateau crossing by the subdivided population is then as fast as that of its fastest deme. We verify our analytical predictions through stochastic simulations. We demonstrate that subdivision can substantially accelerate the crossing of fitness valleys and plateaus in a wide range of parameters extending beyond the optimal window. We study the effect of varying the degree of subdivision of a population, and investigate the trade-off between the magnitude of the optimal speedup and the width of the parameter range over which it occurs. Our results, obtained for fitness valleys and plateaus, also hold for weakly beneficial intermediate mutations. Finally, we extend our work to the case of a population connected by migration to one or several smaller islands. Our results demonstrate that subdivision with migration alone can significantly accelerate the crossing of fitness valleys and plateaus, and shed light onto the quantitative conditions necessary for this to occur.