Probabilistic maturation reaction norms assessed from mark–recaptures of wild fish in their natural habitat

Reaction norms are a valuable tool in evolutionary biology. Lately, the probabilistic maturation reaction norm approach, describing probabilities of maturing at combinations of age and body size, has been much applied for testing whether phenotypic changes in exploited populations of fish are mainly plastic or involving an evolutionary component. However, due to typical field data limitations, with imperfect knowledge about individual life histories, this demographic method still needs to be assessed. Using 13 years of direct mark–recapture observations on individual growth and maturation in an intensively sampled population of brown trout (Salmo trutta), we show that the probabilistic maturation reaction norm approach may perform well even if the assumption of equal survival of juvenile and maturing fish does not hold. Earlier studies have pointed out that growth effects may confound the interpretation of shifts in maturation reaction norms, because this method in its basic form deals with body size rather than growth. In our case, however, we found that juvenile body size, rather than annual growth, was more strongly associated with maturation. Viewed against earlier studies, our results also underscore the challenges of generalizing life‐history patterns among species and populations.     

Parasites and human evolution

Our understanding of human evolutionary and population history can be advanced by ecological and evolutionary studies of our parasites. Many parasites flourish only in the presence of very specific human behaviors and in specific habitats, are wholly dependent on us, and have evolved with us for thousands or millions of years. Therefore, by asking when and how we first acquired those parasites, under which environmental and cultural conditions we are the most susceptible, and how the parasites have evolved and adapted to us and we in response to them, we can gain considerable insight into our own evolutionary history. 1 , 2 As examples, the tapeworm life cycle is dependent on our consumption of meat,³ the divergence of body and head lice may have been subsequent to the development of clothing, 4 , 5 and malaria hyperendemicity may be associated with agriculture. 6 Thus, the evolutionary and population histories of these parasites are likely intertwined with critical aspects of human biology and culture. Here I review the mechanics of these and multiple other parasite proxies for human evolutionary history and discuss how they currently complement our fossil, archeological, molecular, linguistic, historical, and ethnographic records. I also highlight potential future applications of this promising model for the field of evolutionary anthropology.     

Picking personalities apart: estimating the influence of predation,sex and body size on boldness in the guppy Poecilia reticulata

Predation is a strong selective force in most natural systems, potentially fueling evolutionary changes in prey morphology, life history and behaviour. Recent work has suggested that contrasting predation pressures may lead to population differentiation in personality traits. However, there are indications that these personality traits also differ between sexes and not necessarily in a consistent way between populations. We used an integrative approach to quantify boldness (latency to emerge from a shelter) in wild‐caught guppies in relation to predation pressure, population origin, sex and size. In addition we quantified the repeatability of these personality traits. We show that predation regime had significant effects on emergence time. In general, fish from high predation localities emerged sooner from the shelter compared to those from low predation localities. We found strong sex differences; males were significantly bolder than females. The relationship between emergence time and body size was non‐significant in all populations. We discuss what responses to expect from predator‐naïve versus predator‐experienced individuals and how this can be linked to the shyness–boldness continuum.     

Evolutionary divergence in life history traits among populations of the Lake Malawi cichlid fish Astatotilapia calliptera

During the early stages of adaptive radiation, populations diverge in life history traits such as egg size and growth rates, in addition to eco‐morphological and behavioral characteristics. However, there are few studies of life history divergence within ongoing adaptive radiations. Here, we studied Astatotilapia calliptera, a maternal mouthbrooding cichlid fish within the Lake Malawi haplochromine radiation. This species occupies a rich diversity of habitats, including the main body of Lake Malawi, as well as peripheral rivers and shallow lakes. We used common garden experiments to test for life history divergence among populations, focussing on clutch size, duration of incubation, egg mass, offspring size, and growth rates. In a first experiment, we found significant differences among populations in average clutch size and egg mass, and larger clutches were associated with smaller eggs. In a second experiment, we found significant differences among populations in brood size, duration of incubation, juvenile length when released, and growth rates. Larger broods were associated with smaller juveniles when released and shorter incubation times. Although juvenile growth rates differed between populations, these were not strongly related to initial size on release. Overall, differences in life history characters among populations were not predicted by major habitat classifications (Lake Malawi or peripheral habitats) or population genetic divergence (microsatellite‐based F_ST). We suggest that the observed patterns are consistent with local selective forces driving the observed patterns of trait divergence. The results provide strong evidence of evolutionary divergence and covariance of life history traits among populations within a radiating cichlid species, highlighting opportunities for further work to identify the processes driving the observed divergence.     

Recent parallel divergence in body shape and diet source of alewife life history forms

Recent work suggests that juvenile alewives (Alosa pseudoharengus) share similar phenotypes among independently derived landlocked (freshwater resident) populations. Based on this observation, it is possible that the alewife life history forms represent a case of parallel adaptive divergence. To further evaluate this hypothesis, we describe patterns of body shape divergence between anadromous and landlocked alewife life history forms using geometric morphometrics. Our results suggest that body shape differs significantly between juveniles of the alewife forms: anadromous fish were more robust, with larger heads and deeper caudal peduncles, while landlocked fish from three independently isolated populations were more fusiform with thinner caudal peduncles and smaller heads. These differences matched population level dietary patterns, which suggest that anadromous fish consumed more littoral resources than landlocked fish. Finding consistent differences across populations of the same form supports the notion that landlocked alewives have diverged from their anadromous ancestors in a parallel manner, in response to pressures associated with being isolated in freshwater lakes. Comparing alewife phenotypes to expectations from the literature suggests that neither migration distance of the population, nor the relative availability of habitats in each lake, are likely drivers of the pattern we report. Instead, the pattern is consistent with the hypothesis that divergence between alewife forms results from the distinct effects of each form on its zooplankton prey.     

Reversal of evolutionary downsizing caused by selective harvest of large fish

Evolutionary responses to the long-term exploitation of individuals from a population may include reduced growth rate, age at maturation, body size and productivity. Theoretical models suggest that these genetic changes may be slow or impossible to reverse but rigorous empirical evidence is lacking. Here, we provide the first empirical demonstration of a genetically based reversal of fishing-induced evolution. We subjected six populations of silverside fish (Menidia menidia) to three forms of size-selective fishing for five generations, thereby generating twofold differences among populations in mean weight and yield (biomass) at harvest. This was followed by an additional five generations during which size-selective harvest was halted. We found that evolutionary changes were reversible. Populations evolving smaller body size when subjected to size-selective fishing displayed a slow but significant increase in size when fishing ceased. Neither phenotypic variance in size nor juvenile survival was reduced by the initial period of selective fishing, suggesting that sufficient genetic variation remained to allow recovery. By linear extrapolation, we predict full recovery in about 12 generations, although the rate of recovery may taper off near convergence. The recovery rate in any given wild population will also depend on other agents of selection determined by the specifics of life history and environment. By contrast, populations that in the first five generations evolved larger size and yield showed little evidence of reversal. These results show that populations have an intrinsic capacity to recover genetically from harmful evolutionary changes caused by fishing, even without extrinsic factors that reverse the selection gradient. However, harvested species typically have generation times of 3–7 years, so recovery may take decades. Hence, the need to account for evolution in managing fisheries remains.     

The role of fish life histories in allometrically scaled food‐web dynamics

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Trawl selectivity‐induced evolution effects on age structure and size‐at‐age of largehead hairtail (Trichiurus lepturus) Linnaeus, 1758 in the East China Sea,China

Increasing evidence has demonstrated that the life history traits of fishes have changed in many exploited populations, caused principally by intense fishing mortality and size‐selectivity of the fishing gear. Broad and intensive trawl fishing over an extended period has the enormous potential to change the biological characters of exploited fish populations. An individual‐based model was developed to explore the interactions between trawl fishing and evolutionary changes in length‐at‐age and age structure of an exploited fish population. A perennial fish population was simulated with a multiple age structure in the model to examine the effects of long‐term trawl fishing on hairtail, Trichiurus lepturus, in the East China Sea. The results revealed that distribution of the body length‐at‐age and the age structure of the fish population were irreversibly changed under long‐term trawl fishing. The simulated results confirm that the length‐at‐age is increasing shorter, the younger individuals dominate, the influence of trawl selectivity on the biological traits of the fish population is highly significant, and that these changes have potentially evolutionary consequences on the fish body length‐at‐age.     

Population Variation in the Life History of a Land Fish,Alticus arnoldorum,and the Effects of Predation and Density

Life history variation can often reflect differences in age-specific mortality within populations, with the general expectation that reproduction should be shifted away from ages experiencing increased mortality. Investigators of life history in vertebrates frequently focus on the impact of predation, but there is increasing evidence that predation may have unexpected impacts on population density that in turn prompt unexpected changes in life history. There are also other reasons why density might impact life history independently of predation or mortality more generally. We investigated the consequences of predation and density on life history variation among populations of the Pacific leaping blenny, Alticus arnoldorum. This fish from the island of Guam spends its adult life out of the water on rocks in the splash zone, where it is vulnerable to predation and can be expected to be sensitive to changes in population density that impact resource availability. We found populations invested more in reproduction as predation decreased, while growth rate varied primarily in response to population density. These differences in life history among populations are likely plastic given the extensive gene flow among populations revealed by a previous study. The influence of predation and density on life history was unlikely to have operated independently of each other, with predation rate tending to be associated with reduced population densities. Taken together, our results suggest predation and density can have complex influences on life history, and that plastic life history traits could allow populations to persist in new or rapidly changing environments.     

Simulated trapping and trawling exert similar selection on fish morphology

Commercial fishery harvest can influence the evolution of wild fish populations. Our knowledge of selection on morphology is however limited, with most previous studies focusing on body size, age, and maturation. Within species, variation in morphology can influence locomotor ability, possibly making some individuals more vulnerable to capture by fishing gears. Additionally, selection on morphology has the potential to influence other foraging, behavioral, and life‐history related traits. Here we carried out simulated fishing using two types of gears: a trawl (an active gear) and a trap (a passive gear), to assess morphological trait‐based selection in relation to capture vulnerability. Using geometric morphometrics, we assessed differences in shape between high and low vulnerability fish, showing that high vulnerability individuals display shallower body shapes regardless of gear type. For trawling, low vulnerability fish displayed morphological characteristics that may be associated with higher burst‐swimming, including a larger caudal region and narrower head, similar to evolutionary responses seen in fish populations responding to natural predation. Taken together, these results suggest that divergent selection can lead to phenotypic differences in harvested fish populations.     

Contribution of comparative fish studies to general endocrinology: structure and function of some osmoregulatory hormones

Fish endocrinologists are commonly motivated to pursue their research driven by their own interests in these aquatic animals. However, the data obtained in fish studies not only satisfy their own interests but often contribute more generally to the studies of other vertebrates, including mammals. The life of fishes is characterized by the aquatic habitat, which demands many physiological adjustments distinct from the terrestrial life. Among them, body fluid regulation is of particular importance as the body fluids are exposed to media of varying salinities only across the thin respiratory epithelia of the gills. Endocrine systems play pivotal roles in the homeostatic control of body fluid balance. Judging from the habitat-dependent control mechanisms, some osmoregulatory hormones of fish should have undergone functional and molecular evolution during the ecological transition to the terrestrial life. In fact, water-regulating hormones such as vasopressin are essential for survival on the land, whereas ion-regulating hormones such as natriuretic peptides, guanylins and adrenomedullins are diversified and exhibit more critical functions in aquatic species. In this short review, we introduce some examples illustrating how comparative fish studies contribute to general endocrinology by taking advantage of such differences between fishes and tetrapods. In a functional context, fish studies often afford a deeper understanding of the essential actions of a hormone across vertebrate taxa. Using the natriuretic peptide family as an example, we suggest that more functional studies on fishes will bring similar rewards of understanding. At the molecular level, recent establishment of genome databases in fishes and mammals brings clues to the evolutionary history of hormone molecules via a comparative genomic approach. Because of the functional and molecular diversification of ion-regulating hormones in fishes, this approach sometimes leads to the discovery of new hormones in tetrapods as exemplified by adrenomedullin 2.     

Life‐history strategy varies with the strength of competition in a food‐limited ungulate population

Fluctuating population density in stochastic environments can contribute to maintain life‐history variation within populations via density‐dependent selection. We used individual‐based data from a population of Soay sheep to examine variation in life‐history strategies at high and low population density. We incorporated life‐history trade‐offs among survival, reproduction and body mass growth into structured population models and found support for the prediction that different life‐history strategies are optimal at low and high population densities. Shorter generation times and lower asymptotic body mass were selected for in high‐density environments even though heavier individuals had higher probabilities to survive and reproduce. In contrast, greater asymptotic body mass and longer generation times were optimal at low population density. If populations fluctuate between high density when resources are scarce, and low densities when they are abundant, the variation in density will generate fluctuating selection for different life‐history strategies, that could act to maintain life‐history variation.     

INVESTIGATING EVOLUTIONARY TRADE‐OFFS IN WILD POPULATIONS OF ATLANTIC SALMON (SALMO SALAR): INCORPORATING DETECTION PROBABILITIES AND INDIVIDUAL HETEROGENEITY

Evolutionary trade‐offs among demographic parameters are important determinants of life‐history evolution. Investigating such trade‐offs under natural conditions has been limited by inappropriate analytical methods that fail to address the bias in demographic estimates that can result when issues of detection (uncertain detection of individual) are ignored. We propose a new statistical approach to quantify evolutionary trade‐offs in wild populations. Our method is based on a state‐space modeling framework that focuses on both the demographic process of interest as well as the observation process. As a case study, we used individual mark–recapture data for stream‐dwelling Atlantic salmon juveniles in the Scorff River (Southern Brittany, France). In freshwater, juveniles face two life‐history choices: migration to the ocean and sexual maturation (for males). Trade‐offs may appear with these life‐history choices and survival, because all are energy dependent. We found a cost of reproduction on survival for fish staying in freshwater and a survival advantage associated with the “decision” to migrate. Our modeling framework opens up promising prospects for the study of evolutionary trade‐offs when some life‐history traits are not, or only partially, observable.     

Evidence for small scale variation in the vertebrate brain: mating strategy and sex affect brain size and structure in wild brown trout (Salmo trutta)

The basis for our knowledge of brain evolution in vertebrates rests heavily on empirical evidence from comparative studies at the species level. However, little is still known about the natural levels of variation and the evolutionary causes of differences in brain size and brain structure within‐species, even though selection at this level is an important initial generator of macroevolutionary patterns across species. Here, we examine how early life‐history decisions and sex are related to brain size and brain structure in wild populations using the existing natural variation in mating strategies among wild brown trout (Salmo trutta). By comparing the brains of precocious fish that remain in the river and sexually mature at a small size with those of migratory fish that migrate to the sea and sexually mature at a much larger size, we show, for the first time in any vertebrate, strong differences in relative brain size and brain structure across mating strategies. Precocious fish have larger brain size (when controlling for body size) but migratory fish have a larger cerebellum, the structure in charge of motor coordination. Moreover, we demonstrate sex‐specific differences in brain structure as female precocious fish have a larger brain than male precocious fish while males of both strategies have a larger telencephalon, the cognitive control centre, than females. The differences in brain size and structure across mating strategies and sexes thus suggest the possibility for fine scale adaptive evolution of the vertebrate brain in relation to different life histories.     

Experimental evidence of gradual size‐dependent shifts in body size and growth of fish in response to warming

A challenge facing ecologists trying to predict responses to climate change is the few recent analogous conditions to use for comparison. For example, negative relationships between ectotherm body size and temperature are common both across natural thermal gradients and in small‐scale experiments. However, it is unknown if short‐term body size responses are representative of long‐term responses. Moreover, to understand population responses to warming, we must recognize that individual responses to temperature may vary over ontogeny. To enable predictions of how climate warming may affect natural populations, we therefore ask how body size and growth may shift in response to increased temperature over life history, and whether short‐ and long‐term growth responses differ. We addressed these questions using a unique setup with multidecadal artificial heating of an enclosed coastal bay in the Baltic Sea and an adjacent reference area (both with unexploited populations), using before‐after control‐impact paired time‐series analyses. We assembled individual growth trajectories of ~13,000 unique individuals of Eurasian perch and found that body growth increased substantially after warming, but the extent depended on body size: Only among small‐bodied perch did growth increase with temperature. Moreover, the strength of this response gradually increased over the 24 year warming period. Our study offers a unique example of how warming can affect fish populations over multiple generations, resulting in gradual changes in body growth, varying as organisms develop. Although increased juvenile growth rates are in line with predictions of the temperature–size rule, the fact that a larger body size at age was maintained over life history contrasts to that same rule. Because the artificially heated area is a contemporary system mimicking a warmer sea, our findings can aid predictions of fish responses to further warming, taking into account that growth responses may vary both over an individual's life history and over time.     

Heterogeneity of genetic architecture of body size traits in a free‐living population

Knowledge of the underlying genetic architecture of quantitative traits could aid in understanding how they evolve. In wild populations, it is still largely unknown whether complex traits are polygenic or influenced by few loci with major effect, due to often small sample sizes and low resolution of marker panels. Here, we examine the genetic architecture of five adult body size traits in a free‐living population of Soay sheep on St Kilda using 37 037 polymorphic SNPs. Two traits (jaw and weight) show classical signs of a polygenic trait: the proportion of variance explained by a chromosome was proportional to its length, multiple chromosomes and genomic regions explained significant amounts of phenotypic variance, but no SNPs were associated with trait variance when using GWAS. In comparison, genetic variance for leg length traits (foreleg, hindleg and metacarpal) was disproportionately explained by two SNPs on chromosomes 16 (s23172.1) and 19 (s74894.1), which each explained >10% of the additive genetic variance. After controlling for environmental differences, females heterozygous for s74894.1 produced more lambs and recruits during their lifetime than females homozygous for the common allele conferring long legs. We also demonstrate that alleles conferring shorter legs have likely entered the population through a historic admixture event with the Dunface sheep. In summary, we show that different proxies for body size can have very different genetic architecture and that dense SNP helps in understanding both the mode of selection and the evolutionary history at loci underlying quantitative traits in natural populations.     

Demography and genome divergence of lake and stream populations of an East African cichlid fish

Disentangling the processes and mechanisms underlying adaptive diversification is facilitated by the comparative study of replicate population pairs that have diverged along a similar environmental gradient. Such a setting is realized in a cichlid fish from southern Lake Tanganyika, Astatotilapia burtoni, which occurs within the lake proper as well as in various affluent rivers. Previously, we demonstrated that independent lake and stream populations show similar adaptations to the two habitat regimes. However, little is known about the evolutionary and demographic history of the A. burtoni populations in question and the patterns of genome divergence among them. Here, we apply restriction site‐associated DNA sequencing (RADseq) to examine the evolutionary history, the population structure and genomic differentiation of lake and stream populations in A. burtoni. A phylogenetic reconstruction based on genome‐wide molecular data largely resolved the evolutionary relationships among populations, allowing us to re‐evaluate the independence of replicate lake–stream population clusters. Further, we detected a strong pattern of isolation by distance, with baseline genomic divergence increasing with geographic distance and decreasing with the level of gene flow between lake and stream populations. Genome divergence patterns were heterogeneous and inconsistent among lake‐stream population clusters, which is explained by differences in divergence times, levels of gene flow and local selection regimes. In line with the latter, we only detected consistent outlier loci when the most divergent lake–stream population pair was excluded. Several of the thus identified candidate genes have inferred functions in immune and neuronal systems and show differences in gene expression between lake and stream populations.     

Variation in life history and genetic traits of Hawaiian mosquitofish populations

Mosquitofish (Gambusia affinis) were collected from 17 reservoirs on three islands in Hawaii, USA. Genetic and life history traits for adult females from these populations were used to evaluate hypotheses concerning short-term evolutionary divergence of populations recently established from a common ancestral source. The effects of founder events and drift on genetic variability and population differentiation were also examined. Significant differences in life history characteristics, allele frequencies, and multi-locus heterozygosities (H) were found among fish populations collected from different reservoirs and between reservoirs classified as stable or fluctuating on the basis of temporal fluctuation in water level. Females from stable reservoirs exhibited greater standard length (35.1 vs 32.8 mm), lower fecundity (11.9 vs 15.2 embryos), lower reproductive allocation (18.2% vs 22.8%), but larger mean embryo size (1.95 vs 1.67 mg) than females from fluctuating reservoirs. Consistency in means among replicates of each reservoir class and concordance in direction and magnitude of differences reported here and results of sampling conducted from these same locations 10 years previously (Stearns, 1983a) suggest that ecological factors intrinsic to these two environments are important in determing population life history traits. Females from stable reservoirs exhibited lower heterozygosity than females from fluctuating reservoirs (0.134 vs 0.158, respectively). Levels and direction od differences in heterozygosity, the high proportion of polymorphic loci and lack of fixation of alternative alleles argue against a purely stochastic explanation for genetic and life history variation among reservoir populations. Levels of genetic variability and interpopulation differentiation were similar to those observed in mainland populations of this species. A high proportion of the genetic diversity was apportioned between populations and within populations due to differences between juveniles and adults. Significant genotypic differences between adult and juvenile age classes suggest that the genetic divergence of local populations may occur over short periods of time.     

Adaptations of Cave Fishes with some Comparisons to Deep-sea Fishes

I provide my retrospective and prospective views on adaptations of cave fishes. I emphasize the history of my insights into cave adaptation from 45 years of research using surface, cave-spring, and cave species of amblyopsid fishes. My approach has been to use natural experiments and to always consider multiple hypotheses. To clarify evolutionary adaptations, I show the importance of a broad comparative approach which includes studies of morphology, metabolic physiology, foraging behavior, life history, and ecology. And I show that the most important agents of selection, of darkness and attendant low food supply, are best understood in the context of rigor, variability, and predictability. I also present my insights from what I consider the most insightful contributions on deep-sea fishes. The contributions are those of Marshall in studies of morphology in relation to energy economy of pelagic and benthic species, Childress in studies of physiological and biochemical adaptations with depth for pelagic species, and Koslow in studies on population biology and life history of bathybenthic and benthic sea-mount species. Compared to caves, I suggest that the extremes of metabolic and life history adaptations of deep-sea fish are explained by a longer evolutionary history and a much greater habitat range, food supply, and predation risk. Finally, I take a retrospective view of what we have learned about cave fishes. I discuss possible evolutionary mechanisms that can explain the trends with increasing cave adaptation in amblyopsid fishes, especially progenesis and the pleiotropic effects of the stress resistance syndrome. Finally, based on insights from deep-sea fishes, and emerging new techniques, I suggest what cave fish biologists should do in the future.     

Inter‐island divergence within Drosophila mauritiana,a species of the D. simulans complex: Past history and/or speciation in progress?

Speciation with gene flow may be more common than generally thought, which makes detailed understanding of the extent and pattern of genetic divergence between geographically isolated populations useful. Species of the Drosophila simulans complex provide a good model for speciation and evolutionary studies, and hence understanding their population genetic structure will increase our understanding of the context in which speciation has occurred. Here, we describe genetic diversity and genetic differentiation of two distant populations of D. mauritiana (Mauritius and Rodrigues Islands) at mitochondrial and nuclear loci. We surveyed the two populations for their mitochondrial haplotypes, eight nuclear genes and 18 microsatellite loci. A new mitochondrial type is fixed in the Rodrigues population of D. mauritiana. The two populations are highly differentiated, their divergence appears relatively ancient (100,000 years) compared to the origin of the species, around 0.25MYA, and they exhibit very limited gene flow. However, they have similar levels of divergence from their sibling, D. simulans. Both nuclear genes and microsatellites revealed contrasting demographic histories between the two populations, expansion for the Mauritius population and stable population size for the Rodrigues Island population. The discovery of pronounced geographic structure within D. mauritiana combined to genetic structuring and low gene flow between the two island populations illuminates the evolutionary history of the species and clearly merits further attention in the broad context of speciation.