The pace of modern life II: from rates of contemporary microevolution to pattern and process

We compiled a database of microevolution on contemporary time scales in nature (47 source articles; 30 animal species), comprising 2649 evolutionary rates in darwins (proportional change per million years) and 2151 evolutionary rates in haldanes (standard deviations per generation). Here we demonstrate how quantitative rate measures can provide general insights into patterns and processes of evolution. The frequency distribution of evolutionary rates was approximately log-normal, with many slow rates and few fast rates. Net selection intensities estimated from haldanes were on average lower than selection intensities commonly measured directly in natural populations. This difference suggests that natural selection could easily accomplish observed microevolution but that the intensities of selection typically measured in nature are rarely maintained for long (otherwise observed evolutionary rates would be higher). Traits closely associated with fitness (life history traits) appear to evolve at least as fast as traits less closely tied to fitness (morphology). The magnitude of evolutionary difference increased with the length of the time interval, particularly when maximum rates from a given study were considered. This pattern suggests a general underlying tendency toward increasing evolutionary diversification with time. However, evolutionary rates also tended to decrease with time, perhaps because longer time intervals average increasingly disparate rates over time, or because evolution slows when populations approach new optima or as genetic variation is depleted. In combination, our results suggest that macroevolutionary transitions may ultimately arise through microevolution occasionally writ large but are perhaps temporally characterized by microevolution writ in fits and starts.     

Population variation in sexual selection and its effect on size allometry in two dung fly species with contrasting sexual size dimorphism

Body size is one of the most important quantitative traits under evolutionary scrutiny. Sexual size dimorphism (SSD) in a given species is expected to result if opposing selection forces equilibrate differently in both sexes. We document variation in the intensity of sexual and fecundity selection, male and female body size, and thus SSD among 31 and 27 populations of the two dung fly species, Scathophaga stercoraria and Sepsis cynipsea, across Switzerland. Whereas in S. cynipsea females are larger, the SSD is reversed in S. stercoraria. We comprehensively evaluated Fairbairn and Preziosi's (1994) general, three-tiered scenario, hypothesizing that sexual selection for large male size is the major driving force of SSD allometry within these two species. Sexual selection intensity on male size in the yellow dung fly, S. stercoraria, was overall positive, greater, and more variable among populations than fecundity selection on females. Also, sexual selection intensity in a given population correlated positively with mean male body size of that population for both the field-caught fathers and their laboratory-reared sons, indicating a response to selection. In S. cvnipsea, sexual selection intensity on males was lower overall and significantly positive, about equal in magnitude, but more variable than fecundity selection on females. However, there was no correlation between the intensity of sexual selection and mean male body size among populations. In both species, the laboratory-reared offspring indicate genetic differentiation among populations in body size. Despite fulfillment of all key prerequisites, at least in S. stercoraria, we did not find hypoallometry for SSD (Rensch's rule, i.e., greater evolutionary divergence in male size than female size) for the field-caught parents or the laboratory-reared offspring: Female size was isometric to male size in both species. We conclude that S. cynipsea does not fit some major requirements of Fairbairn and Preziosi's (1994) scenario, whereas for S. stercoraria we found partial support for it. Failure to support Rensch's rule within the latter species may be due to phylogenetic or other constraints, power limitations, erroneous estimates of sexual selection, insufficient genetic isolation of populations, or sex differences in viability selection against large size.     

Sexual selection and sexual size dimorphism in animals

Sexual selection is often considered as a critical evolutionary force promoting sexual size dimorphism (SSD) in animals. However, empirical evidence for a positive relationship between sexual selection on males and male-biased SSD received mixed support depending on the studied taxonomic group and on the method used to quantify sexual selection. Here, we present a meta-analytic approach accounting for phylogenetic non-independence to test how standardized metrics of the opportunity and strength of pre-copulatory sexual selection relate to SSD across a broad range of animal taxa comprising up to 95 effect sizes from 59 species. We found that SSD based on length measurements was correlated with the sex difference in the opportunity for sexual selection but showed a weak and statistically non-significant relationship with the sex difference in the Bateman gradient. These findings suggest that pre-copulatory sexual selection plays a limited role for the evolution of SSD in a broad phylogenetic context.     

Mainland size variation informs predictive models of exceptional insular body size change in rodents

The tendency for island populations of mammalian taxa to diverge in body size from their mainland counterparts consistently in particular directions is both impressive for its regularity and, especially among rodents, troublesome for its exceptions. However, previous studies have largely ignored mainland body size variation, treating size differences of any magnitude as equally noteworthy. Here, we use distributions of mainland population body sizes to identify island populations as ‘extremely’ big or small, and we compare traits of extreme populations and their islands with those of island populations more typical in body size. We find that although insular rodents vary in the directions of body size change, ‘extreme’ populations tend towards gigantism. With classification tree methods, we develop a predictive model, which points to resource limitations as major drivers in the few cases of insular dwarfism. Highly successful in classifying our dataset, our model also successfully predicts change in untested cases.     

Shrinking before our isles: the rapid expression of insular dwarfism in two invasive populations of guttural toad (Sclerophrys gutturalis)

Island ecosystems have traditionally been hailed as natural laboratories for examining phenotypic change, including dramatic shifts in body size. Similarly, biological invasions can drive rapid localized adaptations within modern timeframes. Here, we compare the morphology of two invasive guttural toad (Sclerophrys gutturalis) populations in Mauritius and Réunion with their source population from South Africa. We found that female toads on both islands were significantly smaller than mainland counterparts (33.9% and 25.9% reduction, respectively), as were males in Mauritius (22.4%). We also discovered a significant reduction in the relative hindlimb length of both sexes, on both islands, compared with mainland toads (ranging from 3.4 to 9.0%). If our findings are a result of natural selection, then this would suggest that the dramatic reshaping of an amphibian''s morphology—leading to insular dwarfism—can result in less than 100 years; however, further research is required to elucidate the mechanism driving this change (e.g. heritable adaptation, phenotypic plasticity, or an interaction between them).     

Spatiotemporal variation in selection on body size in the dung fly Sepsis cynipsea

Standardized measures of the strength of selection on a character allow quantitative comparisons across populations in time and space. Spatiotemporal variation in selection influences patterns of adaptation and the evolution of characters and must therefore be documented. For the dung-breeding fly Sepsis cynipsea, we document patterns of variation in sexual, fecundity and larval and adult viability selection on body size at several spatiotemporal scales: between-populations, over the season, over the day and between dung pats. Adult viability selection based on residual physiological survivorship in the laboratory was nil or weakly negative. In contrast, larval viability selection in two laboratory environments was weakly positive for males at low competition and females at high competition. Fecundity selection was positive and strong at all times and in all populations. Sexual selection reflecting pairing success was overall strongly positive (about three times stronger than fecundity selection), while selection reflecting male reproductive success via the clutch size of his mate (i.e. assortative mating) was essentially nil. Only sexual selection varied significantly at coarse (between populations and seasonally) but not at fine (within a day or between pats on a pasture) spatial and temporal scales. Quadratic and correlational selection differentials were low and inconsistent in all episodes except for fecundity selection, where there was some evidence that clutch size reaches an asymptote at large body sizes, implying weaker selection for large size as females get bigger. Implications of these results for the evolution of body size and body size dimorphism are discussed.     

Seasonal change in the opportunity for sexual selection

Environmental and population parameters that influence the strength of sexual selection may vary considerably over the course of the reproductive season. However, the potential for sexual selection frequently fails to translate into variation in reproductive success among individuals. We investigated seasonal changes in variation in reproductive success, measured as the opportunity for sexual selection, using parentage analysis in 20 experimental populations of the European bitterling (Rhodeus amarus, Cyprinidae), a small freshwater fish with a promiscuous, resource-based mating system. We showed that although the largest males sired most offspring over the entire reproductive season, variation in reproductive success and hence the opportunity for sexual selection was low at the start of the season but increased significantly at its end. This seasonal difference probably arose from the superior competitive endurance of large males and from a higher temporal clustering of reproductively active females at the start of the breeding season than later in the season. The spatial distribution of oviposition sites had a negligible effect on the variation in reproductive success. We discuss the potential implications of our results for the importance and strength of sexual selection in natural populations.     

Intra- and interspecific morphological variation in the field mouse species Apodemus argenteus and A. speciosus in the Japanese archipelago: the role of insular isolation and biogeographic gradients

Two species of field mice, Apodemus argenteus and A. speciosus, occur in sympatry across the Japanese archipelago. The inter- and intraspecific patterns of morphological differentiation have been evaluated, using a Fourier analysis of the mandible outline. The relative importance of the effect of insular isolation and latitudinal climatic gradient on the size and shape of the two species was assessed by a comparison of the populations from the large island of Honshu and the surrounding small-island populations. The size variation in A. argenteus is correlated with the climatic gradient whilst the shape variation corresponds mainly to a random differentiation of the small-island populations from a Honshu-like basic morphological pattern. A. speciosus displays increased size on small islands, and its shape variation is related to both the climatic gradient and insularity. Finally, the two species are differentiated by both the size and shape of the mandible across the Japanese archipelago, suggesting that interspecific competition between both species is reduced via niche partitioning. Our results emphasize the importance of insular isolation on shape differentiation, but a part of the morphological differentiation is also related to the latitudinal climatic gradient. Isolation on small islands could have favoured such a response to environmental factors by lowering the gene flow that prevents almost any significant differentiation within Honshu populations.     

Body size evolution in insular vertebrates: generality of the island rule

Aim My goals here are to (1) assess the generality of the island rule – the graded trend from gigantism in small species to dwarfism in larger species – for mammals and other terrestrial vertebrates on islands and island‐like ecosystems; (2) explore some related patterns of body size variation in insular vertebrates, in particular variation in body size as a function of island area and isolation; (3) offer causal explanations for these patterns; and (4) identify promising areas for future studies on body size evolution in insular vertebrates. Location Oceanic and near‐shore archipelagos, and island‐like ecosystems world‐wide. Methods Body size measurements of insular vertebrates (non‐volant mammals, bats, birds, snakes and turtles) were obtained from the literature, and then regression analyses were conducted to test whether body size of insular populations varies as a function of body size of the species on the mainland (the island rule) and with characteristics of the islands (i.e. island isolation and area). Results The island rule appears to be a general phenomenon both with mammalian orders (and to some degree within families and particular subfamilies) as well as across the species groups studied, including non‐volant mammals, bats, passerine birds, snakes and turtles. In addition, body size of numerous species in these classes of vertebrates varies significantly with island isolation and island area. Main conclusions The patterns observed here – the island rule and the tendency for body size among populations of particular species to vary with characteristics of the islands – are actually distinct and scale‐dependent phenomena. Patterns within archipelagos reflect the influence of island isolation and area on selective pressures (immigration filters, resource limitation, and intra‐ and interspecific interactions) within particular species. These patterns contribute to variation about the general trend referred to as the island rule, not the signal for that more general, large‐scale pattern. The island rule itself is an emergent pattern resulting from a combination of selective forces whose importance and influence on insular populations vary in a predictable manner along a gradient from relatively small to large species. As a result, body size of insular species tends to converge on a size that is optimal, or fundamental, for a particular bau plan and ecological strategy.     

Mandibles and molars of the wood mouse, Apodemus sylvaticus (L.): integrated latitudinal pattern and mosaic insular evolution

Aim The distinct nature of island populations has traditionally been attributed either to adaptation to particular insular conditions or to random genetic effects. In order to assess the relative importance of these two disparate processes, insular effects were addressed in the European wood mouse, Apodemus sylvaticus (Linnaeus, 1758). Location Wood mice from 33 localities on both mainland and various Atlantic and western Mediterranean islands were considered. This sampling covers only part of the latitudinal range of A. sylvaticus but included the two main genetic clades identified by previous studies. Islands encompass a range of geographical conditions (e.g. small islands fringing the continent through large and isolated ones). Methods The insular syndrome primarily invokes variations in body size, but ecological factors such as release from competition, niche widening and food availability should also influence other characters related to diet. In the present study, the morphology of the wood mice was quantified based on two characters involved in feeding: the size and shape of the mandibles and first upper molars. The size of the mandible is also a proxy for the body size of the animal. Patterns of morphological differentiation of both features were estimated using two‐dimensional outline analysis based on Fourier methods. Results Significant differences between mainland and island populations were observed in most cases for both the mandibles and molars. However, molars and mandibles displayed divergent patterns. Mandible shape diverged mostly on islands of intermediate remoteness and competition levels, whereas molars exhibited the greatest shape differentiation on small islands, such as Port‐Cros and Porquerolles. A mosaic pattern was also displayed for size. Body and mandible size increased on Ibiza, but molar size remained similar to mainland populations. Mosaic patterns were, however, not apparent in the mainland populations. Congruent latitudinal variations were evident for the size and shape of both mandibles and molars. Main conclusions Mosaic evolution appears to characterize insular divergence. The molar seems to be more prone to change with reduced population size on small islands, whereas the mandible could be more sensitive to peculiar environmental conditions on large and remote islands.     

Comparing measures of breeding inequality and opportunity for selection with sexual selection on a quantitative character in bighorn rams

The reliability and consistency of the many measures proposed to quantify sexual selection have been questioned for decades. Realized selection on quantitative characters measured by the selection differential i was approximated by metrics based on variance in breeding success, using either the opportunity for sexual selection I_s or indices of inequality. There is no consensus about which metric best approximates realized selection on sexual characters. Recently, the opportunity for selection on character mean OSM was proposed to quantify the maximum potential selection on characters. Using 21 years of data on bighorn sheep (Ovis canadensis), we investigated the correlations between seven indices of inequality, I_s, OSM and i on horn length of males. Bighorn sheep are ideal for this comparison because they are highly polygynous and sexually dimorphic, ram horn length is under strong sexual selection, and we have detailed knowledge of individual breeding success. Different metrics provided conflicting information, potentially leading to spurious conclusions about selection patterns. I_δ, an index of breeding inequality, and, to a lesser extent, I_s showed the highest correlation with i on horn length, suggesting that these indices document breeding inequality in a selection context. OSM on horn length was strongly correlated with i, I_s and indices of inequality. By integrating information on both realized sexual selection and breeding inequality, OSM appeared to be the best proxy of sexual selection and may be best suited to explore its ecological bases.     

Population dynamics and evolutionary processes: the manifold roles of habitat selection

Summary Any character that has a substantial effect on a species' distribution and abundance can exert a variety of indirect effects on evolutionary processes. It is suggested that an organism's capacity for habitat selection is just such a character. Habitat selection can constrain the selective environment experienced by a population. Habitat selection can also indirectly influence the relative importance of natural selection, drift, and gene flow, through its effect on population size and growth rate. In many circumstances (but not all), habitat selection increases population size and growth rate, and thereby makes selection in a local environment more effective than drift and gene flow.     

Of mice and mammoths: evaluations of causal explanations for body size evolution in insular mammals

Aim We investigated the hypothesis that the insular body size of mammals results from selective forces whose influence varies with characteristics of the focal islands and the focal species, and with interactions among species (ecological displacement and release). Location Islands world‐wide. Methods We assembled data on the geographic characteristics (area, isolation, maximum elevation, latitude) and climate (annual averages and seasonality of temperature and precipitation) of islands, and on the ecological and morphological characteristics of focal species (number of mammalian competitors and predators, diet, body size of mainland reference populations) that were most relevant to our hypothesis (385 insular populations from 98 species of extant, non‐volant mammals across 248 islands). We used regression tree analyses to examine the hypothesized contextual importance of these factors in explaining variation in the insular body size of mammals. Results The results of regression tree analyses were consistent with predictions based on hypotheses of ecological release (more pronounced changes in body size on islands lacking mammalian competitors or predators), immigrant selection (more pronounced gigantism in small species inhabiting more isolated islands), thermoregulation and endurance during periods of climatic or environmental stress (more pronounced gigantism of small mammals on islands of higher latitudes or on those with colder and more seasonal climates), and resource subsidies (larger body size for mammals that utilize aquatic prey). The results, however, were not consistent with a prediction based on resource limitation and island area; that is, the insular body size of large mammals was not positively correlated with island area. Main conclusions These results support the hypothesis that the body size evolution of insular mammals is influenced by a combination of selective forces whose relative importance and nature of influence are contextual. While there may exist a theoretical optimal body size for mammals in general, the optimum for a particular insular population varies in a predictable manner with characteristics of the islands and the species, and with interactions among species. This study did, however, produce some unanticipated results that merit further study – patterns associated with Bergmann’s rule are amplified on islands, and the body size of small mammals appears to peak at intermediate and not maximum values of latitude and island isolation.     

The quick and the dead: correlational selection on morphology, performance, and habitat use in island lizards

Natural selection is an important driver of microevolution. Yet, despite significant theoretical debate, we still have a poor understanding of how selection operates on interacting traits (i.e., morphology, performance, habitat use). Locomotor performance is often assumed to impact survival because of its key role in foraging, predator escape, and social interactions, and shows strong links with morphology and habitat use within and among species. In particular, decades of study suggest, but have not yet demonstrated, that natural selection on locomotor performance has shaped the diversification of Anolis lizards in the Greater Antilles. Here, we estimate natural selection on sprinting speed and endurance in small replicate island populations of Anolis sagrei. Consistent with past correlational studies, long-limbed lizards ran faster on broad surfaces but also had increased sprint sensitivity on narrow surfaces. Moreover, performance differences were adaptive in the wild. Selection favored long-limbed lizards that were fast on broad surfaces, and preferred broad substrates in nature, and also short-limbed lizards that were less sprint sensitive on narrow surfaces, and preferred narrow perches in nature. This finding is unique in showing that selection does not act on performance alone, but rather on unique combinations of performance, morphology, and habitat use. Our results support the long-standing hypothesis that correlated selection on locomotor performance, morphology, and habitat use drives the evolution of ecomorphological correlations within Caribbean Anolis lizards, potentially providing a microevolutionary mechanism for their remarkable adaptive radiation.     

The insular herpetofauna of Mexico: Composition,conservation, and biogeographic patterns

We compile a Mexican insular herpetofaunal checklist to estimate endemism, conservation status, island threats, net taxonomic turnover among six biogeographic provinces belonging to the Nearctic and Neotropical regions, and the relationships between island area and mainland distance versus species richness. We compile a checklist of insular herpetofaunal through performing a literature and collection review. We define the conservation status according to conservation Mexican law, the Red List of International Union for Conservation of Nature, and Environmental Vulnerability Scores. We determine threat percentages on islands according to the 11 major classes of threats to biodiversity. We estimate the net taxonomic turnover with beta diversity analysis between the Nearctic and Neotropical provinces. The Mexican insular herpetofauna is composed of 18 amphibian species, 204 species with 101 subspecies of reptiles, and 263 taxa in total. Endemism levels are 11.76% in amphibians, 53.57% in reptiles, and 27.91% being insular endemic taxa. Two conservation status systems classify the species at high extinction risk, while the remaining system suggests less concern. However, all systems indicate species lacking assessment. Human activities and exotic alien species are present on 60% of 131 islands. The taxonomic turnover value is high (0.89), with a clear herpetofaunal differentiation between the two biogeographic regions. The species–area and species–mainland distance relationships are positive. Insular herpetofauna faces a high percentage of threats, with the Neotropical provinces more heavily impacted. It is urgent to explore the remaining islands (3,079 islands) and better incorporate insular populations and species in ecological, evolutionary, and systematic studies. In the face of the biodiversity crisis, islands will play a leading role as a model to apply restoration and conservation strategies.     

The old wasp and the tree: fossils, phylogeny and biogeography in the Orussidae (Insecta, Hymenoptera)

The phylogenetic relationships of the fossil orussid taxa Mesorussus taimyrensis and Minyorussus luzzi are examined by analysing them together with a large data set compiled previously for the extant Orussidae. The fossils are placed in an unresolved trichotomy with the extant Orussidae. The phylogeny is used for evaluating the hypothesis that the ancestors of the family had reduced body size; the results of this analysis are inconclusive. The biogeographical history of the Orussidae is explored. The common ancestor of the family was probably widespread, the initial splitting events taking place prior to or coinciding with the separation of Laurasia from Gondwana. Later putative vicariance events can be correlated with the gradual breakup of Gondwana. However, the biogeographical history of the Orussidae is dominated by speciation within regions and dispersal. The minimum age of the common ancestor of the Orussidae is >180 Myr when estimated from the biogeographical pattern, >95 Myr when estimated from the phylogenetic position of the fossils; the earlier date is considered to be the most likely.  © 2004 The Linnean Society of London , Biological Journal of the Linnean Society , 2004, 82 , 139–160.     

Sexual selection and allometry: a critical reappraisal of the evidence and ideas

One of the most pervasive ideas in the sexual selection literature is the belief that sexually selected traits almost universally exhibit positive static allometries (i.e., within a sample of conspecific adults, larger individuals have disproportionally larger traits). In this review, I show that this idea is contradicted by empirical evidence and theory. Although positive allometry is a typical attribute of some sexual traits in certain groups, the preponderance of positively allometric sexual traits in the empirical literature apparently results from a sampling bias reflecting a fascination with unusually exaggerated (bizarre) traits. I review empirical examples from a broad range of taxa illustrating the diversity of allometric patterns exhibited by signal, weapon, clasping and genital traits, as well as nonsexual traits. This evidence suggests that positive allometry may be the exception rather than the rule in sexual traits, that directional sexual selection does not necessarily lead to the evolution of positive allometry and, conversely, that positive allometry is not necessarily a consequence of sexual selection, and that many sexual traits exhibit sex differences in allometric intercept rather than slope. Such diversity in the allometries of secondary sexual traits is to be expected, given that optimal allometry should reflect resource allocation trade-offs, and patterns of sexual and viability selection on both trait size and body size. An unbiased empirical assessment of the relation between sexual selection and allometry is an essential step towards an understanding of this diversity.