Some theoretical consequences of the allometric growth equations

The rate of growth of an organism may be described as a function of both its mass and of the time. In particular, if the rate of growth is described as a function of mass alone, one derives the so-called principle of “equifinality”, according to which the final mass achieved is independent of any interruption of the growth process. Since this phenomenon is observed in many instances, the assumption that the rate of growth is a function of mass alone has some theoretical justification. On the other hand, it is obvious that in many instances irreversible effects can be achieved by interrupting the growth process, thus indicating that the rate of growth may depend on variables other than mass. The consequences of some of these alternative assumptions are examined. A growth factorG is postulated, which is supposed to decay with time, and whose concentration determines the over-all rate of growth per unit mass. The rate of decay ofG is calculated from the postulated growth equation under two alternative assumptions, (1) thatG is a constituent of all the cells of the organism and (2) thatG is produced separately and distributed through the organism. It is shown that in each case the concentration ofG satisfies a simple differential equation. While in normal growth the results deduced from these hypotheses are indistinguishable from those deduced from the assumption that the rate of growth depends on mass alone, widely different results are obtained in the case of interrupted growth. In particular, the rate at which growth is resumed after interruption and the final mass achieved depend on the time and on the duration of interruption. It appears, therefore, that various assumptions about the nature of the “aging” of the organism lead to various quantitative relations between growth and the times of interruption and resumption of growth.     

Evolutionary consequences of Wolbachia infections

The past decade has revealed the bacterium Wolbachia as the most widespread symbiont of arthropods and nematodes. Behind this evolutionary success is an remarkable variety of effects on host biology, ranging from manipulation of reproduction in favor of females to more classical mutualistic interactions. Here we discuss the potential of Wolbachia for promoting evolutionary changes in its hosts.     

Evolutionary consequences of a search image

Many predators are able to become better at spotting cryptic prey by recognising specific clues, but by concentrating on one prey type they will become worse at spotting other prey types. This phenomenon is known as the formation of a search image for a certain prey by a predator and is related to apostatic selection. Here, we study the evolution of a search image in the predator by formulating and analysing a mathematical model. The predator forages for two prey types and is able to form an independent search image for both prey. The results show that the evolutionary dynamics can be divided into two parts: a fast and a slow part. At first selection pressure will be strong towards a stable ratio of prey, which is the same as the ratio found for the unbeatable prey choice for predators with a Holling type II functional response. Following this, the slow dynamics will keep this ratio constant independent of the trait values, but the predator will slowly evolve towards a stronger search image and ultimately become a specialist predator or slowly evolve towards generalist with a weak search image. In conclusion, the formation of a search image causes the predator to control the prey densities such that the ratio of available prey is kept constant by the predator.     

Evolutionary consequences of indirect genetic effects

Indirect genetic effects (IGEs) are environmental influences on the phenotype of one individual that are due to the expression of genes in a different, conspecific, individual. Historically, work has focused on the influence of parents on offspring but recent advances have extended this perspective to interactions among other relatives and even unrelated individuals. IGEs lead to complicated pathways of inheritance, where environmental sources of variation can be transmitted across generations and therefore contribute to evolutionary change. The existence of IGEs alters the genotype-phenotype relationship, changing the evolutionary process in some dramatic and non-intuitive ways.     

Evolutionary consequences of sperm cell aging

Animal breeding research, reproductive biology, and cellular biogerontology show that fertilization rates and zygote viability critically depend on sperm age. Sexual selection research focuses on differences between male genotypes in sperm performance, such as motility, competitive ability, or compatibility with eggs, but without considering sperm age. A combined view (that the thermodynamically inevitable decline in sperm performance selects for traits in diploid individuals to prevent fertilization with aged sperm) has received very little attention. In this paper, I correct this bias and show that many male and female traits affect sperm aging or the sperm age distribution at any reproductive event. Such traits coincide well with condition-dependent traits considered sexually selected: multiple mating by both sexes, high sperm production rates, the delivery of dense ejaculates containing many sperm (including nonfertilizing types), the packaging of sperm into spermatophores, male and female sperm ejection, sexual coercion, as well as the production of showy antioxidants and various cellular and nuclear repair mechanisms. I conclude that altering the sperm age distribution at any step during reproduction can be an origin of sexually selected traits, and may explain presently observed paternity variation without assuming genetic incompatibility of gametes.     

Evolutionary consequences of asymmetric dispersal rates

We study the consequences of asymmetric dispersal rates (e.g., due to wind or current) for adaptive evolution in a system of two habitat patches. Asymmetric dispersal rates can lead to overcrowding of the "downstream" habitat, resulting in a source-sink population structure in the absence of intrinsic quality differences between habitats or can even cause an intrinsically better habitat to function as a sink. Source-sink population structure due to asymmetric dispersal rates has similar consequences for adaptive evolution as a source-sink structure due to habitat quality differences: natural selection tends to be biased toward the source habitat. We demonstrate this for two models of adaptive evolution: invasion of a rare allele that improves fitness in one habitat but reduces it in the other and antagonistic selection on a quantitative trait determined by five additive loci. If a habitat can sustain a population without immigration, the conditions for adaptation to that habitat are most favorable if there is little or no immigration from the other habitat; the influence of emigration depends on the magnitude of the allelic effects involved and other parameters. If, however, the population is initially unable to persist in a given habitat without immigration, our model predicts that the population will be most likely to adapt to that habitat if the dispersal rates in both directions are high. Our results highlight the general message that the effect of gene flow upon local adaptation should depend profoundly on the demographic context of selection.     

Evolutionary and biomedical consequences of internal melanins

The adaptive function of melanin located in the integument is well known. Although pigments are also deposited in various internal organs, their function is unclear. A review of the literature revealed that ‘internal melanin’ protects against parasites, pollutants, low temperature, oxidative stress, hypoxemia and UV light, and is involved in the development and function of organs. Importantly, several studies have shown that the amount of melanin deposited on the external body surface is correlated with the amount located inside the body. This finding raises the possibility that internal melanin plays more important physiological roles in dark than light‐colored individuals. Internal melanin and coloration may therefore not evolve independently. This further emphasizes the major role played by indirect selection in evolutionary processes.     

Evolutionary consequences of cytoplasmic sex ratio distorters

Summary Computer simulations of diploid genetic models were used to examine the consequences of the spread of a cytoplasmic sex ratio distorter on the frequencies of nuclear sex-determination alleles and the spread of nuclear resistance alleles in female biased populations. The cytoplsmic elements considered here override the expression of the nuclear sex-determination genes, turning genetic males into females. When homozygous male genotypes are viable, a cytoplasmic sex ratio historter spreads in a population if the proportion of daughters produced by infected females exceeds the proportion of daughters produced by uninfected females. The equilibrium frequency of male phenotypes is the proportion of uninfected progeny produced by infected females. When homozygous male genotypes are lethal, the conditions for the spread of the cytoplasmic element are more stringent. The spread of a cytoplasmic sex ratio distorter causes an increase in the frequency of nuclear male sex-determination alleles as a result of the unusual combinations of genotypes which mate in infected populations. Eventually, a cytoplasmic element may replace the nuclear gene as the sex-determination mechanism. This occurs without selection. Nuclear genes conferring resistance to cytoplasmic sex ratio distorters generally increase in female biased populations and often restore a 11 sex ratio despite continual selection on the cytoplasmic element to increase its transmission efficiency.     

Evolutionary consequences of dating the Yixian Formation

The Yixian Formation of northeastern China has yielded important new fossils that are fuelling debates on the origin of angiosperms, on the early radiation of birds and of mammals, and on the origin of feathers. Although these fossils provide a wealth of detailed anatomical information, knowledge of the absolute age of the Yixian Formation is crucial if we are to understand their true evolutionary significance. The age of the Yixian Formation has been disputed, but recent evidence provides strong support for an Early Cretaceous rather than a Late Jurassic age.     

Evolutionary genetic consequences of facultative sex and outcrossing

Explaining the selective forces that underlie different reproductive modes forms a major part of evolution research. Many organisms are facultative sexuals, with the ability to reproduce both sexually and asexually. Reduced sequencing costs means it is now possible to start investigating genome sequences of a wider number of these organisms in depth, but teasing apart the genetic forces underlying the maintenance of facultative sexual reproduction remains a challenge. An analogous problem exists when determining the genetic consequences of a degree of outcrossing (and recombination) in otherwise self‐fertilizing organisms. Here, I provide an overview of existing research on the evolutionary basis behind different reproductive modes, with a focus on explaining the population genetic effects favouring low outcrossing rates in either partially selfing or asexual species. I review the outcomes that both self‐fertilization and asexuality have on either purging deleterious mutations or fixing beneficial alleles, and what empirical data exist to support these theories. In particular, a greater application of mathematical models to genomic data has provided insight into the numerous effects that transitions to self‐fertilization from outcrossing have on genetic architecture. Similar modelling approaches could be used to determine the forces shaping genetic diversity of facultative sexual species. Hence, a further unification of mathematical models with next‐generation sequence data will prove important in exploring the genetic influences on reproductive system evolution.     

Evolutionary consequences of ecological release in Caribbean Anolis lizards

On the large islands of the Greater Antilles, multi-species communities of Anolis lizards are composed of species specialized to use particular habitats; similar sets of specialized species have evolved independently on each island. We studied species of anoles found on small Caribbean islands. Because these islands contain at most only one other species of anole, we predicted that species on these islands should not be as specialized as Greater Antillean species; rather, they might be expected to exhibit a generalized morphology and a greater breadth of habitat use. Our findings, however, do not confirm these predictions. Lesser Antillean species do not exhibit greater breadth of habitat use than Greater Antillean species, nor do they exhibit a generalized morphology. Most species are ecologically and morphologically similar to specialized trunk-crown anoles of the Greater Antilles, although some species exhibit morphologies unlike those seen in Greater Antillean species. Among descendants of specialized Greater Antillean species occurring on one-or two-species islands, most descendants of trunk-crown species have diverged relatively little, whereas several descendants of trunk-ground anoles have diverged considerably. Consequently, we propose that ancestral species in the Greater Antilles may have been trunk-crown anoles.     

Evolutionary consequences of altered atmospheric oxygen in Drosophila melanogaster

Twelve replicate populations of Drosophila melanogaster, all derived from a common ancestor, were independently evolved for 34+ generations in one of three treatment environments of varying PO(2): hypoxia (5.0-10.1 kPa), normoxia (21.3 kPa), and hyperoxia (40.5 kPa). Several traits related to whole animal performance and metabolism were assayed at various stages via "common garden" and reciprocal transplant assays to directly compare evolved and acclimatory differences among treatments. Results clearly demonstrate the evolution of a greater tolerance to acute hypoxia in the hypoxia-evolved populations, consistent with adaptation to this environment. Greater hypoxia tolerance was associated with an increase in citrate synthase activity in fly homogenate when compared to normoxic (control) populations, suggesting an increase in mitochondrial volume density in these populations. In contrast, no direct evidence of increased performance of the hyperoxia-evolved populations was detected, although a significant decrease in the tolerance of these populations to acute hypoxia suggests a cost to adaptation to hyperoxia. Hyperoxia-evolved populations had lower productivity overall (i.e., across treatment environments) and there was no evidence that hypoxia or hyperoxia-evolved populations had greatest productivity or longevity in their respective treatment environments, suggesting that these assays failed to capture the components of fitness relevant to adaptation.     

Dispersal limitation: Evolutionary origins and consequences in arthropods

Niche and dispersal ability are key traits for explaining the geographical structuring of species into discrete populations, and its evolutionary significance. Beyond their individual effects, the interplay between species niche and its geographic limits, together with the evolutionary lability of dispersal ability, can underpin trait diversification and speciation when exposed to gradients of selection. In this issue of Molecular Ecology, two complementary papers demonstrate how evolutionary lability for dispersal ability linked to niche shift can drive such a model in a context that includes selection. Both papers investigate the evolution of dispersal limitation in arthropods across altitudinal gradients, but using taxa with contrasting ecologies. McCulloch et al. (2019) investigate the evolution of wing loss at higher altitudes in stoneflies, a taxon inhabiting freshwater systems. Suzuki et al. (2019) report a similar phenomenon, but involving wing reduction at higher altitudes in scorpionflies, a taxon associated with moist terrestrial habitats. Here, we compare and contrast the results of both studies to explore their broader implications for understanding diversification and speciation within arthropods.     

Evolutionary causes and consequences of sequential polyandry in anuran amphibians

Among anuran amphibians (frogs and toads), there are two types of polyandry: simultaneous polyandry, where sperm from multiple males compete to fertilize eggs, and sequential polyandry, where eggs from a single female are fertilized by multiple males in a series of temporally separate mating events, and sperm competition is absent. Here we review the occurrence of sequential polyandry in anuran amphibians, outline theoretical explanations for the evolution of this mating system and discuss potential evolutionary implications. Sequential polyandry has been reported in a limited number of anurans, but its widespread taxonomic and geographic distribution suggests it may be common. There have been no empirical studies that have explicitly investigated the evolutionary consequences of sequential polyandry in anurans, but species with this mating pattern share an array of behavioural, morphological and physiological characteristics, suggesting that there has been common sexual selection on their reproductive system. Sequential polyandry may have a number of adaptive benefits, including spreading the risk of brood failure in unpredictable environments, insuring against male infertility, or providing genetic benefits, either through good genes, intrinsic compatibility or genetic diversity effects. Anurans with sequential polyandry provide untapped opportunities for innovative research approaches that will contribute significantly to understanding anuran evolution and also, more broadly, to the development of sexual‐selection and life‐history theory.     

Evolutionary consequences of gender plasticity in genetically dimorphic breeding systems

A functional view of gender helps evolutionary biologists evaluate the mechanisms underlying breeding-system evolution. Evolutionary pathways from hermaphroditism to dioecy include the intermediate breeding systems of gynodioecy and androdioecy. These pathways start with the invasion of unisexual mutants, females or males, respectively, followed by alteration of the hermaphrodites to allocate more to the sexual function that the unisexuals lack. Eventually, hermaphrodites become unisexual and dioecy has evolved. Some species evolving along these pathways stop short of completing this second step, or even revert back from dioecy. We evaluate the hypothesis that gender plasticity is involved in these transitions to and from dioecy. Evidence from studies of subdioecious species that have evolved along the gynodioecy pathway suggests that gender plasticity occurs and stabilizes subdioecy by lowering the cost of producing seed. Factors influencing species evolving toward androdioecy, or reverting to androdioecy from dioecy, appear to be more varied and include reproductive assurance, herbivory and gender plasticity. In general, gender specialization appears to be favored in resource-poor environments regardless of which pathway is taken to dioecy.     

Evolutionary origins and ecological consequences of endophyte symbiosis with grasses

Over the past 20 yr much has been learned about a unique symbiotic interaction between fungal endophytes and grasses. The fungi (Clavicipitaceae, Ascomycota) grow intercellularly and systemically in aboveground plant parts. Vertically transmitted asexual endophytes forming asymptomatic infections of cool-season grasses have been repeatedly derived from sexual species that abort host inflorescences. The phylogenetic distribution of seed-transmitted endophytes is strongly suggestive of cocladogenesis with their hosts. Molecular evidence indicates that many seed-transmitted endophytes are interspecific hybrids. Superinfection may result in hyphal fusion and parasexual recombination. Most endophytes produce one or more alkaloid classes that likely play some role in defending the host plant against pests. Hybridization may have led to the proliferation of alkaloid-production genes among asexual endophytes, favoring hybrids. The ergot alkaloid ergovaline, lolitrems, and lolines are produced by only a single sexual species, Epichlo? festucae, but they are common in seed-transmitted endophytes, suggesting that E. festucae contributed genes for their synthesis. Asexual hybrids may also be favored by the counteracting of the accumulation of deleterious mutations (Muller's rachet). Endophyte infection can provide other benefits, such as enhanced drought tolerance, photosynthetic rate, and growth. Estimates of infection frequency have revealed variable levels of infection with especially high prevalence in the subfamily Pooideae. Longitudinal studies suggest that the prevalence of seed-transmitted endophytes can increase rapidly over time. In field experiments, infected tall fescue suppressed other grasses and forbs relative to uninfected fescue and supported lower consumer populations. Unlike other widespread plant/microbial symbioses based on the acquisition of mineral resources, grass/endophyte associations are based primarily on protection of the host from biotic and abiotic stresses.     

Evolutionary consequences of climate‐induced range shifts in insects

Range shifts can rapidly create new areas of geographic overlap between formerly allopatric taxa and evidence is accumulating that this can affect species persistence. We review the emerging literature on the short‐ and long‐term consequences of these geographic range shifts. Specifically, we focus on the evolutionary consequences of novel species interactions in newly created sympatric areas by describing the potential (i) short‐term processes acting on reproductive barriers between species and (ii) long‐term consequences of range shifts on the stability of hybrid zones, introgression and ultimately speciation and extinction rates. Subsequently, we (iii) review the empirical literature on insects to evaluate which processes have been studied, and (iv) outline some areas that deserve increased attention in the future, namely the genomics of hybridisation and introgression, our ability to forecast range shifts and the impending threat from insect vectors and pests on biodiversity, human health and crop production. Our review shows that species interactions in de novo sympatric areas can be manifold, sometimes increasing and sometimes decreasing species diversity. A key issue that emerges is that climate‐induced hybridisations in insects are much more widespread than anticipated and that rising temperatures and increased anthropogenic disturbances are accelerating the process of species mixing. The existing evidence only shows the tip of the iceberg and we are likely to see many more cases of species mixing following range shifts in the near future.