Predictable adaptive trajectories of sexual coloration in the wild: Evidence from replicate experimental guppy populations

Female preference for rare males is maintained by indirect selection in Trinidadian guppies

When females prefer mates with rare phenotypes, sexual selection can maintain rather than deplete genetic variation. However, there is no consensus on why this widespread and frequently observed preference might evolve and persist. We examine the fitness consequences of female preference for rare male color patterns in a natural population of Trinidadian guppies, using a pedigree that spans 10 generations. We demonstrate (i) a rare male reproductive advantage, (ii) that females that mate with rare males gain an indirect fitness advantage through the mating success of their sons, and (iii) the fitness benefit that females accrue through their "sexy sons" evaporates for their grandsons as their phenotype becomes common. Counter to prevailing theory, we show that female preference can be maintained through indirect selection.

Using fish to understand how cities affect sexual selection before and after mating

Urbanization transforms natural and agricultural areas into built landscapes. Such profound habitat alteration imposes strong pressure on phenotypic trait changes through processes related to natural and/or sexual selection. Evidence of how natural selection drives changes to traits in urban biota is increasing, but little is known about the role of sexual selection. In this study, we assessed the effect of urbanization on the expression and interaction of males' pre-mating traits (body size and color) and a post-mating trait (sperm load). We used a widespread invasive species, the guppy (Poecilia reticulata), which is a wellknown model for studying sexual selection, but have never been studied in urban systems for this purpose. We found that urbanization did not affect mean body size or condition, but it resulted in size-dependent reductions in the expression of orange and iridescent colors, as well as sperm load. The orange color was reduced in small urban guppies, while the iridescent colors were reduced in large urban guppies compared to non-urban guppies. The difference in sperm load was only found in large males, with lower sperm load in urban guppies. The relationship between orange color and sperm load was positive in urban guppies but negative in non-urban guppies, while the association between iridescent color and sperm load followed the opposite pattern. Our findings suggest that sexual selection on pre- and post-mating traits is weaker in urban than in non-urban systems and that interactions between such traits are context dependent. These responses can be related to the pollution and altered visual environment of urban systems and provide an opportunity to advance our understanding of the mechanisms determining adaptation in cities.

Compiling forty years of guppy research to investigate the factors contributing to (non)parallel evolution

Examples of parallel evolution have been crucial for our understanding of adaptation via natural selection. However, strong parallelism is not always observed even in seemingly similar environments where natural selection is expected to favour similar phenotypes. Leveraging this variation in parallelism within well-researched study systems can provide insight into the factors that contribute to variation in adaptive responses. Here we analyse the results of 36 studies reporting 446 average trait values in Trinidadian guppies, Poecilia reticulata, from different predation regimes. We examine how the extent of predator-driven phenotypic parallelism is influenced by six factors: sex, trait type, rearing environment, ecological complexity, evolutionary history, and time since colonization. Analyses show that parallel evolution in guppies is highly variable and weak on average, with only 24.7% of the variation among populations being explained by predation regime. Levels of parallelism appeared to be especially weak for colour traits, and parallelism decreased with increasing complexity of evolutionary history (i.e., when estimates of parallelism from populations within a single drainage were compared to estimates of parallelism from populations pooled between two major drainages). Suggestive - but not significant - trends that warrant further research include interactions between the sexes and different trait categories. Quantifying and accounting for these and other sources of variation among evolutionary 'replicates' can be leveraged to better understand the extent to which seemingly similar environments drive parallel and nonparallel aspects of phenotypic divergence.

Rapid genomic convergent evolution in experimental populations of Trinidadian guppies (Poecilia reticulata)

Although rapid phenotypic evolution has been documented often, the genomic basis of rapid adaptation to natural environments is largely unknown in multicellular organisms. Population genomic studies of experimental populations of Trinidadian guppies (Poecilia reticulata) provide a unique opportunity to study this phenomenon. Guppy populations that were transplanted from high-predation (HP) to low-predation (LP) environments have been shown to evolve toward the phenotypes of naturally colonized LP populations in as few as eight generations. These changes persist in common garden experiments, indicating that they have a genetic basis. Here, we report results of whole genome variation in four experimental populations colonizing LP sites along with the corresponding HP source population. We examined genome-wide patterns of genetic variation to estimate past demography and used a combination of genome scans, forward simulations, and a novel analysis of allele frequency change vectors to uncover the signature of selection. We detected clear signals of population growth and bottlenecks at the genome-wide level that matched the known history of population numbers. We found a region on chromosome 15 under strong selection in three of the four populations and with our multivariate approach revealing subtle parallel changes in allele frequency in all four populations across this region. Investigating patterns of genome-wide selection in this uniquely replicated experiment offers remarkable insight into the mechanisms underlying rapid adaptation, providing a basis for comparison with other species and populations experiencing rapidly changing environments.

Revisiting old truths: The evolution of male coloration in guppies as a function of predation

The Trinidadian guppy, Poecilia reticulata, is a long-studied model for the evolution of trade-offs in male nuptial coloration as a function of female mating preferences versus predation risk. Previous work suggests that female mating preferences favor the evolution of increased conspicuous male coloration in low predation populations. In contrast, high predation risk shifts the balance towards reduced conspicuous coloration. In a From the Cover article in this issue of Molecular Ecology, Yong, Croft, Troscianko, Ramnarine, and Wilson (2021) use visual detection models to estimate the 'conspicuousness' of male color patterns as seen by guppies and their predators. The study fails to find robust patterns of increased conspicuousness in low predation populations. Only one of eight measures of conspicuousness showed parallel changes between high and low predation regimes, forcing us to re-consider the validity and repeatability of this classic example of parallel evolution.

Differential habitat use and recruitment facilitate coexistence in a community with intraguild predation

Theory predicts that species engaged in intraguild predation (IGP) can only coexist under limited conditions, yet IGP is common in nature. Habitat complexity can promote coexistence by reducing encounter rates, but little information is known about the contribution of differential habitat use. We hypothesized that differential use of alternative habitats promotes coexistence of an intraguild (IG) predator and prey. We evaluated predictions of this hypothesis with an experimental introduction of an IG predator fish into four natural stream communities that previously contained only the IG prey fish. We monitored the development of this IGP over the course of four years to determine how each species used alternative stream habitats. The introduced species preferred pool habitats while the resident species was more evenly distributed across pools and riffles. The density of the resident decreased in the pool habitat preferred by the invader, accompanied by a local increase in the mean of the resident size distribution. Selective predation by the invader on hatchling residents appears to impact the residents' demographic response. The continued recruitment of resident juveniles in riffles, where the introduced species is rare, facilitated the persistence of the resident. This differential use of habitats was not accompanied by a change in the resident's growth rates in either habitat. Our results showed that differential habitat selection and recruitment promoted persistence during an invasion involving IGP, which helps to bridge the gap between theory and observation in explaining coexistence in IGP systems.

Using Delaunay triangulation to sample whole-specimen color from digital images

Color variation is one of the most obvious examples of variation in nature, but biologically meaningful quantification and interpretation of variation in color and complex patterns are challenging. Many current methods for assessing variation in color patterns classify color patterns using categorical measures and provide aggregate measures that ignore spatial pattern, or both, losing potentially important aspects of color pattern.Here, we present Colormesh, a novel method for analyzing complex color patterns that offers unique capabilities. Our approach is based on unsupervised color quantification combined with geometric morphometrics to identify regions of putative spatial homology across samples, from histology sections to whole organisms. Colormesh quantifies color at individual sampling points across the whole sample.We demonstrate the utility of Colormesh using digital images of Trinidadian guppies (Poecilia reticulata), for which the evolution of color has been frequently studied. Guppies have repeatedly evolved in response to ecological differences between up- and downstream locations in Trinidadian rivers, resulting in extensive parallel evolution of many phenotypes. Previous studies have, for example, compared the area and quantity of discrete color (e.g., area of orange, number of black spots) between these up- and downstream locations neglecting spatial placement of these areas. Using the Colormesh pipeline, we show that patterns of whole-animal color variation do not match expectations suggested by previous work.Colormesh can be deployed to address a much wider range of questions about color pattern variation than previous approaches. Colormesh is thus especially suited for analyses that seek to identify the biologically important aspects of color pattern when there are multiple competing hypotheses or even no a priori hypotheses at all.

Deep-time convergent evolution in animal communication presented by shared adaptations for coping with noise in lizards and other animals

Convergence in communication appears rare compared with other forms of adaptation. This is puzzling, given communication is acutely dependent on the environment and expected to converge in form when animals communicate in similar habitats. We uncover deep-time convergence in territorial communication between two groups of tropical lizards separated by over 140 million years of evolution: the Southeast Asian Draco and Caribbean Anolis. These groups have repeatedly converged in multiple aspects of display along common environmental gradients. Robot playbacks to free-ranging lizards confirmed that the most prominent convergence in display is adaptive, as it improves signal detection. We then provide evidence from a sample of the literature to further show that convergent adaptation among highly divergent animal groups is almost certainly widespread in nature. Signal evolution is therefore curbed towards the same set of adaptive solutions, especially when animals are challenged with the problem of communicating effectively in noisy environments.

Sensory-based quantification of male colour patterns in Trinidadian guppies reveals no support for parallel phenotypic evolution in multivariate trait space

Parallel evolution, in which independent populations evolve along similar phenotypic trajectories, offers insights into the repeatability of adaptive evolution. Here, we revisit a classic example of parallelism, that of repeated evolution of brighter males in the Trinidadian guppy (Poecilia reticulata). In guppies, colonisation of low predation habitats is associated with emergence of 'more colourful' phenotypes since predator-induced viability selection for crypsis weakens while sexual selection by female preference for conspicuousness remains strong. Our study differs from previous investigations in three respects. First, we adopted a multivariate phenotyping approach to characterise parallelism in multitrait space. Second, we used ecologically-relevant colour traits defined by the visual systems of the two selective agents (i.e., guppy, predatory cichlid). Third, we estimated population genetic structure to test for adaptive (parallel) evolution against a model of neutral phenotypic divergence. We find strong phenotypic differentiation that is inconsistent with a neutral model but very limited support for the predicted pattern of greater conspicuousness at low predation. Effects of predation regime on each trait were in the expected direction, but weak, largely nonsignificant, and explained little among-population variation. In multitrait space, phenotypic trajectories of lineages colonising low from high predation regimes were not parallel. Our results are consistent with reduced predation risk facilitating adaptive differentiation, potentially by female choice, but suggest that this proceeds in independent directions of multitrait space across lineages. Pool-sequencing data also revealed SNPs showing greater differentiation than expected under neutrality, among which some are found in genes contributing to colour pattern variation, presenting opportunities for future genetic study.

The complex ecology of genitalia: Gonopodium length and allometry in the Trinidadian guppy

Male genitalia present an extraordinary pattern of rapid divergence in animals with internal fertilization, which is usually attributed to sexual selection. However, the effect of ecological factors on genitalia divergence could also be important, especially so in animals with nonretractable genitalia because of their stronger interaction with the surrounding environment in comparison with animals with retractable genitalia. Here, we examine the potential of a pervasive ecological factor (predation) to influence the length and allometry of the male genitalia in guppies. We sampled guppies from pairs of low-predation (LP) and high-predation (HP) populations in seven rivers in Trinidad, and measured their body and gonopodium length. A key finding was that HP adult males do not have consistently longer gonopodia than do LP adult males, as had been described in previous work. However, we did find such divergence for juvenile males: HP juveniles have longer gonopodia than do LP juveniles. We therefore suggest that an evolutionary trend toward the development of longer gonopodia in HP males (as seen in the juveniles) is erased after maturity owing to the higher mortality of mature males with longer gonopodia. Beyond these generalities, gonopodium length and gonopodium allometry were remarkably variable among populations even within a predation regime, thus indicating strong context dependence to their development/evolution. Our findings highlight the complex dynamics of genitalia evolution in Trinidadian guppies.

Behavioral Variation in the Pygmy Halfbeak Dermogenys collettei: Comparing Shoals With Contrasting Ecologies

Variation in biotic and abiotic factors among populations affects individual behaviors by transforming the social landscape and shaping mating systems. Consequently, describing behaviors in natural populations requires consideration of the biological and physical factors that different individuals face. Here, we examined variation in socio-sexual and locomotor behaviors in a small, livebearing, freshwater fish, the pygmy halfbeak Dermogenys collettei, across natural populations in Singapore. The pygmy halfbeak is a surface feeding fish that spends most of the time near the water surface, making it ideal for non-invasive behavioral observations. We compared behaviors between sexes among 26 shoals while simultaneously accounting for environmental variation. We demonstrated that sexual interactions and locomotor behaviors differed among shoals with varying levels of canopy cover and water flow. Specifically, in areas with greater canopy cover, sexual interactions decreased, whereas time spent in a stationary position increased. Sexual interactions were more numerous in still water, where fish spent less time swimming. Variation in the expression of socio-sexual and locomotor behaviors were not associated with differences in the amount of aquatic vegetation, water depth or halfbeak shoal size. Agonistic interactions were robust to environmental effects, showing little variation among environments. However, there were strong sex effects, with males performing more agonistic behaviors and spending less time in a stationary position compared to females, regardless of the environment. Moreover, sexual interactions, measured as actively performed by males and passively received by females, were on average more frequent in males than in females. Our findings help us explore the proximal causes of intraspecific behavioral variation and suggest that fundamental information on socio-sexual behaviors from wild populations can lead to a better understanding of how sexual selection operates when the strength of natural selection varies across environments.

Divergence and Remarkable Diversity of the Y Chromosome in Guppies

The guppy sex chromosomes show an extraordinary diversity in divergence across populations and closely related species. In order to understand the dynamics of the guppy Y chromosome, we used linked-read sequencing to assess Y chromosome evolution and diversity across upstream and downstream population pairs that vary in predator and food abundance in three replicate watersheds. Based on our population-specific genome assemblies, we first confirmed and extended earlier reports of two strata on the guppy sex chromosomes. Stratum I shows significant accumulation of male-specific sequence, consistent with Y divergence, and predates the colonization of Trinidad. In contrast, Stratum II shows divergence from the X, but no Y-specific sequence, and this divergence is greater in three replicate upstream populations compared with their downstream pair. Despite longstanding assumptions that sex chromosome recombination suppression is achieved through inversions, we find no evidence of inversions associated with either Stratum I or Stratum II. Instead, we observe a remarkable diversity in Y chromosome haplotypes within each population, even in the ancestral Stratum I. This diversity is likely due to gradual mechanisms of recombination suppression, which, unlike an inversion, allow for the maintenance of multiple haplotypes. In addition, we show that this Y diversity is dominated by low-frequency haplotypes segregating in the population, suggesting a link between haplotype diversity and female preference for rare Y-linked color variation. Our results reveal the complex interplay between recombination suppression and Y chromosome divergence at the earliest stages of sex chromosome divergence.

The Size, Symmetry, and Color Saturation of a Male Guppy's Ornaments Forecast His Resistance to Parasites

AbstractSexually selected ornaments range from highly dynamic traits to those that are fixed during development and relatively static throughout sexual maturity. Ornaments along this continuum differ in the information they provide about the qualities of potential mates, such as their parasite resistance. Dynamic ornaments enable real-time assessment of the bearer's condition: they can reflect an individual's current infection status, or they can reflect resistance to recent infections. Static ornaments, however, are not affected by recent infection but may instead indicate an individual's genetically determined resistance, even in the absence of infection. Given the typically aggregated distribution of parasites among hosts, infection is unlikely to affect the ornaments of the vast majority of individuals in a population: static ornaments may therefore be the more reliable indicators of parasite resistance. To test this hypothesis, we quantified the ornaments of male guppies (Poecilia reticulata) before experimentally infecting them with Gyrodactylus turnbulli. Males with more left-right symmetrical black coloration and those with larger areas of orange coloration, both static ornaments, were more resistant. However, males with more saturated orange coloration, a dynamic ornament, were less resistant. Female guppies often prefer symmetrical males with larger orange ornaments, suggesting that parasite-mediated natural and sexual selection act in concert on these traits.

Early-life social environment predicts social network position in wild zebra finches

Early-life experience can fundamentally shape individual life-history trajectories. Previous research has suggested that exposure to stress during development causes differences in social behaviour later in life. In captivity, juvenile zebra finches exposed to elevated corticosterone levels were less socially choosy and more central in their social networks when compared to untreated siblings. These differences extended to other aspects of social life, with 'stress-exposed' juveniles switching social learning strategies and juvenile males less faithfully learning their father's song. However, while this body of research suggests that the impacts of early-life stress could be profound, it remains unknown whether such effects are strong enough to be expressed under natural conditions. Here, we collected data on social associations of zebra finches in the Australian desert after experimentally manipulating brood sizes. Juveniles from enlarged broods experienced heightened sibling competition, and we predicted that they would express similar patterns of social associations to stress-treated birds in the captive study by having more, but less differentiated, relationships. We show striking support for the suggested consequences of developmental stress on social network positions, with our data from the wild replicating the same results in 9 out of 10 predictions previously tested in captivity. Chicks raised in enlarged broods foraged with greater numbers of conspecifics but were less 'choosy' and more central in the social network. Our results confirm that the natural range of variation in early-life experience can be sufficient to predict individuals' social trajectories and support theory highlighting the potential importance of developmental conditions on behaviour.

Experimental translocations to low predation lead to non-parallel increases in relative brain size

Predation is a near ubiquitous factor of nature and a powerful selective force on prey. Moreover, it has recently emerged as an important driver in the evolution of brain anatomy, though population comparisons show ambiguous results with considerable unexplained variation. Here, we test the reproducibility of reduced predation on evolutionary trajectories of brain evolution. We make use of an introduction experiment, whereby guppies (Poecilia reticulata) from a single high predation stream were introduced to four low predation streams. After 8-9 years of natural selection in the wild and two generations of common garden conditions in the laboratory, we quantified brain anatomy. Relative brain region sizes did not differ between populations. However, we found a general increase and striking variation in relative brain size of introduced populations, which varied from no change to a 12.5% increase in relative brain weight, relative to the ancestral high predation population. We interpret this as evidence for non-parallel evolution, which implies a weak or inconsistent association of relative brain size with fitness in low predation sites. The evolution of brain anatomy appears sensitive to unknown environmental factors, or contingent on either chance events or historical legacies of environmental change.

Experimental Studies of Evolution and Eco-Evo Dynamics in Guppies (Poecilia reticulata)

Guppies in Trinidad range across aquatic environments with fish communities that vary in risk of predation. These communities are often discrete, separated by waterfalls, with high-predation communities downstream and low-predation communities upstream. This gradient is repeated in many rivers; in each one, we see the same divergence between guppy populations in life history, behavior, morphology, and physiology. We have shown that the agent of selection on the life history, behavior, and physiology in low-predation communities is high population density and the cascade of ecological effects that stems from it. In effect, guppy populations modify their ecosystem and, in so doing, impose selection on themselves and shape their own evolution, which further changes the ecosystem. Evolution unfolds rapidly in this system, which has enabled us to study the dynamics of the process, not just its end points. Those studies enable us to answer some very general questions in ecology and evolutionary biology.

Convergent evolution of coloration in experimental introductions of the guppy (Poecilia reticulata)

Despite the multitude of examples of evolution in action, relatively fewer studies have taken a replicated approach to understand the repeatability of evolution. Here, we examine the convergent evolution of adaptive coloration in experimental introductions of guppies from a high-predation (HP) environment into four low-predation (LP) environments. LP introductions were replicated across 2 years and in two different forest canopy cover types. We take a complementary approach by examining both phenotypes and genetics. For phenotypes, we categorize the whole color pattern on the tail fin of male guppies and analyze evolution using a correspondence analysis. We find that coloration in the introduction sites diverged from the founding Guanapo HP site. Sites group together based on canopy cover, indicating convergence in response to light environment. However, the axis that explains the most variation indicates a lack of convergence. Therefore, evolution may proceed along similar phenotypic trajectories, but still maintain unique variation within sites. For the genetics underlying the divergent phenotypes, we examine expression levels of color genes. We find no evidence for differential expression, indicating that the genetic basis for the color changes remains undetermined.