Dimensionality and modularity of adaptive variation: Divergence in threespine stickleback from diverse environments

In nature, populations are subjected to a wide variety of environmental conditions that affect fitness and induce adaptive or plastic responses in traits, resulting in phenotypic divergence between populations. The dimensionality of that divergence, however, remains contentious. At the extremes, some contend that populations diverge along a single axis of trait covariance with greatest availability of heritable variation, even if this does not lead a population directly to its fitness optimum. Those at the other extreme argue that selection can push populations towards their fitness optima along multiple phenotype axes simultaneously, resulting in divergence in numerous dimensions. Here, we address this debate using populations of threespine stickleback (Gasterosteus aculeatus) in the Cook Inlet region of southern Alaska from lakes with contrasting ecological conditions. We calculated effective dimensionality of divergence in several trait suites (defensive, swimming, and trophic) thought to be under correlated selection pressures, as well as across all traits. We also tested for integration among the trait suites and between each trait suite and the environment. We found that populations in the Cook Inlet radiation exhibit dimensionality of phenotype high enough to preclude a single axis of divergence.

Climate and habitat loss interactively restructure trait composition and covariance across a human-modified landscape

Species traits influence their response to environmental conditions and the match between phenotypes and environment mediates spatial changes in species composition. These trait-environment linkages can be disrupted in human-modified landscapes. Human land-use creates habitat fragments where dispersal limitation or edge effects can exclude species that may otherwise suit a given macro-scale environment. Furthermore, stressful micro-environments in fragments may limit viable trait combinations resulting in stronger trait covariance compared to contiguous forest, especially in harsher macroenvironments. In a wet tropical forest landscape in the Western Ghats Biodiversity Hotspot of peninsular India, I compared fragments with adjacent contiguous forest for signatures of trait-mediated assembly of tree communities along macroenvironmental gradients. Using four key plant traits—seed size, specific leaf area (SLA), wood density, and maximum height—I evaluated changes in trait-mediated abundances and trait covariance across environmental gradients. Trait-mediated abundances primarily changed along the elevation gradient in contiguous forest, smaller-seeded, shorter, thinner-leaved species increased at higher elevations. In fragments, higher SLA species increased in more seasonal climate and decreased with higher precipitation, and larger seeds decreased at warmer sites. However, traits only weakly predicted abundances and only contiguous forests experienced significant compositional change via traits, driven by trait syndromes varying along a composite environmental gradient defined by elevation, water deficit, and soil C:N ratio. Covariance of seed size and maximum height along gradients of precipitation and temperature revealed divergent constraints on viable phenotypes in fragments and contiguous forest. Notably, local biotic conditions (functional diversity) had stronger effects than environment in explaining trait covariance. Overall, the results imply that trait syndromes and trait covariance, rather than single traits, determine the phenotypes best suited to different macroenvironmental conditions and should inform management or restoration goals in fragments.

Multivariate analysis of morphology, behaviour, growth and developmental timing in hybrids brings new insights into the divergence of sympatric Arctic charr morphs

Background Studying the development of fitness related traits in hybrids from populations diverging in sympatry is a fundamental approach to understand the processes of speciation. However, such traits are often affected by covariance structures that complicate the comprehension of these processes, especially because the interactive relationships between traits of different nature (e.g. morphology, behaviour, life-history) remain largely unknown in this context. In a common garden setup, we conducted an extensive examination of a large suit of traits putatively involved in the divergence of two morphs of Arctic charr (Salvelinus alpinus), and investigated the consequences of potential patterns of trait covariance on the phenotype of their hybrids. These traits were measured along ontogeny and involved growth, yolk sac resorption, developmental timing (hatching and the onset of exogeneous feeding), head morphology and feeding behaviour. Results Growth trajectories provided the strongest signal of phenotypic divergence between the two charr. Strikingly, the first-generation hybrids did not show intermediate nor delayed growth but were similar to the smallest morph, suggesting parental biases in the inheritance of growth patterns. However, we did not observe extensive multivariate trait differences between the two morphs and their hybrids. Growth was linked to head morphology (suggesting that morphological variations in early juveniles relate to simple allometric effects) but this was the only strong signal of covariance observed between all the measured traits. Furthermore, we did not report evidence for differences in overall phenotypic variance between morphs, nor for enhanced phenotypic variability in their hybrids. Conclusion Our study shed light on the multivariate aspect of development in a context of adaptive divergence. The lack of evidence for the integration of most traits into a single covariance structure suggested that phenotypic constraints may not always favour nor impede divergence toward ecological niches differing in numerous physical and ecological variables, as observed in the respective habitats of the two charr. Likewise, the role of hybridization as a disruptive agent of trait covariance may not necessarily be significant in the evolution of populations undergoing resource polymorphism.

Multivariate genome-wide covariance analyses of literacy, language and working memory skills reveal distinct etiologies

Several abilities outside literacy proper are associated with reading and spelling, both phenotypically and genetically, though our knowledge of multivariate genomic covariance structures is incomplete. Here, we introduce structural models describing genetic and residual influences between traits to study multivariate links across measures of literacy, phonological awareness, oral language, and phonological working memory (PWM) in unrelated UK youth (8–13 years, N = 6453). We find that all phenotypes share a large proportion of underlying genetic variation, although especially oral language and PWM reveal substantial differences in their genetic variance composition with substantial trait-specific genetic influences. Multivariate genetic and residual trait covariance showed concordant patterns, except for marked differences between oral language and literacy/phonological awareness, where strong genetic links contrasted near-zero residual overlap. These findings suggest differences in etiological mechanisms, acting beyond a pleiotropic set of genetic variants, and implicate variation in trait modifiability even among phenotypes that have high genetic correlations.

Developmental instability and phenotypic evolution in a small and isolated bear population

We explored fluctuating asymmetry (FA) and morphological integration (MI) in the skull of the small, highly inbred and divergent Apennine bear ( Ursus arctos marsicanus ), to explore its uniqueness and investigate any potential effects of inbreeding depression. We used 3D geometric morphometrics contrasting Apennine bears with other two large outbred bear populations from Scandinavia and Kamchatka as controls. Shape divergence and variability were explored by a principal component analysis on aligned coordinates of 39 landmarks. Procrustes ANOVA, morphological disparity and the global integration index were used to explore FA, shape variance and MI. By remarking Apennine bears as a highly divergent phenotype, we recorded the highest FA and deviation from self-similarity compared with the other two control populations. We conclude that Apennine bears are likely facing developmental instability as a consequence of inbreeding depression, whereas the divergent trait covariance pattern may represent a potential source of evolutionary novelties. We discuss the implications for the conservation and management of this imperiled taxon.

Multivariate distributions of behavioural, morphological, and ontogenetic traits in hybrids bring new insights into the divergence of sympatric Arctic charr morphs

Studying the development of fitness related traits in hybrids from populations diverging in sympatry is a fundamental approach to understand the processes of speciation. However, such traits are often affected by covariance structures that complicate the comprehension of these processes, especially because the interactive relationships between traits of different nature (e.g. morphology, behaviour, life-history) remain largely unknown in this context. In a common garden setup, we conducted an extensive examination of phenotypic traits suspected to be involved in the divergence of two recently evolved morphs of Arctic charr (Salvelinus alpinus), and investigated the consequences of potential patterns of trait covariance on the phenotype of their hybrids. We observed differences among morphs in overall phenotypic variance and in trait correlations. Phenotypic contrainsts also tended to be reduced in the hybrids, which corroborates the narrative of hybridization facilitating adaptive divergence by relaxing trait covariance. However, the hybrids were associated with reduced phenotypic variance at different scales (i.e. at the scale of the entire P matrix and in different parts of the multivariate space), and we identified stronger correlations between several ontogenetic and morphological traits in the hybrids than in both morphs. These findings suggest a limited potential for hybridization to generate phenotypic novelty, and emphasise the need for multivariate approaches conciliating ontogenetic, morphological and behavioural processes to study the processes of adaptive divergence and speciation.

Animal personality adds complexity to the processes of adaptive divergence and speciation

Divergent selection is a powerful driver of speciation and has been widely studied in relation to the physical characters of organisms. Because evolution of behavioural traits may contribute to evolutionary processes, we explored how consistent variation in behaviours may affect the process of adaptive divergence and speciation. We studied whether two sympatric morphs of Arctic charr (Salvelinus alpinus) have recently evolved genetically-based differences in personality that conform to their respective ecological niches, and whether these differences contribute to reproductive isolation by generating maladaptive hybrid behaviours. Studying three aspects of behavioural variation (average trait value, consistent individual differences and trait covariance), we assessed the sociality and risk-taking propensity of hybrid and pure-morph offspring reared in common conditions. Contrary to expectations, the two morphs did not differ in the average values of these traits but showed different behavioural syndromes (trait covariances). While the hybrids did not differ from either morph in their average behavioural responses, they showed less individual consistency in these behaviours and a different set of behavioural syndromes. Differences between morphs and their hybrids in other behavioural aspects than their average behavioural responses suggest that our understanding of speciation processes can benefit from an integrative view of behavioural variation.

Plant trait covariance and nonlinear averaging: a reply to Koussoroplis et al.

SADIE (Spatial Analysis by Distance Indices) is designed specifically to quantify patterns in spatially-referenced count-based data. It was developed for dealing with data that can be considered ‘patchy’. Such distributions are commonly found, for example, in insect populations where discrete patches of individuals are often evident. The distributions of such populations have ‘hard edges’, with patches and gaps occurring spatially. In these cases variance of abundance does not vary smoothly, but discontinuously. In this paper we outline the use of SADIE and provide free access to the SADIE software suite, establishing Rethinking Ecology as its permanent home. Finally, we review the use of SADIE and demonstrate its use in a wide variety of sub-disciplines within the general field of ecology.

Sex-specific pace-of-life syndromes

The pace-of-life syndrome (POLS) hypothesis considers an animal’s behavior, physiology, and life history as nonindependent components of a single integrated phenotype. However, frequent deviations from the expected correlations between POLS traits suggest that these relationships may be context, and potentially, sex dependent. To determine whether the sexes express distinct POLS trait covariance structures, we observed the behavior (mobility, latency to emerge from a shelter), physiology (mass-specific metabolic rate), and life history (life span, development time) of male and female Australian field crickets (Teleogryllus oceanicus). Path analysis modeling suggested that POLS trait covariation differed between the sexes. Although neither sex displayed the complete integration of traits predicted by the POLS hypothesis, females did display greater overall integration with a significant negative correlation between metabolic rate and risk-taking behavior but with life-history traits varying independently. In males, however, there was no clear association between traits. These results suggest that T. oceanicus do indeed display sex-specific trait covariance structures, emphasizing the importance of acknowledging sex in assessments of POLS.