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bioRxiv2017

Parasite escape through trophic specialization in a species flock

In adaptive radiations species diversify rapidly to occupy an array of ecological niches. In these different niches, species might be exposed to parasites through different routes and at different levels. If this is the case, adaptive radiations should be accompanied by a turnover in parasite communities. How the adaptive radiation of host species might be entangled with such a turnover of parasite communities is poorly documented in nature. In the present study, we examined the intestinal parasite faunas of eleven species belonging to the tribe Tropheini, one of several adaptive radiations of cichlid fishes in Lake Tanganyika. The most parsimonious ancestral foraging strategy among Tropheini is relatively unselective substrate ingestion by browsing of aufwuchs. Certain lineages however evolved more specialized foraging strategies, such as selective combing of microscopic diatoms or picking of macro-invertebrates. We found that representatives of such specialized lineages bear reduced infection with intestinal acanthocephalan helminths. Possibly, the evolution of selective foraging strategies entailed reduced ingestion of intermediate invertebrate hosts of these food-web transmitted parasites. In Tropheini, trophic specialization is therefore intertwined with divergence in parasite infection. We conclude that the study of parasite communities could improve our understanding of host evolution, ecological speciation and the origin of adaptive radiations.

evolutionary biology↗

bioRxiv2017

Density-dependent selection in evolutionary genetics: a lottery model of Grime’s triangle

Selection is commonly described by assigning constant relative fitness values to genotypes. Yet population density is often regulated by crowding. Relative fitness may then depend on density, and selection can change density when it acts on a density-regulating trait. When strong density-dependent selection acts on a density-regulating trait, selection is no longer describable by density-independent relative fitnesses, even in demographically stable populations. These conditions are met in most previous models of density-dependent selection (e.g. \"K-selection\" in the logistic and Lotka-Volterra models), suggesting that density-independent relative fitnesses must be replaced with more ecologically explicit absolute fit-nesses unless selection is weak. Here we show that density-independent relative fitnesses can also accurately describe strong density-dependent selection under some conditions. We develop a novel model of density-regulated population growth with three ecologically intuitive traits: fecundity, mortality, and competitive ability. Our model, unlike the logistic or Lotka-Volterra, incorporates a density-dependent juvenile \"reproductive excess\", which largely decouples density-dependent selection from the regulation of density. We find that density-independent relative fitnesses accurately describe strong selection acting on any one trait, even fecundity, which is both density-regulating and subject to density-dependent selection. Pleiotropic interactions between these traits recovers the familiar K-selection behavior. In such cases, or when the population is maintained far from demographic equilibrium, our model offers a possible alternative to relative fitness.

evolutionary biology↗

bioRxiv2017

Colour vision models: a practical guide, some simulations, and colourvision R package.

O_LIHuman colour vision differs from the vision of other animals. The most obvious differences are the number and type of photoreceptors in the retina. E.g., while humans are insensitive to ultraviolet (UV) light, most non-mammal vertebrates and insects have a colour vision that spans into the UV. The development of colour vision models allowed appraisals of colour vision independent of the human experience. These models are now widespread in ecology and evolution fields. Here I present a guide to colour vision modelling, run a series of simulations, and provide a R package - colourvision - to facilitate the use of colour vision models.\nC_LIO_LII present the mathematical steps for calculation of the most commonly used colour vision models: Chittka (1992) colour hexagon, Endler & Mielke (2005) model, and Vorobyev & Osorio (1998) linear and log-linear receptor noise limited models (RNL). These models are then tested using identical simulated and real data. These comprise of reflectance spectra generated by a logistic function against an achromatic background, achromatic reflectance against an achromatic background, achromatic reflectance against a chromatic background, and real flower reflectance data against a natural background reflectance.\nC_LIO_LIWhen the specific requirements of each model are met, between model results are, overall, qualitatively and quantitatively similar. However, under many common scenarios of colour measurements, models may generate spurious values and/or considerably different predictions. Models that log-transform data and use relative photoreceptor outputs are prone to generate unrealistic results when the stimulus photon catch is smaller than the background photon catch. Moreover, models may generate unrealistic results when the background is chromatic (e.g. leaf reflectance) and the stimulus is an achromatic low reflectance spectrum.\nC_LIO_LIColour vision models are a valuable tool in several ecology and evolution subfields. Nonetheless, knowledge of model assumptions, careful analysis of model outputs, and basic knowledge of calculation behind each model are crucial for appropriate model application, and generation of meaningful and reproducible results. Other aspects of vision not incorporated into these models should be considered when drawing conclusion from model results.\nC_LI

animal behavior and cognition↗

bioRxiv2017

Iroki: automatic customization for phylogenetic trees

Phylogenetic trees are an important analytical tool for evaluating community diversity and evolutionary history. In the case of microorganisms, the decreasing cost of sequencing has enabled researchers to generate ever-larger sequence datasets, which in turn have begun to fill gaps in the evolutionary history of microbial groups. However, phylogenetic analyses of these types of datasets create complex trees that can be challenging to interpret. Scientific inferences made by visual inspection of phylogenetic trees can be simplified and enhanced by customizing various parts of the tree. Yet, manual customization is time-consuming and error prone, and programs designed to assist in batch tree customization often require programming experience or complicated file formats for annotation. Iroki, a user-friendly web interface for tree visualization, addresses these issues by providing automatic customization of large trees based on metadata contained in tab-separated text files. Irokis utility for exploring biological and ecological trends in sequencing data was demonstrated through a variety of microbial ecology applications in which trees with hundreds to thousands of leaf nodes were customized according to extensive collections of metadata. The Iroki web application and documentation are available at https://www.iroki.net or through the VIROME portal (http://virome.dbi.udel.edu). Irokis source code is released under the MIT license and is available at https://github.com/mooreryan/iroki.

bioinformatics↗

bioRxiv2017

Between semelparity and iteroparity: empirical evidence for a continuum of modes of parity

The number of times an organism reproduces (i.e. its mode of parity) is a fundamental life-history character, and evolutionary and ecological models that compare the relative fitness of strategies are common in life history theory and theoretical biology. Despite the success of mathematical models designed to compare intrinsic rates of increase between annual-semelparous and perennial-iteroparous reproductive schedules, there is widespread evidence that variation in reproductive allocation among semelparous and iteroparous organisms alike is continuous. This paper reviews the ecological and molecular evidence for the continuity and plasticity of modes of parity--that is, the idea that annual-semelparous and perennial-iteroparous life histories are better understood as endpoints along a continuum of possible strategies. I conclude that parity should be understood as a continuum of different modes of parity, which differ by the degree to which they disperse or concentrate reproductive effort in time. I further argue that there are three main implications of this conclusion: (1) That seasonality should not be conflated with parity; (2) that mathematical models purporting to explain the evolution of semelparous life histories from iteroparous ones (or vice versa) should not assume that organisms can only display either an annual-semelparous life history or a perennial-iteroparous one; and (3) that evolutionary ecologists should examine the physiological or molecular basis of traits underlying different modes of parity, in order to obtain a general understanding of how different life history strategies can evolve from one another.

evolutionary biology↗

bioRxiv2017

Cohesion: A method for quantifying the connectivity of microbial communities

The ability to predict microbial community dynamics lags behind the quantity of data available in these systems. Most predictive models use only environmental parameters, although a long history of ecological literature suggests that community complexity should also be an informative parameter. Thus, we hypothesize that incorporating information about a communitys complexity might improve predictive power in microbial models. Here, we present a new metric, called community \"cohesion,\" that quantifies the degree of connectivity of a microbial community. We validate our approach using long-term (10+ year) phytoplankton datasets, where absolute abundance counts are available. As a case study of our metrics utility, we show that community cohesion is a strong predictor of Bray-Curtis dissimilarity (R2 = 0.47) between phytoplankton communities in Lake Mendota, WI, USA. Our cohesion metrics outperform a model built using all available environmental data collected during a long-term sampling program. The result that cohesion corresponds strongly to Bray-Curtis dissimilarity is consistent across the five lakes analyzed here. Our cohesion metrics can be used as a predictor for many community-level properties, such as phylogenetic diversity, nutrient fluxes, or ecosystem services. We explain here the calculation of our cohesion metrics and their potential uses in microbial ecology.\n\nConflict of InterestThe authors declare no conflict of interest.

microbiology↗

bioRxiv2017

Random Drift With A Determined Outcome: A Parsimonious Null Model OfNeanderthal Replacement By Modern Humans Via Neutral Species Drift

The processes that led to the demise of the Neanderthals and their replacement by modern humans have been the object of speculation, research, and heated debate. Most hypotheses fall into one of two categories: one highlights the role of climate change, epidemics, or other environmental pressures in the Neanderthals demise, and the other attributes it to direct or indirect competition with modern humans, who seem to have occupied the same ecological niche. The latter are based on the assumption that modern humans benefited from some selective advantage over Neanderthals that led to the latters extinction. We show that a scenario that includes migration and selectively neutral species drift can explain the Neanderthals replacement and is in line with the archaeological evidence. Our model offers a parsimonious alternative to those that invoke external factors or selective advantage, and can represent a null hypothesis in assessing such alternatives. We show that for a wide range of parameters this hypothesis cannot be rejected. Moreover, we suggest that although selection and environmental factors may or may not have played a role in the interspecies dynamics of the Neanderthals and modern humans, the eventual outcome of these dynamics, the replacement of the Neanderthals, was the result of the hominid migration dynamics at the end of the middle Paleolithic, namely repeated migration of modern humans from Africa into the Levant and Europe.\n\nSignificance statementMultiple factors have been proposed as possible drivers of the extinction of the Neanderthals and their replacement by modern humans circa 40,000 years ago: climate change, epidemics, and - most prominently - a selective advantage, such as superior cognitive capacity of modern humans over Neanderthals. We propose an alternative model that includes only migration of modern humans out of Africa into the Levant and Europe. We show that, given that the two species occupied a similar ecological niche, modern humans were destined to replace the Neanderthals even under a neutral scenario in which neither species has a selective advantage.

evolutionary biology↗

bioRxiv2017

Geographically structured genetic variation in the Medicago lupulina-Ensifer mutualism

Mutualisms are interspecific interactions affecting the ecology and evolution of species. Patterns of geographic variation in interacting species may play an important role in understanding how variation is maintained in mutualisms, particularly in introduced ranges. One agriculturally and ecologically important mutualism is the partnership between legume plants and rhizobia. Through characterizing and comparing the population genomic structure of the legume Medicago lupulina and two rhizobial species (Ensifer medicae and E. meliloti), we explored the spatial scale of population differentiation between interacting partners in their introduced range in North America. We found high proportions of E. meliloti in southeastern populations and high proportions of E. medicae in northwestern populations. Medicago lupulina and the Ensifer genus showed similar patterns of spatial genetic structure (isolation by distance). However, we detected no evidence of isolation by distance or population structure within either species of bacteria. Genome-wide nucleotide diversity within each of the two Ensifer species was low, suggesting limited introduction of strains, founder events, or severe bottlenecks. Our results suggest that there is potential for geographically structured coevolution between M. lupulina and the Ensifer genus, but not between M. lupulina and either Ensifer species.

evolutionary biology↗

bioRxiv2017

Evolution Of Hierarchy In Bacterial Metabolic Networks

BackgroundIn self-organized systems, the concept of flow hierarchy is a useful way to characterize the movement of information throughout a network. Hierarchical network organizations are shown to arise when there is a cost of maintaining links in the network. A similar constraint exists in metabolic networks, where costs come from reduced efficiency of nonspecific enzymes or from producing unnecessary enzymes. Previous analyses of bacterial metabolic networks have been used to predict the minimal nutrients that a bacterium needs to grow, its mutualistic relationships with other bacteria, and its major ecological niche. Using flow hierarchy, we can also infer the tradeoffs between growth rate and metabolic efficiency that bacteria make given their environmental constraints.\n\nResultsUsing a comparative approach on 2,935 bacterial metabolic networks, we show that flow hierarchy in bacterial metabolic networks tracks a fundamental tradeoff between growth rate and biomass production, and reflects a bacteriums realized ecological strategy. Additionally, by inferring the ancestral metabolic networks, we find that hierarchy decreases with distance from the root of the tree, suggesting the important pressure of increased growth rate relative to efficiency in the face of competition.\n\nConclusionsJust as hierarchical character is an important structural property in efficiently engineered systems, it also evolves in self-organized bacterial metabolic networks, reflects the life-history strategies of those bacteria, and plays an important role in network organization and efficiency.

evolutionary biology↗

bioRxiv2017

Response Of Microbial Community Function To Fluctuating Geochemical Conditions Within A Legacy Radioactive Waste Trench Environment

During the 1960s, small quantities of radioactive materials were co-disposed with chemical waste at the Little Forest Legacy Site (Sydney, Australia) in three-metre-deep, unlined trenches. Chemical and microbial analyses, including functional and taxonomic information derived from shotgun metagenomics, were collected across a six-week period immediately after a prolonged rainfall event to assess how changing water levels impact upon the microbial ecology and contaminant mobility. Collectively, results demonstrated that oxygen-laden rainwater rapidly altered the redox balance in the trench water, strongly impacting microbial functioning as well as the radiochemistry. Two contaminants of concern, plutonium and americium, were shown to transition from solid-iron-associated species immediately after the initial rainwater pulse, to progressively more soluble moieties as reducing conditions were enhanced. Functional metagenomics revealed the potentially important role that the taxonomically-diverse microbial community played in this transition. In particular, aerobes dominated in the first day followed by an increase of facultative anaerobes/denitrifiers at day four. Towards the mid-end of the sampling period, the functional and taxonomic profiles depicted an anaerobic community distinguished by a higher representation of dissimilatory sulfate reduction and methanogenesis pathways. Our results have important implications to similar near-surface environmental systems in which redox cycling occurs.\n\nImportanceThe role of chemical and microbiological factors in mediating the biogeochemistry of groundwaters from trenches used to dispose of radioactive materials during the 1960s is examined in this study. Specifically, chemical and microbial analyses, including functional and taxonomic information derived from shotgun metagenomics, were collected across a six-week period immediately after a prolonged rainfall event to assess how changing water levels influence microbial ecology and contaminant mobility.\n\nResults demonstrate that oxygen-laden rainwater rapidly altered the redox balance in the trench water, strongly impacting microbial functioning as well as the radiochemistry. Two contaminants of concern, plutonium and americium, were shown to transition from solidiron-associated species immediately after the initial rainwater pulse, to progressively more soluble moieties as reducing conditions were enhanced. Functional metagenomics revealed the important role that the taxonomically-diverse microbial community played in this transition. Our results have important implications to similar near-surface environmental systems in which redox cycling occurs.

microbiology↗

bioRxiv2017

Semicircular Canals In Anolis Lizards: Ecomorphological Convergence And Ecomorph Affinities Of Fossil Species

Anolis lizards are a model system for the study of adaptive radiation and convergent evolution. Greater Antillean anoles have repeatedly evolved six similar forms or ecomorphs: crown-giant, grass-bush, twig, trunk, trunk-crown, and trunk-ground. Members of each ecomorph category possess a specific set of morphological, ecological and behavioural characteristics which have been acquired convergently. Here we test whether the semicircular canal system - the organ of balance - is also convergent among ecomorphs, reflecting the shared sensory requirements of their ecological niches. As semicircular canal shape has been shown to reflect different locomotor strategies, we hypothesised that each Anolis ecomorph would have a unique canal morphology. Using 3D semilandmarks and geometric morphometrics, semicircular canal shape was characterised in 41 Anolis species from the Greater Antilles and the relationship between canal shape and ecomorph grouping, phylogenetic history, size, and perch characteristics was assessed. Further, canal morphology of modern species was used to predict the ecomorph affinity of five fossil anoles from the Miocene of the Dominican Republic. Our study recovered ecomorph as the single-most important covariate of canal morphology in modern taxa; although phylogenetic history and size also showed a small, yet significant correlation with shape. Surprisingly, perch characteristics were not found to be significant covariates of canal shape, even though they are important habitat variables. Using posterior probabilities, we found that the fossil anoles have different semicircular canals shapes to modern ecomorph groupings implying extinct anoles may have been interacting with their Miocene environment in different ways to modern Anolis species.

evolutionary biology↗

bioRxiv2017

Intracellular metabolic circuits shape inter-species microbial interactions

Metabolite exchanges in microbial communities give rise to ecological interaction networks that influence ecosystem diversity and stability1-6. These exchanges depend on complex intracellular pathways thus raising the question of whether ecological interactions are inferable from genomes. We address these questions by integrating genome-scale models of metabolism7, to compute the fitness of interacting microbes, with evolutionary game theory, which uses these fitness values to infer evolutionarily stable interactions in multi-species microbial \"games\". After validating our approach using data on sucrose hydrolysis by S. cerevisiae, we performed over 80,000 in silico experiments to evaluate the rise of unidirectional and cross-feeding metabolic dependencies in populations of Escherichia coli secreting 189 amino acid pairs. We found that, despite the diversity of exchanged amino acids, most pairs conform to general patterns of inter-species interactions. However, several amino acid pairs deviate from these patterns due to pleiotropy and epistasis in metabolic pathways. To better understand the emergence of cross-feeding, we performed in silico invasion experiments and found possible evolutionary paths that could lead to such association. Overall, our study provides mechanistic insights into the rise of evolutionarily stable interdependencies, with important implications for biomedicine and microbiome engineering 8-10.

systems biology↗

bioRxiv2017

Uncovering The Drivers Of Animal-Host Microbiotas With Joint Distribution Modeling

In addition to the processes structuring free-living communities, host-associated microbiota are directly or indirectly shaped by the host. Therefore, microbiota data have a hierarchical structure where samples are nested under one or several variables representing host-specific factors, often spanning multiple levels of biological organization. Current statistical methods do not accommodate this hierarchical data structure, and therefore cannot explicitly account for the effect of the host in structuring the microbiota. We introduce a novel extension of joint species distribution models which can straightforwardly accommodate and discern between effects such as host phylogeny and traits, recorded covariates like diet and collection site, among other ecological processes. Our proposed methodology includes powerful yet familiar outputs seen in community ecology overall, including: (i) model-based ordination to visualize and quantify the main patterns in the data; (ii) variance partitioning to asses how influential the included host-specific factors are in structuring the microbiota; and (iii) co-occurrence networks to visualize microbe-to-microbe associations.

microbiology↗

bioRxiv2017

A Maladaptive Combination Of Traits Contributes To The Maintenance Of A Stable Hybrid Zone Between Two Divergent Species Of Drosophila

Geographical areas where two species come into contact and hybridize serve as natural laboratories for assessing mechanisms that limit gene flow between species. The ranges of about half of all closely related Drosophila species overlap, and the genomes of several pairs reveal signatures of past introgression. However, only two contemporary hybrid zones have been characterized in the genus, and both are recently diverged sister species (D. simulans-D. sechellia, Ks = 0.05; D. yakuba-D. santomea, Ks = 0.048). Here we present evidence of a new hybrid zone, and the ecological mechanisms that maintain it, between two highly divergent Drosophila species (Ks = 0.11). On the island of Bioko in west Africa, D. teissieri occupies mostly forests, D. yakuba occupies mostly open agricultural areas, and recently, we discovered that hybrids between these species occur near the interface of these habitats. Genome sequencing revealed that all field-sampled hybrids are F1 progeny of D. yakuba females and D. teissieri males. We found no evidence for either advanced-generation hybrids or F1 hybrids produced by D. teissieri females and D.yakuba males. The lack of advanced-generation hybrids on Bioko is consistent with mark-recapture and laboratory experiments that we conducted, which indicate hybrids have a maladaptive combination of traits. Like D. yakuba, hybrids behaviorally prefer open habitat that is relatively warm and dry, but like D. teissieri, hybrids have low desiccation tolerance, which we predict leaves them physiologically ill-equipped to cope with their preferred habitat. These observations are consistent with recent findings of limited introgression in the D. yakuba clade and identify an ecological mechanism for limiting gene flow between D. yakuba and D. teissieri; namely, selection against hybrids that we have documented, in combination with hybrid male sterility, contributes to the maintenance of this narrow (~30m), stable hybrid zone centered on the forest-open habitat ecotone. Our results show how a deleterious combination of parental traits can result in unfit or maladapted hybrids.

evolutionary biology↗

bioRxiv2017

Migration Harshness Drives Habitat Choice And Local Adaptation In Anadromous Arctic Char: Evidence From Integrating Population Genomics And Acoustic Telemetry

Migration is a ubiquitous life history trait with profound evolutionary and ecological consequences. Recent developments in telemetry and genomics, when combined, can bring significant insights on the migratory ecology of non-model organisms in the wild. Here, we used this integrative approach to document dispersal, gene flow and potential for local adaptation in anadromous Arctic Char from six rivers in the Canadian Arctic. Acoustic telemetry data from 124 tracked individuals indicated asymmetric dispersal, with a large proportion of fish (72%) tagged in three different rivers migrating up the same short river in the fall. Population genomics data from 6,136 SNP markers revealed weak, albeit significant, population differentiation (average pairwise FST = 0.011) and asymmetric dispersal was also revealed by population assignments. Approximate Bayesian Computation simulations suggested the presence of asymmetric gene flow, although in the opposite direction to that observed from the telemetry data, suggesting that dispersal does not necessarily lead to gene flow. These observations suggested that Arctic Char home to their natal river to spawn, but may overwinter in rivers with the shortest migratory route to minimize the costs of migration in non-breeding years. Genome scans and genetic-environment associations identified 90 outlier markers putatively under selection, 23 of which were in or near a gene. Of these, at least four were involved in muscle and cardiac function, consistent with the hypothesis that migratory harshness could drive local adaptation. Our study illustrates the power of integrating genomics and telemetry to study migrations in non-model organisms in logistically challenging environments such as the Arctic.

evolutionary biology↗

bioRxiv2017

Parallel radiations of Cremastosperma and Mosannona (Annonaceae), tropical rainforest trees tracking Neogene upheaval of the South American continent

This preprint has been reviewed and recommended by Peer Community In Evolutionary Biology (http://dx.doi.org/10.24072/pci.evolbiol.100033). Much of the immense present day biological diversity of Neotropical rainforests originated from the Miocene onwards, a period of geological and ecological upheaval in South America. We assess the impact of the Andean orogeny, drainage of lake Pebas, and closure of the Panama Isthmus on two clades of trees (Cremastosperma, c. 31 spp.; and Mosannona, c. 14 spp.; both Annonaceae) found in humid forest distributed across the transition zones between the Andes and Western (lowland) Amazonia and between Central and South America. We inferred phylogenies based on c. 80% of recognised species of each clade using plastid and nuclear encoded sequence markers, revealing similar patterns of geographically restricted clades. Using molecular dating we showed that diversifications in the different areas occurred in parallel, with timing consistent with Andean vicariance and Central American geodispersal. In apparent contradiction of high dispersal abilities of rainforest trees, Cremastosperma clades within Amazonia are also geographically restricted, with a southern/montane clade that appears to have diversified along the foothills of the Andes sister to one of more northern/lowland species that diversified in a region once inundated by lake Pebas. Ecological niche modelling approaches show phylogenetically conserved niche differentiation, particularly within Cremastosperma. Niche similarity and recent common ancestry of Amazon and Guianan Mosannona species contrasts with dissimilar niches and more distant ancestry of Amazon, Venezuelan and Guianan species of Cremastosperma suggesting that this element of the similar patterns of disjunct distributions in the two genera is instead a biogeographic parallelism, with differing origins. The results provide further independent evidence for the importance of the Andean orogeny, the drainage of Lake Pebas, and the formation of links between South and Central America in the evolutionary history of Neotropical lowland rainforest trees.

evolutionary biology↗

bioRxiv2017

Pathogen Dependent Phenotypic But No Genetic Correlation Between Sexual Activity And Immunity In Male Drosophila melanogaster Subjected To Differential Sexual Selection

The theory of trade-off suggest that limited resources should lead to trade-off in resource intensive traits such as immunity related and sexually selected traits in males. Alternatively, sexual exaggerations can also act as an honest indicator of underlying immunocompetence, leading to positive correlations between these traits. Several studies have addressed this question using experimental evolution. However, they have rarely used ecologically relevant pathogens and fitness measurement (e.g., measuring post-infection survivorship) to find correlations between sexual selection and immunity. Here we attempt to address this caveat by evolving populations of Drosophila melanogaster under differential sexual selection. After more than hundred generations, we infected virgin and mated males from each population with three pathogenic bacteria: Pseudomonas entomophila (Pe), Staphylococcus succinus (Ss) and Providentia rettgeri (Pr). Fitness was measured as either post-infection survivorship (Pe and Ss) or bacterial clearance ability (Pr). Contrary to expectations, sexual selection had no evolutionary effect on male fitness against any of the pathogens. Moreover, mating had a beneficial effect against Pe and Pr, but no effect against Ss, suggesting pathogen specific phenotypic correlations between mating and immunity. Following these results, we discuss the significance of using ecologically relevant pathogens and quantifying host fitness while studying sexual selection-immunity correlations.

evolutionary biology↗

bioRxiv2017

Beyond Brain Size

Despite prolonged interest in comparing brain size and behavioral proxies of intelligence across taxa, the adaptive and cognitive significance of brain size variation remains elusive. Central to this problem is the continued focus on hominid cognition as a benchmark, and the assumption that behavioral complexity has a simple relationship with brain size. Although comparative studies of brain size have been criticized for not reflecting how evolution actually operates, and for producing spurious, inconsistent results, the causes of these limitations have received little discussion. We show how these issues arise from implicit assumptions about what brain size measures and how it correlates with behavioral and cognitive traits. We explore how inconsistencies can arise through heterogeneity in evolutionary trajectories and selection pressures on neuroanatomy or neurophysiology across taxa. We examine how interference from ecological and life history variables complicates interpretations of brain-behavior correlations, and point out how this problem is exacerbated by the limitations of brain and cognitive measures. These considerations, and the diversity of brain morphologies and behavioral capacities, suggest that comparative brain-behavior research can make greater progress by focusing on specific neuroanatomical and behavioral traits within relevant ecological and evolutionary contexts. We suggest that a synergistic combination of the bottom up approach of classical neuroethology and the top down approach of comparative biology/psychology within closely related but behaviorally diverse clades can limit the effects of heterogeneity, interference, and noise. We argue this shift away from broad-scale analyses of superficial phenotypes will provide deeper, more robust insights into brain evolution.

animal behavior and cognition↗