Coevolutionary Interactions Research Articles

Historical Contingency Drives Compensatory Evolution and Rare Reversal of Phage Resistance.

Bacteria and lytic viruses (phages) engage in highly dynamic coevolutionary interactions over time, yet we have little idea of how transient selection by phages might shape the future evolutionary trajectories of their host populations. To explore this question, we generated genetically diverse phage-resistant mutants of the bacterium Pseudomonas syringae. We subjected the panel of mutants to prolonged experimental evolution in the absence of phages. Some populations re-evolved phage sensitivity, whereas others acquired compensatory mutations that reduced the costs of resistance without altering resistance levels. To ask whether these outcomes were driven by the initial genetic mechanisms of resistance, we next evolved independent replicates of each individual mutant in the absence of phages. We found a strong signature of historical contingency: some mutations were highly reversible across replicate populations, whereas others were highly entrenched. Through whole-genome sequencing of bacteria over time, we also found that populations with the same resistance gene acquired more parallel sets of mutations than populations with different resistance genes, suggesting that compensatory adaptation is also contingent on how resistance initially evolved. Our study identifies an evolutionary ratchet in bacteria–phage coevolution and may explain previous observations that resistance persists over time in some bacterial populations but is lost in others. We add to a growing body of work describing the key role of phages in the ecological and evolutionary dynamics of their host communities. Beyond this specific trait, our study provides a new insight into the genetic architecture of historical contingency, a crucial component of interpreting and predicting evolution.

The roles of balancing selection and recombination in the evolution of rattlesnake venom.

The origin of snake venom involved duplication and recruitment of non-venom genes into venom systems. Several studies have predicted that directional positive selection has governed this process. Venom composition varies substantially across snake species and venom phenotypes are locally adapted to prey, leading to coevolutionary interactions between predator and prey. Venom origins and contemporary snake venom evolution may therefore be driven by fundamentally different selection regimes, yet investigations of population-level patterns of selection have been limited. Here, we use whole-genome data from 68 rattlesnakes to test hypotheses about the factors that drive genomic diversity and differentiation in major venom gene regions. We show that selection has resulted in long-term maintenance of genetic diversity within and between species in multiple venom gene families. Our findings are inconsistent with a dominant role of directional positive selection and instead support a role of long-term balancing selection in shaping venom evolution. We also detect rapid decay of linkage disequilibrium due to high recombination rates in venom regions, suggesting that venom genes have reduced selective interference with nearby loci, including other venom paralogues. Our results provide an example of long-term balancing selection that drives trans-species polymorphism and help to explain how snake venom keeps pace with prey resistance.

Genetic characterization of potential venom resistance proteins in California ground squirrels (Otospermophilus beecheyi) using transcriptome analyses.

Understanding the molecular basis of adaptations in coevolving species requires identifying the genes that underlie reciprocally selected phenotypes, such as those involved in venom in snakes and resistance to the venom in their prey. In this regard, California ground squirrels (CGS; Otospermophilus beecheyi) are eaten by northern Pacific rattlesnakes (Crotalus oreganus oreganus), but individual squirrels may still show substantial resistance to venom and survive bites. A recent study using proteomics identified venom interactive proteins (VIPs) in the blood serum of CGS. These VIPs represent possible resistance proteins, but the sequences of genes encoding them are unknown despite the value of such data to molecular studies of coevolution. To address this issue, we analyzed a de novo assembled transcriptome from CGS liver tissue-where many plasma proteins are synthesized-and other tissues from this species. We then examined VIP sequences in terms of three characteristics that identify them as possible resistance proteins: evidence for positive selection, high liver expression, and nonsynonymous variation across CGS populations. Based on these characteristics, we identified five VIPs (i.e., α-2-macroglobulin, α-1-antitrypsin-like protein GS55-LT, apolipoprotein A-II, hibernation-associated plasma protein HP-20, and hibernation-associated plasma protein HP-27) as the most likely candidates for resistance proteins among VIPs identified to date. Four of these proteins have been previously implicated in conferring resistance to the venom in mammals, validating our approach. When combined with the detailed information available for rattlesnake venom proteins, these results set the stage for future work focused on understanding coevolutionary interactions at the molecular level between these species.

Enhancing protein contact map prediction accuracy via ensembles of inter-residue distance predictors

Protein contact maps capture coevolutionary interactions between amino acid residue pairs that are spatially within certain proximity threshold. Predicted contact maps are used in many protein related problems that include drug design, protein design, protein function prediction, and protein structure prediction. Contact map prediction has achieved significant progress lately but still further challenges remain with prediction of contacts between residues that are separated in the amino acid residue sequence by large numbers of other residues. In this paper, with experimental results on 5 standard benchmark datasets that include membrane proteins, we show that contact map prediction could be significantly enhanced by using ensembles of various state-of-the-art short distance predictors and then by converting predicted distances into contact probabilities. Our program along with its data is available from https://gitlab.com/mahnewton/ecp.

Egg rejection based on egg size recognition as a specific strategy against parasitic cuckoos.

In the coevolutionary interactions between avian brood parasites and their hosts, egg recognition based on color and/or pattern is a common and effective defense to counter parasitism. However, for egg recognition based on size, only a few studies have found affirmative results, and they do not provide unambiguous evidence that egg size recognition in hosts has evolved as an important and specific anti-parasite adaptation against parasite eggs. We studied the brood parasite system between the Asian emerald cuckoo Chrysococcyx maculatus and its host, the chestnut-crowned warbler Phylloscopus castaniceps. The cuckoo parasitizes the warbler using non-mimetic and larger eggs at a parasitism rate of 12.9%. The warbler nests used in this experiment were built in a dark environment with the nest illuminance near 0 lux. Experiments with 2 types of model eggs with colors and patterns resembling cuckoo eggs of different sizes (cuckoo egg size or host egg size) showed that the warblers were able to reject 63.6% of cuckoo model eggs under these dim light conditions. However, model eggs with the same color and pattern similar to the warbler egg size were always accepted. This study provides strong evidence supporting the theory that egg size recognition can be evolved in hosts as a specific anti-parasite adaptation against cuckoos. We suggest that the egg size recognition of the warbler is an outcome of the tradeoff between the costs of violating the parental investment rule and suffering cuckoo parasitism.

Cophylogeny and convergence shape holobiont evolution in sponge-microbe symbioses.

Symbiotic microbial communities of sponges serve critical functions that have shaped the evolution of reef ecosystems since their origins. Symbiont abundance varies tremendously among sponges, with many species classified as either low microbial abundance (LMA) or high microbial abundance (HMA), but the evolutionary dynamics of these symbiotic states remain unknown. This study examines the LMA/HMA dichotomy across an exhaustive sampling of Caribbean sponge biodiversity and predicts that the LMA symbiotic state is the ancestral state among sponges. Conversely, HMA symbioses, consisting of more specialized microorganisms, have evolved multiple times by recruiting similar assemblages, mostly since the rise of scleractinian-dominated reefs. Additionally, HMA symbioses show stronger signals of phylosymbiosis and cophylogeny, consistent with stronger co-evolutionary interaction in these complex holobionts. These results indicate that HMA holobionts are characterized by increased endemism, metabolic dependence and chemical defences. The selective forces driving these patterns may include the concurrent increase in dissolved organic matter in reef ecosystems or the diversification of spongivorous fishes.

Ehrlich and Raven escape and radiate coevolution hypothesis at different levels of organization: Past and future perspectives.

The classic paper by Ehrlich and Raven on coevolution will soon be 60 years old. Although they were not the first to develop the idea of coevolution, their thought-provoking paper certainly popularized this idea and inspired several generations of scientists interested in coevolution. Here, we describe some of their main contributions, quantitatively measure the impact of their seminal paper on different fields of research, and discuss how ideas related to their original paper might push the study of coevolution forward. To guide our discussion, we explore their original hypothesis into three research fields that are associated with distinct scales/levels of organization: (1) the genetic mechanisms underlying coevolutionary interactions; (2) the potential association between coevolutionary diversification and the organization of ecological networks; and (3) the micro- and macroevolutionary mechanisms and expected patterns under their hypothesis. By doing so, we discuss potentially overlooked aspects and future directions for the study of coevolutionary dynamics and diversification.

Microbiome Transmission During Sexual Intercourse Appears Stochastic and Supports the Red Queen Hypothesis.

Microbes inhabit virtually everywhere on and/or in our bodies, including the seminal and vaginal fluids. They have significant importance in maintaining reproductive health and protecting hosts from diseases. The exchange of microbes during sexual intercourse is one of the most direct and significant microbial transmissions between men and women. Nevertheless, the mechanism of this microbial transmission was little known. Is the transmission mode stochastic, passive diffusion similar to the random walk of particles, or driven by some deterministic forces? What is the microbial transmission probability? What are the possible evolutionary implications, particularly from the perspective of sexual reproduction (selection)? We tackle these intriguing questions by leveraging the power of Hubbell’s unified neutral theory of biodiversity, specifically implemented as the HDP-MSN (hierarchical Dirichlet process approximated multi-site neutral model), which allows for constructing truly multi-site metacommunity models, simultaneously including vaginal and semen microbiomes. By reanalyzing the microbiome datasets of seminal and vaginal fluids from 23 couples both before and after sexual intercourses originally reported by Mändar and colleagues, we found that the microbial transmission between seminal and vaginal fluids is a stochastic, passive diffusion similar to the random walk of particles in physics, rather than driven by deterministic forces. The transmission probability through sexual intercourse seems to be approximately 0.05. Inspired by the results from the HDP-MSN model, we further conjecture that the stochastic drifts of microbiome transmissions during sexual intercourses can be responsible for the homogeneity between semen and vaginal microbiomes first identified in a previous study, which should be helpful for sexual reproduction by facilitating the sperm movement/survival and/or egg fertilization. This inference seems to be consistent with the classic Red Queen hypothesis, which, when extended to the co-evolutionary interactions between humans and their symbiotic microbiomes, would predict that the reproductive system microbiomes should support sexual reproduction.

Antagonistic Coevolution Limits the Range of Host Defense in C. elegans Populations

Host populations often evolve defenses against parasites due to the significant fitness costs imposed by infection. However, adaptation to a specific parasite may alter the effectiveness of the host’s defenses in general. Consequently, the specificity of host defense may be influenced by a host population’s evolutionary history with parasites. Further, the degree of reciprocal change within an interaction may profoundly alter the range of host defense, given that antagonistic coevolutionary interactions are predicted to favor defense against specific parasite genotypes. Here, we examined the effect of host evolutionary history on host defense range by assessing the mortality rates of Caenorhabditis elegans host populations exposed to an array of Serratia marcescens bacterial parasite strains. Importantly, each of the host populations were derived from the same genetic background but have different experimental evolution histories with parasites. Each of these histories (exposure to either heat-killed, fixed genotype, or coevolving parasites) carries a different level of evolutionary reciprocity. Overall, we observed an effect of host evolutionary history in that previously coevolved host populations were generally the most susceptible to novel parasite strains. This data demonstrates that host evolutionary history can have a significant impact on host defense, and that host-parasite coevolution can increase host susceptibility to novel parasites.

Chemical Ecology of the North American Newt Genera Taricha and Notophthalmus.

The North American newt genera Taricha and Notophthalmus (order Caudata) are well known for the combination of potent toxicity, aposematic coloration, and striking defense postures that protects these animals from predation. This suite of traits is centered around the neurotoxin tetrodotoxin, which causes paralysis and death in metazoans by disrupting the initiation and propagation of electrical signals in the nerves and muscles. Tetrodotoxin defends newts from predation across multiple life history stages and its role in generating arms-race coevolution between Taricha newts and garter snake (genus Thamnophis) predators is well studied. However, understanding the broader picture of chemical defenses in Taricha and Notophthalmus requires an expanded comprehension of the defensive chemical ecology of tetrodotoxin that includes possible coevolutionary interactions with insect egg predators, protection against parasites, as well as mimicry complexes associated with tetrodotoxin and aposematic coloration in both genera. Herein the authors review what is known about the structure, function, and pharmacology of tetrodotoxin to explore its evolution and chemical ecology in the North American newt. Focus is made specifically on the origin and possible biosynthesis of tetrodotoxin in these taxa as well as providing an expanded picture of the web of interactions that contribute to landscape level patterns of toxicity and defense in Taricha and Notophthalmus.

A geographic mosaic of coevolution between Eurosta solidaginis (Fitch) and its host plant tall goldenrod Solidago altissima (L.).

A geographic mosaic of coevolution has produced local reciprocal adaptation in tall goldenrod, Solidago altissima (L.), and the goldenrod ball-gall fly, Eurosta solidaginis (Fitch 1855). The fly is selected to induce gall diameters that minimize mortality from natural enemies, and the plant is selected to limit gall growth that reduces plant fitness. We conducted a double reciprocal transplant experiment where S. altissima and E. solidaginis from three sites were grown in gardens at each site to partition the gall morphology variation into fly genotype, plant genotype, and the environment components. The host plant gall diameter induced by each E. solidaginis population was adapted to inhibit local natural enemies from ovipositing on or consuming enclosed larvae. Reciprocally, increasing the gall size induced by the local fly population increased the resistance of the local plant host population to gall growth. Differences among sites in natural enemies produced a mosaic of hotspots of coevolutionary arms races between flies selecting for greater gall diameter and plants for smaller diameters, and coldspots where there is no selection on plant or fly for a change in gall diameter. In contrast, the geographic variations of gall length and gall shape were not due to coevolutionary interactions.

The Perceptual and Cognitive Processes That Govern Egg Rejection in Hosts of Avian Brood Parasites

Hosts of avian brood parasites are under intense selective pressure to prevent or reduce the cost of parasitism. Many have evolved refined egg discrimination abilities, which can select for eggshell mimicry in their parasite. A classic assumption underlying these coevolutionary dynamics is that host egg recognition depends on the perceivable difference between their own eggs and those of their parasite. Over the past two decades, the receptor noise-limited (RNL) model has contributed to our understanding of these coevolutionary interactions by providing researchers a method to predict a host’s ability to discriminate a parasite’s egg from its own. Recent research has shown that some hosts are more likely to reject brown eggs than blue eggs, regardless of the perceived differences to their own. Such responses suggest that host egg recognition may be due to perceptual or cognitive processes not currently predictable by the RNL model. In this perspective, we discuss the potential value of using the RNL model as a null model to explore alternative perceptual processes and higher-order cognitive processes that could explain how and why some hosts make seemingly counter-intuitive decisions. Further, we outline experiments that should be fruitful for determining the perceptual and cognitive processing used by hosts for egg recognition tasks.

Genetic diversity and structure of Oncomelania hupensis hupensis in two eco-epidemiological settings as revealed by the mitochondrial COX1 gene sequences.

Oncomelania hupensis hupensis is the only intermediate host of Schistosoma japonicum, the causative agent of schistosomiasis in China and is therefore of significant medical and veterinary health importance. Although tremendous progress has been achieved, there remains an understudied area of approximately 2.06 billion m2 of potential snail habitats. This area could be further increased by annual flooding. Therefore, an understanding of population genetics of snails in these areas may be useful for future monitoring and control activities. We sampled snails from Hexian (HX), Zongyang (ZY) and Shitai (ST) in Anhui (schistosomiasis transmission control), and from Hengtang (HT), Taicang (TC), Dongsan (DS) and Xisan (XS) in Jiangsu (schistosomiasis transmission interrupted), downstream of Anhui. ST, DS and XS are classified as hilly and mountainous areas, and HX, ZY, TC and HT as lake and marshland areas. The mitochondrial cytochrome c oxidase subunit I gene were sequenced. Out of 115 snails analyzed, 29 haplotypes were identified. We observed 56 (8.72%) polymorphic sites consisting of 51 transitions, four transversions and one multiple mutational change. The overall haplotype and nucleotide diversity were 0.899 and 0.01569, respectively. Snail populations in Anhui had higher genetic diversity than in Jiangsu. 73.32% of total variation was distributed among sites and 26.68% within sites. Snails were significantly separated according to eco-epidemiological settings in both network and phylogenetic analyses. Our results could provide important guidance towards assessing coevolutionary interactions of snails with S. japonicum, as well as for future molluscan host monitoring and control activities.

Pathogen-driven coevolution across the CBP60 plant immune regulator subfamilies confers resilience on the regulator module.

Components of the plant immune signaling network need mechanisms that confer resilience against fast-evolving pathogen effectors that target them. Among eight Arabidopsis CaM-Binding Protein (CBP) 60 family members, AtCBP60g and AtSARD1 are partially functionally redundant, major positive immune regulators, and AtCBP60a is a negative immune regulator. We investigated possible resilience-conferring evolutionary mechanisms among the CBP60a, CBP60g and SARD1 immune regulatory subfamilies. Phylogenetic analysis was used to investigate the times of CBP60 subfamily neofunctionalization. Then, using the pairwise distance rank based on the newly developed analytical platform Protein Evolution Analysis in a Euclidean Space (PEAES), hypotheses of specific coevolutionary mechanisms that could confer resilience on the regulator module were tested. The immune regulator subfamilies diversified around the time of angiosperm divergence and have been evolving very quickly. We detected significant coevolutionary interactions across the immune regulator subfamilies in all of 12 diverse core eudicot species lineages tested. The coevolutionary interactions were consistent with the hypothesized coevolution mechanisms. Despite their unusually fast evolution, members across the CBP60 immune regulator subfamilies have influenced the evolution of each other long after their diversification in a way that could confer resilience on the immune regulator module against fast-evolving pathogen effectors.

Relative Time Constraints Improve Molecular Dating.

Dating the tree of life is central to understanding the evolution of life on Earth. Molecular clocks calibrated with fossils represent the state of the art for inferring the ages of major groups. Yet, other information on the timing of species diversification can be used to date the tree of life. For example, horizontal gene transfer events and ancient coevolutionary interactions such as (endo)symbioses occur between contemporaneous species and thus can imply temporal relationships between two nodes in a phylogeny. Temporal constraints from these alternative sources can be particularly helpful when the geological record is sparse, for example, for microorganisms, which represent the majority of extant and extinct biodiversity. Here, we present a new method to combine fossil calibrations and relative age constraints to estimate chronograms. We provide an implementation of relative age constraints in RevBayes that can be combined in a modular manner with the wide range of molecular dating methods available in the software. We use both realistic simulations and empirical datasets of 40 Cyanobacteria and 62 Archaea to evaluate our method. We show that the combination of relative age constraints with fossil calibrations significantly improves the estimation of node ages. [Archaea, Bayesian analysis, cyanobacteria, dating, endosymbiosis, lateral gene transfer, MCMC, molecular clock, phylogenetic dating, relaxed molecular clock, revbayes, tree of life.]

Prolactin mediates behavioural rejection responses to avian brood parasitism.

Adaptations resulting from co-evolutionary interactions between avian brood parasites and their hosts have been extensively studied, yet the physiological mechanisms underlying antiparasitic host defences remain little known. Prolactin, one of the main hormones involved in the regulation of avian parental behaviour, might play a key role in the orchestration of the host responses to avian brood parasitism. Given the positive association between prolactin and parental behaviour during incubation, decreasing prolactin levels are expected to facilitate egg-rejection decisions. We tested this prediction by implanting Eurasian blackbird (Turdus merula) females with an inhibitor of prolactin secretion, bromocriptine mesylate, to experimentally decrease their plasma prolactin levels. Bromocriptine mesylate-implanted individuals ejected mimetic model eggs at higher rates, and showed shorter latency to egg ejection, than placebo-treated birds. To our knowledge, this is the first experimental evidence that behavioural host defences against avian brood parasitism are mediated by prolactin.

Values-based Physical Education for the intermediate schooling phase in a diverse South African context

Participation in physical activity, Physical Education (PE) and sport has been recognised as a powerful learning tool for education, providing a universal language for contributing to valuable life principles. Values-based education implies that learners are educated about the aspects determining their behaviour. Values-based PE, physical activities and sport have the potential to transcend diversity and achieve cohesion, promote tolerance and trust and affirm respect between individuals and communities. The goal of PE can be to contribute to the acceptance of the infinite qualities of South Africa’s diversity and to claim the country’s diversity as a source of strength that forms a bond of a common set of values. There has been a global change in the interaction of learners with their environment; their lives are shaped by forces that do not necessarily assist them to learn and apply values. A PE programme infused with the values of Olympism and Ubuntuism can offer an investment in individual and societal improvement as the co-evolutionary interaction of these values and how they affect each learner can add to the celebration of human diversity. The question this study set out to answer was how can PE be used as a tool to teach values. Thus, the study aimed to inform the development of a values-based PE programme for the intermediate schooling phase. This qualitative study, from a constructivist paradigm, has enhanced the understanding of individuals’ cultures, beliefs and values, human experiences and situations. Purposeful sampling, of 10 intermediate phase teachers from five different public primary schools sought information-rich cases. The theoretical perspectives of the experiential learning theory were applied to teaching PE during in-service PE teacher training workshops. The process was documented by collecting data from multiple sources. Participatory action research was used, determining how data were collected, analysed and presented on an ongoing, cyclical basis. This study developed material for the intermediate phase PE curriculum that underpins the values of Olympism and Ubuntuism as core values, which were modelled by teachers and guided their work. The PE programme included key elements of and aligned with the study aims of the subject Life Skills. The outcomes of using PE as a tool to teach values propose recommendations to the Department of Basic Education of South Africa, to improve and implement a quality PE curriculum that is applicable to practice and that will optimise the chances of meeting National Curriculum Statement standards. Further research is recommended on the rest of the intermediate phase PE curriculum over the entire year, which includes other movement phenomena infused with values.

Integrative Pre-Breeding for Biotic Resistance in Forest Trees.

Climate change is unleashing novel biotic antagonistic interactions for forest trees that may jeopardize populations’ persistence. Therefore, this review article envisions highlighting major opportunities from ecological evolutionary genomics to assist the identification, conservation, and breeding of biotic resistance in forest tree species. Specifically, we first discuss how assessing the genomic architecture of biotic stress resistance enables us to recognize a more polygenic nature for a trait typically regarded Mendelian, an expectation from the Fisherian runaway pathogen–host concerted arms-race evolutionary model. Secondly, we outline innovative pipelines to capture and harness natural tree pre-adaptations to biotic stresses by merging tools from the ecology, phylo-geography, and omnigenetics fields within a predictive breeding platform. Promoting integrative ecological genomic studies promises a better understanding of antagonistic co-evolutionary interactions, as well as more efficient breeding utilization of resistant phenotypes.

Manduca sexta experience high parasitoid pressures in the field but minor fitness costs of consuming plant secondary compounds.

Plant‐herbivore coevolutionary interactions have led to a range of plant defenses that minimize insect damage and a suite of counter adaptations that allow herbivores to feed on defended plants. Consuming plant secondary compounds results in herbivore growth and developmental costs but can have beneficial effects such as deterrence or harm of parasitoid enemies. Therefore, the role of secondary compounds on herbivore fitness must be considered in the context of the abundance and level of harm from natural enemies and the costs herbivores incur feeding on plant secondary compounds.In this study, I combined field measurements of Cotesia congregata wasp parasitism pressure with detailed measurements of the costs of plant secondary compounds across developmental stages in the herbivore host, Manduca sexta.I show that C. congregata parasitoids exert large negative selective pressures, killing 31%–57% of M. sexta larvae in the field. Manduca sexta developed fastest during instars most at risk for parasitoid oviposition but growth was slowed by consumption of plant secondary compounds. The negative effects of consuming plant secondary compounds as larvae influenced adult size traits but there were no immune, survival, or fecundity costs.These results suggest that developmental costs experienced by M. sexta herbivores consuming defensive compounds are minor in comparison to the strong negative survival pressures from abundant parasitoid enemies.

Coevolution between Himalayan cuckoos and 2 sympatric Pycnonotidae hosts.

Selection due to cuckoo parasitism is responsible for the evolution of anti-parasitism defenses in hosts. Different host species breeding sympatrically with a single parasitic cuckoo may evolve different strategies to reduce the risk of counter cuckoo parasitism, resulting in different interactions between cuckoos and hosts in areas of sympatry. Here, we studied the coevolutionary interactions between Himalayan cuckoos Cuculus saturatus and 2 sympatric and closely related potential hosts belonging to the family Pycnonotidae, the brown-breasted bulbul Pycnonotus xanthorrhous and the collared finchbill Spizixos semitorques. We investigated parasitism rates and nest-site selection (nest height, nest cover, human disturbance, perch height, forest distance, and degree of concealment) related to parasitism risk, nest defense against a cuckoo dummy, and egg rejection against cuckoo model eggs. Bulbuls used specific nest sites that were further away from forests than those of finchbills, and they behaved more aggressively toward cuckoos than finchbills. In contrast, bulbuls possessed moderate egg rejection ability, whereas the finchbill rejected 100% of cuckoo model eggs. We suggest that selection of a nest site away from forests by the bulbul explains the absence of parasitism by Himalayan cuckoos. We suggest that these interspecific differences in nest-site selection and nest defense indicate alternative responses to selection due to cuckoos.