Multifunctionality is often framed as a core constraint of evolution, yet many evolutionary transitions involve traits taking on additional functions. Mouthbrooding, a form of parental care where offspring develop inside a parent’s mouth, increases multifunctionality by adding a major function (reproduction) to a structure already serving other vital functions (feeding and respiration). Despite increasing multifunctionality, mouthbrooding has evolved repeatedly from other forms of parental care in at least seven fish families. We hypothesized that mouthbrooding is more likely to evolve in lineages with feeding adaptations that are already advantageous for mouthbrooding. We tested this hypothesis in Neotropical cichlids, where mouthbrooding has evolved four or five times, largely within winnowing clades, providing several pairwise comparisons between substrate-brooding and mouthbrooding sister taxa. We found that the mouthbrooding transition rate was 15 times higher in winnowing than in nonwinnowing clades and that mouthbrooders and winnowers overlapped substantially in their buccal cavity morphologies, which is where offspring are incubated. Species that exhibit one or both of these behaviors had larger, more curved buccal cavities, while species that exhibit neither behavior had narrow, cylindrical buccal cavities. Given the results we present here, we propose a new conceptual model for the evolution of mouthbrooding, integrating the roles of multifunctional morphology and the environment. We suggest that functional transitions like mouthbrooding offer a different perspective on multifunctionality: increasing constraints in one trait may release them for another, generating new evolutionary opportunities.

Host plant specialization across herbivorous insects varies dramatically, but while the molecular mechanisms of host plant adaptations are increasingly known, we often lack a comprehensive understanding of the selective forces that favor specialization. The milkweed bugs (Heteroptera: Lygaeinae) are ancestrally associated with plants of the Apocynaceae from which they commonly sequester cardiac glycosides for defense, facilitated by resistant Na⁺/K⁺-ATPases and adaptations for transport, storage, and discharge of toxins. Here, we show that three Lygaeinae species independently colonized four novel nonapocynaceous hosts that convergently produce cardiac glycosides. A fourth species shifted to a new source of toxins by tolerating and sequestering alkaloids from meadow saffron (Colchicum autumnale, Colchicaceae). Across three milkweed bug species tested, feeding on seeds containing toxins did not improve growth or speed of development and even impaired growth and development in two species, but sequestration mediated protection of milkweed bugs against two natural predators: lacewing larvae and passerine birds. We conclude that physiological preadaptations and convergent phytochemistry facilitated novel specialized host associations. Since toxic seeds did not improve growth but either impaired growth or, at most, had neutral effects, selection by predators on sequestration of defenses, rather than the exploitation of additional profitable dietary resources, can lead to obligatory specialized host associations in otherwise generalist insects.

Intergenerational effects, also known as parental effects in which the offspring phenotype is influenced by the parental phenotype, can occur in response to factors that occur not only in early but also in late parental life. However, little is known about how these parental life stage–specific environments interact with each other and with the offspring environment to influence offspring phenotypes, particularly in organisms that realize distinct niches across ontogeny. We examined the effects of parental larval starvation and adult reproductive environment on offspring traits under matching or mismatching offspring larval starvation conditions using the holometabolous, haplodiploid insect Athalia rosae (turnip sawfly). We show that parental larval starvation had trait-dependent intergenerational effects on both life history and consumption traits of offspring larvae, partly in interaction with offspring conditions, while there was no significant effect of parental adult reproductive environment. In addition, while offspring larval starvation led to numerous gene- and pathway-level expression differences, parental larval starvation impacted far fewer genes and only the ribosomal pathway. Our findings reveal that parental starvation evokes complex intergenerational effects on offspring life history traits, consumption patterns, and gene expression, although the effects are less pronounced than those of offspring starvation.

The process of adaptation toward novel environments is directly connected to the acquisition of higher fitness relative to others. Such increased fitness is obtained by changes in life history traits that may directly impact population dynamics. From a functional perspective, increased fitness can be achieved through higher resource use or more efficient resource use, each potentially having its own impact on population dynamics. In the first case, adaptation is expected to directly translate into higher population growth. In the second case, adaptation requires less energy and hence may lead to higher carrying capacity. Adaptation may thus lead to changes in ecological dynamics and vice versa. Here, by using a combination of evolutionary experiments with spider mites and a population dynamic model, we investigate how an increase in fecundity (a validated proxy for adaptation) affects a population’s ecological dynamics. Our results show that adaptation can positively affect population growth rate and either positively or negatively affect carrying capacity, depending on the ecological condition leading to variation in adaptation. These findings show the importance of evolution for population dynamics in changing environments, which may ultimately affect the stability and resilience of populations.

Spatial segregation of closely related species is usually attributed to differences in stress tolerance and competitive ability. For both animals and plants, reproductive interactions between close relatives can impose a fitness cost that is more detrimental to the rarer species. Frequency-dependent mating interactions may thus prevent the establishment of immigrants within heterospecific populations, maintaining spatial segregation of species. Despite strong spatial segregation in natural populations, two sympatric California monkeyflowers (Mimulus nudatus and M. guttatus) survive and reproduce in the other’s habitat when transplanted reciprocally. We hypothesized that a frequency-dependent mating disadvantage maintains spatial segregation of these monkeyflowers during natural immigration. To evaluate this hypothesis, we performed two field experiments. First, we experimentally added immigrants in varying numbers to sites dominated by heterospecifics. Second, we reciprocally transplanted arrays of varying resident and immigrant frequencies. Immigrant seed viability decreased with conspecific rarity for M. guttatus but not for M. nudatus. We observed immigrant minority disadvantage for both species, but it was driven by different factors—frequency-dependent hybridization for M. guttatus and competition for resources and/or pollinators for M. nudatus. Overall, our results suggest a major role for reproductive interference in spatial segregation that should be evaluated along with stress tolerance and competitive ability.

By virtue of their niche construction traits, plants play a significant role in shaping landscapes. The resultant outcome could change the selective environment, which then influences the evolution of these same plants. To date, almost all biogeomorphic models have assumed that niche construction traits are invariant in time. Conversely, niche construction studies often assume that independent abiotic changes either are nonexistent or are simply linear. Here, I consider the concomitant evolution of plant niche construction traits during complex landscape development. I construct a geo-evolutionary model that couples a population genetic module with a landscape development module. Allowing plants to evolve always results in landforms different from those that appear when evolution is not included. The topographic difference between cases with and without evolution ranges from a small difference in steady-state elevation (topography) to pronounced differences in landforms. Furthermore, evolution of niche construction traits could introduce alternative stable states and hysteresis, modifying the responses of landscapes to environmental stress. Allowing the landscape to develop while evolution occurs alters evolutionary trajectories for niche construction traits. The system can even develop into states that suppress natural selection. Model results support the need to integrate niche construction theory and biogeomorphology to better understand both.

The emergence of animal societies offers unsolved problems for both evolutionary and ecological studies. Social spiders are especially well suited to address this problem given their multiple independent origins and distinct geographic distribution. On the basis of long-term research on the spider genus Anelosimus, we developed a spatial model that re-creates observed macroecological patterns in the distribution of social and subsocial spiders. We show that parallel gradients of increasing insect size and disturbance (rain, predation) with proximity to the lowland tropical rain forest would explain why social species are concentrated in the lowland wet tropics but absent from higher elevations and latitudes. The model further shows that disturbance, which disproportionately affects small colonies, not only creates conditions that require group living but also tempers the dynamics of large social groups. Similarly simple underlying processes, albeit with different players on a somewhat different stage, may explain the diversity of other social systems.

Predicting temperature effects on species interactions can be challenging, especially for parasitism, where it is difficult to experimentally separate host and parasite thermal performance curves. Prior authors proposed a possible solution based on the metabolic theory of ecology (MTE), using MTE-based equations to describe the thermal mismatch between host and parasite performance curves and account for thermal acclimation responses. Here, we use published infection data, supplemented with experiments measuring metabolic responses to temperature in each species, to show that this modeling framework can successfully describe thermal acclimation effects on two different stages of infection in a tadpole-trematode system. All thermal acclimation effects on host performance manifested as changes in one key model parameter (activation energy), with measurements of host respiration generating similar MTE parameter estimates and acclimation effects compared with measurements of the host’s ability to clear encysted parasites. This result suggests that metabolic parameter estimates for whole-body metabolism can sometimes be used to estimate temperature effects on host and parasite performance curves. However, we found different thermal patterns for measurements of host prevention of initial parasite encystment emphasizing potential challenges when applying MTE-based models to complex parasite-host systems with multiple distinct stages of infection.

A pressing issue is to understand how biological complexity impacts the persistence and adaptation of populations. Natural environments are under unprecedented pressure as a result of climate change and land use change, which makes biological populations and ecological communities vulnerable. Evolution by natural selection—that is, genetic change in response to selection—is one important way species can cope with such changes. Selection often operates on complex traits, and much of selection is due to ecological interactions that, in turn, often form complex networks of species. In this sense, ecological interactions play a dual role: ecological interactions are essential to guarantee the resilience of communities and the functioning of ecosystem services, and they are a source of selection that shapes complex traits. The development of a new integrative framework combining the complexity of selected traits with the complexity of interaction patterns is essential to address potential cascading effects and extinctions. Unfortunately, studies that focus on these two levels of complexity, whether using theoretical or empirical approaches, are still scarce. In this special feature, we bring together articles that contribute to bridging this gap in the study of species coevolution and the evolution of complex traits.

Organismal traits often influence fitness via interactions with multiple species. That selection is not necessarily predictable from pairwise interactions, such as when interactions occur during different life cycle stages. Theoretically, directional selection during two sequential episodes (e.g., pollination and seed survival) can generate quadratic or correlational selection for a set of traits that passes both selective filters. We compared strength of selection during pollination versus seed predation in the field and tested whether interactions with multiple species give rise to nonlinear selection on floral traits. We planted common gardens with seeds of two species of Ipomopsis and hybrids at sites where pollination was primarily by hummingbirds or also included hawk moths. We examined selection on six floral traits, including corolla width, sepal width, color, nectar, and two scent compounds. Female fitness (seeds) was broken down into fitness during (1) pollination (seeds initiated) and (2) seed predation (proportion of seeds escaping fly predation). All traits showed evidence of selection. Directional and quadratic selection were stronger during seed initiation than during seed predation. Correlational selection occurred mostly during seed initiation rather than arising from combining species interactions at two points in the life cycle. These results underscore how multispecies interactions can combine to exert selection on trait combinations.

A current frontier of character displacement research is to determine whether displacement occurs via multiple phenotypic pathways and varies across communities with different species compositions. Here, we conducted the first test for context-dependent character displacement in multimodal floral signals by analyzing variation in floral scent in a system that exhibits character displacement in flower size and that has multiple types of sympatric communities. In a greenhouse common garden experiment, we measured quantitative variation in volatile emission rates of the progeny of two species of Clarkia from replicated parental communities that contain one, two, or four Clarkia species. The first two axes of a constrained correspondence analysis, which explained 24% of the total variation in floral scent, separated the species and community types. Of the 23 compounds that were significantly correlated with these axes, nine showed patterns consistent with character displacement. Two compounds produced primarily by C. unguiculata and two compounds produced primarily by C. cylindrica were emitted in higher amounts in sympatry. Character displacement in some volatiles varied across sympatric parental communities and occurred in parallel with displacement in flower size, demonstrating that this evolutionary process can be context dependent and may occur through multiple pathways.

Ecological interactions link species in networks. Loss of species from or introduction of new species into an existing network may have substantial effects for interaction patterns. Predicting changes in interaction frequency while allowing for rewiring of existing interactions—and hence estimating the consequences of community compositional changes—is thus a central challenge for network ecology. Interactions between species groups, such as pollinators and flowers or parasitoids and hosts, are moderated by matching morphological traits or sensory clues, most of which are unknown to us. If these traits are phylogenetically conserved, however, we can use phylogenetic distances to construct latent, surrogate traits and try to match those across groups, in addition to observed traits. Understanding how important traits and trait matching are, relative to abundances and chance, is crucial to estimating the fundamental predictability of network interactions. Here, we present a statistically sound approach (“tapnet”) to fitting abundances, traits, and phylogeny to observed network data to predict interaction frequencies. We thereby expand existing approaches to quantitative bipartite networks, which so far have failed to correctly represent the nonindependence of network interactions. Furthermore, we use simulations and cross validation on independent data to evaluate the predictive power of the fit. Our results show that tapnet is on a par with abundance-only, matching centrality, and machine learning approaches. This approach also allows us to evaluate how well current concepts of trait matching work. On the basis of our results, we expect that interactions in well-sampled networks can be well predicted if traits and abundances are the main driver of interaction frequency.

Niche packing is one of the prevailing mechanisms underlying the increase in the number of co-occurring species and the extraordinary diversity of tropical ecosystems. However, it is not yet understood whether niche packing is facilitated by higher specialization and reduced niche overlap or, rather, by diffuse competition and increased niche overlap. We combined highly resolved bird-plant interaction networks, bird phylogenies, and plant functional traits to compare dietary niche overlap and foraging frequencies among frugivorous birds at seven sites in the tropical Andes. We quantified niche overlap on the basis of the traits of the plants used by each bird and related it to the degree of niche packing at the different sites. Niche complementarity decreased with increasing niche packing, suggesting that increasingly dense niche packing is facilitated by increased niche overlap. Pairwise niche overlap was mediated by shifts in foraging frequencies away from shared resources, and it decreased with decreasing phylogenetic relatedness and increasing dependence on fruit as resource. Our findings suggest that foraging choices are a key axis of diversification in frugivorous birds and that differences in resource use frequencies are already sufficient to reduce potential competition between ecologically similar species and facilitate niche packing, especially if species differ in their dependence on particular resources.

Studies of coevolution in the wild have largely focused on reciprocally specialized species pairs with striking and exaggerated phenotypes. Textbook examples include interactions between toxic newts and their garter snake predators, long-tongued flies and the flowers they pollinate, and weevils with elongated rostra used to bore through the defensive pericarp of their host plants. Although these studies have laid a foundation for understanding coevolution in the wild, they have also contributed to the widespread impression that coevolution is a rare and quirky sideshow to the day-to-day grind of ecology and evolution. In this perspective, we argue that the focus of coevolution has been biased toward the obvious and ignored the cryptic. We have focused on the obvious—studies of reciprocally specialized species pairs with exaggerated phenotypes—mainly because we have lacked the statistical tools required to study coevolution in more generalized and phenotypically mundane systems. Building from well-established coevolutionary theory, we illustrate how model-based approaches can be used to remove this barrier and begin estimating the strength of coevolutionary selection indirectly using routinely collected data, thus uncovering cryptic coevolution in more typical communities. By allowing the distribution of coevolutionary selection to be estimated across genomes, phylogenies, and communities and over deep timescales, these novel approaches have the potential to revolutionize the way we study coevolution. As we develop a road map to these next-generation approaches, we highlight recent studies making notable progress in this direction.