Trait Conservatism Research Articles

Conservatism and diversification of plant functional traits: Evolutionary rates versus phylogenetic signal

The concepts of niche conservatism and adaptive radiation have played central roles in the study of evolution and ecological diversification. With respect to phenotypic evolution, the two processes may be seen as opposite ends of a spectrum; however, there is no straightforward method for the comparative analysis of trait evolution that will identify these contrasting scenarios. Analysis of the rate of phenotypic evolution plays an important role in this context and merits increased attention. In this article, independent contrasts are used to estimate rates of evolution for continuous traits under a Brownian motion model of evolution. A unit for the rate of phenotypic diversification is introduced: the felsen, in honor of J. Felsenstein, is defined as an increase of one unit per million years in the variance among sister taxa of ln-transformed trait values. The use of a standardized unit of measurement facilitates comparisons among clades and traits. Rates of diversification of three functional traits (plant height, leaf size, and seed size) were estimated for four to six woody plant clades (Acer, Aesculus, Ceanothus, Arbutoideae, Hawaiian lobeliads, and the silversword alliance) for which calibrated phylogenies were available. For height and leaf size, rates were two to approximately 300 times greater in the Hawaiian silversword alliance than in the other clades considered. These results highlight the value of direct estimates of rates of trait evolution for comparative analysis of adaptive radiation, niche conservatism, and trait diversification.

Potential link between plant and fungal distributions in a dipterocarp rainforest: community and phylogenetic structure of tropical ectomycorrhizal fungi across a plant and soil ecotone

*Relatively little is known about diversity or structure of tropical ectomycorrhizal communities or their roles in tropical ecosystem dynamics. In this study, we present one of the largest molecular studies to date of an ectomycorrhizal community in lowland dipterocarp rainforest. *We sampled roots from two 0.4 ha sites located across an ecotone within a 52 ha forest dynamics plot. Our plots contained > 500 tree species and > 40 species of ectomycorrhizal host plants. Fungi were identified by sequencing ribosomal RNA genes. *The community was dominated by the Russulales (30 species), Boletales (17), Agaricales (18), Thelephorales (13) and Cantharellales (12). Total species richness appeared comparable to molecular studies of temperate forests. Community structure changed across the ecotone, although it was not possible to separate the role of environmental factors vs host plant preferences. Phylogenetic analyses were consistent with a model of community assembly where habitat associations are influenced by evolutionary conservatism of functional traits within ectomycorrhizal lineages. *Because changes in the ectomycorrhizal fungal community parallel those of the tree community at this site, this study demonstrates the potential link between the distribution of tropical tree diversity and the distribution of tropical ectomycorrhizal diversity in relation to local-scale edaphic variation.

Phylogenetic trait conservatism and the evolution of functional trade-offs in arbuscular mycorrhizal fungi.

The diversity of functional and life-history traits of organisms depends on adaptation as well as the legacy of shared ancestry. Although the evolution of traits in macro-organisms is well studied, relatively little is known about character evolution in micro-organisms. Here, we surveyed an ancient and ecologically important group of microbial plant symbionts, the arbuscular mycorrhizal (AM) fungi, and tested hypotheses about the evolution of functional and life-history traits. Variation in the extent of root and soil colonization by AM fungi is constrained to a few nodes basal to the most diverse groups within the phylum, with relatively little variation associated with recent divergences. We found no evidence for a trade-off in biomass allocated to root versus soil colonization in three published glasshouse experiments; rather these traits were positively correlated. Partial support was observed for correlated evolution between fungal colonization strategies and functional benefits of the symbiosis to host plants. The evolution of increased soil colonization was positively correlated with total plant biomass and shoot phosphorus content. Although the effect of AM fungi on infection by root pathogens was phylogenetically conserved, there was no evidence for correlated evolution between the extent of AM fungal root colonization and pathogen infection. Variability in colonization strategies evolved early in the diversification of AM fungi, and we propose that these strategies were influenced by functional interactions with host plants, resulting in an evolutionary stasis resembling trait conservatism.

Spatial overlap enhances geographic range size conservatism

Phylogenetic conservatism or heritability of the geographic range sizes of species has been predicted to occur because of the phylogenetic conservatism of niche traits. However, evidence for range size conservatism is mixed, and even when statistically significant is often rather weak and of questionable biological importance. Here, we test the prediction that such conservatism will be more strongly expressed when the amount of spatial overlap between sister species increases. We used the global distributions of 1136 avian species (>10% of extant members of this Class), and tested the conservatism of geographic range sizes using the coefficients of correlation between values for pairs of sister species. We used a null model to test whether the range sizes of sister species were more similar to one another than expected by chance. We found that sister species showed a significant positive relationship between their geographic range sizes whatever the degree of spatial overlap. However, as predicted, the level of conservatism increases with the level of range overlap between sister species. More precisely, the strong increase in the coefficient of correlation between sister species’ range sizes when we add species with some range overlap to the pool of pairs without any such overlap indicates an important threshold effect. These results suggest that niche conservatism is more likely to lead to marked heritability of the range sizes of species when similar niche traits are expressed under more similar environmental conditions. These results have significant implications because they suggest 1) that previous analyses of conservatism of range sizes have been confounded by the level of spatial overlap, and 2) that closely related species experiencing similar conditions may tend to expand or restrict their geographic ranges in parallel when faced with climate change.

More rich means more diverse: Extending the ‘environmental heterogeneity hypothesis’ to taxonomic diversity

It is widely appreciated that increasing environmental heterogeneity is one of the chief determinants of high species richness. An additional outcome that arises from the relationship between environmental heterogeneity and species richness is that species richer areas are usually taxonomically more diverse than species poor areas. For instance, due to the larger niche availability, species that coexist in heterogeneous environments experience a less severe effect of clustering in their functional traits giving rise to assemblages that are more functionally diverse than in more homogeneous areas. On the other hand, due to the conservatism of many species traits during evolutionary change, the ability of species to colonize the same ecological space is thought to depend at least partially on their taxonomic similarity, such that a positive relationship between the species taxonomic relatedness and their trait similarity is expected. In this paper, we tested the relationship between species richness and taxonomic diversity with 11 florae collected in Latium (Central Italy). The significance of the observed association was then verified with a null model assuming a random distribution of species across the landscape.

Common species have lower taxonomic diversity Evidence from the urban floras of Brussels and Rome

ABSTRACTThe species pool hypothesis claims that the large‐scale regional species pool is the chief parameter in determining small‐scale species richness through filtering of species that can persist within a community on the basis of their tolerance of the abiotic environment. Accordingly, different environmental conditions give rise to different species assemblages. From a taxonomic perspective, under the assumption of trait conservatism, co‐occurring species that experience similar environmental conditions are likely to be more taxonomically similar than ecologically distant species. The next step consists in understanding how commonness and rarity of individual species produce the observed taxonomic diversity. In this paper, the importance of environmental filtering in regulating the taxonomic structure of rare and common plant species in the urban floras of Brussels (Belgium) and Rome (Italy) is tested. First, we computed the taxonomic diversity of the rare and common species of Brussels and Rome based on the branching topology of the Linnaean taxonomic trees. Next, using a randomization procedure, we determined whether the taxonomic diversity of the rare species was significantly higher than the diversity of the common species. Results show that, for both urban floras, common species that shape the community matrix and experience similar environmental conditions have a taxonomic diversity that is significantly lower than that of the rare species that represent a relatively incidental set of species of more ‘disperse’ origin. Finally, from a conservation/management perspective our results imply that, given their high taxonomic heterogeneity, the protection of rare species is a central issue for preserving high levels of diversity in urban areas.

Shell surface adaptations in relation to water management in rock‐dwelling land snails, Albinaria (Pulmonata: Clausiliidae)

Land snails display a great diversity of shell structures subject to adaptive explanations. Ribs, which are transverse protrusions on the shell surface, are an example. For the rock‐dwelling snail Albinaria variation of shell sculpture is attributed to water management adaptations. However, detailed experimental tests of these hypotheses are lacking. Various Albinaria species, differing in rib pattern, were examined for trait conservatism and the capacity to adhere and retain water on their shells, and the relation between ribbing and activation time after aestivation. Phylogenetic signal in all traits was random. Differences in shell sculpture affect shell weight and water adhesion but not water loss and activation time. Climatic variables have no influence on rib variation. Therefore, shell sculpture is not sufficient alone to explain the ability of Albinaria to withstand adverse conditions. Thus, additional morphological, behavioural and physiological adaptations must be considered as promising candidate traits for further studies of divergence.

Exploring taxonomic filtering in urban environments

AbstractQuestion: Several mechanisms have been proposed that control the spatio‐temporal pattern of species coexistence. Among others, the species pool hypothesis states that the large‐scale species pool is an important factor in controlling small‐scale species richness through filtering of species that can persist within a species assemblage on the basis of their tolerance of the abiotic environment. Because of the process of environmental filtering, co‐occurring species that experience similar environmental conditions are likely to be more taxonomically similar than ecologically distant species. This is because, due to the conservatism of many species traits during evolutionary diversification, the ability of species to colonize the same ecological space is thought to depend at least partially on their taxonomic similarity. The question for this study is: Under the assumption of trait conservatism, does environmental filtering lead to nonrandom species assemblages with respect to their taxonomic structure?Methods: The significance of taxonomic filtering in regulating species coexistence is tested using data from 15 local species assemblages from the urban flora of Rome (Italy). To find out whether the taxonomic structure of the selected’ local’ species assemblages was significantly different from random, we used a Monte Carlo simulation in which for each local species assemblage, the actual taxonomic diversity was compared to the taxonomic diversity of 1000 virtual species lists of the same size extracted at random from a larger ‘regional’ species pool. Results: We found that in most cases the local species assemblages have a higher degree of taxonomic similarity than would be expected by chance showing a phenomenon of ‘species condensation’ in a small number of higher‐level taxa.Conclusions: Our observations support the species pool hypothesis and imply that environmental filtering is an important mechanism in shaping the taxonomic structure of species assemblages. Therefore, the incorporation of taxonomic diversity into landscape and community ecology may be beneficial for a better understanding of the processes that regulate species coexistence.

THE INFLUENCE OF SPATIAL AND SIZE SCALE ON PHYLOGENETIC RELATEDNESS IN TROPICAL FOREST COMMUNITIES

The relative importance of biotic, abiotic, and stochastic processes in structuring ecological communities continues to be a central focus in community ecology. In order to assess the role of phylogenetic relatedness on the nature of biodiversity we first quantified the degree of phylogenetic niche conservatism of several plant traits linked to plant form and function. Next we quantified the degree of phylogenetic relatedness across two fundamental scaling dimensions: plant size and neighborhood size. The results show that phylogenetic niche conservatism is likely widespread, indicating that closely related species are more functionally similar than distantly related species. Utilizing this information we show that three of five tropical forest dynamics plots (FDPs) exhibit similar scale-dependent patterns of phylogenetic structuring using only a spatial scaling axis. When spatial- and size-scaling axes were analyzed in concert, phylogenetic overdispersion of co-occurring species was most important at small spatial scales and in four of five FDPs for the largest size class. These results suggest that phylogenetic relatedness is increasingly important: (1) at small spatial scales, where phylogenetic overdispersion is more common, and (2) in large size classes, where phylogenetic overdispersion becomes more common throughout ontogeny. Collectively, our results highlight the critical spatial and size scales at which the degree of phylogenetic relatedness between constituent species influences the structuring of tropical forest diversity.

Influence of Phylogeny on Fungal Community Assembly and Ecosystem Functioning

Ecology seeks to explain species coexistence and its functional consequences, but experimental tests of mechanisms that simultaneously account for both processes are difficult. We used an experimental mycorrhizal plant system to test whether functional similarity among closely related species (phylogenetic conservatism) can drive community assembly and ecosystem functioning. Communities were constructed with the same number of fungal species, but after 1 year of growth, realized species richness was highest where the starting species were more distantly related to each other. Communities with high realized species richness also stimulated plant productivity more than those with low realized species richness. Our findings suggest that phylogenetic trait conservatism can promote coexistence because of reduced competition between distinct evolutionary lineages and enhance ecosystem function because of functional complementarity among those same lineages.

Geographical range size heritability: what do neutral models with different modes of speciation predict?

ABSTRACTAim Phylogenetic conservatism or heritability of the geographical range sizes of species (i.e. the tendency for closely related species to share similar range sizes) has been predicted to occur because of the strong phylogenetic conservatism of niche traits. However, the extent of such heritability in range size is disputed and the role of biology in shaping this attribute remains unclear. Here, we investigate the level of heritability of geographical range sizes that is generated from neutral models assuming no biological differences between species.Methods We used three different neutral models, which differ in their speciation mode, to simulate the life‐history of 250,000 individuals in a square lattice of 50 × 50 cells. These individuals can speciate, reproduce, migrate and die in the metacommunity according to stochastic events. We ran each model for 3000 steps and recorded the range size of each species at each step. The heritability of geographical range size was assessed using an asymmetry coefficient between range sizes of sister species and using the coefficient of correlation between the range sizes of ancestors and their descendants.Results Our results demonstrated the ability of neutral models to mimic some important observed patterns in the heritability of geographical range size. Consistently, sister species exhibited higher asymmetry in range sizes than expected by chance, and correlations between the range sizes of ancestor–descendant species pairs, although often weak, were almost invariably positive.Main conclusions Our findings suggest that, even without any biological trait differences, statistically significant heritability in the geographical range sizes of species can be found. This heritability is weaker than that observed in some empirical studies, but suggests that even here a substantial component of heritability may not necessarily be associated with niche conservatism. We also conclude that both present‐day and fossil data sets may provide similar information on the heritability of the geographical range sizes of species, while the omission of rare species will tend to overestimate this heritability.

Biogeographic, environmental, and phylogenetic influences on reproductive traits in subtropical forest trees, South Africa

The distribution of subtropical forests in South Africa is largely a consequence of climatic change during the Quaternary period. We gathered data for 195 canopy tree species from Afrotemperate, scarp, and coastal forests in KwaZulu‐Natal, South Africa and assessed patterns in reproductive traits using a comparative phylogenetic approach. The overlap in species composition among the forest types reflected the geographic position and colonisation history of the forests. Despite a high degree of phylogenetic conservatism in reproductive traits, there were differences in their expression among forest types. Afrotemperate forests had a higher incidence of wind pollination, consistent with the steep topography, seasonally dry environment, and limited resource availability in the habitat. Scarp forests had more dry brown fruits with abiotic (explosive and wind) dispersal. Coastal forests had the most species with fleshy fruits, zoochory, and large seeds, all traits that may have facilitated the colonisation or persistence of species in this more recent and dynamic community. Since many suites of traits occur together in diverse phylogenetic lines, they represent adaptive complexes that are influenced by biogeography and environmental conditions. Overall, this study demonstrates that biogeography, environmental factors, and phylogenetic history all influence the distribution of reproductive traits in these forest communities.

Conservatism of host specificity in parasites

As information becomes available for many groups of organisms a general pattern of phylogenetic conservatism in ecological characters or morphological traits is now widely recognized. Conversely, conservatism of external ecological attributes throughout a lineage is still a contentious theme in ecology. Moreover, the studies exploring this topic have focused on free‐living organisms, and have ignored parasites. The main external ecological attribute of parasite species is certainly their host specificity, which is a key determinant of both their range size and local abundance. We address the subject of conservatism and predictability of host specificity using 2 large databases concerning, respectively, ectoparasites and endoparasites. We found a significant positive relationship between the numbers of host species infested by flea sister species. Moreover, this result was consistent whether we used sympatric or allopatric flea species, suggesting no influence of the mode of speciation on this conservatism of specificity. Additionally, our results showed that congeneric helminth species have more similar host taxonomic diversities than expected by chance, although this conservatism is due mostly to trematodes. Whilst there is evidence of conservatism, the moderate levels preclude robust prediction of host specificity for one species based on that of closely related species.

PHYLOGENETIC STRUCTURE OF FLORIDIAN PLANT COMMUNITIES DEPENDS ON TAXONOMIC AND SPATIAL SCALE

Consideration of the scale at which communities are defined both taxonomically and spatially can reconcile apparently contradictory results on the extent to which plants show phylogenetic niche conservatism. In plant communities in north central Florida, we collected species abundances in 55 0.1-ha plots in several state parks. When communities were defined narrowly to include a single phylogenetic lineage, such as Quercus, Pinus, or Ilex, neighbors tended to be less related than expected (phylogenetic overdispersion) or there was no pattern. If the same communities were defined more broadly, such as when all seed plants were included, neighbors tended to be more related than expected (phylogenetic clustering). These results provide evidence that species interactions among close relatives influence community structure, but they also show that niche conservatism is increasingly evident as communities are defined to include greater phylogenetic diversity. We also found that, as the spatial scale is increased to encompass greater environmental heterogeneity, niche conservatism emerges as the dominant pattern. We then examined patterns of trait evolution in relation to trait similarity within communities for 11 functional traits for a single phylogenetic lineage (Quercus) and for all woody plants. Among the oaks, convergent evolution of traits important for environmental filtering contributes to the observed pattern of phylogenetic overdispersion. At the broader taxonomic scale, traits tend to be conserved, giving rise to phylogenetic clustering. The shift from overdispersion to clustering can be explained by the increasing conservatism of traits at broader phylogenetic scales.

Phylogenetic Relatedness Limits Co‐occurrence at Fine Spatial Scales: Evidence from the Schoenoid Sedges (Cyperaceae: Schoeneae) of the Cape Floristic Region, South Africa

Species co-occurrence at fine spatial scales is expected to be nonrandom with respect to phylogeny because of the joint effects of evolutionary (trait convergence and conservatism) and ecological (competitive exclusion and habitat filtering) processes. We use data from 11 existing vegetation surveys to test whether co-occurrence in schoenoid sedge assemblages in the Cape Floristic Region shows significant phylogenetic structuring and to examine whether this changes with the phylogenetic scale of the analysis. We provide evidence for phylogenetic overdispersion in an alliance of closely related species (the reticulate-sheathed Tetraria clade) using both quantile regression analysis and a comparison between the mean observed and expected phylogenetic distances between co-occurring species. Similar patterns are not evident when the analyses are performed at a broader phylogenetic scale. Examination of six functional traits suggests a general pattern of trait conservatism within the reticulate-sheathed Tetraria clade, suggesting a potential role for interspecific competition in structuring co-occurrence within this group. We suggest that phylogenetic overdispersion of communities may be common throughout many of the Cape lineages, since interspecific interactions are likely intensified in lineages with large numbers of species restricted to a small geographic area, and we discuss the potential implications for patterns of diversity in the Cape.

THE EVOLUTION AND LOSS OF FEEDING LARVAL STAGES OF MARINE INVERTEBRATES.

The feeding larvae of marine invertebrates have interested evolutionists for three reasons. First, the long pelagic larval stage often associated with larval feeding permits dispersal over unusually large distances. This dispersal of larvae affects gene flow and the geographic distribution of species (Mileikovsky, 1971; Scheltema, 1971). Second, many traits of feeding larvae are extremely conservative, so larval forms have played a large part in phylogenetic theories (Garstang, 1951; Hatschek, 1878; Jagersten, 1972; Zimmer, 1973). Third, homologous stages of development can be either planktotrophic (feeding) or lecithotrophic (dependent on reserves in the egg) within many genera or families, and this variation has promoted several models of adaptive strategies concerning parental investment per offspring, dispersal, and other consequences of planktotrophy versus lecithotrophy (Crisp, 1974a; 1974b; Strathmann, 1974a; 1977; Vance, 1973). The conservatism of many traits of feeding larvae within a class or phylum contrasts with the independent loss of a feeding larval stage in many genera and families, yet patterns of evolution and loss of feeding larvae have not been reviewed, and differences in these patterns among higher taxa have not been recognized. The following survey of ten phyla presents a scheme for the evolution and loss of feeding larval stages which is consistent with recent studies on larval feeding mechanisms. These phyla fall into two groups, the oligomera and the spiralia (Table 1). Comparisons among the phyla indicate potential or realized differences in flexibility in reacquiring a planktotrophic larval stage once it is lost. I will present evidence for the oligomera indicating that loss of a planktotrophic larval stage is usually irreversible, that feeding larvae have originated in the remote past, and that feeding larvae have been lost by all members of some orders and classes. A survey of larvae in spiralian phyla indicates that for several reasons the spiralians have been more flexible in acquiring new planktotrophic larval forms. This survey therefore indicates trends and constraints in the evolution of larval planktotrophy which differ among major groups of invertebrates, and some of the reasons for these differences. The survey also can serve as a guide which will prevent erroneous extensions of predictions from optimization models to groups with little variation or evolutionary flexibility. The paper which follows extends these results to indicate progressive vacating of adaptive types in some clades. The acceptance here of a particular set of relationships among phyla may disturb readers who are either unfamiliar with construction of phylogenies or advocate an alternative phylogenetic scheme. The criterion for deciding among phylogenetic schemes, as for any set of alternative hypotheses, is parsimony, and under available evidence other phylogenetic schemes are less parsimonious. (See Garstang to Zimmer, cited above.) New evidence may change views on phylogeny, but phylogenetic schemes could change substantially without affecting the major conclusions of this review. I have therefore not discussed the effect which this or that change in inferred re-

CONSERVATISM AND THE PERCEPTION OF SELF AND OTHERS

A study of the extent to which people are likely to attribute traits to other people rather more than to themselves produced support for the Jones and Nisbett (1972) hypothesis. The level of trait attribution in the present study was, however, higher than that obtained in previous studies. Subjects low in conservatism assigned more traits than did those high in conservatism, but there was no interaction between conservatism and attribution of traits to self versus others. No support was found for the hypothesis that high conservatives make more extreme judgments than low conservatives.