Atwood lecture 2012, Toronto, ON


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  • For those of you who don’t already know me, my name is Rachel and I am a MSc student in Ben Gilbert’s lab. I’ve been here September, and as a result, I don’t have any actual data to show you. But, I will be presenting the conceptual background for the questions that are motivating my current MSc work, framed as more of a critical review for the state of the field at this point.
  • Like most things, Darwin was the first to first to recognize that there is a relationship between the ecology and evolution of species. To draw a quote from the origin of species, Darwin suggested that…
  • This basic concept was followed up by early ecologistssuch as Charles Elton, who first observed that…”…” compared to the number of available. For example, meta-analysis on # studies revealed that 84% of plant genera only have a single species represented locally. Something such as competition must be limiting the ability of closely related species to coexist.This basic observation set the basis for 20 years of work by Harper, Lack, and others, who used taxonomic relationships as proxies for ecological equivalence, often to understand the role competition plays in explaining broad-scale patterns of species co-existenceThe remarkable thing is that although this work is from 1946, we are still asking very similar questions today. But I would argue that 66 years later we aren’t any closer to getting answers.
  • Part of the reason is that most contemporary work is based on this Cam Webb framework, which uses phylogenetic patterns to understand the role of competitive interactions and environmental filtering for species assembly.Explain graph. Environment that selects for a red phenotype.
  • …but what we generally find is that the phylogenetic patterns themselves are either weak, inconsistent, or lack generality. Here is an example by Anderson et al. which falls more onto the “weak” category. Looking at dispersion patterns along a rainfall gradient, where you expect conditions to be more stressful at the lower end of the range Include all points explain nothing.stress gradient from low annual rainfall to more benign, with the expectation that communities switch from under to overdispersed as conditions improve. If you ignore all of the non-significant points and squint your eyes, there appears to be a positive relationship between rainfall and MPD.Explained an underwhelming 3.9% of the data, which actually happens to be slightly significant given the sample size.This is a pretty typical example, failure of phylogeny to explain large scale patterns that have caused many to question the utility of phylogenetic techniques in community ecology at all.So why is there this disconnect between theory and reality?
  • Going back to the floral trait example I showed you earlier, where flower colour was conserved within each clade, but now we look at a situation where things are not so conserved…
  • First issue, which is commonly invoked to explain the failure of PD to predict ecological patterns, is the lack of trait conservatism for ecologically important traits. Problem is, if environment is selecting for species with certain traits, and the trait is convergent across lineages, end up with overdispersed community even though not a reflection of competitive interactions.Convergent across lineages. So if you look at this first case where competitive interactions dominate, only one floral colour can coexist in a local community. Leads to this unpredictable pattern, because which species of the two persists can lead to multiple patterns across local environments. Because can be drawn from anywhere on the phylogeny, resulting in communities with the same trait dispersion but different phylogenetic structure.
  • Competitive interactions give random pattern, whereas environmental filtering results in phylogenetic overdispersionAnd so, most people acknowledge this problem when their data fails to conform to Cam Webb’s dipersion patterns predicted according to competitive interactions and environmental filtering, but there are some added complications in that…
  • …traits within a single lineage can vary in their level of phylogenetic conservatism. Example from Floridian oaks, Environment simultaneously selecting for trait similarity and trait differences, on traits that vary in their level of conservatism. As such, it is unclear what outcomes to expect with respect to phylogeny.Additionally, not all traits contribute to coexistence in the same way. Some traits more important for mitigating competition, whereas others more important for abiotic tolerances.
  • We run into similar problems when we look across lineages, and find that rates of trait `evolution are widely variable. Example from mammalian lineages, where the branch lengths and colours on this phylogeny are weighted to represent the rate of morphological evolution.Rates differ both within and between groups, ranging from a 3-fold decrease to a 52-fold increase in the rates of trait evolution from an ancestral to more derived lineages compared to ancestral rates. This is generally not acknowledged in the PCA literature, but again could have large implications for defining expectations…(body size evolution; scaling factor; shifts in rate of evolution from ancestral to derived lineage varies forma 3-fold decrease to a 52-fold increase)
  • Deconstruct the community into its constituent genera, we would expect that phylogenetic patterns should emerge.…but this is interesting because there is lots of talk in the literature about this issue of taxonomic scale dependency, where generally expect stronger phylogenetic signals at smaller taxonomic scales because traits are more conserved. This is the paper which is most cited with respect to this issue, but does not even present strong evidence for this between three groups at same taxonomic level. There are a number of reasons why lineages differ in taxonomic…keystone innovations or adaptive radiation events which allow rapid diversification for some lineages…
  • Because it evolved to a different environment/resident community, traits much different such that may actually be competing most with more distant than similar species. Have enough of these…introduction of new species may be more likely to compete with..-continuous trait evennlydistd across phylSpecies evolve in response to their biotic and abiotic environments. Different selective regimes and drift.One other reason why rates of evolution vary across lineages is because biogeography may decouple relationship. Species evolve in response to their biotic and abiotic environments. We would expect that species that have co-occurred in sympatry through evolutionary time would have experienced community-wide selective pressures for divergent niches, with simultaneous pressure for habitat niche. By contrast, species that have not co-occurred through time have evolved and diverged wrt an entirely different set of species and potentially environments. When brought together, makes things unpredictable from novel interactions.Human activity novel interactions between species that are widely separated in time, even though live in similar ecotypes. Many of these species invasive because…Species evolve in response to their biotic and abiotic environmentsHigh occurrence of exotic species may obscure phylogenetic patterns.
  • Are phylogenetic patterns useful for understanding ecological processes?Obvious problems given that they are sensitive to trait conservatism between traits and lineages, defining appropriate taxonomic scales, as well as the biogeographic history of species.But at the same time, all complexity and contingency in interpreting these patterns can serve as a starting point for asking more sophisticated questions about the interplay between ecology and evolution. How do traits combine to influence evolution? Coevolution? How to traits combineHow does biogeography alter patternsMove beyond community-level phylogenetic patterns and into empirical examplesUseful in that guiding more mechanistic
  • Independently controlsfor historical interactions, in two environmentsCombine empirical and theoretical approaches to understand the role of species differences, biogeographic origin, and also environmental variability in determining the strength of phylogeny-coexistence relationships.
  • Atwood lecture 2012, Toronto, ON

    1. 1. Evolution of species interactions: finding meaning in community-level phylogenetic patterns Rachel M. Germain and Benjamin Gilbert Department of Ecology & Evolutionary Biology, University of Toronto
    2. 2. “As species of the same genus usuallyhave some similarity in habits andstructure, the struggle will generallybe more severe between species ofthe same genus.” C. Darwin, 1859
    3. 3. 1946: “…the thing that stands out (in ecological surveys) is the high percentage of genera with only one species present.” 84% of plant genera only have a single species represented locally Elton et al. 1946 J of Animal Ecology
    4. 4. Competitive interactions Environmental filtering Over-dispersion Under-dispersion Webb et al. 2002 Annu Rev Ecol Syst
    5. 5. …but evidence is mixed in natural systems = sig over- or under- dispersion = non-sig over -or under- dispersion Anderson et al. 2011 J of Ecology
    6. 6. Mechanistic inferences are contingent on trait conservatism Competitive interactions Environmental filtering
    7. 7. Mechanistic inferences are contingent on trait conservatism Competitive interactions Environmental filtering
    8. 8. Mechanistic inferences are contingent on trait conservatism Competitive interactions Environmental filtering
    9. 9. Mechanistic inferences are contingent on trait conservatism Competitive interactions Environmental filtering No phylogenetic signal Over-dispersion
    10. 10. Phylogenetic conservatism varies from trait-to-trait Cavender-Bares et al. 2006 Ecology
    11. 11. Rates of trait evolution vary widely across lineages Rate of trait evolution Venditti et al. 2011 Nature
    12. 12. Implications for taxonomic scale dependencyQuercus Ilex Pinus Cavender-Bares et al. 2006 Ecology
    13. 13. Biogeographic history influences trait evolution and species interactionsPhylogenetic patterns may be unpredictable between speciesthat have not coevolved through evolutionary time
    14. 14. Are phylogenetic patterns useful for understanding ecological processes?• Problems: – lack of trait conservatism • between traits • between lineages – taxonomic scales – biogeographic history Empirical examples?
    15. 15. My MSc work Mediterranean annual plants from California and Spain Pairwise competition trials based on: – phylogenetic distance – biogeographic origin How do species differences, How do species differences, biogeography, and the biogeography, and the environment combine to regulate environment combine to phylogeny-coexistence regulate phylogeny- relationships? coexistence relationships?
    16. 16. AcknowledgementsMarc Cadotte Chris Blackford Natalie JonesJason Weir Yvonne Chan Alicia Hou S&S SeedBruce Hall HerbiseedAndrew Petrie