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IHPST Philbio 20/03/2008 Ecologie et macroévolution Julien Delord CERES-ERTI Neutral Theories in Ecology and Evolution • Introduction: Do ecological communities exist (and evolve)? • 1. Presentation of the Unified Neutral Theory of Biodiversity (UNTBB) • 2. The (epistemological) non-neutrality of neutral models - Neutrality and falsity (in biology) - Neutrality as a strategy towards generality and unification • 3. Neutrality and the unification of evolutionary theory - Neutral theories in genetics and in ecology - « L-neutrality » and « L+1 neutrality » - « Grand Unification » or « domain unification »? - Communities as evolutionnary entities • Conclusion - Theory change in ecology « Are phenomenological communities causal systems? » (Sterelny 2006) = Are communities structured, functionally organized with effects on the fate of populations they are composed of? (downward causation) 3 criteria: - Boundedness - Internal regulation - Emergent property effects For Sterelny, they fail to meet these criteria… For most ecologists, communities are structured by niche competition and niche assembly rules!!! Dynamiques des communautés végétales Organicisme Frederic CLEMENTS (1874-1945) « Comme un organisme, la formation naît, croît, mûrit et meurt. En outre chaque formation climacique [ayant atteint le climax] est capable de se perpétuer, en reproduisant avec une fidélité absolue les étapes de son développement », 1916. Déterminisme des successions végétales vs. Individualisme Henry GLEASON (1882-1975) « Le phénomène de la végétation dépend complètement des phénomènes des espèces individuelles » Assemblage des végétations au Introduction : Do ecological communities exist and evolve ? A Ecological Community Group of populations from different species interacting with each other and sharing the same spatio-temporal framework. B C D Community 1 A: 25% B: 25% C: 25% D: 25% Species Richness Relative Species Abundance Community 2 A: 80% B: 5% C: 5% D: 10% 27 different models of Species Abundance Distribution McGill Brian J. et al., “Species abundance distributions: moving beyond single prediction theories to integration within an ecological framework”, Ecology Letters, 10, 2007, p. 995-1015. La diversité des communautés dépend de leur structuration Jared Diamond (1975) a été le premier à explorer l’idée selon laquelle il existe des règles qui gouvernent l’assemblage des communautés: Ressources restantes 1. The Unified Neutral Theory of Biodiversity and Biogeography (UNTBB) - How to explain species biodiversity in ecological communities ? - How to explain the curves of relatives species abundance ? -Is the niche concept necessary ? -From case-study rules to general deductive laws ! Courbes SAD (Surface Abundance Distribution) Principles of UNTBB Definition: Ecological community : Group of trophically similar and sympatric species Conservation law: Neutrality : The assumption of a complete identity of ecological interactions affecting community organisms, i. e. a per capita ecological equivalence (in terms of reproduction and death rates + speciation) among all individuals of every species. Dependant Variables: Relative species abundance and species number Parameters: Reproduction rates, death rates, speciation rates, community size (= number of individuals), limits to dispersal. Principles of UNTBB (Cont’d) Hypothesis: « Communities are open, nonequilibrium assemblages of species largely thrown together by chance, history and random dispersal » (Hubbell, 2001, p.8) Process: Ecological drift: demographic stochasticity in a neutral community (~ genetic drift) Result: Curves of relative species abundance at equilibrium between speciation and extinction (stationary dynamics) Markovian process Theoretical developments Random walk of the abundance of ith species under ecological drift (The initial relative species abundance is 0.5 - ergodic )case) A brief reminder of population genetics: how to calculate the proportion of homozygotes at equilibrium between mutation and drift. Theoretical developments How to calculate the probability of drawing two individuals from the same species at equilibrium between speciation and ecological drift. Let be the speciation rate per generation and JM the number of individuals in metacommunity M. Theoretical developments F2 = 1 / 1+ 2 JM Feq = 1 / 1 + 2Nμ = 2 JM is named by Hubbell the fundamental biodiversity number because this dimensionless number controls not only the equilibrium species richness but also the equilibrium relative species abundance in the metacommunity. Some results and predictions of UNT on biodiversity dynamics By generalization, Hubbell extends this rationale to the case of 3 individuals randomly chosen in JM , etc., then J individuals belonging to S species. Etienne R., Alonso D., McKane A. J., « The zero-sum assumption in neutral biodiversity theory », Journal of Theoretical Biology, 248, 2007, p. 522-536. Dornelas M. et al., “Coral reef diversity refutes the neutral theory of biodiversity”, Nature, 440, 2006, p. 80-82. Epistemological questions: Ecology and its recurrent « Physics envy » In search for laws, regularities and generality The concept of generality: from universality (spatio-temporal unboundedness) to invariance (Van Fraassen, Laws and Symmetry, 1991) Invariance as a conservation law (when a certain value is conserved through time despite the changes of the system) Neutral models as a quest for invariance in ecology Generality should enhance unification 2. The (epistemological) non-neutrality of neutral models - Neutrality and falsity (in biology) Conservation law in physics (sub-atomic level) vs. conservation law in ecology (macroscopic world) In particle physics, conservation laws (i. e. conservation of energy) give rules to make sense of the data and to define the ontology of particles. In ecology, the ontology is already defined but the rules are not known. They are interpreted in light of the discrepancy between expected patterns and empirical data. Problems: 1. Theoretical: the underdetermination of theories 2. Empirical: the confirmation of the neutral model 3. Neutrality and the unification of evolutionary theory Neutral theories in genetics and in ecology Comparative approach of neutrality in genetics and in ecology Hu X.-S., He F. & Hubbel S. P. (2006), “Neutral theory in macroecology and population genetics”, Oikos, 113, 3, p. 548-56. NeutralNatural theory selection of evolution Neutral Theory of Biodiversity Community Evolver Niche ??? Evolver Espèce Species Espèce Interactor Individu Individu Individual Type Class of d’Allèle Alleles Type d’Allèle Replicator -Interactor Allèle Allele Allèle Species Class of individuals Individual Replicator -Interactor UNT and phylogeny Last consequence of Hubbell ’s theory for phylogeny (but not the least), UNT models show that the organization of biodiversity at different scales is intrinsically fractal. UNT and phylogeny On the fractal dimension of biodiversity : Hubbell : “ If biodiversity is a perfectly homogeneous fractal, this implies that a satisfactory answer to the question - how many species are there ? – cannot really be answered except operationally. It requires a definition of the scale of aggregation that we call species! » (p.263) This implies a pluralist or arbitrary fixation of the level of “ ranking ”for the species. Are neutrality in genetics and ecology comparable? Historical differences : The neutral theory of molecular evolution appeared as a solution to a problematic in evolutionary genetics and as a consequence of new developments in molecular biology (high protein polymorphism). “The theory of molecular evolution by mutation and random drift led to Kimura’s prediction that those amino acids or nucleotide changes that have the least consequence would evolve faster” (J. Crow, Neutral models in biology, 1987, p. 13). UNTBB as an elaborated null model??? What is selection for UNT ? (Rules of niche assembly ?) Neutrality Neutrality in a hierarchical systems • Neutrality relative to level L+1 (replicator <> interactor) • Neutrality relative to level L (replicator-interactor) Example of « contextual » neutrality: genetic drift and transposons « L neutrality » and « L+1 neutrality » Level L+1 (species) Level L (individuals) Yes No Yes No UNTBB Apparent neutrality « free » replicators Species selection Kimura’s neutral theory was defined as a « L+1 neutrality » with respect to natural selection Hubbell’s neutral theory of biodiversity was defined as a « L neutrality » with respect to differential replication - « Grand Unification » or « domain unification »? Is a neutral synthesis of evolutionary theory possible ? Ecological drift Incompatibility Genetic drift vs deterministic “rules” of niche assembly Incompatibility vs natural selection - Ecological drift requires non-neutral mutations - Ecological drift would induce different speeds of genetic drift Gould’s hierarchical macroevolutionary theory Vrba and Gould « The hierarchical expansion of sorting and selection: sorting and selection cannot be equated », Paleobiology, 12, 1986, p. 217-28. Stephen J. Gould’s approach Species selection: - Emergent species-level trait - Emergent species-level fitness « True » species selection - Aggregate traits - Neutral traits - 2 Confusions : - The nature of extinction (why dissymetry between speciation and extinction???) (p. 669) - Species drifting and clade drifting (p. 718) Gould’s analogy between micro- and macroevolution Gould’s analogy between microand macroevolution Hiérarchies historiques et fonctionnelles « Domain unification » in Evolution UNTBB is both a macroecological and a macroevolutionary theory : Evolution at the community level is ecology !!! (The equivalent of genetics at the community level is ecology) Macroevolution Megaevolution Mesoevolution - Fauna and flora - Geological times -Communities et metacommunities - Ecological times Microevolution Darwinian evolution - Populations - Adaptations Punctuated equilibrium theory, fractal dynamics, etc. UNTBB Interspecific competition between neutral individuals Or Niche assembly Natural selection (mutation, sélection, migration) Intraspecific competition Molecular evolution - Populations - Neutral variations Neutral theory of molecular evolution (mutation, genetic drift) Interallelic competition Conclusion: 2 macro-ecoevolutionary strategies Neutral theory: Ecology explains macroevolution and phylogeny Hierachical sorting and selection theory Macroevolutionary processes changed by environment Iterative and formal Realist and causal Alleles Organism Species Genus or family Conclusion 3 major challenges for the UNTBB: 1. Ecology: • “Neutral theory is limited because it is incapable of predicting which species are rare or abundant” (Tilman, 2004)… Frustration for disappearance of causes, forces, etc: Opportunity for ecologists to move from typological to population thinking!! Challenge of the reconciliation with the niche perspective, like the niche construction: from idealized invariant to ceteris paribus laws. • Dynamics of species substitution Conclusion 2. Evolution: • How to test the neutral nature of species substitution at the community level ? • Community invasion rate as a test for species substitution??? • The unification of macro-and microevolution !!! • More collaboration between paleoevolution and community ecology with regard to species evolution (for a better contribution to the emergent property/emergent fitness debate)????? Bibliography - Dornelas M., Connolly S. R., Hughes T. P., “Coral reef diversity refutes the neutral theory of biodiversity”, Nature, 440, 2006, p. 80-82. - Etienne R., Alonso D., McKane A. J., « The zero-sum assumption in neutral biodiversity theory », Journal of Theoretical Biology, 248, 2007, p. 522-536. - Holt Robert D., “Emergent neutrality”, Trends in Ecology and Evolution, 21, 10, 2006, p. 531533. - Hu X.-S., He F. & Hubbel S. P., 2006, “Neutral theory in macroecology and population genetics”, Oikos, 113, 3, p. 548-56. - Hubbell Stephen P., 2001, The Unified Neutral Theory of Biodiversity and Biogeography, Princeton and Oxford, Princeton University Press. - Leigh Jr. E. G., « Neutral theory : a historical perspective », Journal of Evolutionary Biology, 20, 2007, p. 2075-2091. - McGill Brian J. et al., “Species abundance distributions: moving beyond single prediction theories to integration within an ecological framework”, Ecology Letters, 10, 2007, p. 9951015. - Sterelny, K., Local Ecological Communities, Philosophy of Science, 73, 2006, p. 215-231. - Tilman, D., “Niche tradeoffs, neutrality, and community structure: a stochastic theory of resource competition, invasion, and community assembly”, Proc. Natl. Acad. Sci., 101, 2004, p. 10854–10861. - Volkov Igor, Banavar Jayanth R., Hubbell Stephen P., Maritan Amos, “A neutral theory and relative species abundance in ecology”, Nature, 424, 2003, p. 1035-1037. - Volkov Igor, Banavar Jayanth R., Hubbell Stephen P., Maritan Amos, “Patterns of relative species abundance in rainforests and coral reefs”, Nature, 450, 1 november 2007, p. 45-49.