LECTURE 17 EVOLUTION OF SPECIES INTERACTIONS: COEVOLUTONMAJOR CONCEPTS1) Coevolution involves mutual evolutionary responses by interacting populations.2) Diffuse coevolution may be more common than strict coevolution.3) Constraints restrict evolution of strict mutualisms.4) Coevolution in plant-pathogen systems reveals genotype-genotype interactions and involves a gene-for-gene concept and an ‘evolutionary arms race’.5) Evidence for coevolution can arise from inference, circumstantial evidence, or experimentation.6) Mutualists have complementary functions; they involve trophic, defensive, and dispersive functions.I. Symbioses 289 Intimate, often obligatory association of two species Usually involving coevolution May be parasitic (host-parasite) or mutualistic (algae-fungi in lichens) 1.8II. Coevolution pg. 348 Traits of interacting species evolve in response to each other Each species acts as selective agent on other species Traits of each species affect fitness of other species Traits have genetic variation May be mutualistic (e.g. plant/pollinator) or antagonistic (plant/pathogen) A. Strict coevolution Limited to pair of species Specialized response May be rare and limited to very strong interactions B. Diffuse coevolution Response to many other species Generalized responseIV.Mimicry 350-1 A. Batesian: palatable species mimics more common unpalatable model 17.4 ½ of coevolution equation: response of prey to selection by predator B. Müllerian: multiple unpalatable species resemble each other 17.5V. Gene-for-gene concept and co-evolutionary ‘arms race’ (antagonistic) 346-7; 354-5; 17.1, 17.10 Occurs in plant-pathogen and host-parasite systems Based on single gene conferring resistance to host or virulence to pathogen Back and forth selection between genotype of host and genotype of pathogen Interaction escalates as more and more traits are addedVI. Evidence for coevolution A. Inference from closely related herbivores feeding on closely related plants 360-2; 17.18Suggests long evolutionary history of interaction Based on parallel phylogenetic relationships 17.20 B. Experimentation ICA C. Circumstantial evidence 17.16, 17.17 e.g. character displacement of competing species when in sympatry but not when in allopatry; infer that competition drives coevolutionVII. Mutualism 297-299 Two species specialized to perform positive function for each other A. Trophic: partners complement food/nutrients for each other 1.8 e.g. legume/Rhizobium; plant/mycorrhizal fungi B. Defensive: species receive food and/or shelter in return for defending against natural enemies 14.11; 14.12 C. Dispersive: animal vectors move pollen or seeds (gene flow) in return for food rewards Pollination examples 17.19 Seed dispersal examples D. Mixed systems 17.19 Yucca and its pollinator moth acting as both mutualist and seed predator 362-3; 366 When is it coevolution? Preadaptation: some adaptations present before establishment of mutualism Some adaptations occur in close relatives that are not mutualistsVIII. Constraints on evolution of strict mutualism Community diversity diffuses selection from single species. Changes in species’ ranges or disturbance change selection over time/space. Genetic complexities cause uneven rates of evolution between mutualists.Summary Chap 14: 1-4; 14 Chap 17: 1-5; 7;
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