<p>LECTURE 17 CH 20 COEVOLUTION AND MUTUALISM</p><p>Types of pairwise interspecific interactions Mutualism (+/+) Predation (+/-) Competition (-/-) Commensalism (+/0) Amensalism (-/0) Fluidity in type of relationship</p><p>Coevolution Interacting species evolve in response to each other Traits of each species affect fitness of other species Traits have genetic basis May be mutualistic or antagonistic</p><p>Strict coevolution Limited to pair of species Specialized response May be rare and limited to very strong interactions Diffuse coevolution Response to many other species Generalized response</p><p>Symbioses Intimate, often obligatory association of two species Usually involving coevolution May be parasitic or mutualistic</p><p>Gene-for-gene concept and evolutionary ‘arms race’ 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 added Can reach equilibrium stage of endemic disease Requires pathogen/parasite to need host not to die for transmission</p><p>Evidence for coevolution Inference from closely related herbivores feeding on closely related plants Suggests long evolutionary history of interaction Based on parallel phylogenetic relationships Experimentation Circumstantial evidence e.g. character displacement of competing species when in sympatry but not when in allopatry; infer that competition drives coevolution Mutualism Two species specialized to perform positive function for each other Trophic: partners complement food/nutrients for each other Defensive: species receive food and/or shelter in return for defending against natural enemies Dispersive: animal vectors move pollen or seeds in return for food rewards Pollination examples Seed dispersal examples Mixed systems Yucca and its pollinator moth acting as both mutualist and seed predator When is it coevolution? Preadaptation: some adaptations present before establishment of mutualism Some adaptations occur in close relatives that are not mutualists</p><p>Constraints on evolution of strict mutualism Community diversity diffuses selection from single species. Changes in species’ ranges or disturbance change selection over time. Genetic complexities cause uneven rates of evolution between mutualists. </p>