Download lecture 17 ch 20 coevolution and mutualism

LECTURE 15 COEVOLUTION AND MUTUALISM
pg. 381-383; 385-386; 389-396
MAJOR CONCEPTS
1) 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.
Types of pairwise interspecific interactions
Mutualism (+/+)
Predation (+/-)
Competition (-/-)
Commensalism (+/0)
Amensalism (-/0)
Fluidity in type of relationship
Symbioses
Intimate, often obligatory association of two species
Usually involving coevolution
May be parasitic or mutualistic
Coevolution
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 basis
May be mutualistic or antagonistic
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
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
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
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.