Coevolution & Mutualism

1. Coevolution & Mutualism • Coevolution • Host-parasite systems • Coevolution among competitors • Character displacement • Mutualisms & symbioses

2. Coevolution – evolution of one species in response to characteristics of another • Differs from other types of evolution in two ways:

3. Coevolution involves mutual responses in the two species • predator species evolves to be faster, allowing it to catch more • results in greater selection on speed in the prey • for “regular” evolution, there isn’t any response by the environment

4. Coevolution promotes diversity of adaptations • evolution often converges on the same solution for the same problems • coevolution involves specific, unique responses to specific challenges

5. The confusing definition of coevolution Species A evolves an adaptation in response to species B Species B evolves in response to the adaptation of species A

6. This isn’t coevolution Species A has some trait unrelated to species B Species B evolves in response to that trait in species A

7. Not coevolution…

8. Coevolution in host-parasite systems • Hosts and parasites change evolutionarily in response to each other • Resistance – the ability of the host to combat the parasite • Virulence – the ability of the parasite to harm the host

10. Hosts evolved resistance to the parasitoids

11. Both resistance (in the host) and virulence (in the parasite) are costly • if the host is susceptible, selection favors virulence • this causes selection for resistance in the host… • leading to selection for less virulence in the parasite (because virulence is costly)… • leading to selection for less resistance in the host (because resistance is costly)…

13. Parasites that are too virulent may reduce their own fitness by killing their hosts.

14. Coevolution & Mutualism (Chapter 20) • Host-parasite interactions • Coevolution among competitors • Character displacement • Mutualisms & symbioses

15. Host-parasite interactions are due to genotype-genotype interactions • newly evolved pathogen strains may overcome host resistance • host resistance may be specific to certain genotypes

18. Red Queen Hypothesis • Parasites are constantly evolving into new forms to avoid host resistance • Hosts are constantly under selective pressure to evolve new resistance genes • Result is a “coevolutionary arms race” in which both parasite and host must constantly evolve just to stay in place…

19. Plant-herbivore Coevolution • Plants produce toxic “secondary” chemicals that reduce herbivory • Some herbivores have evolved to detoxify the toxic chemicals. • herbivores may specialize on the hosts whose defenses they have overcome • plants may evolve new defenses, and the cycle continues (Red Queen)

22. Generic defenses against herbivory (spines, cellulose, etc.) and predation (speed, hiding, lack of smell, etc.) are notcoevolution • They don’t involve responses to an adaptation specific predator/herbivore

23. Coevolution among competitors

24. Character Displacement • According to the competitive exclusion law, no two species can share the same niche • When living in allopatry (spatially separated), two species may use the same resources (food, nest sites, etc.) • Where these species live in sympatry (in the same geographic area), they will evolve to use different resources to avoid competition.

25. Greater divergence in morphology in sympatry than in allopatry, is character displacement

28. Mutualism – mutually positive interaction between two species • Commensalism – interaction in which one species benefits and the other is unaffected • Symbiosis – association in which two species live together as one distinctive entity

29. Mutualisms and commensalisms can be • obligate – neither can survive without the other • facultative – nonessential

30. Three Kinds of Mutualisms • Energetic & nutritional – transfer of energy/nutrients from one organism to another • Protective – defense against predators or herbivores • Transport – movement of seeds, pollen, or adult organisms

31. Energetic Mutualisms Gut symbiont • bacteria in animal guts digest plant materials and produce vitamins/amino acids • benefit to bacteria is a stable environment and a steady food source

32. Mycorrhizal fungi • fungi that live intermingled with plant roots • fungi uptake nutrients from soil for plants • plants provide carbohydrates to the fungi • mycorrhizal fungi are found in almost every plant • very important where nutrients are limited

35. Honeyguides (energetic mutualism)

37. Protective Mutualisms Ants/acacias – • acacias provide food, hollow stems for nesting • ants attack herbivores and kill encroaching plants • wrens nest in the acacias and are protected from predators by the ants (commensalism)

39. Transport Mutualisms Pollination • flowers trade nectar for pollen transport • can be general or species-specific

40. Seed dispersal Plants provide nutritious fruit, seeds to encourage dispersal by animals • many seeds require gut passage to germinate • some plants trade-off consumption of some seeds to ensure dispersal

42. Seed dispersal Why does seed dispersal benefit plants? (why not just drop seeds?) • Avoid pathogens that the parent may harbor • Colonize new habitats where conditions may be better than near the parent • Find locations where germination is more likely (ant nests)

43. Facultative mutualisms • mixed-species flocks in birds • clownfish-anemone • associational defense with toxic plant species • Commensalism • birds’ nesting in tree cavities • epiphytes’ growing on trees

44. Symbiosis – “living together” • Lichen – alga and fungus • Coral and zooxanthellae • coral capture food, nutrients • zooxanthellae photosynthesize and provide sugars, energy • coral “signal” the algae to release sugars