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ICA 6 COEVOLUTION/MUTUALISMS
Group Name__________________ Your Name ______________________________
1. Pairwise interspecific interactions Which is: +/-, +/0, +/+, -/-, -/0 ?
Mutualism
Herbivory
Commensalism +,0
Predation
Amensalism
-,0
Parasitism
Competition
Disease
2. Can the type of interaction change over evolutionary time? Yes Give an example.
A seed predator may become a seed disperser…so changes from +/- to +/+
3. What is coevolution? Traits of 2 or more species change in response to selection by
their mutual interactions controlled by those traits.
What must occur for a change in allele frequency in each species? Traits must have
genetic basis. Traits of each species must affect the fitness of individuals of the other
species (and vice versa).
Are all coevolutionary relationships mutualistic? No Explain. They can be mutualisit
or antagonistic (e.g. competitors or predator/prey interactions can cause coevolution).
4. What is the difference between strict = only 2 species interacting
and diffuse = multiple species interact to affect each other’s traits coevolution?
Which is more common? diffuse
5. Use the interaction between wheat rust (a fungus causing disease) and wheat to
illustrate the steps involved in an ‘evolutionary arms race. What is the ‘gene-for-gene
concept? Include genes for resistance and virulence.
1. plant has gene for resistance; selection on fungus for matching gene for virulence.
2. fungus gains virulence; selection on plant for new gene for resistance.
3. plant regains resistance; selection on fungus for new gene for virulence.
4. fungus regains virulence; selection on plant for new gene for resistance. etc…
6. What evidence is used to claim a coevolutionary relationship?
1. inferential e.g. closely related herbivores feed on closely related plants; suggests
long history of each selecting for traits in the other group.
2. experimental e.g. see question 7.
7. Experimental evidence of a coevolved mutualistic relationship: Ants + tree hoppers.
If…treehoppers feed ants and ants protect ‘hoppers from predatory spiders,
then more ‘hoppers will be present when ants present than when ants removed.
Figure 1.What is the independent variable? presence/absence of ants
What is the dependent variable? number of ‘hoppers
Summarize the results. More ‘hoppers were present when ants present than absent.
What is the conclusion? Is this a mutualistic relationship? Yes; the ‘hoppers feed
the ants and the ants protect ‘hoppers from predators.
8. What is a trophic mutualism? 2 or more species specialized in complementary
ways to obtain limited energy and nutrients. (trophic = pertaining to food)
A. Figure 2. Describe the mutualism between legumes and Rhizobium bacteria.
Legume provides shelter and food to bacteria that provide nitrogen to legume.
B. Figure 3. between endomycorrhizae and higher plants Plant provides food to
non-photosynthetic fungi that provide ‘soil mining’ of P for plants.
9. What is a defensive mutualism? Species receive food for shelter in return for defense
by other species against natural enemies.
Figure 4. Describe the relationship between eels + cleaning shrimp Shrimp eat eel’s
parasites (= food) while shrimp rid the eel of its enemy (parasites).
Figure 5. between Acacia and ants Plant provides shelter, nest sites, and food to ants
that provide defense against herbivores (and fire and vines).
Figure 6. What is the evidence that Acacia benefit from ants? Survial, shoot weight,
number of leaves and thorns are less for plants without and than with ants.
What is a possible evolutionary response when a plant, but not the ant, invades an
island? Why? The plant evolves without thorns and food. They are costly to make
and provide no benefit to the plant in the absence of the ant. Perhaps they divert
those energies to a new defense such as chemical defense.
(On the mainland, they are mutualists).
10. What are two types of dispersive mutualisms? 1. pollination 2. seed dispersal
Which type is related to gene flow? both
Can seed dispersal involve no mutualism? How? Yes, if seeds are dispersed
by abiotic forces such as wind, water, hydrostatic explosion.
11. What are three reasons that strict mutualisms are rare?
1. So many species are interacting that selection is diffused from any one species.
2. Species don’t consistently co-occur through space or time.
3. Genetic complexities cause the two species to evolve at different rates; or one
species has more potential to evolve than the other.
12. Figure 7. Describe the complex relationship between the Yucca plant and the Yucca
moth. Moth pollinates plant and also lays eggs in some ovules (potential seeds) of
plant that feeds pollen to moth and provides nourishment to its young. Is the
relationship mutualistic? Explain. It’s a strict (obligatory mutualism. Plant must
have moth for pollination and moth must have plant to raise its offspring. Both have
no relationship with other species for pollination or a place to raise offspring.
Figure 8. When did the adaptations arise? Before establishment of mutualism =
preadaptations in earlier lineages.
Do the adaptations occur in close relatives that are not mutualists? yes; sometimes
the moths are predators only and not pollinators.
What’s the main point here? Some mutualisms today that appear to have coevolved
traits that were actually preadaptations that were critical to the development of the
obligate mutualism.