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CO-EVOLUTIONTheoretical Considerations
Photo © Steven D. Johnson
Barb Sharanowski
Department of Entomology
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Classical Co-evolution
• Darwin: Origin of Species
“ Thus, I can understand how a flower and a bee might slowly become, either simultaneously or one after the other, modified and adapted in the most perfect manner to each other, by continued preservation of individuals presenting mutual and slightly favorable deviations of structure”
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Darwin’s Orchid (Angraecum sesquipedale)
Xanthopan morgani praedicta (Sphingidae)
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Paul Ehrlich and Peter Raven (1964)“ Butterflies and Plants: A Study in Co-evolution”
Brassicaceae (cabbage and mustard family) – Pieridae (whites, sulphurs)
Escape and Radiate
“…as in the occupation of any adaptive zone, the first organisms to enter it have a tremendous advantage and are apt to have the opportunity to become exceedingly diverse before evolution in other organisms sharply restricts their initial advantage (p.604)”
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Time
Evolves new chemical defense:
Free from herbivory
Radiates into new adaptive zone
Free from competition
Cycle Continues
Evolves ability to overcome plant defenses
Radiates into new adaptive zone
“Evolutionary Arms Race”
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The Critics
•Cornelius Muller (1969)• Secondary defensive compounds are metabolic waste products evolved independently of insect pressure
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Arabidopsis
(Carter and Thornburg, 2004)
NEC1 + NEC 5
Hydrogen Peroxide
32 genes
Flower Nectar: Attractant or Defense?
Limits microbial growth
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The CriticsJanzen, 1980:Diffuse Co-evolution/ Guild Co-evolution
- many species, on the same or different trophic levels, exerting selective pressure on each other
vs.
Pairwise Reciprocal Co-evolution
- 2 species evolving in stepwise fashion in response to selective pressure exerted by the other species
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The Critics
• Tibor Jermy (1993)
– suggested insects do not exert any real selective pressure on plants:
1) Herbivory is relatively rare in higher orders and population densities tend to be relatively low
1) Insect herbivory may not effect the reproductive fitness of a plant
1) Outbreaks are rare
1) Conflicting selective pressures (eg. with microorganisms and other plants)
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Evidence for E & R’s Model
For co-evolution to progress between a plant and a herbivore, the following conditions must be met:
there must be genetic variation for characters in both plant and insect that influence the interaction between the species
each species must be a selective force on the other species (i.e., affect the other's fitness)
there must a response to selection in each species.
Berenbaum and Zangerl, 1998
Webworm and wild parsnip
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Most Primitive Form = simple coumarins or hydroxycoumarins
More Derived = linear furanocoumarins
Most derived = angular furanocoumarins
more derived genera of parsnips:
• are defended by angular furanocoumarins
• contain disproportionately higher number of plant species
• support more specialist feeders, than generalists
Derived Plant compounds = Greater Plant Diversity
Berenbaum, 1981; 1983
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Berenbaum and Zangerl, 1998
A tight Relationship between webworm and parsnip phenotypes in four populations
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Geographic Mosaic of Co-evolution
John Thompson, 1999:
• Pairwise reciprocal selection imposes an artificial dichotomy on the study of co-evolution
• Population differentiation is a key component to co-evolution
– Hughes et al. (1997) estimated that species are divided on average into 220 genetically differentiated populations
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Geographic Mosaic of Co-evolution
Co-evolutionary hotspot
Co-evolutionary hotspot
Selection on one species
Selection on one species
Selection on one species
Selection on neither species
Reciprocal selectionGene Flow
Extinction
Genetic Drift
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Geographic Mosaic of Co-evolution
Thompson’s theory predicts:
• Populations will differ in the traits shaped by an interaction
• Traits of interacting species will be matched in some communities and mismatched in others
• There will be few co-evolved traits that are distributed across all populations of a set of interacting species, because few coevolved traits will be favored across all communities
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Non-pollination among Yucca Moths evolved multiple times independently
Pellmyr et al.,
Nature 1996
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Interactions between plant polyploidy and insect herbivores
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Percentage of Seed Capsules attacked by Greya politella in different geographical locations
Thompson et al. (1997)
D = diploid
T = Tetraploid
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Questions?