CO-EVOLUTIONTheoretical Considerations

Photo © Steven D. Johnson

Barb Sharanowski

Department of Entomology

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”

Darwin’s Orchid (Angraecum sesquipedale)

Xanthopan morgani praedicta (Sphingidae)

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)”

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”

The Critics

•Cornelius Muller (1969)• Secondary defensive compounds are metabolic waste products evolved independently of insect pressure

Arabidopsis

(Carter and Thornburg, 2004)

NEC1 + NEC 5

Hydrogen Peroxide

32 genes

Flower Nectar: Attractant or Defense?

Limits microbial growth

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

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)

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

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

Berenbaum and Zangerl, 1998

A tight Relationship between webworm and parsnip phenotypes in four populations

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

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

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

Non-pollination among Yucca Moths evolved multiple times independently

Pellmyr et al.,

Nature 1996

Interactions between plant polyploidy and insect herbivores

Percentage of Seed Capsules attacked by Greya politella in different geographical locations

Thompson et al. (1997)

D = diploid

T = Tetraploid

Questions?