Transcript Coevolution
CO-EVOLUTION Theoretical Considerations Barb Sharanowski Department of Entomology Photo © Steven D. Johnson 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) “…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)” Escape and Radiate Radiates into new adaptive zone Evolves new chemical defense: Radiates into new adaptive zone Free from herbivory Evolves ability to overcome plant defenses Free from competition Cycle Continues “Evolutionary Arms Race” Time The Critics •Cornelius Muller (1969) • Secondary defensive compounds are metabolic waste products evolved independently of insect pressure Flower Nectar: Attractant or Defense? NEC1 + NEC 5 32 genes Hydrogen Peroxide Limits microbial growth Arabidopsis (Carter and Thornburg, 2004) The Critics Janzen, 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. Webworm and wild parsnip Berenbaum and Zangerl, 1998 Derived Plant compounds = Greater Plant Diversity Berenbaum, 1981; 1983 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 A tight Relationship between webworm and parsnip phenotypes in four populations Berenbaum and Zangerl, 1998 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 coevolution – Hughes et al. (1997) estimated that species are divided on average into 220 genetically differentiated populations Geographic Mosaic of Co-evolution Co-evolutionary hotspot Selection on one species Selection on one species Extinction Genetic Drift Selection on one species Co-evolutionary hotspot Gene Flow Selection on neither species Reciprocal selection 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 D = diploid T = Tetraploid Thompson et al. (1997) Questions?