The Geographic Mosaic Theory of Coevolution

1. The Geographic Mosaic Theory of Coevolution Advanced by John N.Thompson in his 1994 book entitled �The Coevolutionary Process�

2. Geographic Mosaic Theory States that most interactions exhibit an evolutionary dynamic geographic mosaic Formed by the interpopulational differences in outcome, adaptation and specialization

3. Geographic Mosaic Theory For example, natural selection on the relationship between a pair or group of species may favor directional selection in one population and polymorphism in another It may favor escalation of antagonism in some environments and reduced antagonism or mutualism in others

4. Geographic Mosaic Theory Parasitism, grazing, predation, competition, symbiotic mutalisms, and nonsymbiotic mutalisms all show dynamic patterns of specialization that are molded in different ways both locally and geographically by natural selection through the community context in which the interactions occur

5. Geographic Mosaic Theory The geographic mosaic approach bridges studies of local populations and of whole species, it also incorporates many aspects of the ecology and genetics of species that we�ve discovered in recent decades

6. An Example Herre, E.A. 1993. Population structure and the evolution of virulence in nematode parasites of fig wasps. Science 259:1442-1445.

7. 1st Some Theoretical Predictions This first example deals with the conditions that favor the evolution of reduced antagonism One of the conditions favoring reduced antagonism in long-term interactions is high partner fidelity and low availability of alternative partners For internal parasites the availability of alternative partners depends upon both the modes and the rates of transmission between hosts

8. Theoretical Predictions Low transmission rates and strictly vertical transmission (parent to offspring) generally appear to favor parasite genotypes that are less detrimental in their effects on host survival High transmission rates and horizontal transmission (between unrelated individuals in a population) favor parasite genotypes that maximize their growth rate in the host and thereby remain highly antagonistic to their host

9. Herre�s (1993) work on fig wasps and their nematode parasites provide strong support for this view as well as support for the Geographic Mosaic Theory

10. Fig Wasps The opportunity for horizontal transmission of the nematode parasite is dependent upon the number of fig wasp females that normally enter a fig inflorescence Each fig species is pollinated by its own species of fig wasp Depending upon the fig species and the population, one or more gravid female fig wasps enter each enclosed inflorescence

11. Fig Wasps The females pollinate and lay eggs within the flowers and then die within the fig The offspring develop within the seeds, eclose as adults, mate, and then the females fly off to lay their eggs in another fig inflorescence

12. Nematodes In the vicinity of the Panama Canal, each of eleven species of fig wasp is parasitized by a distinct species of nematode The nematodes lay their eggs within the figs and the eggs hatch synchronously with the emergence of adult female fig wasps The young nematodes crawl onto a fig wasp, enter the body cavity, and are transported with the female as she flies off in search of a fig where she lays her eggs

13. Nematodes As the nematodes grow, they eventually consume the adult female, emerge from her body, and renew the cycle by laying their eggs in the fig

14. Transmission If only one fig wasp lays her eggs within a fig, the nematodes must rely solely on the offspring of that female as hosts for the next generation. That is, transmission of the nematodes is strictly vertical, from a fig wasp female to her offspring If, however, several fig wasps oviposit into the same fig, the nematodes have the opportunity to attack the offspring of several females (horizontal transmission can take place).

15. Herre�s (1993) Findings That vertical transmission has favored nematodes that have no effect on the number of offspring produced by their host By comparison, horizontal transmission has favored nematodes that are more destructive to their hosts

16. Among the 11 species he studied, there was a near linear relationship between virulence and the proportion of figs colonized by fig wasp females

17. These results suggest that the direction of evolution depends partially upon the structure of populations, which will affect factors such as partner fidelity and transmission

18. How Gene Flow Affects Geographic Mosaics S.E. Forde, J.N. Thompson and B.J.M. Bohannan. 2004. Adaptation varies through space and time in a coevolving host-parasitoid interaction. Nature 431:841-844.

19. Gene Flow & Geographic Mosaic Show that gene flow across a spatially structured landscape alters coevolution of parasitoids and their hosts and that the resulting patterns can fluctuate in both space and time Study used E. coli and a T7 bacteriophage

23. Gene Flow & Geographic Mosaic Results provide evidence that gene flow can alter the dynamics of coevolution and That fragmentation of the natural landscape can alter the evolutionary processes that enable organisms to adapt to changing local conditions slowing coevolutionary changes