An Evolutionary Play
- The stage is set: it is early Spring - the goldenrod plant starts to grow.
- Enter the gall fly. Adult gall flies emerge and lay eggs on the terminal bud of a goldenrod plant.
- The gall fly egg hatches, and the larva burrows into the stem and stimulates the plant to form a ball gall.
- The gall fly larva lives happily in the gall, feeding on the gall tissue and growing.
- The gall also continues to grow.
- Enter the parasitoid wasps. While the fly larva is still in the bud, E. obtusiventris places
its own egg into the fly egg or larva. The strategy of E. gigantea is different. It locates a developing
gall and inserts an egg into the lumen of the gall.
- In both cases, the wasp larva eventually kills the fly larva.
- Enter the beetles. Throughout the growth of the gall, beetles may insert their eggs just under the surface
of the gall. The beetle eggs hatch.
- Burrowing beetle larvae enter the lumen and kill the fly larva.
- Fly larvae that survive through the Fall overwinter in the gall. They will emerge in the Spring to mate and
The plot thickens:
- The ability of the wasps to deposit their eggs depends on gall size. The wasp
must be able to penetrate the gall wall with its ovipositor if the egg is to be deposited in a location where it
- The size of the gall is an inherited trait of both the plant and the gall fly.
It is the result of an interaction between substances secreted by the fly and the response of the plant to those
substances. Genes in the fly control the amount of substances secreted. Genes in the plant mediate the plant response
to the fly substances.
How can the interactions in this evolutionary play lead a change in mean gall size?
Begin the play:
For natural selection to occur, there must be:
- variation in a population
- the variation must be heritable
- selection events must preferentially eliminate individuals in the population based on a trait, or cause
them not to leave offspring in the next generation. If all individuals are
equally likely to be eliminated, selection events are not occuring.
- the change in the trait in the next generation is the population's response to selection
How does the intensity of selection affect the mean and variance (s2) of
the galls in which flies survive? Remember that variance = the square of
the standard deviation (s).
Test your understanding of the plot of the play:
- The type of selection in this play is known as directional selection because it
causes a directional shift in the mean of the population. What would happen if Eurytoma gigantea parasitized
only large galls? Check yourself.
- How would directional selection affect the mean and variance of gall size if selection reduced the number
of small galls? Check yourself
- If the two species of wasps parasitized galls at the opposite ends of the frequency
distribution, stabilizing selection would occur. How would this affect the gall size distribution?
- If beetles preferred galls of intermediate size, disruptive selection would
occur. How would this affect the gall size distribution? Check
Printable summary of the Interactions place cursor
in left frame and print if desired.
The curtain closes ************Restart
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Last edited 9-25-09