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This is a simple simulation of natural selection in a trait controlled by a single gene with two alleles. It is modeled on evolution of color-morphs of the peppered moth Biston betularia.
Peppered moths rest on tree trunks. In England in the early 19th century, the most commonly seen of these moths had light-gray, speckled wings. They were well camouflaged against the light-gray lichens and bark. Occasionally one saw dark-winged morphs, but these were poorly camouflaged and tended to be eaten by birds.
As the Industrial Revolution progressed, air pollution killed many of the lichens and darkened the tree trunks with soot. As the previously cryptic light-winged moths became visible to predators -- and the dark-winged morphs became better camouflaged -- the dark-winged trait became more common.
Each moth has a single-gene, two-allele genotype. There are two phenotypes -- light and dark -- and the user can specify whether light or dark is the dominant trait.
In every generation, the simulation starts with a population of fifty moths. Selection eliminates ten of them, preferentially eliminating dark moths if the bark is light and light moths if the bark is dark.
The surviving moths then pair off and mate. Pairs produce offspring whose genotypes are determined probabilistically by simple Mendelian genetics. Offspring phenotypes follow from their genotypes.
The plot then records the new generation's allele frequencies.
First, SET UP. This creates the first generation of moths. Your first time playing with the simulation, leave all the settings at their defaults.
Click GO ONCE. Watch selection eliminate moths, and note that the poorly-camouflaged moths are preferentially killed. (Selection is indicated by X's. Dark moths have dark-red X's, and light moths have light-red X's, so it's easy to make a quick visual assessment of the selection bias.)
If you have the genotype-labels turned on, watch the genotypes of the offspring as they emerge from the crosses. Either way, get a general feel for whether you see more or fewer dark moths emerging.
Click GO ONCE again, or click GO REPEATEDLY. (Click GO REPEATEDLY again to stop. Note that the simulation won't stop immediately; it will stop at the end of the current generation.) Watch the trajectories of the allele frequencies.
When the bark is light, you should see the dark allele decrease in frequency. When the bark is dark, you should see the light allele decline. It doesn't happen particularly straightforwardly, though. Does one of the elleles decline more quiickly than the other? Why? (Look at http://people.brynmawr.edu/twong/models/completedominance.html for a hint.)
The default settings have the light allele as recessive to dark. Try setting the light as dominant. With light bark, does the dark allele decline more or less quickly than before? (Remember, you can't make conclusions on the basis of single trials. Try running the simulation to fixation several times for each set of settings, and compare averages.)
http://people.brynmawr.edu/twong/models/completedominance.html
http://people.brynmawr.edu/twong/models/incompletedominance.html
http://faculty.ccp.edu/faculty/twong/demos/drift.html
This simulation was built by Ted Wong, November 2009. It is hosted as a web applet at http://faculty.ccp.edu/faculty/twong/demos/simple_selection.html
More detailed information on peppered-moth evolution can be found at http://en.wikipedia.org/wiki/Peppered_moth_evolution