Gametic disequilibrium under both recombination and natural selection

To simulate the combined action of recombination and natural selection on gametic disequilibrium, try the program Populus, which can be obtained by following the link on the text website.

In the Java version of Populus, use the Natural Selection menu to select the Two-Locus Selection simulation. Set pAB = pab = 0.5 and pAb = paB = 0.0 as a case where there is maximum gametic disequilibrium initially. Use fitness values of wAaBb = 1, all others = 0.5 and wAAbb = waaBB = 1, all others = 0.5 to generate strong natural selection (relative fitness values are explained in Chapter 6). Finally, try recombination values of r = 0.5 and 0.05. Focus your attention on the D vs. t plot. What do the two different fitness cases do to levels of gametic disequilibrium and how effective is recombination in opposing or accelerating this effect?

For those who would like to see the details behind the recombination and natural selection model in Populus, a spreadsheet version of this model is available in Microsoft Excel format. The spreadsheet model will allow you to see all the calculations represented by formulas along with a graph of gametic disequilibrium over time.

recombination. In such cases, the population reaches a balance where the action of natural selection to increase D and the action of recombination to decrease D cancel each other out. The point where the two processes are exactly equal in magnitude but opposite in their effects is where gametic disequilibrium will be maintained in a population. It is important to recognize that the amount of steady-state gametic disequilibrium depends on which genotypes have high fitness values, so there are also plenty of cases where natural selection and recombination act in concert to accelerate the decay of gametic disequilibrium more rapidly than just recombination alone.

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