Help me, Lazyweb, you're my only hope

Does anyone have a line on a graphical simulator of genetic trait propagation within a population? I want to illustrate the impact on a species of introducing a trait that directly affects reproduction - ideally, being able to show how the trait is expected to spread if the trait enhances, inhibits, or fails to impact, reproductive rates of the species over a period of generations. ('Illustrate' was an important word in that sentence - I'm trying to tie a graphical representation of 'here is what happens' together with 'here is the math' for folks for whom the mathematical explanation is user-hostile).

While we're at it, I'd be interested in informed comment upon the proposition that 'human artificial genetic modification produces rates of genetic change that are higher / lower / comparable to natural propagation rates.'

As an aside, the article Integrating gene flow, crop biology, and farm management in on-farm conservation of avocado (Persea americana, Lauraceae) from the American Journal of Botany is quite interesting.