I think the green line is average fitness of all iterations and the black line is a logarithmic index of each generation's maximum fitness to the optimal achieved (both relative to time). So the black line pretty quickly gets bounded by an asymptote at the top of the screen, because the improvements in fitness are far more significant early on. Basically an example of diminishing returns...

There is a great discussion of this (and a further mapping of fitness functions and evolutionary algorithms to a 2-dimensional "fitness landscape") in The Origin of Wealth by Eric Beinhocker - http://www.amazon.com/Origin-Wealth-Evolution-Complexity-Eco...

Absolutely phenomenal exploration of complexity economics.