What is the rate of diffusion of the white vinegar into your phenolthelein agar cube? should be distance per unit time

Its because the dark green phenotype benefits the insects’ survival, making them more likely to reproduce.

Explanation:

In a mixed population having different variants of insects, those that can better adapt themselves will survive and will reproduce more offspring or we can say that they will be naturally selected. Natural selection brings evolution. But remember no variant is completely wiped out. One similar example is industrial melanism. Before the industrialization in England, white winged moths were more in number than the dark winged moths. The tree trunks were lighter in color so the predator could not spot the white winged moths against the light back ground but could easily spot the dark winged moths. Hence the predator fed on dark or melanised moths and resulted in the decrease of their number.  

But after industrialization, due to the deposition of smoke and soot on the tree trunks, the white winged moths were easily visible for the predator. This resulted in decrease of number of white winged moths. Hence all this explains about the better survival chances, one with more survival chances will produce more offspring and will increase in number.