The proportion of well-adapted organisms in a population can increase over the generations by the process of evolution by natural selection.

Note that this change was not due to pollution making the moths darker. The dark variety had always existed in small numbers, but only had a selective advantage when the selection pressure changed.

Aa Aa Aa Natural selection, genetic drift, and gene flow are the mechanisms that cause changes in allele frequencies over time. When one or more of these forces are acting in a population ...

Although the study of natural selection ... genetic loci at which selection has likely occurred, and some of the selective pressures involved have been elucidated. Three significant forces that ...