The gene pool is the set of all genes in a species or population. Kettlewell found that dark moths constituted less than 2% of the population prior to 1848.The English moth, Biston betularia, is a frequently cited example of observed evolution. The frequency of the dark morph increased in the years following.Individual organisms do not evolve, they retain the same genes throughout their life.When a population is evolving, the ratio of different genetic types is changing -- each individual organism within a population does not change.This is a brief introduction to evolutionary biology.
The late eighteen hundreds was the time of England's industrial revolution.
Phenotypic changes, like this, induced solely by changes in environment do not count as evolution because they are not heritable; in other words the change is not passed on to the organism's offspring.
Phenotype is the morphological, physiological, biochemical, behavioral and other properties exhibited by a living organism.
[gene: a hereditary unit] So, the change in frequency of dark colored moths represented a change in the gene pool.
[gene pool: the set all of genes in a population] This change was, by definition, evolution.