Biological diversity
Variation already present in a population is what natural selection acts on, and classification groups species by shared ancestry.
A worked example
If 320 of 400 sampled snails have banded shells, the remaining 80 are 80 / 400 = 20% of the sample, all within one species.
The mistake people actually make
Individuals do not change to suit their surroundings during their lives. What shifts over generations is the proportion of each variety in the population, because some individuals leave more offspring than others.