------That's called a change in the allele frequency. In other words, evolution.
No, it's change, but it isn't "evolution." For "evolution" you need (at a minimum) the introduction of new genetic material, not simply the elimination of some characteristics in some individuals. Check your dictionary, see de-vol-u-tion, n. It's interesting that loss of specific traits is so much easier to document than addition of traits, A fact that lends support to the ID folk.
The biologists love to tell us about fish that lost their eyes, moths that lost their color, horses that lost their toes, hippos that lost their legs, birds that lost their teeth, Aunt Sally losing her appendix etc., etc., all to convince us that "little by little, the accumulation (!) of changes has lead to the structures we see today."
What these biomissionaries need is a Freshman Logic course.
You could use a freshman biology course. Evolution is a change in allele frequency in populations.
It is a fact that additional genetic material is sometimes added or subtracted from genomes, but the most common form of evolution is the change in allele frequency. New alleles are introduced all the time by mutation. Animal and plant breeders have been observing this for centuries. Mutations used to be called sports.