Gertie, "selective breeding" happens in nature every day.

Mating rituals help many animals SELECT a breeding partner.

Darwin's law mandates that weak and inferior specimens are "selected out" by predation.

There are examples of snake colonies where certain colorations survive and others fail, resulting over time in that colony becoming all the same color.

The main difference is in a breeders hands it is MAN that does the selecting. In the wild, circumstance selects who is to breed.

The human race is a good example, a couple hundred years ago (yesterday in evolutionary terms) children born with deformities rarely survived, hell some folks just tossed the baby to the wolves.

As our race has "evolved" people with deformities or handicaps can lead a complete life cycle.

In nature, a crippled deer cannot outrun the coyotes, so it gets naturally removed from the breeding program.