Within a species of short, ground-dwelling animals, two parents produced a group of offspring that were 1 cm taller than they were. these offspring were able to reach slightly higher vegetation than their parents and other members of the species. when the offspring grew up, they produced new offspring who were also slightly taller and could also reach the higher vegetation. this process continued over several generations such that offspring became gradually taller with time.

eventually, after many generations, all members of the species were at least 20 cm taller than the original two parents that produced the slightly taller offspring. what does this illustrate?