Dr. Cho should conclude that the growth rate of a specie is still greatly affected by its environment. I think that genetics play a big role, but only in the first few time periods of a life cycle. It is our genetics that tell our physical characteristics such as eye color, hair color, body type or skin tone. But as time progresses and your are exposed to an environment for a long period of time, these characteristic may be affected and can evolve. For example, the hair color of a person is originally black may become brownish if works regularly under the sun. The same is true for bone structure, metabolism and growth rate. External factors such as food could greatly affect a specie's characteristics.