The high temperatures in the desert environment can kill Beetles Large Beedles cannot burrow into the sand to escape the heat. Therefore, fewer of the large Beetles will survive.

Question

The Beetles are affected by a deadly disease that spreads rapidly through the desert. Green beetles are susceptible but brown beetles are resistant

Did the desert environment CAUSE adaptive variations or did it SELECT adaptive variations?

I think it caused?

Answer 1

There are two things here: heat, and disease.
The heat could act to select smaller beetles, but we don't know anything about size of the green vs brown beetles.

The disease: Green beetles are susceptible, brown are not. So apparently, there are no adaptive variants being caused, but brown are being selected because of disease resistance. One hopes they have some mechanism to avoid heat.