Can you send me to a link that explains how genetic variation and environmental pressures can lead to a change in the characteristics of a population.

Answer 1

You can google using keywords
"genetic variation and environmental pressures" to get multiple articles on the subject, and choose the appropriate one(s) for you.

One of the articles I got was:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2859910/