How do I tell if someone likes me?
And How do I get someone to like me?

Ask them out would be the simple answer (this question also depends on if they are male or female) If they respond to messages quickly or in a female's case they will talk in a slightly higher pitched voice they are usually interested in you. There isn't really a sure way to get someone to like you, I would just say don't be awkward and being good at flirting wouldn't hurt.

Explanation:

the correct answer is:

a population changes in response to a change in the environment.