can occur when part of a population of a species becomes separated from the remainder

The correct answer is would be - geographic isolation.

Explanation:

The geographic isolation is also known as the speciation in which biological species experience genetic changes and on the basis of the genetic changes that experienced this separated with other organisms of the species.

It is physical separation or speciation of the biological organisms on the basis of geographical barriers of two population of the same species that keep them from interbreeding and reproducing.

Thus, the correct answer is - geographical barriers.

i think its the first one

it's diluted or concentrated

in a diluted solution, you can add a lot more solute but in concentrated only a little more can be added