Why are some people concerned about using vaccines?

Because they never know what things like illnesses they can get if they take the vaccine

small groups of individuals leave their home population and establish new settlements, mating only among themselves.

Explanation:

The founder effect is what happens when a small group of individuals leave their home population in order to establish new settlements. They will then mate only amongst themselves and colonize the area without mixing with the natives.

As a result of the founder effect, there is less genetic variation in the new population than in the home one because the smaller number of individuals who left would not possibly be able to carry all the genes in the home population. The only genes that would be passed on are those of the small individuals.