Populations are often small when they have just been founded by a group of individuals that have moved to a new location. the allele frequencies in the new populations are likely to be different from what they were in the source population. what is this called?

This is called the founder effect.

Explanation:

Founder Effect Definition

The founder effect refers to a situation in which a larger group breaks into smaller groups as people of that group move to other locations or spread out to live in different places or states.

The traits of the original group get lost along the way , the smaller group tend to inherit the traits from the individual who original founded the smaller group.

As the group get smaller the more it loses the traits they had from the original larger group.

If the individuals who were the first to isolate from original large group did not have the same alleles carried by most of the members in the original large group then the new smaller group will not have those alleles which yields the founder effect.

The family end up being genetically different, the founder effect is caused by geographic isolation which create a gap that restrict an interaction that may bring back the former alleles. It can also due to mutation.

the correct labels are as follows:

a: yellow river

b: himalayan mountains

c: north china plain

d: taklimakan desert