An inherited characteristic that, through natural selection, increases in a population because it to solve a survival problem at the time it emerges is called

The first wave mainly included wealthy people and the latter was poor people.
Early immigrants arrived at larger ships, and later groups arrived at smaller ships' fleets.
The first wave of immigrants included a very diverse population, but the second immigrant was more homogeneous.
The first wave mainly came from the North and Central Europe. The second wave came mainly from Southeastern Europe - I guess this.