Darwinian thought affected the social sciences mainly because it helped to bring about the idea of Social Darwinism.
As Darwinian thought became more popular, social scientists started to think of societies as living organisms. They started to think about how various aspects of society played certain roles just as various organs of the body play certain roles. At the same time, they started to think about societies, or parts of societies, as living beings that competed with one another.
When they came to think in this way, Social Darwinism was born. Social Darwinism held that there is a struggle within society that leads to the "survival of the fittest." This means that whoever comes out "on top" in society must clearly be the most fit. This led to the conclusion that inequality was justified because those who became rich and powerful clearly deserved it on the basis of their superior talents.