Social Darwinism begins with the work of Charles Darwin and his ideas of evolution and natural selection, ideas that were later co-opted and applied to society.
It’s important to note that these ideas have been mostly discredited in academia in sociology. Sociologists don't call themselves Social Darwinists. Despite this, these ideas are a very important part of Western thought.
Charles Darwin spent a lot of time in South America and the Galapagos Islands. He argued that evolution in animals happens because of natural selection. Random genetic mutations occur within individuals that give them an advantage over other members of the species; these mutated individuals are therefore more likely to reproduce and pass on their genes.
It follows, then, that he's going to eat more and will be more likely to reproduce and pass on those genes to his offspring, who themselves may have longer beaks. In this way, over many millennia, genes like this get spread about the population, and eventually, all the birds have beaks that are that length.
This is how the theory of natural selection works. The bird with the longer beak was selected because it had a mutation that was advantageous. This mutation allowed it to outrival other birds.
Herbert Spencer (1820-1903) was an English philosopher, a mathematician, and a natural and a social scientist; he wore many hats in his career, as many of the early thinkers did. A contemporary of Darwin, Spencer took Darwin’s theories, particularly natural selection and competition, and applied them to society, formulating the idea of Social Darwinism.
Spencer called evolutionary competition ‘survival of the fittest’. He thought that the same kind of competition that occurs in nature also happens in society. Spencer argued that societies pass through a process of natural selection, and only the most fit societies will win the competition and advance.
Compared to these tribal societies, Englishmen like Spencer felt that their own societies were more evolved, more refined. They had already passed through the stages that the primitive societies were currently in.
Given this diversity of human life all over the globe, Spencer felt assured in his belief that societies progress from simplicity towards complexity, similar to Darwin’s argument that the natural organism passes from simplicity to complexity. They thought that this was a law--a rule-bound way that societies will develop. Societies will compete and progress or they will falter.
The idea of the progress of social evolution was extremely influential among Europeans in the late 19th and early 20th centuries, although today it is widely discredited. At that time, though, the scientific community was obsessed with discovering laws of society that would cause society to evolve and progress towards a more harmonious state.
Spencer maintained that more simple forms of social organization were outcompeted and eliminated. This way of thinking has completely fallen out of fashion because it's easy to see how dangerous it can be for a society.
EXAMPLEHitler was motivated by this line of thinking when he advanced the idea that we could develop an Aryan race that was somehow better or more progressed than the rest of society.
Source: This work is adapted from Sophia author Zach Lamb.