Darwin's theory revolved around survival of the fittest. This meant that only the organisms that were better adapted to the environment would survive, and those that didn't died (and maybe become extinct). The desired characteristics that helped the particular organism to survive, would then be classed as the dominant allele. This could now be passed onto offsprings.
People didn't know much about genetics we only really began to understand it 50 years after the theory was published. Darwin's theory started to help scientists understand how certain traits and diseases could be spread from parent to offspring. Darwin used natural selection to evaluate that humans are descendants from apes. This was problematic because many people thought that God created humans, many scientists argued that Darwin didn't have enough evidence to support his theory.
