In the mid-1800s, which two men concluded that variation of traits might provide better odds of survival in certain environments?

The correct answer is "Charles Darwin and Alfred Russel Wallace".

Explanation:

Charles Darwin and Alfred Russel Wallace are two of the most prominent scientists regarding evolutionary biology. In 1859, the famous book "The origin of the species" by Charles Darwin explained that species develop new traits along several generations, where the traits that are advantageous for the species remain. Alfred Russel Wallace, as well as Charles Darwin, was a naturalist that performed many research expeditions. These two men shared many letters and both concluded that variation of traits might provide better odds of survival in certain environments, where natural selection works as a selector of traits.