The theory of evolution by natural selection is now widely accepted. When Darwin proposed it, nobody knew how characteristics were passed on. We now know that characteristics are inherited by offspring in genes, so the process that natural selection relies on has an explanation.
There is further evidence in the fossil record. Fossils show how organisms have changed over millions of years, and they fit the idea that new species develop gradually from earlier ones.
Evidence also comes from antibiotic resistance in bacteria. Within a population of bacteria, a mutation can make some bacteria resistant to an antibiotic. When the antibiotic is used, the resistant bacteria survive and reproduce, and the population becomes mainly resistant. This is natural selection that can be seen happening within a human lifetime.
Data from all of these sources supports the theory, which is why scientists accept it.
