darwin hypothesized that new species could appear gradually through small changes in an ancestral species…

Brief Explanations:

Darwin conducted experiments on pigeon breeding. By selectively breeding pigeons, he observed how small, incremental changes could lead to significant variations over time, which supported his hypothesis about the gradual appearance of new species through small changes in an ancestral species. The finches were mainly observed in the Galapagos for their beak variations related to natural selection but not in a controlled breeding experiment by Darwin to test this specific hypothesis. Observing humans or dog breeds was not the key experiment he did for this particular hypothesis.

Answer:

the breeding of pigeons