Darwin performed hybridization experiments that demonstrated the principles, and the topic appeared as a section in Origin of Species. However, Darwin's experiments lacked statistical rigor -- he didn't do enough variations to make statistical assertions. This gap was later adeptly filled by Mendel. Freeman Dyson did a wonderful lecture on this topic right before his passing, which covered various forms of biological & cultural evolution spanning Darwin to present:

https://www.ias.edu/news/in-the-media/freeman-dyson-biologic...

If you want an alternate source:

https://www.nature.com/articles/s41437-019-0289-9