Galapagos IslandCharles Darwin explored the Galapagos Islands where he made the observation that the bird's beaks and the shells of the tortoises on the island were different than the ones on the mainland. All other features were virtually identical which led him to speculate as to what would make the two different.
(Keynes, Richard. “Charles Darwin's Beagle Diary.” Darwin Online, 2001, http://darwin-online.org.uk/content/frameset?viewtype=text&itemID=F1925&pageseq=388.)
Selective Breeding and Natural SelectionDarwin observed the agrarian life of animals and denoted that breeding certain animals for the best possible stock lead him to conclude this led to the genetic variants in DNA. He also surmised this as the reason why immunities and other faults of the human genome get weeded out.
(Wyhe, John. “MIND THE GAP: DID DARWIN AVOID PUBLISHING HIS THEORY FOR MANY YEARS?” Darwin Online, 2007, http://darwin-online.org.uk/content/frameset?viewtype=text&itemID=A544&pageseq=1.)
EvolutionBased off his research from his voyage on The Beagle, Darwin creates the Theory of Evolution through a paper he wrote called On the Origins of Species which used his theory of natural selection as a foundation.
(Desmond, Adrian; Moore, James (1991). Darwin. London: Michael Joseph, Penguin Group. ISBN 978-0-7181-3430-3.)
Natural Selection EcologyThe Fertilization of Orchids was published and elaborated on how natural selection does not just occur in humans and animals but in plant life and entire ecosystems in general. These theories were now recognized and accepted academically while the church held condemnations against his work.
(Wyhe, John. “Fertilisation of Orchids.” Darwin Online, 1999, http://darwin-online.org.uk/EditorialIntroductions/Freeman_FertilisationofOrchids.html.)