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He found that the birds, tortoises, iguanas, and many plants he saw on the Galapagos were found nowhere else in the world He did not publish his book, On the Origin of Species, because it challenged the beliefs of that time, the findings stunned him, and he was disturbed by them He did not publish it until 25 years after his findings In his book, he presented evolution as natural selection, that it has been taking place for millions of years, and continues with all living things He saw that members of each species vary from one another in important ways He spoke with animal & plant breeders that convinced him variation existed in nature & farm Ex., some plants in the same species bear bigger fruits than others, some cows produce more milk than others He learned this was heritable variation- differences passed from parent to offspring He didn’t know how heredity worked, now we know its controlled by genes Darwin said farmers and animal breeders used heritable variation to improve crops and livestock They would only breed the large hogs, the fastest horses, etc. He called this artificial selection: nature provides the variation, & humans select the variations they find useful Artificial selection worked in nature High birth rates and shortages of life’s basic needs would force organisms to compete for these resources Struggle for existence: members of each species compete for food, living space, etc. Fast predators can catch their prey Prey that are better camouflaged or better protected could avoid being caught Darwin observed how well an organism fit into its environment Fitness: ability of an individual to survive/reproduce in its environment Fitness is the result of adaptations: any inherited characteristic that increases an organism’s chance of survival Adaptations can be structural or anatomical Ex., porcupine’s sharp quills, plants able to perform photosynthesis, how some animals hunt in groups Individuals that are not well-suited to their environments die or leave few offspring and can become extinct Individuals well suited, survive and reproduce more successfully Darwin called this survival of the fittest Because of its similarities to artificial selection, Darwin referred to survival of the fittest as natural selection Over time, natural selection results in changes in the inherited characteristics of a population. Natural selection acts directly on phenotypes (the way things look) He also said that today species look different from their ancestors Each living species has descended with changes from their ancestors over time: descent with modification This also means that all living things are related to one another: common descent; a single tree of life links all living things