As he traveled from island to island, Darwin noticed that finches on each island had unique bill adaptations that allowed them to take advantage of the food sources of each island. Darwin proposed that the finches had all arrived on the islands at the same time, presumably as members of the same flock blown off course by a storm. There they were able to survive because within the original group there were bill variations that allowed each finch to establish an ecological niche and take advantage of different ways of life. Finches best suited for survival on each island produced offspring with the same bill variation. Overtime, the finches diverged from the original ancestral finch line and became 14 new finch species.

Today, Darwin's finches are found only on the Galapagos Islands with one species on the Cocos Island. They are differentiated from each other by the variations in their bills. What is unusual is that mainland finches are usually distinguished from each other by variations in plumage not body form.

There are few bird species on the Galapagos Islands. When the original finches arrived on the islands, they met little competition and were able to exploit the ecological niches available on each island. Ground finches have stout, strong bills that are good for cracking seeds. Tree finches have pointed bills and eat insects from under tree bark. Another type of finch has a woodpecker-like bill that drills into wood for insects. Unlike, the woodpecker, the finch has a short tongue, so it uses a twig to pull out the insect. Some are able to use cactus needles for this purpose.

The finches have two main predators: the Galapagos hawk and short-eared owl. With human introduction of domestic cats, the ground finches have been vulnerable to attack from this new predator. For the most part, the finches were able to adapt to the island food supply with little threat from predators. They also developed in isolation. This is a necessary condition for changes to take place that lead to new species.

In his studies, Darwin noticed that individuals shared characteristics common to a species, but within a species there could be individual variation. These variations are the result of subtle differences in the genes of individuals. Slight changes in the DNA of an individual can occur through mutation. He speculated that while some mutations might be harmful, others might be beneficial and give an organism a selective advantage in survival.

Darwin also observed that many species produce far more eggs or seeds than could ever develop into adults and be supported by the ecosystem. For example, a single female horseshoe crab may 100,000 eggs during the breeding season, but only 1 in 130,000 eggs survives to adulthood. Many eggs are lost to predators before they ever hatch. Predators also thin out hatchlings as soon as they reach the ocean waters. Darwin speculated that the overproduction of offspring was necessary to ensure that there would be enough adults to continue the species.

Darwin observed that members of the same species competed for food, water, shelter, space, and mates. Those individuals best suited for survival had an advantage during harsh conditions or competition for scarce necessities. Darwin noticed that on the Galapagos Islands tortoise populations included individuals with long necks and short necks. During times of adequate rainfall, vegetation was available for both types of tortoises. However, in dry conditions, tortoises with long necks were favored for survival since they could reach leaves that were higher off the ground. Therefore, their variation was better adapted to the environmental conditions and led to survival.

Finally, individuals whose characteristics were best suited for survival were the most likely to achieve adulthood and reproduce. Through reproduction, the individual's genes were passed onto the next generation. Over time, in isolation, a new species might arise. This is how Darwin explained the 14 different species of finches who were differentiated by their beak type from one another, but who otherwise each strongly resembled an extinct ancestral finch from the South American mainland. This is called adaptive radiation and will be covered in detail in high school biology.

Although Darwin's theory explained his observations, he lacked experimental evidence to support his hypothesis that new species come from existing species through a process of natural selection in which certain genetic characteristics provide a survival advantage under isolated conditions. The experimental evidence would come later during Dr. Kettlewell's experiments with peppered moths in England during the Industrial Revolution.