Forming a Theory

Darwin began to realize that the birds’ isolation on the islands may have resulted in special adaptation. These changes happened slowly over time until new species were formed. He believed that the finches and the mockingbirds each came from a common ancestor in South America.

Darwin wasn’t the only scientist to be formulating this theory. Several naturalists had already developed this idea by the end of the 18th century. Darwin’s great contribution to science was that he was able to explain how and why evolution occurred.

Española cactus finch

large ground finch

All of Darwin’s thinking came together in 1859. It was then that he published a book called On the Origin of Species. In it, he proposed that species evolve and that all living things can trace their descent back to a common ancestor.

Darwin also suggested a mechanism for evolution: natural selection. This says that some plants and animals have traits that work best in a particular place. These living things are the ones that will survive and reproduce. They pass their traits to their offspring.

Over time, a population changes, or evolves, to better fit where it is. It may evolve in small ways. For example, size or fur color may change. However, with enough time, an entirely new species can emerge.

The samples Darwin collected on the Beagle expedition and the thinking that came from his observations contributed to significant changes in how people saw the natural world. His contributions to science will always be remembered.