Charles Darwin (1809-1882) was a British naturalist and scientist who developed theories about evolution and natural selection and is credited with being the father of evolutionary theory. Darwin began to develop his ideas on a historical 5 year long scientific voyage on the HMS Beagle.
While in the Galapagos Islands he noted that each island had finches that were closely related but each island's finch population had differing physical characteristics. This lead Darwin to the idea that would eventually become the concept of natural selection.
Natural selection occurs because animals can have different traits from one another within the same species. The animals that have traits that are most advantageous for a particular environment are more successful and more likely to reproduce. They pass their beneficial traits on to their offspring who continue to be successful and pass the traits along to their offspring. Gradually the species will evolve so that the beneficial traits occur in all of the organisms. The finches that Darwin saw at Galapagos had different kinds of beaks. The beaks were more advantageous for the food on the particular island they were on and over generations the finches with the 'better' beaks got more food and had more offspring until the birds with the less advantageous beaks faded away in the population.
Darwin worked on his theories for 20 years and collaborated with a man named Alfred Wallace who had independently reached many of the same conclusions. In 1859 Darwin published his major work "On the Origin of Species by Means of Natural Selection." This work was considered revolutionary at the time because it defined humans as simply another animal who evolved from less complex organisms. Darwin's theories have been developed over time and are still widely used in many areas of science and beyond.