The premise of Darwin's theory of evolution by natural selection is that all life, from mammals to single celled organisms, is related through descent with modification from common ancestral stock. The mechanism he proposed to explain descent with modification was natural selection.
Natural selection: the engine of evolution
Darwin unveiled his theory in 1859 in his book On the Origin of Species by Means of Natural Selection. He was certainly not the first to theorise that man was descended from animals - Greek philosophers such as Anaximander and Empedocles had advanced this view as early as the 6th century BC. However through his keen insights as a biologist, Darwin was able to demonstrate the scientific basis for evolution: a concept which he called 'natural selection'. By observing different species, Darwin saw that there is variation in every population and that within these groups there is competition for limited resources such as food, water and shelter from predators. The creatures that survive this 'struggle for existence' pass on their favourable heritable traits to their offspring, and because more offspring are produced than can survive, this process of 'natural selection' continues. The process later became popularly known as 'the survival of the fittest'.
It was during Darwin's journeys on the British survey ship HMS Beagle that he saw the variations in different species that led him to develop the idea of natural selection. Darwin's experiences on The Galápagos Islands were a catalyst for his thinking about evolution.
The species on these islands - birds, plants, insects and reptiles - resembled those on the South American mainland, but they were also different in many subtle ways. After his journey back to England, Darwin began to develop the idea that the species from the mainland had reached the Galápagos, and then changed, adapting to their new environment. Until this point it was universally accepted that species did not change yet Darwin's emerging theory was about to challenge this in a radical way.
An evolving idea
Unlike today's scientists with their sophisticated lab techniques and computer equipment, Darwin was heavily reliant on observation and deduction; yet one of the amazing things about Darwin's theory is that it has remained at the heart of scientific thinking about the origins and development of life. Through our understanding of DNA we now have a much more sophisticated view of genetically inherited traits, which provides an even stronger scientific basis for Darwin's 150 year old theory.
Like to know more?
If you'd like to explore the science behind Darwin's theory and much more besides, why not join over 20,000 others who study science with the Open University every year.