Natural Selection

Have you ever heard the phrase, survival of the fittest? When most people hear this phrase, they think of physical fitness features such as speed, strength and endurance. But what do biologists really mean by fitness?

It all started when Charles Darwin published The Origin of Species in 1859 which aroused much debate. Darwin's Theory of Natural Selection, which he defined as, '…the principle by which each slight variation, if useful, is preserved.' sparked the interest of a prominent 19th Century thinker, Herbert Spencer. In his book, Principles of Biology, Spencer coined the phrase, 'survival of the fittest', to explain Darwin's Natural Selection. This phrase resonated with many including Darwin himself and the term fitness became popular.

However, when biologists think of an organism's fitness, they don't think about its strength, speed or endurance. Instead, they simply consider fitness as the reproductive success of an organism relative to others. Take these Snowshoe Hares for example. Organisms with greater relative fitness have adaptations that allow them to have more offspring that survive and reproduce. These adaptations are called phenotypes defined simply as a visual expr