Charles Darwin: The Theory of Natural Selection
Charles Darwin (1809-1882)
In 1859, Charles Darwin (1809-1882) published On the Origin of Species, which detailed a new mechanism whereby evolution could take place. During his now infamous 1831-1836 voyage aboard the Beagle, Darwin concluded that species were not fixed and could change but, at the time, he had no idea what could cause the change. Then after reading Thomas Malthus's Essay on the Principle of Population which explains how population growth is kept in check by the availability of resources and infant mortality, Darwin began to question whether these processes could influence the evolution of a species.
Darwin reasoned that if plant and animal breeders could, through a process known as artificial selection, alter the characteristics of a given species or breed by selecting those traits that are desirable to the breeders then why couldn't nature do the same.
This idea was further solidified when Darwin read in Mathus' essay that far more animals are born than reach maturity. What would determine whether an animal lives or dies? Darwin, surmised that those individual that survived to adulthood must have traits that better allow them to compete for resources.
Darwin stated that all organisms within a given population possess heritable traits such as size, speed, and color. Some of those variation would be more favorable than others in a given environment and give the individual who possess those favorable traits a competitive edge in acquiring resources, avoiding predators, and eventually passing those traits on to descendants. Natural selection would therefore work on the existing variation within a given population.
Take for example Lamark's giraffes. Darwin would argue that the neck length of giraffe increased over time because the original population of giraffe contained some individuals with longer necks than others. As environmental conditions changed and trees became taller those individuals with slightly longer necks were favored and could out compete shorter necked individuals for food. As a result longer necked giraffe were more likely to survive into adulthood and pass on thier traits.
Critics of Natural Selection were quick to point out that Darwins natural selection could not account for the development of new traits within a population and could only work on existing variation. Some reasoned that new traits that did arise would simply blend with other existing traits. Darwin himself even acknowledged potential problems with the theory stating, ""If it could be demonstrated that any complex organ existed which could not possibly have been formed by numerous, successive, slight modifications, my theory would absolutely break down." Some critics have used this to discredit natural selection by asserting that some organs, such as eyes, are too complex to have been formed through natural selection since any form other than their current would be useless. This, however, is faulty logic considering even the most basic eye--a simple light sensitive cell would be of more use than none at all. The answers to all of these critisms had actually already been answered but would remain unknown until 1900.