It is now generally accepted that new species develop by adaptation of existing forms. Present-day species are the result of gradual evolution over many generations from earlier forms by the process of natural selection (or survival of the fittest). They are still evolving, and this is only made possible by genetic variation, which results from gene mutation and sexual reproduction.
Variation in a population has great survival value because it increases the chance of at least some individuals being adapted to changing conditions. Gene mutations are uncommon, but ultimately they are the origin of variation. Mutations are usually harmful or have no obvious effect, but occasionally they may lead to a survival advantage.
It was Charles Darwin (1809 - 1882) who proposed the currently accepted view that natural selection is the mechanism for evolutionary change, discussed in his famous book The origin of species by means of natural selection, published in 1859. The fittest organisms, in the biological sense, are those best suited (adapted) to their environment, and therefore most likely to survive and produce offspring. The adaptive features may be very small (such as slightly quicker reflexes, or slightly increased immunity to disease), but as a consequence these organisms are much more likely to pass their genes on to the next generation, and over many generations useful adaptive features become predominant within the population.
All organisms are adapted in such a way as to maintain all the processes fundamental to living things. They do not adapt to fulfil their own needs, but are adapted by virtue of the features they have inherited. Land animals did not grow legs because they wanted to walk; legs gradually developed over many generations through chance mutations, enabling the creatures to exploit the land, giving them a competitive advantage and increasing their chances of survival. Some adaptations may be more obvious than others, they are not always visible features. Familiar examples of adaptations are: methods of feeding (e.g. birds beaks for different kinds of food; the long proboscis of a bee to reach nectar); defence from attack (e.g. the stinging hairs on a nettle plant, camouflage in moths), response to changes in the environment (e.g. hibernation in hedgehogs), successful competition for resources; and living long enough to reproduce.
In 1809, the French naturalist Lamark proposed that organisms adapt to their environment by developing new adaptive features during their own life times, and these adaptations are inherited by their offspring (i.e. inheritance of acquired characteristics). He suggested for example that water birds extended their toes so much in an effort to swim that the skin stretched between them and started forming webbed feet. This altered condition was then inherited by their offspring who developed it further until, after many generations, they had fully webbed feet. This Lamarkian view is no longer scientifically accepted, but it is fairly straightforward to understand, and it remains a common view among children and adults alike.
ContentsDiversity of organisms