Charles Darwin was not the first person to consider that life had evolved from one form to another; that species were not immutable. Among those to suggest as much, were Lamarck and his own grandfather, Erasmus. What Charles Darwin succeeded in doing – where those before him had failed – was coming up with a mechanism that explained how evolution worked.
The Mechanism of Natural Selection
There are three key components to Darwin’s concept of Natural Selection:
• The are more individuals produced in a given population than can possibly survive in that environment, thereby setting up a struggle for survival.
• Variation exists between members of the population and those that possess the most favourable traits (i.e. those best suited to the environment and circumstances) will survive best and reproduce more: that is, survival of the fittest.
• Traits possessed by individuals are heritable, so that, over time, those with traits best suited to the environment will leave proportionately more offspring, which will in turn also possess the favoured traits. This differential reproductive success will lead to a change in the frequency of individuals possessing such traits and – to an outsider – it will appear that the form of the species has changed over time.
The Formation of New Species
While Natural Selection will lead to a change in the distribution of traits within a species, the formation of new species (i.e. animals that freely interbreed and can produce viable offspring) typically involves something extra: reproductive isolation.
This occurs when a species becomes separated into at least two populations that are reproductively isolated (meaning that something stops them from breeding together: it could be a mountain range, a river system, a difference in seasons, or whatever). Each population goes on changing –adapting to its particular environment through the process of Natural Selection (as outlined above) – until the populations are sufficiently distinct that were they brought back together they could no longer freely interbreed.