Darwin's theory: variation and natural selection

The starting observation: variation

Charles Darwin, a 19th-century English naturalist, starts from a simple observation: within a single species, individuals are never exactly identical. This variation is natural: some individuals run a bit faster, have a slightly longer beak, or withstand cold a bit better.

The struggle for resources

Darwin also noted that resources (food, space, mates) are limited, while species generally produce far more offspring than the environment can support. This results in constant competition among individuals.

Natural selection

From this variation and this competition arises the central mechanism proposed by Darwin, natural selection: in a given environment, individuals whose characteristics give them an advantage (surviving longer, reproducing more) pass on those characteristics to their offspring more often. Generation after generation, advantageous characteristics therefore become more frequent in the population.

An essential clarification

Natural selection never acts on a single individual during its lifetime: an individual does not transform itself to adapt. What changes is the frequency of certain characteristics within the population, over generations, because the individuals carrying them reproduce more than others.

A common misconception to correct once and for all

"The giraffe stretched its neck to eat the high leaves" is a false explanation. In reality: in an ancestral population of giraffes, some individuals already had, through natural variation, a slightly longer neck than others. During shortages of low-lying food, these individuals had an advantage in feeding and reproducing more. Generation after generation, long necks became dominant in the population. No individual ever stretched its own neck.

Remember the key point

Variation + natural selection + differential reproduction = evolution of the population, never the transformation of an individual.