Masai giraffe (Giraffa camelopardalis tippelskirchi), four giraffes stand in savannah, Kenya, Masai Mara National Park
blickwinkel/F. Stober/Alamy
Natural selection is the process by which some organisms in a population survive and reproduce, while others do not, based on their bodies and behaviour. It is one of the processes by which species change from generation to generation, and is a crucial element of the theory of evolution.
A classic example of natural selection at work is the origin of giraffes’ long necks. The ancestors of modern giraffes were animals similar to deer or antelope, with necks of ordinary length. However, because the trees in their habitat were tall, those giraffes with slightly longer necks had an advantage over their shorter-necked fellows. The longer-necked giraffes reproduced more, so in the next generation longer necks were more common. Over many generations this process produced giraffes as they are today.
This process of natural selection was first described by Charles Darwin in 1859 in On the Origin of Species. It helps explain how the many varied species on Earth could be descended from a single ancestral species.
Natural selection is sometimes summed up as “survival of the fittest”. This is true but can be misleading. The word “fittest” does not necessarily refer to physical fitness. Rather, it means how well-suited an organism is to its environment and lifestyle. The fittest organisms are not necessarily the fastest or strongest: often they are the most cooperative.
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Natural selection can produce surprising results. It can produce more complex organisms, for example creating multicellular organisms from single-celled ones, but it can also simplify: for example, fish species that live in dark caves lose their eyes. Furthermore, when circumstances change natural selection can swiftly reverse course.
However, natural selection has its limits. In particular, because it is not guided by a consciousness, it has no foresight and can lead species to evolve down paths that seem advantageous but actually lead to extinction.
There is also more to evolution than natural selection. Species can change in a more undirected way by a process called genetic drift, in which certain genetic variants become more common despite not having any particular advantage. When a species is not under strong selection, genes can vary more freely and this sometimes leads to the emergence of remarkable new traits. There is also sexual selection, in which animals choose their potential mates not because of their actual fitness, but on the basis of showy ornaments like peacock’s tails or complex songs. Finally, many organisms have cultural behaviours such as tool use, and these feed back onto traditional evolutionary processes like natural selection.
Evolution is also chaotic, meaning that the changes it produces are not always predictable. This is partly because it relies on random mutations to produce the raw material on which natural selection can act. More controversially, it has been argued that natural selection has a kind of memory that allows it to swiftly recreate old solutions when they are needed.
Nowadays there is also a new force in evolution: humans. We are exerting new selection pressures on many species, changing them in unpredictable ways.
