What is the process by which organisms become better suited to their environment to survive and reproduce?

The process by which some organisms in a population survive and reproduce, while others do not, based on their bodies and behaviour

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.

Living organisms are adapted to their environment. This means that the way they look, the way they behave, how they are built, or their way of life makes them suited to survive and reproduce in their habitats. For example, giraffes have very long necks so that they can eat tall vegetation, which other animals cannot reach. The eyes of cats are like slits. That makes it possible for the cat's eyes to adjust to both bright light, when the slits are narrow, and to very dim light, when the slits are wide open.

Behavior is also an important adaptation. Animals inherit many kinds of adaptive behavior. In southern Africa there are small animals called meerkats, which live in large colonies. The meerkats take turns standing on their hind legs, looking up at the sky to spot birds of prey. Meanwhile, the meerkats in the rest of the colony go about their lives. You can probably think of many other features of body or behavior that help animals to lead a successful life.

In biology, an ecological niche refers to the overall role of a species in its environment. Most environments have many niches. If a niche is "empty" (no organisms are occupying it), new species are likely to evolve to occupy it. This happens by the process of natural selection. By natural selection, the nature of the species gradually changes to become adapted to the niche. If a species becomes very well adapted to its environment, and if the environment does not change, species can exist for a very long time before they become extinct.

An excellent example of an animal evolving to fill a niche is seen in the evolution of horses. Many fossils of different kinds of horses have been discovered, and paleontologists think that the earliest ancestor of the modern horse lived in North America more than 50 million years ago. This animal was a small padded-foot forest animal about the size of a dog. If you saw one next to a modern horse, you might not even think the two were related! As time passed, the climate of North America became drier, and the vast forests started to shrink. Grasses were evolving, and the amount grassland was increasing. Horses adapted to fill this new grassland niche. They grew taller, and their legs and feet became better adapted to sprinting in the open grasslands. Their eyes also adapted to be further back on their heads to help them to see more of the area around them. Each of these adaptations helped the evolving grassland horses to avoid predators. Their teeth also changed to be better adapted to grinding tough grassland vegetation.

Have you ever wondered what purpose the "dew" claw on the inside of a dog's paw serves? The claw is the dog's thumb. Because a dog runs on the balls of its feet and four digits, the claw no longer serves a purpose. Organs or parts of the body that no longer serve a function are called vestigial structures. They provide evidence that the species is still changing. Even humans have vestigial structures. The human appendix is one such example. It used to store microbes that helped to digest plant matter, but it is no longer needed in the human.

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