What do we call the process by which organisms that are best adapted to their environment will be the ones that 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.

Video transcript

- [Dr. Samuel] Hi everybody, Dr. Samuel here, your friendly neighborhood entomologist. And I was hoping that we could take a few minutes to talk about adaptation. What comes to mind when you think about adaptation? You might think of cryptic morphology that helps organisms hide from predators, scaly armor or spines to protect organisms in a scuffle, or increased melanin pigmentation in the skin to shield an organism from the damaging impacts of harsh sunlight in really sunny areas. And if this is the sort of thing that you thought of, you're right, these are all adaptations. But what people often miss about adaptations is that they all help organisms survive and reproduce in a particular environment. The word adaptation can refer to a trait that makes an organism more suited to its environment. But it can also mean the process by which a population becomes dominated by organisms that are suited to their environment. The point is that adaptation happens in a population as its organisms accumulate adaptations. And this all happens through the process of natural selection, which you may have heard about. With natural selection, we see organisms with beneficial heritable traits, increasing their share of the gene pool in the population because they're more likely to mate and produce offspring. This means these well-adapted organisms have higher fitness, a measure of an organism's ability to survive and reproduce in its specific environment. These traits or adaptations make the organisms better suited to survive and reproduce because they're better suited to the specific environmental context that they live in. That is our green caterpillars blend in, and thus, survive better on the green leaves, and the orange caterpillars on the orange leaves. As you can see, natural selection is intimately intertwined with the environment in which it takes place. And this is why natural selection can lead to adaptation. Even the concept of adaptation is diverse. Adaptations can be anatomical, changes to physical structures, or physiological, changes in the function of these structures, or adaptations can be changes to behavior. We see this with honeybees adapted to life in Africa, sometimes called by the suboptimal names, Africanized honeybees or killer bees. They're the same species as and look identical too. Populations of honeybees adapted to North America, but their behaviors diverge dramatically. Because the African populations are adapted to life in a hot, tropical environment, sometimes, they construct nests right out in the open air, nests in open air. This is extremely rare for populations adapted to the temperate climate of North America, where the temperature range is far too broad between winter and summer for open air nests to be successful. So, North American honeybee populations much prefer to live inside of protected structures like inside of the trunks of hollow trees or the boxes that beekeepers provide, nests in protected structures. And in Africa, there are also more large, fearsome predators interested in getting into the bees' nest. So, the frequency of traits related to hyper defensiveness is much more common than in populations adapted to North America, where there are fewer aggressive predators, heightened defensiveness, low to moderate defensiveness. The success of honeybees adapted to life in Africa left a lot of people worried that they would be similarly successful in North America and replace the honeybees there. But those concerns ignored the connection between adaptation and the environment. Traits that helped those bees survive in Africa and even helped them in similar environments found in many regions of tropical, South and Central America, well, those same exact traits were liabilities in much of North America. Okay, so, this is all fascinating, but you may be wondering, if natural selection is passing out adaptations, why doesn't it choose better stuff? Fish tend to get eaten by birds and bears a lot. And that's gone on for literally millions of years. So, why I haven't fish populations developed something like laser eyes or teleportation? Instead, when a population of fish does evolve defense, it's typically something like a few bones, that it already has, getting longer and sharper and protruding out of their fins, like in the case of the three-spined stickleback fish. These spines protect the stickleback from birds and other predators, but only if the spines are long enough to make it difficult for that specific predator to swallow them. Birds with wider throats are still able to eat even fully grown fish with fully elongated spines. Laser eyes really do seem like they would be more effective. It's almost like natural selection is just tinkering with whatever it has lying around instead of going out and buying something new and perfectly suited to the task. And that is exactly what's happening. Populations don't adapt by accessing a bottomless well of awesome problem-solving options. Quite the contrary, with natural selection as the driver, adaptations can be pretty limited. You might already be tracking with the idea that if an increase in fitness is what natural selection is after, you don't need an adaptation that allows you to incinerate all of your enemies. You just need one good enough to solve whatever problem is keeping others in your population from surviving in mating. If that's a kingfisher bird eating the other fish, the resolution might be just a spine that's only a half a centimeter longer than the other fishes. That's the reason why we see adaptations that look like natural selection just grabbed whatever happened to be lying around and modified it because it kinda did. A turtle's remarkably protective shell may look fancy and novel, but it's literally the turtle's overgrown ribs fused with its spine. A narwhal's face spear, that there is just a tooth that grew all the way out of its face. Natural selection only has the options currently in the genes of a population with the occasional addition of new genetic sequences through mutation to work with when a problem arises. And you might be thinking, "Oh great! Mutations, those will help! That's how my favorite superhero got her powers." Well, unlike what we see in movies, most mutations either have no real effect or result in problematic gene expression or nonfunctional proteins which is pretty bad news for living things. So, adaptations are not likely to be super weapons that ensure survival in all circumstances. Instead, they're useful traits that help increase an organism's fitness as the environmental conditions apply pressure. So, adaptation is a remarkable system, but one with some pretty pronounced limitations. Natural selection has to work with what an organism has. But consider for a moment what it has given us despite those limitations. Pistol shrimp have the ability to slam their claws shut so quickly that the water around them collapses in on itself. It implodes knocking nearby fish unconscious. Some honeybees can bundle together and vibrate their flight muscles so rapidly that they can create a biological convection oven which can cook predators to death. And there are parasitic wasps that have evolved to live inside of parasitic wasps that live inside of parasitic wasps that live inside of caterpillars, like tiny, strange, little Russian nesting dolls. And don't even get me started on the hornets that have yellow patches on their abdomen that absorb solar energy to supplement the energy provided by their diet. They have solar panels! I'd say natural selection has done pretty well for itself.

What is it called when organisms adapt to their environment?

What is it called when the best adapted organisms survive and reproduce?