Natural selection how does it work

With a little help from Darwin himself, we're going to learn about natural selection and how it created the astonishing complexity and diversity of life on planet Earth.

Evolution is the result of the tendency for some organisms to have better reproductive success than others -- natural selection. It's important to remember that differences between individuals, even individuals from different generations, don't constitute evolution. Those are just variations of traits. Traits are characteristics that are inheritable -- they can be passed down from one generation to the next. Not all traits are physical -- the ability to tolerate close contact with humans is a trait that evolved in dogs.

Here's an example that helps explain these concepts:. Now imagine that some conditions arise that make it more likely for jockeys to reproduce successfully than basketball players. Jockeys have children more frequently, and these children tend to be short. Basketball players have fewer children, so there are fewer tall people. After a few generations, the average height of humans decreases.

Humans have evolved to be shorter. Evolution is all about change, but what is the mechanism that causes these changes? Every living thing has everything about its construction encoded in a special chemical structure called DNA. Within the DNA are chemical sequences that define a certain trait or set of traits. These sequences are known as genes. The part of each gene that results in the varying expression of traits is called an allele.

Because a trait is an expression of an allele, the tendency of a certain trait to show up in a population is referred to as allele frequency. In essence, evolution is a change in allele frequencies over the course of several generations. Sexual reproduction itself is a product of natural selection -- organisms that blend genes in this way gain access to a greater variety of traits, making them more likely to find the right traits for survival.

For more detailed information on evolution, head over to How Evolution Works. A population is a defined group of organisms. In terms of evolutionary science, a population usually refers to a group of organisms that have reproductive access to each other.

For example, zebras that live on the plains of Africa are a population. If other zebras lived in South America none do, but let's pretend they do for the sake of the example , they would represent a different population because they're too far away to mate with the African zebras. Lions that live on the plains of Africa are a different population as well, because lions and zebras are biologically unable to mate with each other.

Fitness is the key to natural selection. We're not talking about how many reps a sea otter can burn through at the gym -- biological fitness is an organism's ability to successfully survive long enough to reproduce. Beyond that, it also reflects an organism's ability to reproduce well. It isn't enough for a tree to create a bunch of seeds.

Those seeds need the ability to end up in fertile soil with enough resources to sprout and grow. Fitness and natural selection were first explained in detail by Charles Darwin , who observed wildlife around the world, took copious notes, then sought to understand what he had seen. Natural selection is probably best explained in his words, taken from his landmark work "On the Origin of Species. Organisms show variation of traits. No one supposes that all the individuals of the same species are cast in the same actual mould.

More organisms are born than could ever possibly be supported by the planet's resources. Therefore, all organisms must struggle to live. Some traits offer advantages in the struggle. Organisms that have those traits are more likely to successfully reproduce and pass the traits on to the next generation.

Successful variations accumulate over the generations as the organisms are exposed to population pressure. The ultimate result is that each creature tends to become more and more improved in relation to its conditions.

The process of natural selection can be sped up immensely by strong population pressures. Population pressure is a circumstance that makes it harder for organisms to survive. There's always some kind of population pressure, but events like floods , droughts or new predators can increase it. Under high pressure, more members of a population will die before reproducing. This means that only those individuals with traits that allow them to deal with the new pressure will survive and pass along their alleles to the next generation.

This can result in drastic changes to allele frequencies within one or two generations. Here's an example -- imagine a giraffe population with individuals that range in height from 10 feet to 20 feet tall. One day, a brush fire sweeps through and destroys all the vegetation below 15 feet. Only the giraffes taller than 15 feet can reach the higher leaves to eat. Giraffes below that height are unable to find any food at all.

Most of them starve before they can reproduce. In the next generation, very few short giraffes are born. The population's average height has gone up by several feet. There are other ways to quickly and drastically affect allele frequency. One way is a population bottleneck. In a large population, alleles are evenly distributed across the population. If some event, such as a disease or a drought, wipes out a large percentage of the population, the remaining individuals may have an allele frequency very different from the larger population.

By pure chance, they may have a high concentration of alleles that were relatively rare before. As these individuals reproduce, the formerly rare traits become the average for the population. The founder effect can also bring about rapid evolution.

This occurs when a small number of individuals migrate to a new location, "founding" a new population that no longer mates with the old population. Just as with a population bottleneck, these individuals may have unusual allele frequencies, leading subsequent generations to have very different traits from the original population that the founders migrated from. The difference between slow, gradual changes over many generations gradualism and rapid changes under high population pressure interspersed with long periods of evolutionary stability punctuated equilibrium is an ongoing debate in evolutionary science.

So far, we've looked at natural selection as an agent of change. When we look around the world, however, we see many animals that have remained relatively unchanged for tens of thousands of years -- in some cases, even millions of years. Sharks are one example. They will best know the preferred format. When you reach out to them, you will need the page title, URL, and the date you accessed the resource. If a media asset is downloadable, a download button appears in the corner of the media viewer.

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Any interactives on this page can only be played while you are visiting our website. You cannot download interactives. The theory of natural selection was explored by 19th-century naturalist Charles Darwin. Natural selection explains how genetic traits of a species may change over time. This may lead to speciation, the formation of a distinct new species. Select from these resources to teach your classroom about this subfield of evolutionary biology. Artificial selection is the identification by humans of desirable traits in plants and animals, and the steps taken to enhance and perpetuate those traits in future generations.

Artificial selection works the same way as natural selection, except that with natural selection it is nature, not human interference, that makes these decisions. Charles Darwin and Alfred Wallace developed the idea of evolution through natural selection.

But this idea was not accepted by scientists until more evidence came along. Use this infographic to explore how Darwinism and genetics came together to explain what we know today about evolution. Join our community of educators and receive the latest information on National Geographic's resources for you and your students. Skip to content. Photograph by Ian Forsyth via Getty Images. Twitter Facebook Pinterest Google Classroom.

Encyclopedic Entry Vocabulary. Media Credits The audio, illustrations, photos, and videos are credited beneath the media asset, except for promotional images, which generally link to another page that contains the media credit. Media If a media asset is downloadable, a download button appears in the corner of the media viewer. CC google Hyla versicolor LeConte, Cryptic colored adults clinging to a tree trunk. On a tree in a wooded area, the Gray Treefrog is much harder to detect.

Natural selection is the process in nature by which organisms better adapted to their environment tend to survive and reproduce more than those less adapted to their environment. For example, treefrogs are sometimes eaten by snakes and birds. Gray Treefrogs blend well in dark wooded areas on tree bark and Green Treefrogs blend in well with green vegetation found in marshes and swamps. A Green Treefrog on the bark of a tree is easier for a predator to find, compared to a Green Treefrog on a green leaf.

So, Green Treefrogs that go into habitats where they are not camouflaged are more likely to be eaten by predators. Since Treefrogs that have been eaten do not live to have any more baby Treefrogs, natural selection has favored Treefrogs that live in habitats in which they are more camouflaged.

This explains the distribution of Gray and Green Treefrogs. In the area in which Gray and Green Treefrogs overlap, both habitats occur but in different places. However, natural selection does not always go to the optimum. It only goes to what works. For instance, rabbits are herbivores, which have hind gut fermentation fermentation of food after passing through the stomach. They have a special organ called a caecum which helps them digest their food.

Unlike other animals, the caecum of the rabbit is located too far down the intestines of the rabbit for the rabbit to get all the nutrients out of its food.