Evolution

Evolution is the way living things change over many generations. These changes happen because of things like natural selection and genetic drift. Over time, some traits become more common, while others disappear. This explains why there are so many different kinds of plants and animals on Earth and how they all come from a common ancestor.

The idea of evolution was first explained by Charles Darwin and Alfred Russel Wallace in the 1800s. They saw that animals often have more babies than can survive. Each animal is a little different from others. These differences can decide if an animal lives long enough to have its own babies. When animals with helpful traits have more babies, those traits can spread through the group over time.

Today, scientists know these changes are linked to DNA, which carries information from one generation to the next. Things like mutations and gene flow can also change the DNA in a group. By studying these changes, scientists have learned about the history of life on Earth, how new species form, and how all living things are connected.

Evolution helps us understand many important things, from how diseases work to how to grow better crops. It is a big idea that touches many areas of science and helps explain the amazing variety of life we see around us.

Heredity

Further information: Introduction to genetics, Genetics, and Heredity

DNA structure. Bases are in the centre, surrounded by phosphate–sugar chains in a double helix.

Evolution happens when the traits we get from our parents change over time. For example, in humans, the colour of our eyes is something we get from our parents. If one parent has brown eyes, you might have brown eyes too. These traits are controlled by tiny parts inside our cells called genes. All the genes together in our body make up our genome.

We can see these traits in how our bodies look and act, which is called our phenotype. Some traits come from our genes, while others come from things around us, like the sun. For example, getting a suntanned skin colour comes from both your genes and how much sun you get. Some people, like those with albinism, don’t tan at all because of their genes. These traits are passed from one generation to the next through DNA, which carries the instructions that make each living thing unique.

Sources of variation

Main article: Genetic variation

Further information: Genetic diversity and Population genetics

Evolution can happen when there are differences in the genes of living things. These differences come from changes in DNA called mutations, the mixing of genes during sexual reproduction, and animals or plants moving between groups (gene flow). Even with new differences, most DNA in a group stays very similar.

Mutations are changes in the DNA sequence and are the main source of genetic differences. They can change how a gene works. For example, the human eye uses several genes that came from one original gene. In animals like wild boar, mutations can change their colors.

Evolutionary forces

Main article: Natural selection

Evolution is when the traits of plants or animals change over time. This can happen in a few ways.

Natural selection is when traits that help an organism survive and have babies become more common. For example, if birds have different sized beaks and bigger beaks help them eat tough seeds, birds with bigger beaks may have more babies. Over time, bigger beaks become more common. This happens because nature favors traits that help an organism live and reproduce.

Genetic drift is when random changes happen in traits by chance, especially in small groups. Imagine flowers where blue and red flowers are equally common. If, by chance, only red flowers survive and reproduce one year, the next generation might have more red flowers even if blue flowers are just as good at surviving. Over many years, this can change the mix of traits in a group.

Natural outcomes

Evolution changes how living things look and act. It helps them find food, stay safe from predators, and find mates. Some animals work together, like helping family or sharing help with other species through symbiosis. Over long periods, evolution can create new types of animals and plants.

Evolution does not have a plan. It does not try to make things more complex. Even though some complex animals have evolved, simple ones are still the most common. Simple life has been the main type of life on Earth for a very long time.

Adaptation

Adaptation makes animals and plants better fit for where they live. It can mean a special trait that helps them survive, like a horse’s teeth for chewing grass. Adaptations happen because of natural selection. Sometimes animals lose old features or gain new ones.

Adaptations can change over time. Some body parts may stop doing their old job but become useful for something new. These are called exaptations.

A baleen whale skeleton. Letters a and b label flipper bones, which were adapted from front leg bones, while c indicates vestigial leg bones, both suggesting an adaptation from land to sea.

Coevolution

Animals and plants can change together. For example, a pathogen and a host or a predator and its prey can each change because of the other. This back-and-forth change is called coevolution.

Cooperation

The common garter snake has evolved resistance to the defensive substance tetrodotoxin in its amphibian prey.

Some animals and plants help each other. Plants work with mycorrhizal fungi that live on their roots and help them get food from the soil.

Animals of the same kind can also work together. Social insects like bees, termites, and ants live in groups called a colony. Some insects in the group cannot have babies but help take care of the ones that can.

Speciation

Speciation is when one group of animals or plants splits into two or more new groups. To become new groups, they must no longer breed with each other. Scientists have seen this happen in labs and in nature.

There are four main ways this can happen. The most common way for animals is allopatric speciation, where groups get separated by distance. Peripatric speciation happens when a small group ends up in a new place. Parapatric speciation is another way. In sympatric speciation, new groups form without moving to a new place.

Extinction

Extinction is when a whole group of animals or plants disappears. This happens often because new groups form and old ones disappear. Almost all the animals and plants that ever lived are now gone. Mass extinction events happen sometimes when many groups disappear quickly. People are now causing many animals and plants to go extinct, and global warming may make this happen faster.

Applications

The ideas behind how living things change over time, like natural selection, have many useful uses. People have used artificial selection for thousands of years to pick traits they like in plants and animals. This is how we got different kinds of dogs and crops.

We now use these ideas in medicine and technology, too. In medicine, learning how diseases change helps scientists make better treatments. In computers, programs called evolutionary algorithms copy natural processes to solve tough problems faster.

Evolutionary history of life

The hominoids are descendants of a common ancestor.

The Earth is about 4.54 billion years old. The earliest signs of life go back to at least 3.5 billion years ago. Scientists think life started with simple, single-celled organisms. These early life forms were tiny microbes. Over billions of years, they changed and became the many different plants, animals, and other organisms we see today.

All living things on Earth share a common ancestor. This means every plant, animal, and microbe comes from the same original group of organisms from long ago. Through a process called evolution, these organisms changed and adapted. This led to the wide variety of life we see now. Scientists study fossils, compare the structures of living things, and look at DNA to understand how life has evolved and how different species are related to each other.

History of evolutionary thought

Main article: History of evolutionary thought

Further information: History of speciation

The idea that one kind of living thing could come from another goes back to early thinkers in Greece, like Anaximander and Empedocles. Later, during Roman times, the writer Lucretius talked about these ideas in his book De rerum natura.

During the Middle Ages, many people thought nature followed a fixed plan. But some scholars from Arabia, like Ibn Khaldun, thought humans came from earlier life forms.

In the 1600s, the Scientific Revolution began. Scientists started looking for natural reasons for how things work. John Ray and Carl Linnaeus grouped living things together but still thought each kind stayed the same.

The big change came with Charles Darwin and Alfred Wallace. They said that species change over time through natural selection. Darwin explained this in his well-known book, On the Origin of Species. He showed how animals and plants change to fit their environment, creating the many kinds of life we see today.

Social and cultural responses

When Charles Darwin published his book On the Origin of Species in 1859, his idea that all living things change over time caused many discussions. Some people were excited, but others felt it challenged their beliefs. Today, most scientists agree that evolution explains how life changes, but some religious groups still disagree.

In some places, like the United States, there have been debates about whether schools should teach evolution or ideas like creationism. These conversations continue as people try to balance science with their personal beliefs.