Evolution

Evolution is the process by which the characteristics of living things change over time in groups, or populations. These changes happen because of several processes, such as natural selection and genetic drift, which act on differences in genes among individuals. Over many generations, these changes can lead to new types of plants, animals, and other living things, creating the amazing variety of life we see today.

The idea of evolution was first explained in detail by Charles Darwin and Alfred Russel Wallace in the 1800s. They noticed that animals often produce more babies than can survive, that each animal is slightly different from others, and that some differences help an animal live better in its environment. These helpful differences can then be passed to the next generation.

Today, scientists know that the instructions for these traits are stored in DNA molecules. Changes in DNA, called mutations, can create new traits, and movements of individuals between groups, called gene flow, can also change populations. All living things on Earth share a common ancestor that lived billions of years ago, and through evolution, we have many different species today.

Studying evolution helps us understand not only biology but also fields like medicine and computer science. By looking at the evidence from fossils, DNA, and observations in nature, scientists continue to learn more about how life changes and adapts over time.

Heredity

Further information: Introduction to genetics, Genetics, and Heredity

Evolution happens when the traits that we inherit from our parents change over time. For example, in humans, eye colour is something we inherit. If one of your parents has brown eyes, you might get that trait too. These traits come from tiny parts called genes, which are found in every cell of our body. All the genes together make up our genotype.

We also have a phenotype, which is what we look like and how we behave because of our genes and our environment. For example, getting a suntan depends on both our genes and how much sunlight we get. Some people, like those with albinism, don’t tan at all. These differences show how genes can affect what we see in ourselves and others.

Sources of variation

Main article: Genetic variation

Further information: Genetic diversity and Population genetics

Evolution can happen when there is genetic variation in a group of organisms. This variation comes from changes in DNA called mutations, the mixing of genes during sexual reproduction, and the movement of genes between groups (gene flow). Even though new variations keep appearing, most of the genes in a species stay very similar across all individuals.

Mutations are changes in the DNA sequence and are the ultimate source of genetic variation. These changes can alter how a gene works, stop it from working, or have no effect. Sometimes, mutations can create extra copies of genes, which can lead to the development of new genes with new functions. For example, the human eye uses several genes that all came from one original gene.

In asexual organisms, genes are passed on together, but in sexual organisms, offspring get a mix of genes from both parents. This mixing increases genetic variation and can speed up evolution. Gene flow happens when genes move between different groups of organisms, either through the movement of individuals or through processes like the transfer of pollen. This can introduce new variations into a group.

Some changes that can be passed down are not changes in DNA sequences but are due to other factors, such as changes in how genes are turned on or off. These are called epigenetic changes and can also affect how organisms evolve over time.

Evolutionary forces

From a neo-Darwinian perspective, evolution happens when the frequencies of alleles—different versions of genes—change in a population of organisms over time. This can occur through mechanisms like natural selection, genetic drift, and mutation bias.

Natural selection is the process where traits that help an organism survive and reproduce become more common in future generations. This happens because organisms with helpful traits are more likely to live longer and have more offspring. For example, if a moth has a color that helps it hide from predators, more of its offspring will also have that color. Natural selection can also work in different ways, such as favoring extreme traits or keeping traits within a certain range.

Genetic drift is the random change in allele frequencies from one generation to the next. It can cause some alleles to become more common or even disappear just by chance, especially in small populations. This is different from natural selection, which depends on how helpful or harmful a trait is for survival.

Mutation bias refers to differences in how often certain types of mutations occur. These biases can influence how genes change over time, although natural selection often plays a bigger role in shaping traits.

Natural outcomes

Evolution influences every aspect of the form and behaviour of organisms. Most prominent are the specific behavioural and physical adaptations that are the outcome of natural selection. These adaptations increase fitness by aiding activities such as finding food, avoiding predators or attracting mates. Organisms can also respond to selection by cooperating with each other, usually by aiding their relatives or engaging in mutually beneficial symbiosis. In the longer term, evolution produces new species through splitting ancestral populations of organisms into new groups that cannot or will not interbreed.

A common misconception is that evolution has goals or long-term plans; realistically, evolution has no long-term goal and does not necessarily produce greater complexity. Although complex species have evolved, simple forms of life still remain more common in the biosphere. The evolution of microorganisms is particularly important to evolutionary research since their rapid reproduction allows the study of experimental evolution and the observation of evolution and adaptation in real time.

Adaptation

Adaptation is the process that makes organisms better suited to their habitat. Adaptations are produced by natural selection. Adaptation may cause either the gain of a new feature, or the loss of an ancestral feature. Structures with similar internal organisation may have different functions in related organisms. During evolution, some structures may lose their original function and become vestigial structures.

Coevolution

Interactions between organisms can produce both conflict and cooperation. When the interaction is between pairs of species, such as a pathogen and a host, or a predator and its prey, these species can develop matched sets of adaptations. This cycle of selection and response is called coevolution.

Cooperation

Not all co-evolved interactions between species involve conflict. Many cases of mutually beneficial interactions have evolved. Coalitions between organisms of the same species have also evolved. An extreme case is the eusociality found in social insects, such as bees, termites and ants.

Speciation

Speciation is the process where a species diverges into two or more descendant species. Barriers to reproduction between two diverging sexual populations are required for the populations to become new species. Speciation has been observed multiple times under both controlled laboratory conditions and in nature.

Extinction

Extinction is the disappearance of an entire species. Extinction is not an unusual event, as species regularly appear through speciation and disappear through extinction. The Cretaceous–Paleogene extinction event, during which the non-avian dinosaurs became extinct, is the most well-known, but the earlier Permian–Triassic extinction event was even more severe. The Holocene extinction event is an ongoing mass extinction associated with humanity's expansion across the globe over the past few thousand years.

Applications

Evolutionary ideas help us in many ways. For example, we can choose traits we like in plants and animals, a process called artificial selection, which people have done for thousands of years. This same idea is now used in science to create new medicines and useful proteins.

Understanding evolution helps doctors treat diseases. Germs like bacteria and viruses can change over time to avoid medicines, so learning about evolution helps scientists create better treatments. The same idea is used in computers, where evolution-like processes help solve tricky problems and design new technology.

Evolutionary history of life

See also: Timeline of the evolutionary history of life

The Earth is about 4.54 billion years old. The earliest signs of life on Earth date back to at least 3.5 billion years ago. Scientists have found tiny fossils and special materials in very old rocks that show life existed a very long time ago. Over time, life changed and evolved into the many different plants and animals we see today.

All living things on Earth share a common ancestor. This means every living thing comes from one original group of organisms that lived a very long time ago. As time passed, these organisms changed and formed new species. We can see evidence of this shared history in many ways, such as similarities in body shapes and the chemicals inside our cells. Even the tiny instructions inside our cells, called DNA, show how closely related different animals are.

History of evolutionary thought

Main article: History of evolutionary thought

Further information: History of speciation

The idea that one type of living thing could come from another dates back to early Greek thinkers like Anaximander and Empedocles. Later, during Roman times, the poet Lucretius explored these ideas in his work De rerum natura.

During the Middle Ages, many believed that all nature followed a divine plan, where every creature had a specific role. However, some Arab scholars, like Ibn Khaldun, suggested that humans might have evolved from earlier forms of life.

In the 17th century, the Scientific Revolution began to challenge old ideas. Scientists started looking for natural explanations rather than divine ones. John Ray and later Carl Linnaeus helped organize living things into groups, though they still thought each group was fixed and unchanging.

Many scientists before Charles Darwin wondered if species could change over time. For example, Jean-Baptiste Lamarck suggested that animals could pass on traits they acquired during their lifetime to their offspring.

The big change came with Darwin and Alfred Wallace. They both proposed that species change through a process called natural selection. In nature, more animals are born than can survive. Those with helpful traits are more likely to live and have babies. Over many generations, this can lead to new species. Darwin wrote about this in his famous book, On the Origin of Species. His ideas changed how we understand the natural world.

Main article: Modern synthesis (20th century)

In the 1920s and 1930s, scientists brought together Darwin’s ideas with new discoveries about genes. This became known as the modern synthesis. It explained how natural selection works with genes to shape life over time.

Since then, scientists have kept learning more. For example, we now know how DNA carries genetic information. There are also new ideas about how evolution works, like studying how changes during an animal’s growth affect future generations.

Social and cultural responses

When Charles Darwin published his book On the Origin of Species in 1859, the idea that all living things change over time caused big debates. Many people wondered what this meant for religion and society. Today, most scientists agree with evolution, but some people still have questions about it.

Some religions have found ways to believe in both their teachings and evolution. Others believe that evolution goes against their religious stories and raise concerns about it. One big debate happened in the United States, where some people wanted to teach creationism instead of evolution in schools. Courts have decided that creationism and a later idea called intelligent design cannot be taught as science in public schools. These discussions have not been as big in places like China.