Natural selection

Natural selection is the process by which some animals and plants survive and reproduce better than others because of their traits. These helpful traits are then passed down to their babies. Over time, this changes the traits that are common in a group of animals or plants. This idea was made famous by Charles Darwin, who compared it to artificial selection, where people choose which animals or plants to breed.

For Darwin, natural selection happened because of three main things: parents passing on traits to their babies, differences in traits among individuals, and the struggle to survive and reproduce. Even within a group, some traits help an animal or plant live better and have more babies. These traits become more common if the environment keeps favoring them. If the environment changes, new traits may become helpful, leading to changes in the group or even new species.

Natural selection is a key idea in biology today. Darwin and Alfred Russel Wallace shared this idea in 1858, and Darwin wrote about it in his famous 1859 book On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life. Even though scientists didn’t fully understand genetics when Darwin wrote his book, later discoveries in genetics helped explain how natural selection works. Today, scientists continue to study and learn more about how natural selection shapes life on Earth.

Historical development

Main article: History of evolutionary thought

Pre-Darwinian theories

Many thinkers in the old world had ideas about how nature creates different kinds of animals. Some believed that nature makes many creatures randomly, and only those that can survive and have babies continue to live. One important thinker, Aristotle, thought that animals changed in very specific ways for a purpose. He did not fully agree with the idea of random changes, but he did allow that very rare and unusual animals could appear.

Later writers, like an Arabic scientist in the 800s, talked about how animals might struggle to find food and space. In the 1500s, a famous artist named Leonardo da Vinci studied fossils and thought animals change shapes over time to fit better in their world.

By the 1700s, some scientists began to think that very small changes happening over long periods could add up to big differences between species. In the early 1800s, a scientist named Jean-Baptiste Lamarck suggested that animals could pass on traits they gained during their lives to their babies.

Darwin's theory

Main articles: Inception of Darwin's theory and Development of Darwin's theory

Further information: Coloration evidence for natural selection

In 1859, Charles Darwin shared his big idea about how animals and plants change over time. He called this idea "natural selection." Darwin noticed that animals that were better suited to where they lived were more likely to survive and have babies. If these animals passed on the traits that helped them, over many generations, the whole group could change. This is how new species can form.

Darwin got his ideas from watching animals on his travels and from reading a book by an economist named Thomas Malthus. Malthus wrote that animals can have more babies than the food can support, so there is always a "struggle for existence." Darwin realized that in this struggle, animals with helpful traits would survive better.

Darwin worked hard to gather evidence for his idea before sharing it. Another scientist, Alfred Russel Wallace, came up with the same idea around the same time. Both of their work was shared together in 1858, and Darwin published a full book called On the Origin of Species in 1859.

Darwin compared natural selection to how farmers pick the best plants or animals to breed, which he called "artificial selection." He thought nature did the same thing. Even though he didn't know about genes, Darwin knew that natural selection was just one part of how species change. He also talked about how animals might change because of what they eat or how they live.

The modern synthesis

Main article: Modern synthesis (20th century)

Natural selection needed the idea of genes to be fully understood. A scientist named Gregor Mendel, who lived at the same time as Darwin, discovered the basic rules of how traits are passed down, but his work was not found until 1900. In the 1900s, scientists put together Darwin's ideas with Mendel's rules, and this became known as the "modern synthesis."

Different scientists added important pieces to this idea. One showed how math could explain natural selection. Another explained how changes in genes provide the material that natural selection works on.

A second synthesis

Main article: Evolutionary developmental biology § History

In the mid-1900s, a scientist named Ernst Mayr explained how groups of animals can become separate species. Later, another scientist introduced the idea that animals might help each other survive. By the end of the 1900s, new discoveries about genes and how embryos grow led to a new field called evolutionary developmental biology. This field looks at how changes in the genes that control baby development can change the shape of adult animals.

21st century developments

Main article: Extended evolutionary synthesis

Today, scientists are learning more about how animals themselves play a role in their own evolution, not just their genes or the environment. This has led to new ways of thinking about natural selection.

Terminology

The term natural selection means that some animals survive and have babies more easily because of their traits. These traits can be passed to their babies. Traits that help animals have more babies are called selected for. Traits that make it harder to have babies are called selected against.

Mechanism

Heritable variation, differential reproduction

Main article: Phenotype

Natural selection happens when some animals are better suited to their environment than others. If these animals pass on their traits to their young, more of them will survive in the next generation. Over time, this leads to changes in the group of animals.

For example, peppered moths in England changed color during the Industrial Revolution. When trees got darker from smoke, dark moths were better hidden from birds. This meant more dark moths survived and had babies. Later, when the air got cleaner, light moths became more common again.

Fitness

Main article: Fitness (biology)

Fitness in nature means how well an animal can have babies and raise them. An animal might not live very long but if it has lots of babies, its genes will be passed on. Fitness is about having enough babies to survive to adulthood, not just living a long time.

Competition

Main article: Competition (biology)

Animals often compete for food, water, or space to live. This competition helps decide which animals are better suited to survive and have babies. Some animals have many babies with a low chance of surviving, while others have fewer babies but give them more care to help them survive.

Social species

Main article: Cooperation (evolution)

Some animals work together to help each other survive. For example, birds might warn each other of danger, or animals might hunt together. Working together can help the whole group survive better. Darwin thought that being social helped animals develop many human-like traits such as thinking and talking.

Classification

Natural selection can affect any trait that can be passed down and can be influenced by the environment, such as competition with others. It doesn't always make things better; sometimes it just keeps things the same by removing weaker traits.

We can group natural selection in different ways: by how it changes a trait, by how it affects genetic variety, by the stage of life it affects, by what it acts on (like genes or groups), and by the resource being fought over. For example, selection can keep a trait stable, favor extreme traits, or split traits in different directions. It can also remove genetic variety or keep it around. Depending on the stage of life, it can help with survival or reproduction. Sometimes, selection acts on individuals, genes, or even groups. Finally, selection can happen because of competition for mates or other resources in the environment.

Arms races

Natural selection can be observed when microorganisms develop resistance to antibiotics. Since penicillin was discovered in 1928, antibiotics have been used to treat bacterial infections. However, misuse of these drugs has led to bacteria becoming resistant, creating challenges for doctors. For example, a type of bacteria called methicillin-resistant Staphylococcus aureus (MRSA) is hard to treat with common medicines. Scientists work to create new drugs, but bacteria keep finding ways to resist them. This back-and-forth is called an evolutionary arms race. Similar resistance happens with pesticides used on plants and insects.

Arms races can also happen in nature without human help. One example is a butterfly on the island of Samoa that changed quickly to resist harmful bacteria.

Further information: Antimicrobial resistance

Evolution by means of natural selection

Main articles: Evolution and Darwinism

Natural selection is a big idea that helps explain how animals and plants change over many generations. It works because animals in the same group are not all exactly the same — some are slightly different. These differences can affect how well they survive and have babies.

For natural selection to work, there needs to be differences that can be passed from parent to child. These differences come from changes in the instructions inside cells. Most of these changes are small and may not do much, but sometimes they can help or hurt an animal. If a change helps an animal survive better, then animals with that change are more likely to have babies, and that helpful change will become more common in the group over time.

Genetic basis

Genotype and phenotype

Main article: Genotype–phenotype distinction

Natural selection happens because an organism's traits, or what we can see about it, affect how well it can have babies. These traits are not only decided by genes but also by how the organism grows and behaves in its home environment. When different organisms in a group have different versions of a gene that affects a trait, each version is called an allele. These genetic differences can change how well an organism survives and reproduce, leading to new adaptations and even new species over time.

Some traits are decided by just one gene, but most are influenced by many genes working together. A small change in one gene might not do much, but when many genes work together, they can create a range of possible traits.

Directionality of selection

Main article: Directional selection

When a trait can be passed down, selection changes how common different gene versions become. There are three types of selection based on how they change these gene versions: directional, stabilizing, and disruptive selection. Directional selection happens when one gene version is better, so it becomes more common until everyone has it. Stabilizing selection removes gene versions that are not helpful, keeping the helpful ones. Disruptive selection favors traits that are very different from the average, which can lead to new groups of organisms.

Some selection keeps gene versions at medium levels in a group. This can happen when organisms with one copy of a gene version are better off than those with two copies, like humans who are resistant to malaria because they have one copy of a gene linked to sickle-cell anemia. Balancing selection can also happen when the helpfulness of a trait depends on what other traits are common in the group.

Selection, genetic variation, and drift

Main articles: Genetic variation and Genetic drift

Some genetic differences do not affect an organism's traits or how well it survives. These neutral differences can change by chance, especially in small groups. When genetic differences do not affect survival, natural selection cannot change how common they are. However, new mutations keep happening, creating a balance between new changes and natural selection removing unhelpful ones.

When genes are close together on a chromosome, they tend to be passed down together. If one gene becomes common because it is helpful, the genes near it can also become common, even if they are not helpful. This is called a selective sweep. Background selection is the opposite, where harmful genes and their neighbors are removed, leading to less variation in that area of the genome.

Impact

Main article: Universal Darwinism

Charles Darwin’s ideas changed how people thought in the 1800s. He suggested that simple life forms could slowly change over time into many different, complex life forms just through a few basic rules. This idea was exciting to some but upsetting to others, as it challenged old beliefs about humans’ special place in the world.

Darwin’s idea of natural selection has spread far beyond biology. It is now used in many areas, such as computer science, economics, and even studying how ideas change over time. This wide use of the idea is sometimes called universal Darwinism.