Evolution

Evolution is the change in the traits that living things pass on to their young over many generations. It happens when individuals in a group differ in their genes and in how many of their young survive. Mutation creates new genetic differences. Processes such as genetic drift, gene flow, and natural selection make some of these variants more common or less common over time. Evolution is often described as a change in how common certain alleles (the versions of a gene found at a specific place on a chromosome) are in a population.

In everyday speech, evolution often means a change in how an living thing looks or acts (phenotype). However, because some alleles do not show up in visible ways, evolution can happen even when the way an organism looks stays the same. The process of evolution has produced biodiversity at every level of biological organisation.

The scientific theory of evolution by natural selection was developed separately by two British naturalists, Charles Darwin and Alfred Russel Wallace, in the mid-1800s. It helps explain why living things are suited to where they live. Darwin explained the theory in detail in his book On the Origin of Species. Natural selection is supported by things we can observe. More young are often born than can survive, traits vary among individuals, some traits help an animal survive and have young, and traits can be passed from parents to their children. Over time, members of a group are more likely to be replaced by the offspring of parents whose traits fit their surroundings.

In the early 1900s, competing ideas of evolution were refuted, and evolution was joined with Mendelian inheritance and population genetics to form modern evolutionary theory. In this synthesis, inherited information is carried in DNA molecules, which pass information from one generation to the next. All life on Earth, including humanity, shares a last universal common ancestor (LUCA), which lived about 3.5 to 3.8 billion years ago. Scientists keep studying evolution by forming and testing hypotheses and building theories based on evidence from nature and laboratories, as well as on data from mathematical and theoretical biology. Their discoveries have also helped fields such as farming, medicine, and computer science.

For related articles, see Outline of evolution. For other uses, see Evolution (disambiguation). For a more accessible and less technical introduction to this topic, see Introduction to evolution.

Heredity

Further information: Introduction to genetics, Genetics, and Heredity

Evolution happens through changes in heritable characteristics. These are traits that an organism inherits from its parents. In humans, for example, eye colour is an inherited trait. A person might inherit the "brown-eye trait" from a parent. Inherited traits are controlled by genes. All of the genes in an organism's genome, which is its genetic material, make up its genotype.

The traits you can see, such as an organism's body shape and behaviour, are called its phenotype. Some of these traits come from both the genotype and the environment. Others are not inherited. For example, a suntanned skin colour comes from a mix of a person's genotype and sunlight. Because of this, a suntan is not passed on to children. Inherited traits move from one generation to the next through DNA, a molecule that stores genetic information. DNA is a long biopolymer made of four types of bases. The order of these bases works like letters that spell out a message. Inside cells, each long strand of DNA is called a chromosome. The exact spot where a piece of DNA sits on a chromosome is its locus. When the DNA at a locus is different between individuals, the different forms are called alleles. Mutations can create new alleles. Most traits are shaped by several genes working together in a quantitative or epistatic way. People with inherited albinism do not tan at all, and they are very sensitive to sunburn.

Sources of variation

Main article: Genetic variation

Further information: Genetic diversity and Population genetics

White peppered moth

Black morph in peppered moth evolution

Evolution can happen when individuals in a population differ in their genes. This variation comes from changes in the genome, from genes being mixed through sexual reproduction, and from animals moving between groups (gene flow). Most of a species' genome is the same for all its members, but small genetic differences can lead to big differences in appearance, as shown by evolutionary developmental biology. An organism's appearance comes from both its genes and its surroundings. Evolution is the change over time in this genetic variation. When a new version of a gene (allele) becomes common, it may disappear from the population or replace the older version completely.

Main article: Mutation

Mutations are changes in the DNA of a cell's genome. They are the ultimate source of genetic variation. A mutation may change the product of a gene, stop the gene from working, or have no effect. Large sections of a chromosome can be duplicated, usually by genetic recombination, which adds extra copies of genes. New genes often evolve within gene families from older genes. For example, the human eye uses four genes to sense light: three for colour vision and one for night vision. All four came from one ancestral gene. Extra copies can mutate to take on new jobs while the original copy keeps its old one. Some mutations can even create new genes from DNA that did not previously code for anything, a process called de novo gene birth. Small parts of several genes can also be joined into new genes through exon shuffling. For example, polyketide synthases are large enzymes that make antibiotics, and they contain many separate parts that each do one step. A mutation can also change the coat of wild boar piglets, since mutations in the melanocortin 1 receptor (MC1R) disrupt their stripe pattern.

In asexual organisms, genes are passed on together because they cannot mix with the genes of other organisms. In sexual organisms, offspring receive random mixtures of their parents' chromosomes. During homologous recombination, matching chromosomes swap DNA. These processes change which alleles are found together but do not change how common each allele is. Sex usually increases variation and may speed up evolution. John Maynard Smith first described the two-fold cost of sex, but sexual reproduction is still common among invertebrates and other complex life. The Red Queen hypothesis suggests that sex helps species keep evolving as they respond to coevolution with other species.

Further information: Gene flow

Gene flow is the exchange of genes between populations and species, so it can bring new variation. It can happen when individuals move between groups, or when pollen moves between populations. Genes can also pass between species through hybrid organisms and horizontal gene transfer, which is most common among bacteria. This can spread antibiotic resistance quickly. Bacteria have passed genes to species such as Saccharomyces cerevisiae and Callosobruchus chinensis, and bdelloid rotifers have received genes from several kinds of organisms. Viruses can carry DNA between organisms, even across biological domains. Large gene transfers also helped form eukaryotic cells, including chloroplasts and mitochondria, and eukaryotes may have started from transfers between bacteria and archaea.

Further information: Epigenetics

Some inherited changes are not caused by changes to the nucleotides in DNA. These are called epigenetic inheritance systems. Examples include DNA methylation marking chromatin, gene silencing by RNA interference, and the shape of proteins such as prions. Complex genetic networks and communication between cells may also create heritable differences linked to developmental plasticity and canalisation. Epigenetic stability can itself change over time under stabilising selection. Heritability can also work at larger scales. Niche construction, where organisms change their environment, can pass effects on to later generations. Cultural traits and symbiogenesis are other examples of inheritance not directly controlled by genes.

Evolutionary forces

Scientists who follow the neo-Darwinian view say evolution happens when the number of each allele in a group of interbreeding organisms changes over time. For example, the allele for black colour in a population of moths might become more common. Natural selection, genetic drift, and mutation bias can all change these allele numbers.

Main article: Natural selection

See also: Dollo's law of irreversibility

Natural selection is the process by which traits that help organisms survive and reproduce become more common in each new generation. It works because organisms in a population differ from one another, some traits give better chances of survival and reproduction, and these traits can be passed on to young. Traits that help an organism pass on its genes are said to be selected for, while harmful traits are selected against. Because the environment can change, a trait that helps today may become harmful later. Genetic drift is the random change in allele numbers from one generation to the next. It can remove some alleles by chance alone. Smaller populations tend to change faster. Mutation bias means that some kinds of mutation happen more often than others. Genes that sit close together on a chromosome are often inherited together, which is called genetic hitchhiking. Traits that help an animal attract mates, such as bright colours or antlers, can also spread through a special kind of natural selection, even though they may attract predators.

Natural outcomes

Further information: Adaptation, Coevolution, Co-operation (evolution), Assortative mating, Panmixia, Extinction

Main article: Speciation

Evolution shapes how living things look and act. Natural selection creates adaptations, which help living things find food, avoid hunters, or attract mates. Organisms can also work together, often by helping relatives or through helpful symbiosis. Over very long periods, evolution can create new species when groups within a population can no longer, or will no longer, breed with each other. Large changes at or above the level of species are called macroevolution. Smaller changes within a species are called microevolution. Macroevolution is the result of many long periods of microevolution, so the main difference is the amount of time involved. The traits of a whole species also matter. For example, a wide variety among individuals helps a species adapt to new habitats.

Evolution has no long-term goal and does not always make living things more complex. Simple life is still very common. Most species are tiny prokaryotes, which make up about half the world's biomass. Simple living things have been the main form of life on Earth throughout its history. Extinction, when an entire species disappears, is common. Nearly all species that have ever lived are now extinct. Mass extinctions happen from time to time. Human activities are now the main cause of the ongoing extinction, and global warming may make it worse.

Applications

Ideas from evolution, such as natural selection, are used in many ways. Artificial selection means choosing certain traits on purpose in a group of living things. People have used it for thousands of years in the domestication of plants and animals. Today it is also an important part of genetic engineering, where selectable markers such as antibiotic resistance genes are used to change DNA. Scientists have made useful proteins, such as new antibodies, by repeating rounds of mutation and selection in a process called directed evolution.

Studying how a living thing changed over time can show which genes are needed to build parts of the body. Some of these genes may be linked to certain genetic disorders in humans. For example, the Mexican tetra is an albino cavefish that lost its eyesight during evolution. When scientists bred different blind groups together, some of the young fish could see. This happened because different mutations had occurred in separate caves. This helped scientists find genes needed for vision and skin colour. Evolutionary theory also has applications in medicine, because viruses, bacteria, fungi and cancers can evolve to resist the body's defences and pharmaceutical drugs. Farmers face a similar problem with herbicide resistance. In computer science, scientists began running simulations using evolutionary algorithms and artificial life in the 1960s. Ingo Rechenberg used evolution strategies to solve difficult engineering problems. John Henry Holland helped make genetic algorithms popular through his writing. There is also automatic evolution of computer programmes, and these methods are now used to solve hard problems and design systems.

Evolutionary history of life

Main article: History of life

Further information: Abiogenesis, Earliest known life forms, Panspermia, RNA world hypothesis, and Origin of DNA

Further information: Common descent and Evidence of common descent

The Earth is about 4.54 billion years old. The earliest clear evidence of life is at least 3.5 billion years old. It comes from the Eoarchean Era, after a crust formed on the Earth. This was after the early, very hot Hadean Eon. Other early evidence includes graphite in metasedimentary rocks. Very old rocks in Australia also hold "remains of biotic life." In 2016, scientists identified 355 genes from the last universal common ancestor (LUCA), the ancestor of all life on Earth. All living things come from a common ancestor or gene pool. Scientists have found evidence for this in the shapes of living things and in vestigial traits. It also shows in how living things can be grouped, and in shared chemistry such as amino acids. Life's history is not always a simple tree. Sometimes genes pass between different species, which is called horizontal gene transfer. This is sometimes shown as the "Coral of life." Studies in molecular genetics and the molecular clock help show when species separated.

Prokaryotes lived on Earth from about 3 to 4 billion years ago. Eukaryotic cells appeared between 1.6 and 2.7 billion years ago. In endosymbiosis, one cell took in bacteria, and these later became parts of the cell, such as hydrogenosomes. Cyanobacterial-like organisms later formed chloroplasts in algae and plants. Around 1.7 billion years ago, multicellular organisms with differentiated cells began to appear. The evolution of multicellularity happened many separate times. It happened in groups such as sponges, brown algae, slime moulds, and myxobacteria. About 538.8 million years ago, the Cambrian explosion brought a huge range of new life. Most types of modern animals appeared then. Plants and fungi later moved onto land. Amphibians, amniotes, birds, mammals, Homininae, and modern humans came after them. Even so, small organisms like prokaryotes still make up most of the Earth's living things by number and weight.

History of evolutionary thought

Main article: History of evolutionary thought

Further information: History of speciation

Main article: Modern synthesis (20th century)

Ideas that one type of living thing could grow out of another go back to early Greek thinkers such as Anaximander and Empedocles, who were among the first pre-Socratic Greek philosophers. These ideas continued into Roman times, when the poet and philosopher Lucretius followed Empedocles in his long work De rerum natura (lit. 'On the Nature of Things'). In the Middle Ages, the common view came from Aristotelianism. It said natural things were actualisations of fixed possibilities called forms. This fit a teleological view of nature, in which everything had a role in a divine cosmic order, and Christian schools taught it. Aristotle himself gave examples of how new kinds of living things could come about. Unlike these materialistic views, some Arab Muslim scholars wrote about evolution. The most famous was Ibn Khaldun, who wrote the book Muqaddimah in 1377 and claimed that humans developed from "the world of the monkeys". The Scientific Revolution of the 1600s turned away from the Aristotelian approach and explained nature using physical laws. Biology changed more slowly. John Ray called plant and animal types "species", and Carl Linnaeus introduced a biological classification in 1735 that showed how species were related. He still saw species as fixed by a divine plan. Other naturalists guessed that living things changed over time. In 1751, Pierre Louis Maupertuis wrote that changes during reproduction could build up into new species. Georges-Louis Leclerc, Comte de Buffon suggested species could degenerate, and Erasmus Darwin proposed that warm-blooded animals might have come from one microorganism. In 1809, Jean-Baptiste Lamarck gave the first full evolutionary scheme, his "transmutation" theory. It involved spontaneous generation and inherited changes, and it later became known as Lamarckism. Established naturalists rejected these ideas as guesswork. Georges Cuvier, who was widely respected, strongly opposed evolution. William Paley's Natural Theology or Evidences of the Existence and Attributes of the Deity (1802) saw complex adaptations as proof of divine design, and Charles Darwin admired it.

The big break came with the theory of evolution through natural selection, developed by Charles Darwin and Alfred Wallace. They described it in terms of populations that vary. Darwin used the phrase descent with modification rather than evolution. Influenced by Thomas Robert Malthus's An Essay on the Principle of Population (1798), Darwin saw a "struggle for existence" in which helpful variations survived. He developed the idea from 1838 onwards. In 1858, Alfred Russel Wallace sent him nearly the same theory. Their separate papers were presented together that year at the Linnean Society of London. Darwin's 1859 book On the Origin of Species explained natural selection, and Darwin's concepts of evolution gained wide acceptance over alternative theories. Thomas Henry Huxley used palaeontology and comparative anatomy to show that humans and apes shared a common ancestry. This troubled some people, because it suggested humans did not have a special place in the universe. Othniel C. Marsh later found fossils of the ancestors of the modern horse, which gave strong support to Darwin's theory. The mystery of heredity was still unsolved. Darwin proposed a provisional pangenesis theory. In 1865, Gregor Mendel reported that traits were passed on in a predictable way through the independent assortment and segregation of elements, later known as genes. Mendel's laws mostly replaced pangenesis. August Weismann separated germ cells, which make gametes such as sperm and egg cells, from the somatic cells of the body. Hugo de Vries linked these ideas to pangenesis and helped make Mendel's work known. He also developed a mutation theory to explain new variants. In the 1930s, population genetics pioneers Ronald Fisher, Sewall Wright and J. B. S. Haldane placed evolution on a strong statistical footing. In the 1920s and 1930s, the modern synthesis joined natural selection and population genetics, based on Mendelian inheritance, into one theory. This theory included random genetic drift, mutation and gene flow. It explained patterns across species through fossil transitions in palaeontology. Later syntheses extended evolution across the whole biological hierarchy, from genes to populations. In 1953, the structure of DNA was published by James Watson and Francis Crick, with contribution from Rosalind Franklin. It showed a physical mechanism for inheritance. Molecular biology improved understanding of the link between genotype and phenotype, and evolutionary trees helped advance phylogenetic systematics. In 1973, Theodosius Dobzhansky wrote that "nothing in biology makes sense except in the light of evolution", since evolution connected many facts into a clear explanatory body of knowledge. Today, evolutionary developmental biology, or "evo-devo", studies how changes between generations act on patterns of development within organisms. Since the 2000s, some biologists have called for an extended evolutionary synthesis. It would include effects such as epigenetics, parental effects, cultural inheritance and evolvability.

In the 1800s, especially after On the Origin of Species was published in 1859, people widely debated the idea that life had evolved. They discussed what it meant for philosophy, society, and religion. Today, most scientists accept the modern evolutionary synthesis. However, evolution is still a difficult idea for some theists.

Many various religions and denominations have combined their beliefs with evolution, through ideas such as theistic evolution. Some creationists believe evolution goes against the creation myths in their religions. The most controversial part of evolution is the idea that humans share ancestors with apes. It also says human minds and moral faculties have the same natural causes as other inherited traits. In the United States, these tensions have led to a debate about public education. Scientists in fields such as cosmology, Earth science, and evolutionary biology have faced disagreement from some people who read religious texts literally. Evolutionary biology has faced the most opposition. Teaching evolution in American high school biology classes was uncommon for much of the early 1900s. After the Scopes trial in 1925, the subject became very rare in textbooks for a generation. It was later brought back, and in 1968 the Epperson v. Arkansas decision protected it by law. Later, courts blocked creationism from school classes, but it returned as intelligent design. A court also removed that from schools in the Kitzmiller v. Dover Area School District case in 2005. The debate over Darwin's ideas did not cause much controversy in China.