Evolution is the change in groups of living things over time. Living things (organisms) have children (offspring) which differ from their parents in minor random ways. Many of these differences, called traits, can be passed down to future generations of offspring during reproduction. Evolution is the process of these inheritable differences becoming more common or rare within large groups (populations) of organisms.

Evolution occurs in two different ways. The first way is random — when a population's traits change by chance. The second way is called selection. Selection happens when a trait helps an organism to have more offspring, such as by keeping the organism from dying young. This helpful trait will tend to become more common in the population, because organisms with the trait produce more offspring — who may inherit the same trait.^[1]

Selection and random change can cause more and more differences to accumulate in a population, eventually resulting in new species.^[2] Every living thing is distantly related — every organism is part of an enormous family tree. This means that all differences between species have arisen through this gradual process of change, as different populations have evolved in different ways.^[3]

What evolution is not

In clearly defining evolution, it is helpful to also clarify what evolution is not.

Evolution is not a theory, as defined in biology. Evolution is an observed process and natural phenomenon in the world, akin to gravity or aging. As such, it is a scientific fact. The word evolution is also sometimes used as shorthand for "theory of evolution", a well-supported scientific theory which describes and explains how the observed process of evolution occurs.^[4][5] To avoid ambiguity, the term evolution will here signify the observed fact, and not the broader theory.

Evolution is not progress. Evolution is not "improvement" — it is simply change. These changes can be positive, negative, or neutral, depending on the situation. Evolution may seem progressive at times, because beneficial traits tend to out-compete less helpful traits under selection. However, evolution does not aspire toward any goal; there is no such thing as 'backward evolution' or 'de-evolution' because there is also no 'forward evolution' — evolution does not move in any particular direction. Even natural selection is not progress, since a trait that is helpful in one environment may be harmful after the environment changes.

Evolution is not sudden. Evolution is occurring all the time, as an extremely gradual, incremental process. Evolution usually requires millions of years to cause obvious or dramatic changes, and any major evolutionary change requires thousands of intermediary transitional forms. However, an organism can only be called "transitional" in retrospect, looking back over countless millennia; evolution does not make long-term plans, and is only guided by short-term, opportunistic selection on an individual level.^[6]

Evolution, then, is a directly observed occurrence. Although selection can result in adaptations which help organisms flourish in their environment, most evolution is not particularly helpful. And although evolution can eventually result in dramatic changes, in the short term it is a meandering process of minor modifications.

How does evolution work?

Evolution is the process of organisms' inherited traits changing from generation to generation. As such, evolution follows from two simple facts:

1. Variation: Organisms in a population are different from each other.
2. Heredity: Some of these differences are passed on to the next generation.

This means that populations can change — and that they do change, whenever a different trait increases or decreases in commonness. This change is evolution. Traits can even vanish entirely, and new traits can appear.

Variation

There are two ways that organisms can be different from each other: genetically and environmentally. These are equivalent to "nature" and "nurture", respectively. If an organism's environment changes, it will develop differently. However, on its own this change is not evolution: Dogs will grow bigger if they are fed more, but this size increase will not be passed on to offspring. The "nature" part of organisms — what they are born with — is what can evolve.

This type of variation, genetic diversity, is directly passed down to offspring during reproduction. Although this diversity is encoded in a tiny molecule called DNA stored in each cell of an organism, it has a major impact on the visible traits of an organism. The chemical properties of DNA cause it to build other molecules which make up the organism's body. Because every organism in a population has slightly different DNA, every organism also has a slightly different body.

Heredity

When parents produce offspring during reproduction, the offspring will be very similar to the parents. This is because parents create copies of their DNA during reproduction. These copies are modeled after the parent's DNA, so the newly-made DNA molecule will build a nearly identical new organism.

However, evolution can only occur because offspring are not perfect clones of their parents: Parents and children are different. If the parent's DNA and the offspring's DNA were always exactly alike, it would be impossible for populations to change — and such change is evolution.

There are several reasons why DNA is not perfectly copied, and small differences can arise. DNA is very complex, and even tiny errors in copying (mutations) can lead to differences in the new organism. However, the source of most genetic diversity in plants and animals is sexual reproduction. Through this process, multiple DNA molecules are combined in a random fashion, resulting in a "deliberately" unique new organism. Female and male sex cells each have their different DNA strands split in half during meiosis, and one half from each is combined to form a brand new DNA molecule. As a result, sexual species have much more diverse populations than asexual species do. This in turn makes it easier for sexual species to evolve quickly.

Fitness

If all the different traits which DNA could produce were equally beneficial, evolution would be a strictly random process. However, since some traits are helpful and some are harmful, this is not the case.

Fitness is the ability of an organism to reproduce. It has little to do with physical fitness, since, depending on the environment, a big and strong organism may be worse at reproducing than a small and stealthy organism. A trait can make an organism more fit by increasing how many viable offspring it has, by improving how well it cares for these offspring, or by simply ensuring that the organism survives long enough to reproduce.

A trait can only be "fit" or "unfit" within the context of a certain environment. If the environment changes, or if the organism migrates to a new environment or ecological niche, the fitness (i.e., the usefulness) of many traits is likely to change. This is because different survival strategies will be effective in different environments.

Selection

Selection is the phenomenon of fit traits becoming more common, and unfit traits becoming less common. When some traits are helpful and other traits are unhelpful for producing new organisms (fitness), such traits become common or uncommon within the population (selection).

Selection is simply the consequence of organisms having some traits that affect how many viable offspring they have. Because of this, the number of offspring with such traits will change in each new generation, as traits that are helpful for survival and reproduction are copied through more offspring, and traits that are harmful are copied less. When a trait becomes so common that every organism in a population consistently has it, the trait has reached fixation.

Selection can occur in many different ways. When humans select for certain traits in other species, it is called artificial selection. Otherwise, it is called natural selection. In sexual organisms, one form of natural selection is sexual selection. In sexual selection, a trait is favored and becomes more common because it makes the organisms which have it more desirable to the opposite sex, allowing such organisms to reproduce more. In such cases, traits can be fit even if they would otherwise slightly hinder an organism's survival, such as the peacock's attractive but cumbersome tail. This is because merely being able to survive is useless if an individual cannot attract a mate and reproduce.^[7]

The vast majority of evolutionary changes are random, not selected. This is because most inheritable variations are neutral, neither helping nor hindering survival and reproductive success. Most such variations have no effect on the organism and are only detectable at the molecular level. Although these variations have little short-term effect, they can randomly increase or decrease in commonness, resulting in evolution.

What does evolution result in?

Adaptation

The result of a trait being selected for many generations is adaptation. Populations adapt whenever they become more fit, especially when they do so in response to an environmental change.

Because traits that help organisms survive in their environment become more common over time via selection, organisms tend to evolve so as to take maximum advantage of their ecological niche. Organisms can evolve to become more specialized, in which case they put all their resources into being very good at just a few things. This is a very efficient strategy, but it is not very adaptable, which leaves it vulnerable to any ecosystem disruptions, such as new predators or drought. Generalist species such as omnivores tend to be more successful in new or unstable environments, because they can switch between multiple survival strategies. However, the price for this generalization is a lack of exceptional ability in any particular area.

Organisms can become either more or less complex when they adapt.^[8][9][10] For example, when species migrate to islands, they often evolve to lose many of their former defenses against predators. This is because, when there are no major risks to a population's health, selection favors traits for spending as much energy as possible on reproduction rather than self-preservation. Because evolution only acts on each generation at a time, and never acts with foresight or long-term planning, populations can adapt to short-term environmental changes in ways that harm them in the long run. The dodo migrated to a predator-free island and lost its wings, which were no longer necessary. However, this adaptation proved deadly when humans introduced predators to the island, causing dodos to go extinct before they had time to adapt once again.

Although adaptation is usually an extremely slow process, it can be observed in the evolution of antibiotic and pesticide resistances. When humans seek to exterminate a pest or disease species, they are unintentionally selecting for resistance to the method of extermination. This makes each new generation less and less susceptible to that method, as only resistant individuals survive to reproduce.

Competition and cooperation

The environment of an organism does not only consist of the local climate and terrain. Other organisms also make up part of the environment. Every species in an ecosystem is a part of every other species' environment, and vice versa. As a result, organisms influence each others' evolution: If one species happens to change, this will alter the living environment of every species in the area. Because the environment has changed, different traits may be selected for, causing other species to change as well. As a result, species continually evolve and adapt to each other, even when their nonliving environment remains stable.^[11]

Individuals within a species also influence each others' fitness. Organisms of the same species form groups (populations) in which both competition and cooperation take place. Because organisms in the same population are very similar, there is often a fierce competition for the same limited pool of resources. However, this similarity also means that particularly closely related individuals, especially close family members, will be likely to share much of their DNA. In such cases, selection can favor DNA which codes for the trait of altruism, or helping other organisms without benefiting oneself in the process. This is because DNA does not "care" which body it gets passed down in; if siblings are likely to have copies of the same piece of DNA, then that piece of DNA will be more fit if it helps the siblings reproduce a lot than if it helps its own organism reproduce a little bit.

Organisms often cooperate by forming colonies, groups that live together for mutual benefit. The more similar individuals within a colony are, the more likely they are to make sacrifices for each other. Altruistic traits in clone colonies and ant societies, for example, can have high fitness because the DNA which codes such traits will be nearly identical in other members of the colony.

The most extreme example of this is a multicellular organism. Billions of years ago, each organism had only one cell. However, groups of cells that formed complex colonies had high fitness, so much so that the cells eventually lost the ability to survive as separate organisms. Just as worker ants in an ant colony are sterile, and live to help the queen survive rather than to reproduce themselves, so do body cells in most multicellular organisms survive only to help that organism's sex cells reproduce. The DNA that codes for making new worker ants is passed down by the queen, just as the DNA that codes for making new body cells is passed down by the sex cells.

Competition and cooperation also commonly take place between organisms of different species. When different species compete, small improvements in one competitor's fitness will increase the selective pressure on the other species. This can result in an evolutionary arms race in which each species' evolution forces the other species to continue evolving. Different species also cooperate; when a random difference arises in one species which causes it to help both itself and another species, the other species will often evolve to further encourage the mutually beneficial behavior. Thus, flowers and bees have both evolved to cooperate in a mutualism in which the flowers feed the bees, and in exchange the bees help the flowers reproduce. Such co-evolution does not imply that flowers and bees ever chose or planned to cooperate. Rather, small DNA changes across populations result in cooperative traits which, being useful for reproduction, have slightly higher chances of being passed on to the next generation. Over time, small successive changes result in the complex relationships seen in ecosystems today.

New species

A species is a set of very similar organisms. In sexual organisms, a species is all the organisms that can reproduce with each other to produce viable offspring. Different populations, even if they do not look exactly alike, are the same species if they are still similar enough to possibly interbreed and make fertile offspring.^[12]

Populations frequently split up, forming two new populations. If the two populations are separated for a long time, they will lose the ability to reproduce with each other, becoming separate species.^[13] Moreover, if the two populations live in different environments, they will also adapt in distinct ways, thus evolving to become very different from each other.^[14][15]

All populations are constantly evolving in minor random ways. This is why separated populations eventually lose the ability to interbreed.^[14] The DNA of each population mutates in different ways, which makes it more difficult for the DNA of individuals from different populations to properly link up during reproduction. This makes the individuals' offspring less able to reproduce, and eventually makes it impossible for the individuals to have offspring together at all. Horses and donkeys, for example, were once a single species. However, they are considered separate species now, because even though they are still closely related enough to produce offspring, these offspring are infertile mules.

Because evolution is slow and gradual, new species also arise gradually. This is why it is rare to directly observe a new species developing.^{[16][17][18][19]} This also means that when a species is in the middle of splitting into two separate species, it can be very difficult to say where one species ends and another begins. A population of Ensatina salamanders expanded around California's Central Valley, forming 19 different populations in a circle. Every population is able to successfully reproduce with its neighbors on either side, because these populations remain so similar. However, when the 19th population completed the species "ring" and met the 1st population, these two neighbors were not able to reproduce together. Even though every population was only slightly different from its neighbor, by the time the ring was complete, the last population had accumulated too many differences to be able to breed with the first. This should qualify them as separate species, but it is impossible to say where the new species begins, because the variation between the populations is continuous, and not broken into two obvious pieces. Species can only be clearly defined after populations have been separated long enough to no longer have any connecting populations.

How has life evolved in the past?

When organisms first arose from self-replicating molecules around 4 billion years ago, they were extremely simple and had very little diversity. Evolutionary theory does not describe the origin of life, but it does explain how life has branched out into many different complex groups since then. Random mutations caused separated populations to become distinct species. Competition for the resources needed to reproduce then selectively favored the organisms which developed new adaptations, becoming more complex. As a result of this evolutionary arms race, even the simplest bacteria alive today are vastly more complex than the first organisms.

Common descent

Common descent is the idea that all living things are related as distant family members. Every organism has distant ancestors in common. This explains why all living things use many of the same arbitrary molecules, when other molecules could have worked equally well. While evolution explains the differences between organisms, common descent explains the similarities.

Taxonomy is the branch of biology that names and classifies all living things. Scientists use morphological and genetic similarities to assist them in categorizing life forms based on ancestral relationships. For example, orangutans, gorillas, chimpanzees, and humans all belong to the same taxonomic grouping referred to as a family – in this case the family called Hominidae. These animals are grouped together because of similarities that come from common ancestry (called homology).^[20]

How is ancient evolution studied?

Scientific evidence for evolution comes from many aspects of biology, and includes fossils, homologous structures, and molecular similarities between species' DNA.

Research in the field of paleontology, the study of fossils, supports the idea that all living organisms are related. Fossils provide evidence that accumulated changes in organisms over long periods of time have led to the diverse forms of life we see today. A fossil itself reveals the organism's structure and the relationships between present and extinct species, allowing paleontologists to construct a family tree for all of the life forms on earth.^[21]

The comparison of similarities between organisms of their form or appearance of parts, called their morphology, has long been a way to classify life into closely related groups. This can be done by comparing the structure of adult organisms in different species or by comparing the patterns of how cells grow, divide and even migrate during an organism's development.

Evolutionary biology

Darwin and Mendel

Evolutionary biology became accepted as scientifically valid following the 1859 publication of Charles Darwin's On the Origin of Species in which he set out his evidence for evolution and his theory of natural selection, which had also been independently discovered by Alfred Russell Wallace. Evolutionary common descent was accepted in science, but there were doubts about natural selection and Darwin was unable to explain the inheritance of variations. Investigations into hereditary patterns led to the rediscovery around 1900 of Gregor Mendel's work with plants first published in 1865, and the new genetics helped to explain the mechanisms of inheritance. Around the 1940s the development of the modern evolutionary synthesis integrated Mendelian inheritance with Darwin's natural selection as the basis of modern evolutionary theory.^[22][23] Further discoveries on how genes mutate, together with advances in population genetics, have explained more details of how evolution occurs. Scientists now have a good understanding of the origin of new species (speciation), and they have observed the speciation process happening both in the laboratory and in the wild. This modern view of evolution is the principal theory that scientists use to understand life.

Genetics

The missing information needed to help explain how new features could pass from a parent to its offspring was provided by the pioneering genetics work of Gregor Mendel. Mendel’s experiments with several generations of pea plants demonstrated that inheritance works by separating and reshuffling hereditary information during the formation of sex cells and recombining that information during fertilization. This is like mixing different hands of cards, with an organism getting a random mix of half of the cards from one parent, and half of the cards from the other. Mendel called the information factors; however, they later became known as genes. Genes are the basic units of heredity in living organisms. They contain the information that directs the physical development and behavior of organisms.

Genes are made of DNA, a long molecule that carries information. This information is encoded in the sequence of nucleotides in the DNA, just as the sequence of the letters in words carries information on a page. The genes are like short instructions built up of the "letters" of the DNA alphabet. Put together, the entire set of these genes gives enough information to serve as an "instruction manual" of how to build and run an organism. The instructions spelled out by this DNA alphabet can be changed, however, by mutations, and this may alter the instructions carried within the genes. Within the cell, the genes are carried in chromosomes, which are packages for carrying the DNA, with the genes arranged along them like beads on a string. It is the reshuffling of the chromosomes that results in unique combinations of genes in offspring.

Every living organism (with the possible exception of RNA viruses) contains molecules of DNA, which carries genetic information. Genes are the pieces of DNA that carry this information, and they influence the properties of an organism. Genes determine an individual's general appearance and to some extent their behavior. If two organisms are closely related, their DNA will be very similar.^[11] On the other hand, the more distantly related two organisms are, the more differences they will have. For example, brothers are closely related and have very similar DNA, while cousins share a more distant relationship and have far more differences in their DNA. Similarities in DNA are used to determine the relationships between species in much the same manner as they are used to show relationships between individuals. For example, comparing chimpanzees with gorillas and humans shows that there is as much as a 96 percent similarity between the DNA of humans and chimps. Comparisons of DNA indicate that humans and chimpanzees are more closely related to each other than either species is to gorillas.^[24][25]