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This article is about biological evolution. For other possible meanings, see Evolution (disambiguation).

Evolution generally refers to any process of change over time. However, in the context of the life sciences, evolution is a change in the genetic makeup of a group—a population of interbreeding individuals within a species. Such a population shares a gene pool and members exhibit a degree of genetic relatedness. Since the emergence of modern genetics in the 1940s, evolution was defined more specifically as a change in the frequency of alleles from one generation to the next. Evolution's two-stage process involves, first, the production and redistribution of genetic variation (see Differential survival of traits); and, second, natural selection acting on this variation.

Natural selection (see section below), being so crucial to the modern perspective of evolution, is, therefore, often infused with the word evolution. Hence evolution is used as shorthand for the modern theory of evolution of species based upon Darwin's theory of natural selection. This theory states that all species today are the result of an extensive process of evolution that began several billion years ago with simple single-celled organisms. Thus, evolution via natural selection accounts for the great diversity of life, extinct and extant.

As the theory of evolution has become widely accepted in the scientific community, it has displaced other explanations for the origins and diversity of life, such as spontaneous generation of complex organisms and creationism.

History of life

Main article: Timeline of evolution

Scientists estimate the Earth is approximately 4.6 billion years old. Soon after the crust cooled, single celled life appeared. Within a billion years, oxygenic photosynthesis emerged and radically changed the Earth's atmosphere, providing the conditions necessary for the development of cellular respiration. Over the next two billion years, all of the basic cellular processes developed. At that time viruses probably made their first appearance. In the last billion years, simple multicellular plants and animals appeared in the oceans. Soon after the emergence of the first animals, a period called the Cambrian explosion saw the creation of all the major body plans (phyla) of modern animals. About 500 million years ago, plants and fungi colonized the land, and were soon followed by arthropods and other animals, leading to the development of land ecosystems that we are familiar with.

Information about the early development of life includes input from the fields of geology and planetology. These sciences provide information about the history of the Earth and the changes produced by life. Much information about the early Earth has been destroyed by time. Fossils are important for estimating when various lineages developed. Fossil evidence of life's evolution only exists for relatively recent developments. As fossilization is a rather rare occurrence, this only provides sparse information about the evolution of life.

Since metabolic processes do not leave fossils, research into the evolution of the basic cellular processes is done largely by comparison of existing organisms. Many lineages diverged at different stages of development, so it is possible to determine when certain metabolic processes appeared by comparing the traits of the descendants of a common ancestor. However, not even comparative biology can shed much light on the earliest development of life since all existing organisms share certain traits, including the cellular structure, infection by viruses, and the genetic code. Most scientists interpret this to mean that all existing organisms share a common ancestor, which had already developed the most fundamental cellular processes, but there is no scientific consensus on the relationship of the three domains of life (Archea, Bacteria, Eukaryota), the origins of viruses, or the origin of life. Attempts to shed light on the earliest history of life generally focus on the behavior of macromolecules, particularly RNA, and the behavior of complex systems.

Scientific theory

Currently, the modern synthesis is by scientific consensus the best theory of the evolution of species. This is the synthesis of Darwin's theory of evolution by natural selection and Mendel's theory of the gene made possible by population genetics. This theory conceives of evolution as any change in the frequency of an allele within a gene pool. In modern synthesis, change may be caused by a number of different mechanisms, such as natural selection or genetic drift. The genetic isolation of two populations, which allows their gene pools to diverge, results in speciation.

The commonly accepted scientific theory about how life has changed since it originated has three major aspects:

  1. The common descent of all organisms from (more or less) a single ancestor.
  2. The origin of novel traits in a lineage.
  3. The mechanisms that cause some traits to persist while others perish.

Ancestry of organisms

Main articles: Common descent, Origin of life

A central assumption of evolutionary theory is that life on Earth had a single point of origin; all subsequent life-forms are descendents of this progenitor organism. This is called the theory of common descent.

Evidence for common descent may be found in shared traits between living organisms. For example, all living things make use of nucleic acids as their genetic material, and use the same twenty amino acids as the building blocks for proteins. Furthermore all organisms use the same genetic code (with some extremely rare minor deviations) to translate nucleic acid sequences into proteins. Because the selection of these traits is somewhat arbitrary, their universality strongly suggests common ancestry.

Phylogeny, the study of the ancestry of species, has revealed that biological structures with radically different internal organizations can bear a superficial resemblance and perform similar functions. These examples of analogous structures show that there are many ways to perform the same actions. For example, the eye was evolved independently in radically different ways in organisms such as humans and octopuses. Likewise, other structures with similar internal organisation may perform divergent functions. Vertebrate limbs are a favorite example of such homologous structures. Other vestigial structures may exist without purpose in one organism, though they have a clear purpose in others. The human wisdom teeth and appendix are common examples.

Further evidence of the universal ancestry of life is that abiogenesis has never been observed under controlled conditions, indicating that the origin of life from non-life, is either very rare or only happens under conditions that are not at all like those of modern Earth.

Fossil evidence

Fossil evidence of prehistoric organisms has been found all over the Earth. The age of fossils can often be deduced based upon the geologic context in which they are found. Some fossils bear a resemblance to organisms alive today, while others are radically different. Fossils have been used to determine at what time a lineage developed, and can be used to demonstrate the continuity between two different lineages through transitional forms. Paleontologists investigate evolution largely through analysis of fossils.

Genetic evidence

Comparison of the genetic sequence of organisms reveals that organisms that are phylogenetically close have a higher degree of sequence similarity than organisms that are phylogenetically distant. For example, human genes are more than 99% identical to their nearest genetic relative, chimpanzees, slightly less so for gorillas, and only 80% identical to baboons. Sequence comparison is considered such a robust measure that it is sometimes used to correct mistakes in the phylogenetic tree, in instances where other evidence is scarce.

Further evidence for common descent comes from genetic detritus such as pseudogenes, regions of DNA which are orthologous to a gene in a related organism, but are no longer active and appear to be undergoing a steady process of degeneration.

The emergence of novel traits

Geneticists have studied how traits emerge and how they are passed to succeeding generations. In Darwin's time, there was no widely accepted mechanism for inheritance. Today most inherited variation is traced to discrete, persistent entities called genes. Genes are encoded in linear molecules called DNA. Changes in DNA are commonly called mutations. Furthermore, mutations may have little phenotypic effect in isolation but create new traits when combined in an organism through genetic recombination. Genetic recombination is produced both by the fusion of cells of opposite sexes, and by the transfer of genetic material into an intact cell, which occurs in bacterial conjugation and transformation.

Researchers are also investigating heritable variation that is not connected to variations in DNA sequences that influence standard DNA replication. The processes that produce these variations leave the genetic information intact and are often reversible. This is called epigenetic inheritance and may include phenomena such as DNA methylation, prions, and structural inheritance. Investigations continue into whether these mechanisms allow for the production of specific beneficial heritable variation in response to environmental signals. If this is shown to be the case, then some instances of evolution would lie outside of the framework that Darwin established, which avoided any connection between environmental signals and the production of heritable variation.

In addition to the mechanisms described above, the origin of novel traits may also be attributable to self-organizing properties at the level of the physics and chemistry of the organism (which some hold to be a violation of "strict" Darwinism). Self-organization in this context would refer to traits that were not directly encoded in the genome but rather would always be expected to be present in a wide class of particular biological systems. In this view, as expressed by Stuart Kauffman, natural selection "selects" only particular classes of systems, which happen to include systems which generate such "order for free" (Kauffman also calls this property "anti-chaos"). Several specific mechanisms to enable "order for free" such as the robustness of genetic regulatory networks, the spontaneous self-sustaining order of chemical reactions as autocatalytic sets and the properties of the RNA genotype-to-phenotype map (in this case, the RNA-sequence-to-RNA-shape mapping), have been cautiously incorporated as part of a workable theory as it applies to evolution. However, the entire program as outlined by Kauffman remains a matter for debate.

Differential survival of traits

Differential survival of traits that arise in the population means that some characteristics will become more frequent while others may be lost. Four processes are generally thought to contribute to the survival of a characteristic:

The production and redistribution of variation is produced by the first of these three forces, or agents, of evolution: mutation, genetic drift, and gene flow. Natural selection, in turn, acts on the variation produced by these agents.

Mutation

Main article: Mutation

A mutation occurs when there is a so-called error during duplication or translation of genetic material. Mutation provides natural selection with variation; and is the sole source of new genetic material. Mutations are not always deleterious, having beneficial or neutral effects also.

Genetic drift

Main article: Genetic drift

Genetic drift describes changes in gene frequency that cannot be ascribed to selective pressures, but are due instead to events that are unrelated to inherited traits. This is especially important in small mating populations, which simply cannot have enough offspring to maintain the same gene distribution as the parental generation. Such fluctuations in gene frequency between successive generations may result in some genes disappearing from the population. Two separate populations that begin with the same gene frequency might, therefore, "drift" by random fluctuation into two divergent populations with different gene sets (for example, genes that are present in one have been lost in the other). Rare sporadic events (volcanic explosion, meteor impact, etc.) might contribute to genetic drift by altering the gene frequency outside of "normal" selective pressures.

Unlike natural selection, genetic drift is the random fluctuation of gene frequencies from generation to generation in a small, relatively isolated population. Its chief mechanism of operation is chance within small populations. The term small population is relative, however. Thus, genetic drift occurs when N <= 0.5s, N <= 0.5µ, N <= 0.5m where N is the population numbered in the hundreds, s is the selective value of the allele s, µ is mutation pressure, and m is gene flow.

Gene flow

Main article: Gene flow

Gene flow or gene admixture is the only one of the agents that makes populations closer genetically while building larger gene pools. Migration of one population into another area occupied by a second population can result in genetic admixture. Gene flow operates when geography and culture are not obstacles.

Natural selection

Main article: Natural selection

Natural selection, the last of the four forces, is based on three principles: a) there is variation within a species and this variation is heritable; b) parents have more offspring than can survive; and c) surviving offspring have favorable traits. The mechanisms by which it operates is termed survival of the fitter meaning differential mortality and fertility. Differential mortality is the survival rate of individuals before their reproductive age. If they survive, they are then selected further by differential fertility--that is, their total genetic contribution to the next generation.

Natural selection can be categorised into ecological selection – due to differential survival – and sexual selection – due to selection of mates with desirable characteristics.

  • Ecological selection occurs when organisms that survive and reproduce increase the frequency of their genes in the gene pool over those that do not survive.
  • Sexual selection occurs when organisms that are more attractive to the opposite sex because of features reproduce more and hence increase the frequency of those features in the gene pool.

Natural selection also provides for a mechanism by which life can sustain itself over time. Since environments always change, successive generations have to develop adaptations which allow them to survive and reproduce, or the species will die out as their biological niches disappear. Therefore, life can persist over great spans of time, in the form of evolving species. The central role of natural selection in evolutionary theory has created a strong connection between that field and the study of ecology. The probable mutation effect is the proposition that a gene that is not under selection will be destroyed by accumulated mutations. This is an aspect of genome degradation.

Selection by humans of organisms for desirable characteristics, e.g. for agriculture or pets, is called artificial selection.

Microevolution

Main article: Microevolution

Microevolution refers to small scale changes in gene frequencies in a population over the course of a few generations. These changes may be due to a number of processes: mutation, gene flow, genetic drift, as well as natural selection. Population genetics is the branch of biology that provides the mathematical structure for the study of the process of microevolution.

Macroevolution

Main article: Macroevolution

Macroevolution refers to large-scale changes in gene-frequencies in a population over a long period of time, and is usually taken to refer to events that result in speciation, the evolution of a new species. While microevolution has been demonstrated in the laboratory to the satisfaction of most observers, macroevolution has to be inferred from the fossil record and the traits of extant organisms. Its precise mechanisms are an active topic of discussion amongst scientists.

Speciation

Main article: Speciation

Speciation is the creation of two or more separate species from a single one. There are various mechanisms by which this may take place. Allopatry begins when subpopulations of a species become isolated geographically, for example by habitat fragmentation or migration. Sympatry is when new species emerge in the same geographic area. Another mechanism is known as parapatry, a middle ground between the extremes of allopatry and sympatry.

Extinction

Main article: Extinction

Extinction is the disappearance of species, (i.e. gene pools). The moment of extinction is generally considered to be the death of the last individual of that species. Extinction is not an unusual event in geological time—species are created by speciation, and disappear through extinction.

Evolutionary biology

Main article: Evolutionary biology

The study of evolution and the development of related theory is called evolutionary biology. Notable contributors to evolutionary biology include:

Notable popularizers of evolution whose primary research isn't within evolutionary biology include:

Evolutionary thought

Present status

When talking about biological evolution, there is often confusion about whether modern organisms have evolved and are continuing to evolve from older ancestral organisms. There are also questions about the mechanism of the observed changes.

In popular usage, the theory of evolution refers to any Darwinian theories. Within this framework there are still differences of opinion, for example between punctuated equilibrium and strict gradualism or regarding the relative importance of natural selection and genetic drift. As scientists have uncovered more information about the basic operations of life, theories of evolution have changed. The general trend has been not to overturn well supported theories, but instead supplant them with more detailed and therefore more complex ones.

History of evolutionary thought

Main article: History of evolutionary thought

The idea of biological evolution has existed since ancient times, but the modern theory wasn't established until the 18th and 19th centuries, with scientists such as Lamarck and Charles Darwin. Darwin greatly emphasized the difference between his two main points: establishing the fact of evolution, and proposing the theory of natural selection to explain the mechanism of evolution.

While transmutation of species was accepted by a sizeable number of scientists before 1859, it was the publication of Charles Darwin's The Origin of Species which provided the first cogent mechanism by which evolutionary change could persist: his theory of natural selection. The evolutionary timeline outlines the major steps of evolution on Earth as expounded by this theory's proponents.

Following the dawn of molecular biology, it became clear that a major mechanism for variation within a population is the mutagenesis of DNA. An essential component to evolutionary theory is that during the cell cycle, DNA is copied faithfully. When rare copying errors occur, they are said to introduce genetic mutations of three general consequences relative to the current environment: beneficial, detrimental, or neutral. By definition, individuals with beneficial mutations will be more likely to propagate, individuals with detrimental mutations will have less of a chance at successful reproduction, and those carrying neutral mutations will have neither an advantage nor a disadvantage. These definitions assume that the environment remains stable. Considered at the level of a single gene, these variations just described represent different genetic alleles. Following environmental change, alleles may retain their classification of beneficial, detrimental, or neutral, or may shift into one of the other categories. Individuals carrying alleles formerly classified as neutral may now be beneficial as they bear favourably adaptive mutations. Since neutral alleles can accumulate in the population without consequence while an environment is stable, they create a considerable reservoir for adaptability.

Social effect of evolutionary theory

Main article: Social effect of evolutionary theory

As the scientific explanation of life's diversity has developed, it has displaced the explanations held by a significant portion of humanity. As the theory of evolution includes an explanation of humanity's origins, it has had a profound impact on human societies. Some social conservatives have vigorously opposed acceptance of the scientific explanation due to perceived religious implications.

The theory of evolution by natural selection has also been adopted as a foundation for various ethical systems such as social Darwinism, although scientists emphasize that their work is intended purely as a description of nature. The notion that humans share ancestors with other animals has also impacted how some people view the relationship between humans and other species. The theory has also been incorporated into other fields of knowledge, creating hybrids such as evolutionary psychology and sociobiology.

Evolution and religion

Main articles: Creationism, Evolutionary creationism

Throughout most of history, religions have typically discounted or condemned claims that life is the result of an evolutionary process. Literal, or authoritative, interpretation of scripture demands that a supreme supernatural being directly created humans and other animals as separate species. This view is commonly referred to as creationism. Some of those who reject the scientific theory of evolution have proffered what they believe to be physical proof of the impossibility of macroevolution in particular; this viewpoint does not bar the idea of microevolution.

In the United States creationism has a much broader base than only Christian fundamentalists, who are a minority in the American population. A recent poll showed that over half of American voters supported the teaching of creationism in public schools.[1], and in some areas of the United States, creationists have occasionally convinced state governments to give equal time to their views in the science classroom.

In response to the wide scientific acceptance of the the theory of evolution, some have informally synthesized the scientific and religious viewpoints. They may conclude that God has provided a divine spark to ignite the process of evolution, and possibly guided evolution in one way or another; or that Darwinian evolution is essentially God's default method of creation, perhaps with critical reservations, such as stipulating that human souls are created directly by God. These views fall under the umbrella of evolutionary creationism. The claim that life shows evidence of intelligent design is sometimes presented as supporting these views.

See also

Bibliography

Main article: list of popular science books on evolution

Several popular science books are available and include:

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