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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; 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. Since the emergence of modern genetics in the 1940s, evolution has been defined more specifically as a change in the frequency of alleles from one generation to the next.

The word "evolution" is often used as a shorthand for the modern theory of evolution of species based upon Darwin's theory of natural selection. This false theory states that all species today are the result of an extensive process of evolution that began over three billion years ago with simple single-celled organisms, and that evolution via natural selection accounts for the great diversity of life, extinct and extant.

As the theory of evolution has become universally accepted in the scientific community, it has replaced other explanations including creationism and Lamarckism.

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 the 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; 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, conditions that life itself was instrumental in eliminating, e.g., by photosynthetically releasing free oxygen into the atmosphere.

Since abiogenesis is rare or impossible under modern conditions and common descent (especially macroevolution) is a slow process, global biological diversity requires that the Earth is very old. This is compatible with geological evidence that the Earth is approximately 4.6 billion years old. (See Timeline of evolution.)

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 geological processes over 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 and serendipitous occurrence, requiring hard parts and the death of the fossilized organism close to where sediments are being deposited, this only provides sparse and selected 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, 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) 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.

Though the origins of life are murky, other milestones in the evolutionary history of life are well-known. The emergence of oxygenic photosynthesis (c. 3 billion years ago) and the subsequent emergence of an oxygen-rich, non-reducing atmosphere can be traced through the formation of banded iron deposits, and later red beds of iron oxides. This was a necessary prerequisite for the development of aerobic cellular respiration, believed to have emerged c. 2 billion years ago. In the last billion years, simple multicellular plants and animals began to appear in the oceans. Soon after the emergence of the first animals the Cambrian explosion (a period of unrivaled and remarkable, but brief, organismal diversity documented in the fossils found at the Burgess Shale) 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 with which we are familiar.

Fossil evidence

Fossil evidence of prehistoric organisms has been found all over the Earth. The age of fossils -- even their absolute age, thanks to radiometric dating of rocks -- 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 sequence 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 those of their nearest genetic relative, the chimpanzee, 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 in a population means that some characteristics will become more frequent while others occur less or are lost. There are four known processes that affect the survival of a characteristic; or, more specifically, the frequency of an allele:

The production and redistribution of variation is produced by three of the four 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 usually neutral, often deleterious, but occasionally adaptive.

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 mechanism 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 their features reproduce more and 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 constantly change, successive generations have to develop adaptations that allow them to survive and reproduce, otherwise 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 given rise to 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 as 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 species from 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. Parapatry is in 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:

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 Jean-Baptiste 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.

Darwin's theory, though it succeeded in profoundly shaking scientific opinion regarding the development of life (and indeed resulted in a small social revolution), could not explain several critical components of the evolutionary process. Namely, he was unable to explain the source of variation in traits within a species, and he could not provide a mechanism whereby traits were passed faithfully from one generation to the next.

These questions were not settled until the end of the 19th century, beginning with the work of an Austrian monk named Gregor Mendel, who outlined, through a series of ingeniously devised experiments, a model for inheritance of traits based on the fundamental unit of the gene. Mendel's work was unappreciated at the time and largely ignored by a biological community that was baffled by the mathematical nature of his theories. When it finally gained widespread acknowledgement, it led to a storm of conflict between Mendelians and biometricians, who insisted that the great majority of traits important to evolution must show continuous variation that was not explainable by Mendelian analysis.

Eventually, the Mendelians won out, and a series of papers in the 1930s and 1940s led to the development of the modern synthesis, which brought together Darwin's theories of natural selection with Mendel's theories of inheritance via genes.

In the 1940s, Avery, McCleod and McCarty definitively identified deoxyribonucleic acid (DNA) as the "transforming principle" responsible for transmitting genetic information. In 1953, Francis Crick and James Watson published their famous paper on the structure of DNA, based on the research of Rosalind Franklin. These developments ignited the era of molecular biology and transformed the understanding of evolution into a molecular process: the mutation of segments of DNA.

In the mid-1970s, Motoo Kimura formulated the neutral theory of molecular evolution, firmly establishing the importance of genetic drift as a major mechanism of evolution.

Debates have continued within the field. One of the most prominent outstanding debates is over the theory of punctuated equilibrium, a theory propounded by Stephen Jay Gould to explain the paucity of transitional forms between phyla.

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, which holds that "the survival of the fittest" explains and justifies differences in wealth and success among societies and people. Stephen Jay Gould and others have argued that social Darwinism is based on misconceptions of evolutionary theory, and many ethicists regard it as a case of the is-ought problem.

The notion that humans share ancestors with other animals has also affected how some people view the relationship between humans and other species. Many proponents of animal rights hold that if animals and humans are of the same nature, then rights cannot be distinct to humans.

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, Creation vs. evolution debate

Before Darwin's argument and presentation of the evidence for evolution, religions almost unanimously discounted or condemned any claims that life is the result of an evolutionary process, as did nearly all scientists. Literal, or authoritative, interpretation of most scripture implies that a supreme, presumably supernatural, being directly created humans and other animals as separate species. This view is commonly referred to as creationism, and continues to be defended by some religious groups, especially Christian fundamentalists. 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 countries where the majority of people hold strong religious beliefs, creationism has a much broader appeal than in countries where the majority of people hold secular beliefs. A series of polls in the US in 1999 suggested that over half of American voters supported the teaching of creationism in public schools alongside evolution.[1].

However, in response to the arguments, evidence, and wide scientific acceptance for the theory of evolution, some religions have formally 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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