The Evolution reference article from the English Wikipedia on 24-Jul-2004
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Evolution

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

Evolution is any process of growth or development that entails change, but most commonly refers to biological evolution. The theory of evolution is the single unifying paradigm of biology — the current consensus among biologists is that modern life is the result of an extensive process of evolution that began several billion years ago with simple single-celled organisms. While the idea of biological evolution has existed for thousands of years, it only developed a scientific foundation in the middle of the 19th century. As the theory of evolution has become more widely accepted, it has displaced other explanations for the origins and diversity of life, such as spontaneous generation of complex organisms and creationism. Often the word evolution is used as a shorthand for the modern theory of evolution of species based upon Darwin's idea of natural selection.

The current dominant theory of evolution is known as the "modern evolutionary synthesis" (or simply "modern synthesis"), referring to the synthesis of Darwin's theory of evolution by natural selection and Mendel's theory of the gene. Within this theory, evolution may be defined as the change in the frequency of an allele within a gene pool, caused by natural selection and/or genetic drift. Such changes over long periods of time lead to major changes in phenotype. According to this theory, the fundamental event of speciation is the genetic isolation of two populations, which allows their gene pools to diverge. Since the modern synthesis, biological evolution has been defined as changes in allele frequencies in a population from one generation to another.

Table of contents
1 History of life
2 Scientific theory
3 Evolutionary biology
4 History of evolutionary thought
5 Social effect of evolutionary theory
6 See also
7 Bibliography
8 External links

History of life

Main article: Timeline of evolution

The Earth is approximately 4.55 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, and viruses had probably appeared by that time. 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 the land ecosystems that we are familiar with.

Our information about the early development of life includes much 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. 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. However, fossils are important for estimating the date that various lineages developed.

Since metabolic processes do not leave fossils, research into the evolution of the basic cellular processes is done largely by comparison of existing organisms, since many lineages diverged at different stages of development of these basic cellular processes. 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

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, with no exceptions, 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. Since there is an element of arbitrariness in these traits, their universality strongly suggests common ancestry.

The study of the ancestry of species (phylogeny) has revealed that biological structures with radically different internal organisations can bear a superficial resemblance and perform similar functions. These examples of analogous structures show that there are many ways to solve most problems (for example, the eye was evolved independently in radically different ways in different organisms). 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 (i.e. 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 relative, chimpanzees, and 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 scant.

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.

Microevolution and macroevolution

Main articles: Microevolution, Macroevolution

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

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, i.e. 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.

The emergence of novel traits

Geneticists have studied how traits emerge and are passed to succeeding generations. In Darwin's time, there was no widely accepted in-depth mechanism for heritability. 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, DNA variants 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 cellss of opposite mating types (such as human sex), and by the transfer of material into an intact cell (such as bacterial conjugation and transformation).

Researchers are also investigating heritable variation that is not connected to variations in DNA sequence that influence standard DNA replication. The processes that produce this variation leave the genetic information intact and are often reversible. These are often referred to as 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 general, Darwin knew little about the nature or source 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 (see the section Neo-structuralist themes in evolutionary theory in the current article). In this view, most cogently 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.

The foregoing potential sources of novel traits are not mutually exclusive, and most biologists would accept that each mechanism discussed has been demonstrated to be a possible way to generate such traits; however each would most likely assign different degrees of importance to each of the different mechanisms.

Differential survival of traits

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

Natural selection

Main article:
Natural selection

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

Natural selection also provides for a mechanism by which life can sustain itself over time. Since, in the long run, environments always change, if successive generations did not develop adaptations which allowed them to survive and reproduce, species would simply die out as their biological niches die out. Therefore, life is allowed to 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.

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 (i.e. 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.

Gene flow

Main article: Gene flow

Genes may flow between subpopulations. This is called gene flow, and – by changing the frequencies of the alleles within those subpopulations – causes evolution.

Macroevolutionary trends

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 which include allopatry, which begins when subpopulations of a species become isolated geographically (for example, by habitat fragmentation or migration) and sympatry, by which new species emerge in the same geographic area. Another mechanism is known as parapatry, a middleground 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 theory is called evolutionary biology. Notable evolution researchers include:

History of evolutionary thought

Present status

When talking about biological evolution, there is often confusion about the question of whether or not modern organisms have evolved (and are continuing to change) from older ancestral organisms and there are questions about the mechanism of the observed changes.

It is a fact that the frequency of traits changes in populations of organisms. Most biologists believe that this process can account for the differences between existing species, but the relative importance of the various mechanisms continues to be debated. The commonly accepted scientific theory today is known as modern synthesis (or the Neo-Darwinian synthesis), based primarily on Charles Darwin's theory of natural selection, but updated with newer discoveries in biology and genetics, in particular Mendelian inheritance. Population genetics is the branch of biology that provides the mathematical structure for the modern synthesis.

In popular usage, "the" theory of evolution refers to this or other Darwinian theories. However, 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.

History 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 inputs: establishing the fact of evolution, and proposing a theory, natural selection, to explain the mechanism of evolution.

As science has uncovered more and more information about the basic operations of life, such as genetics and molecular biology, theories of evolution have changed. The general trend has been not to overturn well-supported theories, but to supplant them with more detailed and therefore more complex ones.

While transmutation 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 mechanism 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 very, but not entirely, faithfully. When these rare copying errors occur, they are said to introduce genetic mutations of three general consequences relative to the current environment: good, bad, or neutral. By definition, individuals with "good" mutations will be more likely to propagate, individuals with "bad" 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 good, bad, or neutral, or may shift into one of the other categories. Individuals carrying alleles formerly classified as neutral may now be "good" 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.

De Chardin's and Huxley's theories

Pierre Teilhard de Chardin and Julian Huxley formulated theories describing the gradual development of the Universe from subatomic particles to human society, considered by Teilhard as the last stage. (see Gaia theory). These are not generally recognized as scientifically rigorous.

Nine levels are described (scheme), the "classical" biological stages being levels 6, 7 & 8 of the universal evolution. Stages 1 to 5 are grouped into the Lithosphere, also called Geosphere or Physiosphere, where (the progress of) the structure of the organisms is ruled by structure, mechanical laws and coincidence. Stages 6 to 8 are grouped into the Biosphere, where (the progress of) the structure of the organisms is ruled by genetical mechanisms. The actual stage, stage 9, is called the Noosphere, where (the progress of) the structure of human society (socialization) is ruled by psychological, informational and communicative processes.

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 persons view the relationship between humans and other species.

The theory of evolution by natural selection has also been incorporated into other fields of knowledge, creating hybrids such as evolutionary psychology and sociobiology.

Evolution and religion

Creationism

Main article: Creationism

Ever since Darwin provided the first cogent mechanism for evolution, religious fundamentalists have claimed that the theory is false. Due to their literal interpretation of scripture, they posit that a supreme supernatural being (God) intentionally and directly created the universe as a whole, and each species in particular. Christians and Jews who reject evolution point to the the Bible, and Muslims additionally point the Quran, to support their views; some have offered what they believe to be physical proof of the impossibility of macroevolution in particular. This view is commonly referred to as "creationism".

In response to world-wide scientific acceptance of the evolutionary process as a fact, more moderate views have emerged where God only provides a divine spark to make evolution happen (evolutionary creationism). Others claim that life shows evidence of intelligent design. These arguments and their presentation as science are not accepted by the mainstream scientific community. In spite of this, creationists in the United States have succeeded in convincing some state governments to give "equal time" to these views in the classroom. (See for example Disclaimer Adopted by Oklahoma Textbook Commission. As of 2004, a very similar notice is being pasted into biology textbooks in Alabama as well. See also Cambrian Explosion for a discussion of issues raised by the notice.)

Evolutionary creationism

Main article: Evolutionary creationism

The defense of creationism notwithstanding, many other religious people have concluded that the theory of evolution is correct, or is apparently so, with some reservations. For example, in 1996 Pope John Paul II stated that "new findings lead us toward the recognition of evolution as more than an hypothesis," but, referring to previous papal writings, concluded that "if the origin of the human body comes through living matter which existed previously, the spiritual soul is created directly by God."

See also

Bibliography

Main article:
list of popular science books on evolution

Several popular science books are available and include (a more comprehensive list is in the main article listed above):

External links

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