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  Wikipedia: Evolution

Wikipedia: Evolution
From Wikipedia, the free encyclopedia.

This article is about evolution in the biological sense. For other article subjects named evolution see evolution (disambiguation).
Evolution is any process of growth, change or development. The word stems from the Latin evolutio meaning "unfolding" and before the late 19th century was confined to referring to goal-directed, pre-programmed processes such as embryological development. A pre-programmed task, as in a military maneuver, using this definition, may be termed an "evolution." One can also speak of stellar evolution, chemical evolution, cultural evolution or the evolution of an idea. Other kinds of evolution include evolutionary algorithms (which include genetic algorithms) which attempt to mimic processes similar to biological evolution in a computer program, most frequently as an optimization technique and as an experimental framework for the computational modelling of evolution.

Modern usage

In the 19th century the word "evolution" was identified with improvement. It was clear to European thinkers at that time -- in the wake of the Enlightenment and the French Revolution -- that human societies evolved; many people have claimed the same about the evolution of biological species. In the 20th century, most social scientists came to reject the strict identification of social and cultural change with improvement (see also social evolution and cultural evolution); Most interpretations of Darwin's account of evolution similarly argue against identifying biological changes with improvement.

Since the 19th century "evolution" is generally used in reference to biological evolution, the development of the different varieties of living things over generations. Often it is shorthand for the modern theory of evolution of species based upon Darwin's idea of natural selection. Since the modern synthesis, biological evolution has been defined as changes in allele frequencies in a population from one generation to another. The remainder of this article addresses biological evolution.

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

Most biologists believe in common descent: that all life on Earth is descended from one common ancestor. This conclusion is based upon the fact that many traits of living organisms, such as the genetic code, seem arbitrary yet are shared by all organisms. Some have suggested that life may have had more than one origin, but the high degree of commonality argues strongly against multiple origins.

The study of the ancestry of species is phylogeny. Phylogeny has revealed that organs with radically different internal structures can bear a superficial resemblance and perform similar functions. These examples of analogous structures show that there are multiple ways to solve most problems and make it difficult to believe that the universal traits of life are all necessary. Likewise other organs with similar internal structures will perform radically different functions. Vertebrate limbs are a favorite example of homologous structures, organs on two organisms that share a basic structure that had existed in the last common ancestor of the organisms. 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. According to this theory, the fundamental event of speciation is the genetic isolation two populations, which allows their gene pools to diverge.

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.

The emergence of novel traits

If life is to change, then new traits must emerge at some point. 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. However, it is now known that most inherited variation can be traced to discrete, persistent entities called "genes", which are aspects of a linear molecule called DNA. Alterations in DNA, known as mutations, have been observed to alter traits. 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 cells 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 phenomenon 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 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 (the sequence-to-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.

Microevolution and Macroevolution

Microevolution refers to small-scale changes in gene-frequencies in a population over 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 may culminate in the evolution of new species). The difference between the two is hard to distinguish because, over time, successive tiny mutations like those evidenced in microevolution could build up in isolated populations and eventually create entirely new species, which is known as macroevolution. These two terms are often used by certain creationists, who claim that microevolution can and does happen but that macroevolution cannot. This is based on the supposition that microevolution may occur with an existing gene pool, whereas macroevolution requires the introduction of newly-evolved genes.

The study of macroevolution addresses such questions such as;

  • Why did the major groups of animals suddenly appear in the fossil record (known as the Cambrian Explosion)?
  • Similarly, why are there missing links in the fossil record, and a scarcity of fossils for transitional species?
  • Why have no new major groups of living things appeared in the fossil record for a long time?
  • Why does evolution apparently occur in spurts, with many species undergoing long periods of stasis with little evolutionary change (punctuated equilibrium)?
  • What process leads to speciation?

There are two main ways in which "macroevolution" may occur. The first way is through the extrapolation of microevolutionary processes. Tiny microevolutions, over sufficient time, add up and accumulate in isolated populations and eventually result in new species. The second way in which "macroevolution" occurs is through sudden and rapid changes. This theory, punctuated equilibrium, put forth by Stephen Jay Gould, is based on the fact that there are critical genes (such as the homeobox) in all living organisms, and a small change in them could cause drastic changes in the organism, resulting in a new species quite rapidly.

Single small mutations are sometimes the main difference between one species and another. Scientists have discovered very important genes, such as the homeobox, which regulate the growth of animals in their embryonic state. Scientists have managed to create new species of fly by irradiating the homeobox gene, causing a radical mutation in the development of the segments of the body. The fly may grow an extra thorax, or grow legs out of its eyestalks, all due to a single base pair alteration! The additional information needed for these structures did not arise from the mutation, of course, but existed elsewhere in the animal's DNA and was replicated at the novel location. It has been proposed that centipedes and millipedes originated from insect precursors, but their homeobox gene mutated and they ended up growing dozens of body segments instead of just one. A very small change, and an entire species is formed.

It must be noted that many mutations are common and unexpressed, particularly when it involves toggling of the third base sequence in a codon. Most deleterious mutations are not seen simply because they do not result in viable reproduction.

Mutations of the homeobox and other critical genes are sometimes called macromutations, which cause the addition of body segments among the Arthropoda. One major problem lies in the scales of resolution offered by biological techniques. The fossil record cannot record events that happened in less than a million years, which allows it to clearly show slow speciation events that are the result of accumulated mutations over a long time, but records sudden "jumps" in species that are most likely the result of mutations in the critical regulatory genes in only a few generations. Macromutations are probably the best explanation of the Cambrian Explosion that occurred 550 million years ago.

Some proponents of creationism accept that microevolution occurs in the short term, whereas macroevolution, specifically leading to speciation, is expressly rejected. They claim that known sources of variation can only account for variation within species, and can not account for the variation between larger taxonomic groups, thus making macroevolution impossible.

Microevolution can easily be demonstrated in the laboratory to the satisfaction of most observers. Whilst speciation events have been demonstrated in the laboratory and observed in the field, really dramatic differences between species do not usually occur in directly observable timescales (it occurs too quickly for the process to be shown in the fossil record.) Some creationists have argued that, since macroevolution can not be confirmed by a controlled experiment, it cannot be considered to be part of a scientific theory. However, evolutionists counter that astronomy, geology, archaeology and the other historical sciences, like macroevolution, have to check hypotheses through natural experiments. They confirm hypotheses by finding out if they conform or fit with the physical or observational evidence and can make valid predictions. In this way, macroevolution is testable and falsifiable.

Scientists consider large gaps between taxonomic groups to be explainable by ecological/evolutionary factors, such as extinctions, population bottlenecks, and the emergence of unoccupied ecological niches. Macroevolution is simply the result of microevolution over a longer period of time. According to the modern synthesis, no distinction needs to be drawn between different kinds of evolution because all are caused by the same factors.

Creation versus Evolution

Main article: creationism

Ever since the concept of evolution was codified into a scientific theory by Charles Darwin, religious fundamentalists have claimed that the theory is false and that a supreme being (usually referred to as "God") was responsible for creation of the species. 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). Christians, Jews and Muslims who reject evolution point to the Bible and the Quran to support their views and have offered what they believe to be proof of the impossibility of macroevolution in particular. 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 their views in the classroom.

These people say that the questions mentioned above under Microevolution and Macroevolution should be raised on a more general level and the explanatory power of the theory of evolution should be evaluated critically.

(See for example Disclaimer Adopted by Oklahoma Textbook Commission)

Differential survival of traits

Differential survival of characteristics that arise in the population mean 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

Darwinism, and its descendant theories, state that biological evolution results through natural selection. Since natural selection is so important to Darwinism and modern theories of evolution, a very short summary of its main points follows:

  • Organisms have children which inherit genes from their parents. These genes code for different characteristics in a person. Genetically, a child has 50% the DNA of each parent. Depending on how the genotypes are inherited though, the phenotypes may be manifested in different ways. The genotype is the basic code of the gene, and the phenotype is what is expressed in the individual. Two brown-eyed parents may be heterozygous for the eye color allele and end up having a child with the blue eyed phenotype. In plain English, kids are like mom and dad, though the mechanisms through which this occurs can get very complicated.
  • Organisms have differing reproductive (sexual) success based on their traits in a given environment. In plain English, animals (or plants) that are good at what they do are more likely to survive and have kids.
  • Therefore, over time, the types of organisms that have traits better adapted to their environment will tend to become the dominant ones in an environment, while organisms poorly adapted to their environment will become extinct.

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.

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.

Development of evolutionary theories

Status of evolution as a theory

When talking about biological evolution, there is often a confusion 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.

Biologists consider the existence of biological evolution to be a fact, but the relative role of the various mechanisms continue to be debated. The commonly accepted scientific theory today is known as the modern synthesis (also known as 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.

Fossil evidence

The theory of evolution tells us that differential reproduction rates result in very small but persistant changes in the phenotype of a population. Given enough time an isolated population will become an entirely new species. All species, as described by this model, are in a state of transition. However, it is extremely difficult to find examples of gradual change in the fossil record. The work of Mendel, Darwin and Watson/Crick makes an elegant argument for evolution. The lack of fossil evidence adds to complexity and fascination.

History of the idea

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 fairly, 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 have a stronger propensity 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.

Recent developments in evolutionary theory


Another extension to the standard modern synthesis, advocated by Lynn Margulis, is symbiogenesis. Symbiogenesis argues that acquisition and accumulation of random mutations or genetic drift are not sufficient to explain how new inherited variations occur in evolution. This theory states that species arise from the merger of independent organisms through symbiosis. Symbiogenesis emphasizes the impact of cooperation rather than Darwinian competition. This commonly occurs in multigenomic organisms throughout nature.

Neo-structuralist themes in evolutionary theory

In the 1980s and 1990s there was a renewal of structuralist themes in evolutionary biology by biologists such as Brian Goodwin, that incorporates ideas from cybernetics and systems theory, and that emphasizes the role of self-organized processes as being at least as important as the role of natural selection. Some extreme variants consider natural selection as the result of biological evolution and not its cause, a tenet not held by most of the biologists working under the neo-structuralist rubric. This line of inquiry is criticised by classic Darwinists such as Richard Dawkins and Daniel Dennett, who perceive the intrusion of certain kinds of ordering in an evolutionary scenario a step away from strict materialism based on random, non-directed processes.

The evolution of altruism

Main article: Altruism

Altruism has been one of the last (and most deeply embedded) thorns in the side of evolutionary theory, but recent developments in game theory have suggested explanations with an evolutionary context. If humans evolved, then so did human minds, and if minds evolved, then so does behaviour - including, according to these models, altruistic tendencies.

Theories of eusociality and the undoubted advantages of kin selection have made good progress in this direction, but they are far from unproblematic. Some writers have pointed out that the conscience is just another aspect of our mental behaviour, and propose an evolutionary explanation for the existence of conscience and therefore altruism. One recent suggestion, expressed most eloquently by the philosopher Daniel Dennett, was initially developed when considering the problem of so-called 'free riders' in the tragedy of the commons, a larger-scale version of the Prisoner's Dilemma.

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.

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