Evolution

Note: This is only a very rough draft, with notes that may be useful in developing the article. Please do not edit this article until the actual article is complete — i.e., when this notice is removed. You may add comments on what you would like to see included. Rick Swarts 00:05, 28 Sep 2005 (UTC)

This article is about evolution in the field of life science, for other uses please see Evolution (disambiguation).

Charles Darwin, father of the theory of evolution by natural selection.

File:Alfred Russel Wallace.jpg

Alfred Russel Wallace

• The following are some notes that can be used in improving this quite flawed Wikipedia article. Parts of the Wikipedia article are useable, but need to establish the fundamental distinction between descent with modification, which has a lot of evidence and is backed up by Unification Thought, and natural selection, which lacks evidence on the macroevolutionary level. However, most of these notes refer to an article on Evolution and REligion, not on Evolution per se. This article will need a substantial rewrite.

1. Components of evolutionary theory

In Darwin's comprehensive theory of evolution, there can actually be elucidated at least five major, largely independent theories. The two basic theories, and the ones which I will treat here, are: (1) the theory of evolution by common descent, and (2) the theory of modification through natural selection. The first is a kinematic theory which deals with non-causal relations between things — it deals with the pattern of evolution. The latter is a dynamic theory which deals with mechanisms and causal relationships B it deals with the process. Other theories offered by Darwin deal with (3) evolution as such (the fact of evolution), (4) the gradualness of evolution, and (5) populational speciation.

The "theory of descent with modification" essentially postulates that all organisms have descended from common ancestors by a continuous process of branching. In other words, all life evolved from one kind of organism or from a few simple kinds, and each species arose in a single geographic location, from another species that preceded it in time. Evolutionists have marshaled substantial evidence for the theory of descent with modification. That is, the "pattern of evolution" is well documented by the fossil record, the distribution patterns of existing species, methods of dating fossils, and comparison of homologous structures. Interestingly, all of the classical arguments for evolution are fundamentally arguments for imperfections that reflect history. They fit the pattern of observing that the leg of Reptile B is not the best for walking, because it evolved from Fish A. In other words, why would a rat run, a bat fly, a porpoise swim and a man type all with the same structures utilizing the same bones unless inherited from a common ancestor?

Evidence is so overwhelming for the theory of descent with modification that only religious fundamentalists have attempted to challenge this theory. Among these are the Ascientific creationists.@ Scientific creationists@ are a specific group of creationists who maintain that modern organisms did not descend from common ancestors, and that their only historical connectedness is in the mind of God. Instead, scientific creationists promulgate the view that living organisms are immutable, and were all created by God in a short time period, on a earth whose age is generally measured in 1000s of years. The substantial fossil record is dismissed in various ways, including as a trick of God and as an artifact from the Great Flood (with some organisms sinking faster than others and thus on a lower fossil plane). Although some individual presentations by scientific creationists are quite sophisticated, the overall theory of scientific creationism runs counter to an enormous body of evidence and thus is strongly criticized by most of the scientific community.

The second theory of Darwin, the "theory of modification through natural selection," is one explanation offered for how evolution might have occurred, i.e, the "process" by which evolution took place and arrived at the pattern. This theory of natural selection was the most revolutionary and controversial concept advanced by Darwin. While the theory of descent with modification was accepted soon after its introduction, the theory of natural selection took until the mid-1900s to be accepted by the scientific community. By providing a purely non-teleogical, materialistic explanation for all phenomenon of living nature, it was said it "dethroned God."

According to this theory, natural selection is the directing or creative force of evolution. Natural selection is considered far more than just a minor force for weeding out unfit organisms. Even Paley and other natural theologians accepted natural selection, albeit as a devise for removing unfit organisms, rather than as a directive force for creating new species and new designs. Natural selection had three radical components— (a) purposelessness (no higher purpose, just the struggle of individuals to survive and reproduce); (b) philosophical materialism (matter is seen as the ground of all existence with spirit and mind being produced by or a function of the material brain); and (c) the view that evolution is not progressive from lower to higher, but just an adaptation to local environments; it could form a man with his superior brain or a parasite, but no one could say which is higher or lower.

Concrete evidence for the theory of modification by natural selection is limited to microevolution, such as seen in the systematic color change in the peppered moth, Biston betularia which was observed over a 50-year period in England, or through artificial selection, whereby various breeds of animals and varieties of plants have been produced which are different in some respect from their ancestors. The evidence that natural selection directs the major transitions between species and originates new designs (macroevolution) necessarily involves extrapolation from these evidences on the microevolutionary level. That is, it is inferred that if moths can change their color in 50 years, then new designs or entire new genera can originate over millions of years. If geneticists see population changes for fruit flies in laboratory bottles, then given eons of time, birds can be built from reptiles and fish with jaws from jawless ancestors. One of Darwin's chief purposes in publishing the Origin of Species was to show that natural selection had been the chief agent of the change presented in the theory of descent with modification. The validity of making this extrapolation has recently come under strong challenge from top evolutionists.

2. The evolution/creation dichotomy

There is a wide variety of religious viewpoints with respect to evolution: from the specific doctrine of Ascientific creationism,@ which stands in opposition to evolution, to views which accept the pattern observed in creation but not the process, to views which attribute a primacy to natural selection. Millions of religious adherents do successfully juxtapose the two viewpoints of evolution and creationism. As eminent evolutionary geneticist Theodosius Dobzhansky stated, AIt is wrong to hold creation and evolution as mutually exclusive alternatives. I am a creationist and an evolutionist. Evolution is God=s or Nature=s, method of creation.@

In particular, the theory of descent with modification would seem to pose no difficulty whatever to most religious adherents, since it is neutral with respect to the process. The mechanism that gives rise to the pattern could occur by natural selection or it could occur by the directive force of a supreme being.

Some of the confusion in the dialogue between evolutionists and creationists is what is being referred to by the term Aevolution@ or Atheory of evolution.@ For evolutionists, a working definition of the term "evolution" is generally descent with modification or a change of gene frequencies in populations.

Since there is considerable experimental and observational evidence of populations systematically changing over time, evolutionists speak of "the fact of evolution." There is evidence on the microevolutionary level (change in gene frequencies within populations), in terms of artificial selection or the change in the color of the peppered moths. On a macroevolutionary level (large-scale events such as speciation and origin of new designs), various evidences such as fossil records, biogeography, and studies of homologies have strongly supported the view that all organisms have descended from common ancestors. In fact, renowned evolutionist Mayr contends that Athe facts of biogeography posed some of the most insoluble dilemmas for the creationists and were eventually used by Darwin as his most convincing evidence in favor of evolution.@

Darwin helped to establish the "fact of evolution." In 1859, most scientists and laymen believed that the world was constant. The massive evidence that Darwin presented was so convincing that within a few years every biologist became an evolutionist, believing that the world was the product of a continuing process of change. For most biologists today, evolution is no longer a theory but simply a fact. They may disagree with the mechanisms, but that evolution takes place — that there is a systematic change in populations — is unquestioned.

The statement that Aevolution is a fact,@ draws the ire of scientific creationists, of course. However, scientific creationists represent only a small body of those individuals that do believe in a creation by a supreme being. Nonetheless, other religious adherents likewise often speak of opposition to evolution, despite having a belief system which allows descent with modification and change in gene frequencies in populations. There are a couple of ready explanations for this.

For one, there is the case of terminological confusion. When some individuals and religious adherents use the term Aevolution,@ they are not referring to simply a systematic change in populations over time — which is a highly established fact — but are instead treating the word Aevolution@ as synonymous with the specific Darwinian theory of evolution by natural selection — a theory with which even some eminent evolutionists find troublesome as the sole explanation for observed changes. Thus, religious adherents may reject Aevolution@ since they see the concept of randomness in natural selection as counter to their belief that a Supreme Being directs changes

Furthermore, popular writings often tend to create an artificial dichotmy B either belief in a Creator is correct or evolution is correct B an Aeither-or dichotomy@ which tends to foster an erroneous view of the relationship between evolution and religion. By such means, evolution and religion (specifically creation by a God) are presented as if mutually exclusive alternatives. Thus, many religious adherents reject evolution out of hand, not wishing to reject God.

Textbook authors have often confused the dialogue on evolution by treating the term as if it signified one unified whole — not only the fact of evolution having occurred, but also the specific Darwinian and neo-Darwinian theories regarding natural selection, gradualism, speciation, and so forth. Certain textbook authors, in particular, have exacerbated this terminological confusion by lumping Aevidences of evolution@ into a section placed immediately after a comprehensive presentation on Darwin's overall theory — thereby creating the misleading impression that the evidences are supporting all components of Darwin's theory, including natural selection. In reality, the confirming information is invariably limited to the phenomenon of evolution having occurred (descent from a common ancestor or change of gene frequencies in populations), or perhaps including evidence of natural selection within populations.

3. Punctuational Models

Recent evolutionary theories have actually drawn creationists and evolutionists closer. Among the chief of these are the various punctuational models.

Historically, the view of gradualism dominated. Gradulism is a view of evolution as proceeding by means of slow accumulation of very small changes, with the evolving population passing through all the intermediate stages B sort of a "march of frequency distributions" through time. This Darwinian and Neo-Darwinian emphasis on gradualism has been subject to re-examination on several levels B the levels of speciation, origin of new designs, and major evolutionary trends.

3a. Punctuational speciation

A common misconception about evolution is that the development of new species requires millions of years.

Indeed, the gradualist view that speciation involved a slow, steady, progressive transformation of an ancestral population into a new species has dominated much of evolutionary thought from the time of Darwin. Such a transformation was generally viewed as involving large numbers of individuals, being even and slow, and occurring over all or a large part of the ancestral species geographic range. The absence of a gradually graded sequence of intermediary forms in the fossil record was attributed to the imperfection of the geological record.

However, the fossil record is considerably more complete than it was at the time of Darwin, and it still yields the same two points: (1) the sudden appearance of species; and (2) long periods where species do not change much. Indeed, the principle feature of individual species within the fossil record is that they do not change. Species first appear in the fossil record looking much the same as when they disappear. One observes a sudden appearance of fully formed species in the geological record.

The theory of punctuated equilibria ascribes that the fossil record accurately reflects evolutionary change. That is, it posits that macroevolutionary patterns of species are typically ones of morphological stability during their existence, and that most evolutionary change is concentrated in events of speciation— with the origin of a new species usually occurring during geologically short periods of time when the long-term stasis of a population is punctuated by this rare and rapid event of speciation. The sudden transitions between species are sometimes measured on the order of 100s or 1000s of years relative to their millions of years of existence. Although the theory of punctuated equilibria originally generated a lot of controversy, it is now viewed highly favorably in the scientific community, and has even become a part of recent textbook orthodoxy.

The theory of punctuated equilibria has been embraced by many scientific creationists as evidence that the fossil record does not support Darwinian theory. However, the founders and supporters of punctuated equilibria emphasize their view that the pattern of punctuated equilibria (stasis and rapid evolution) is the natural expectation from the now-generally accepted scientific model for speciation, involving evolution within peripherally-isolated local populations.

What can be emphasized is that punctuated equilibria merely addresses the pattern of evolution and is not tied to any one mode of speciation. Although occurring in a brief period of time, the species formation can go through all the stages, or can proceed by leaps. It is even agnostic with respect to natural selection. However, this theory has brought into acceptability a theistic view previously disparaged, that the fossil record supports the relatively sudden appearance of a species, and its morphological stability during its existence. Those who believe in a creator Supreme Being can posit that it is God who directs the sudden changes.

I am not aware of the Unification Thought view on such an evolutionary trend. However, in the late 1970s, I did note in one of the writings of Dr. Sang Han Lee the assertion that the teachings of Rev. Moon on evolution lead to the prediction that evolutionary change would have to be step-wise. That is, if the fossil record were complete, it would have to show that each species remains virtually the same throughout its existence, and then there would be a sudden change or splitting to create a new species from the existing species. At the time, this presentation on evolutionary theory ran counter with the overwhelming, prevailing orthodoxy of evolutionary theory. However, this view is near identical with this new orthodoxy of punctuated equilibria.

3b. Punctuated origin of new designs

There is also the issue of the origin of new designs Bsuch as the vertebrate eye, feathers, or jaws in fishes. Such issues have often been used by critics to counter Darwinian theory. To most observers, the development of such sophisticated new designs via such a random process as natural selection seems inconceivable. However, evolutionary theory has dealt with such criticisms since the time of Darwin, offering three basic scenarios for how natural selection crafted such new designs.

Complicated new designs have historically been explained as developing very gradually, involving numerous, tiny, imperceptible steps, with each step being advantageous and developed by natural selection. This style of argument follows Darwin's famous resolution proposed for the origin of the vertebrate eye.

The origin of many other features are not as easily explained along the lines postulated for the vertebrate eye. For example, Darwin's most cogent critic, St. George Mivart, argued that Darwinism must fail because it cannot explain "the incipient stages of useful structures" B those structures which become useful only when they are fully formed. For example, bird feathers evolved from reptilian scales. If they are for flight, what possible benefit could they have conferred in their early stages? A scale transformed 5% of the way into a feather would be useless in flight; so, how could such an "incipient stage" arise by natural selection? What about the jaws of fishes? What good is half a jaw?

For such origins as feathers and jaws, this thorny issue is generally resolved by evolutionists using the principle of preadaptation, a gradualist approach. Preadaptation holds that intermediate stages in the development of major evolutionary novelties often perform functions different from those of final stages. By such explanations, a gradual transition can be proposed for structures which cannot function in a certain way until they are fully formed. In other words, various structures functioned in one role for ancestors, but by good fortune prove well suited after transformation to perform a very different role for descendants. Thus, feathers may have served originally for heat regulation or catching prey, and only late in development were converted to usage in flight. Likewise, the bony support for jaws may have originally served as a gill arch.

However, another solution for origin of new designs, which is gaining renewed attention among evolutionists, is that the full sequence of intermediate forms need not have existed at all, and instead some major novelties may have arisen rapidly, discontinuously. This in not to suggest that the first complete bird hatched from a fully reptilian egg or that a jawed fish arose all at once, fully formed. However, this view does question, in the evolution of the jaw, for example, whether one can really believe that the front set of gill arch bones lost their connection to the gills and migrated slowly forward, a fraction of a millimeter per generation, until they surrounded the mouth and took on their new function. Instead, it questions, is it not more likely that a genetic change resulted in the transition as a kind of switch: the bones are either back as gill supports or forward as mouth support?

This view of a punctuational origin of key features arose because of: (1) the persistent problem of the lack of fossil evidence for intermediate stages between major designs, with transitions between major groups being characteristically abrupt; and (2) the inability, even in one's imagination, to even construct functional intermediates in many cases. Prominent evolutionist Stephen Jay Gould, for example, cites the fur-lined pouches of pocket gophers and the maxillary bone of the upper jaw of certain genera of boid snakes being split into front and rear halves:

How can a jawbone be half broken? . . . What good is an incipient groove or furrow on the outside? Did such hypothetical ancestors run about three-legged while holding a few scraps of food in an imperfect crease with their fourth leg?

The recent support among prominent evolutionists for the origin of major designs via rapid transitions aids theistic critiques countering gradual, natural selection as the creative force in evolution. For one, such a punctuational model recognizes the lack of intermediates in the fossil record and advances the difficulty of even imagining such intermediates. It also posits a scenario whereby natural selection could be seen as having only a secondary role — eliminating unfit organisms — rather than the main creative role. For such reasons, several prominent evolutionists have denounced the view of punctuational origins, and labeled such views non-Darwinian.

Indeed, Darwin himself had stated, immediately after his discussion of the evolution of the eye: "If it could be demonstrated that any complex organ existed, which could not possibly have been formed by numerous, successive, slight modifications, my theory would absolutely break down."

It should be noted, however, that the main proponents of punctuational origin are ardent evolutionists, who consider this theory to be within the Darwinian framework and, indeed, are careful to present the theory in a manner that supports the primacy of natural selection in evolution.

In biology, evolution is the process by which populations of organisms acquire and pass on novel traits from generation to generation. One of the first theories of biological evolution was proposed in the early 19th century by Jean-Baptiste Lamarck, although his fundamentally flawed idea was that individual organisms acquire traits during their lifetimes that they pass on to offspring. With the publication of Charles Darwin and Alfred Russel Wallace's joint paper in 1858 – followed by Darwin's book Origin of Species in 1859 – the theory of evolution by natural selection became firmly established within the scientific community. In the 1930s work by a number of scientists combined Darwinian natural selection with the re-discovered theory of heredity (proposed by Gregor Mendel) to create the modern evolutionary synthesis. In the modern synthesis, "evolution" means a change in the frequency of an allele within a gene pool from one generation to the next. This change may be caused by a number of different mechanisms: natural selection, genetic drift or changes in population structure (gene flow).

Scientific theory

The theory underlying the modern synthesis has three major aspects:

1. The common descent of all organisms from a single ancestor.
2. The manifestation of novel traits in a lineage.
3. The mechanisms that cause some traits to persist while others perish.

The modern synthesis, like its Mendelian and Darwinian antecedents, is a scientific theory. In plain English, people use the word "theory" to signify "conjecture", "speculation", or "opinion". In contrast, a scientific theory is a model of the world (or some portion of it) from which falsifiable hypotheses can be generated and be verified through empirical observation. In this sense, "theory" and "fact" do not stand in opposition, but rather exist in a reciprocal relationship — for example, it is a "fact" that an apple dropped on earth will fall towards the center of the planet in a straight line, and the "theory" which explains it is the current theory of gravitation. Currently, the modern synthesis is the most powerful theory explaining variation and speciation, and within the science of biology it has completely replaced earlier accepted explanations for the origin of species, including creationism and Lamarckism.

Ancestry of organisms

Pre-Cambrian stromatolites in the Siyeh Formation, Glacier National Park. In 2002, William Schopf of UCLA published a controversial paper in the journal Nature arguing that formations such as this possess 3.5 billion year old fossilized algae microbes. [1] If true, they would be the earliest known life on earth.

File:PhylogeneticTree.jpg A phylogenetic tree of all living things, based on rRNA gene data, showing the separation of the three domains bacteria, archaea, and eukaryotes as described initially by Carl Woese. Trees constructed with other genes are generally similar, although they may place some early branching groups very differently, presumably owing to rapid rRNA evolution. The exact relationships of the three domains are still being debated.

Genetic testing has shown that humans and chimpanzees have most of their DNA in common. In a study of 90,000 base pairs, Wayne State University's Morris Goodman found humans and chimpanzees share 99.4% of their DNA.[2] [3].

This is a NASA recreation of the famous Miller-Urey experiment. In 1953, Stanley Miller and Harold Urey sealed the chemical precursors to life in a closed environment, and subjected them to conditions similar to primordial earth. The results of the experiment suggest that the chemicals necessary for life did tend to arise under those circumstances, supporting the theories of Abiogenesis

For more details on this topic, see Common descent.

A group of organisms is said to have common descent if they have a common ancestor. In biology, the theory of universal common descent proposes that all organisms on Earth are descended from a common ancestor or ancestral gene pool.

Evidence for common descent may be found in traits shared between all living organisms. In Darwin's day, the evidence of shared traits was based solely on visible observation of morphologic similarities, such as the fact that all birds — even those which do not fly — have wings. Today, the theory of evolution has been strongly confirmed by the science of DNA genetics. For example, every living thing makes use of nucleic acids as its genetic material, and uses the same twenty amino acids as the building blocks for proteins. All organisms use the same genetic code (with some extremely rare and minor deviations) to translate nucleic acid sequences into proteins. Because the selection of these traits is somewhat arbitrary, their universality strongly suggests common ancestry.

In addition, abiogenesis — the generation of life from non-living matter — has never been observed, indicating that the origin of life from non-life is either extremely rare or only happens under conditions very unlike those of modern Earth. The 1953 Miller-Urey experiment suggests that conditions on the ancient earth may have permitted abiogenesis.

Since the evolutionary process is exceedingly slow, the diversity and complexity of modern life requires that the Earth be very old, on the order of billions of years. 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 planetary science. These sciences provide information about the history of the Earth and the changes produced by life. A great deal of information about the early Earth has been destroyed by geological processes over the course of time.

Evidence of evolution

Morphological evidence

Fossils are important for estimating when various lineages developed. As fossilization is an uncommon occurrence, usually requiring hard parts (like bone) and death near a site where sediments are being deposited, the fossil record only provides sparse and intermittent information about the evolution of life. Fossil evidence of organisms without hard body parts, such as shell, bone, and teeth is sparse but exists in the form of ancient microfossils and the fossilization of ancient burrows and a few soft-bodied organisms.

Nevertheless, fossil evidence of prehistoric organisms has been found all over the Earth. The age of fossils can often be deduced from the geologic context in which they are found; and their absolute age can be verified with radiometric dating. 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 transitional fossils can be used to demonstrate continuity between two different lineages. Paleontologists investigate evolution largely through analysis of fossils.

Phylogeny, the study of the ancestry of species, has revealed that structures with similar internal organization may perform divergent functions. Vertebrate limbs are a common example of such homologous structures. A vestigial organ or structure may exist with little or no purpose in one organism, though they have a clear purpose in others. The human wisdom teeth and appendix are common examples.

Genetic sequence evidence

Comparison of the genetic sequence of organisms reveals that phylogenetically close organisms have a higher degree of sequence similarity than organisms that are phylogenetically distant. For example, neutral human DNA sequences are approximately 1.2% divergent (based on substitutions) from those of their nearest genetic relative, the chimpanzee, 1.6% from gorillas [4], and 6.6% from baboons[5]. Sequence comparison is considered a measure robust enough to be 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[6].

Since metabolic processes do not leave fossils, research into the evolution of the basic cellular processes is also done largely by comparison of existing organisms. Many lineages diverged at different stages of development, so it is theoretically possible to determine when certain metabolic processes appeared by comparing the traits of the descendants of a common ancestor.

Origin of life

Not much is known about the earliest development of life. However, all existing organisms share certain traits, including cellular structure, and genetic code. Most scientists interpret this to mean 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.

History of life

Though the origins of life are murky, other milestones in the evolutionary history of life are well-known. The emergence of oxygenic photosynthesis (around 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 around 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, or phyla, of modern animals; this event is now believed to have been triggered by the development of Hox genes. 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.

Emergence of novel traits

Mutation

Darwin did not know the source of variations in individual organisms, but observed that it seemed to be by chance. Later work pinned much of this variation onto mutations. Mutations are permanent, transmissible changes to the genetic material (usually DNA or RNA) of a cell, and can be caused by "copying errors" in the genetic material during cell division and by exposure to radiation, chemicals, or viruses, or can occur deliberately under cellular control during processes such as meiosis or hypermutation. In multicellular organisms, mutations can be subdivided into germline mutations, which can be passed on to progeny and somatic mutations, which (when accidental) often lead to the malfunction or death of a cell and can cause cancer.

Mutations serve to introduce novel genetic variation, upon which selection may (or may not, see Neutral mutations) act. Neutral mutations do not affect the organism's chances of survival in its natural environment and can accumulate over time.

Survival of traits

Mechanisms of inheritance

In Darwin's time, scientists did not share broad agreement on how traits were inherited. Today most inherited traits are traced to discrete, persistent entities called genes, encoded in linear molecules called DNA. Though by and large faithfully maintained, DNA is both variable across individuals and subject to a process of change or mutation (described below).

However, other non-DNA based forms of heritable variation exist. 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 typical Darwinian framework, which avoids any connection between environmental signals and the production of heritable variation.

There are factors that influence the frequency of existing alleles. These factors mean that some characteristics will become more frequent while others diminish or are lost entirely. There are three known processes that affect the survival of a characteristic; or, more specifically, the frequency of an allele:

• Natural selection
• Gene flow
• Genetic drift

Natural selection

Natural selection is survival and reproduction as a result of the environment. Differential mortality is the survival rate of individuals to their reproductive age. Differential fertility is the total genetic contribution to the next generation. The central role of natural selection in evolutionary theory has given rise to a strong connection between that field and the study of ecology.

Natural selection can be subdivided into two categories:

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

Natural selection also operates on mutations in several different ways:

• Purifying or background selection eliminates deleterious mutations from a population.
• Positive selection increases the frequency of a beneficial mutation.
• Balancing selection maintains variation within a population through a number of mechanisms, including:
  • Overdominance or heterozygote advantage, where the heterozygote is more fit than either of the homozygous forms (exemplified by human sickle cell anemia conferring resistance to malaria)
  • Frequency-dependent selection, where the rare variants have a higher fitness.
• Stabilizing selection favors average characteristics in a population, thus reducing gene variation but retaining the mean.
• Directional selection favors one extreme of a characteristic; results in a shift in the mean in the direction of the extreme.
• Disruptive selection favors both extremes, and results in a bimodal distribution of gene frequency. The mean may or may not shift.

Mutations that are not affected by natural selection are called neutral mutations. Their frequency in the population is governed entirely by genetic drift and gene flow. It is understood that an organism's DNA sequence, in the absence of selection, undergoes a steady accumulation of neutral mutations. 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.

• Baldwinian evolution refers to the way human beings, as cultured animals capable of symbolic (extrasomatic) learning, can change their environment, or the environment of any species, in such a way as to result in new selective forces.

Gene flow

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

Genetic drift

Genetic drift describes changes in allele frequency from one generation to the next due to sampling variance. The frequency of an allele in the offspring generation will vary according to a probability distribution of the frequency of the allele in the parent generation.

Many aspects of genetic drift depend on the size of the population (generally abbreviated as N). This is especially important in small mating populations, where chance fluctuations from generation to generation can be large. Such fluctuations in allele frequency between successive generations may result in some alleles disappearing from the population. Two separate populations that begin with the same allele frequency might, therefore, "drift" by random fluctuation into two divergent populations with different allele sets (for example, alleles that are present in one have been lost in the other).

The relative importance of natural selection and genetic drift in determining the fate of new mutations also depends on the population size and the strength of selection: when N times s (population size times strength of selection) is small, genetic drift predominates. When N times s is large, selection predominates. Thus natural selection is 'more efficient' in large populations, or equivalently, genetic drift is stronger in small populations. Finally, the time for an allele to become fixed in the population by genetic drift (that is, for all individuals in the population to carry that allele) depends on population size, with smaller populations requiring a shorter time to fixation.

Adaptation

Through the process of natural selection, species become better adapted to their environments. Adaptation is any evolutionary process that increases the fitness of the individual, or sometimes the trait that confers increased fitness, e.g. a stronger prehensile tail or greater visual acuity. Note that adaptation is context-sensitive; a trait that increases fitness in one environment may decrease it in another.

Most biologists believe that adaptation occurs through the accumulation of many mutations of small effect. However, macromutation is an alternative process for adaptation which involves a single, very large scale mutation.

Speciation and extinction

Speciation is the creation of two or more species from one. There are various mechanisms by which this may take place. Allopatric speciation begins when subpopulations of a species become isolated geographically, for example by habitat fragmentation or migration. Sympatric speciation occurs when new species emerge in the same geographic area. Ernst Mayr's peripatric speciation is a type of speciation that exists in between the extremes of allopatry and sympatry. Peripatric speciation is a critical underpinning of the theory of punctuated equilibrium.

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

Evolutionary biology is a subfield of biology concerned with the origin and descent of species, as well as their change over time. Evolutionary biology is a kind of meta field because it includes scientists from many traditional taxonomically oriented disciplines. For example, it generally includes scientists who may have a specialist training in particular organisms such as mammalogy, ornithology, or herpetology but use those organisms as systems to answer general questions in evolution.

Evolutionary biology as an academic discipline in its own right emerged as a result of the modern evolutionary synthesis in the 1930s and 1940s. It was not until the 1970s and 1980s, however, that a significant number of universities had departments that specifically included the term evolutionary biology in their titles.

Evolutionary developmental biology

Evolutionary developmental biology is an emergent subfield of evolutionary biology that looks at genes of related and unrelated organisms. By comparing the explicit nucleotide sequences of DNA/RNA, it is possible to experimentally determine and trace timelines of species development. For example, gene sequences support the conclusion that chimpanzees are the closest primate ancestor to humans, and that arthropods (e.g., insects) and vertebrates (e.g., humans) have a common biological ancestor.

History of evolutionary thought

File:Origin of Species.jpg The 1859 edition of On the Origin of Species

File:Stephen Jay Gould.png Stephen Jay Gould, who, along with Niles Eldredge proposed the theory of punctuated equilibrium in 1972

The idea of biological evolution has existed since ancient times, notably among Hellenists such as Epicurus, but the modern theory was not established until the 18th and 19th centuries, by scientists such as Jean-Baptiste Lamarck and Charles Darwin. 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 occur: his theory of natural selection. Darwin was motivated to publish his work on evolution after receiving a letter from Alfred Russel Wallace, in which Wallace revealed his own discovery of natural selection. As such, Wallace is sometimes given shared credit for the theory of evolution.

Darwin's theory, though it succeeded in profoundly shaking scientific opinion regarding the development of life, could not explain the source of variation in traits within a species, and Darwin's proposal of a hereditary mechanism (pangenesis) was not compelling to most biologists. It was not until the late 19th and early 20th centuries that these mechanisms were established.

When Gregor Mendel's work regarding the nature of inheritance in the late 19th century was "rediscovered" in 1900, it led to a storm of conflict between Mendelians (Charles Benedict Davenport) and biometricians (Walter Frank Raphael Weldon and Karl Pearson), who insisted that the great majority of traits important to evolution must show continuous variation that was not explainable by Mendelian analysis. Eventually, the two models were reconciled and merged, primarily through the work of the biologist and statistician R.A. Fisher. This combined approach, applying a rigorous statistical model to Mendel's theories of inheritance via genes, became known in the 1930s and 1940s as the modern evolutionary synthesis.

In the 1940s, following up on Griffith's experiment, 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 and Maurice Wilkins. These developments ignited the era of molecular biology and transformed the understanding of evolution into a molecular process: the mutation of segments of DNA (see molecular evolution).

George C. Williams' 1966 Adaptation and natural selection: A Critique of some Current Evolutionary Thought marked a departure from the idea of group selection towards the modern notion of the gene as the unit of selection.

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 public debates was over the theory of punctuated equilibrium, proposed in 1972 by paleontologists Niles Eldredge and Stephen Jay Gould to explain the paucity of transitional forms between phyla in the fossil record.

References

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• Darwin, Charles November 24 1859. On the Origin of Species by means of Natural Selection or the Preservation of Favoured Races in the Struggle for Life. London: John Murray, Albemarle Street. 502 pages. Reprinted: Gramercy (May 22, 1995). ISBN 0517123207
• Zimmer, Carl. Evolution: The Triumph of an Idea. Perennial (October 1, 2002). ISBN 0060958502
• Larson, Edward J. Evolution: The Remarkable History of a Scientific Theory (Modern Library Chronicles). Modern Library (May 4, 2004). ISBN 0679642889
• Mayr, Ernst. What Evolution Is. Basic Books (October, 2002). ISBN 0465044263
• Gigerenzer, Gerd, et al., The empire of chance: how probability changed science and everyday life (New York: Cambridge University Press, 1989).
• Williams, G.C. (1966). Adaptation and Natural Selection: A Critique of some Current Evolutionary Thought. Princeton, N.J.: Princeton University Press.
• Sean B. Carroll, 2005, Endless Forms Most Beautiful: The New Science of Evo Devo and the Making of the Animal Kingdom, W. W. Norton & Company. ISBN 0393060160

See also

anagenesis argument from evolution atavism behavioral ecology catagenesis cladogenesis Convergent evolution evolutionary medicine endosymbiont evolutionary algorithm Evolutionary art evolution of sex evolutionary tree experimental evolution

fitness landscape genetic algorithm gradualism modern evolutionary synthesis natural science neutral theory of molecular evolution niche construction Nonhuman animals ethics parallel evolution quantum evolution quasispecies model scientific method sexual selection teratogenesis

Social implications of the theory of evolution

Creation-evolution controversy

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