Evolution

In biology, evolution is the process by which populations of organisms acquire and pass on novel traits from generation to generation, affecting the overall makeup of the population and even leading to the emergence of new species.

Scientific theory

The theory underlying the modern synthesis has three major aspects:

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• The common descent of all organisms from a single ancestor.
• The manifestation of novel traits in a lineage.
• 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 Lamarckism and the creationism of the 18th and 19th centuries.

Related Topics:
Scientific theory - Falsifiable - Hypotheses - Empirical observation - Apple - Gravitation - Powerful - Science - Biology - Lamarckism - Creationism

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Ancestry of organisms

http://www.reasons.org/resources/apologetics/humans_chimps_same_genus.shtml.]]

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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.

Related Topics:
Common descent - Biology

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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.

Related Topics:
Morphologic - DNA - Nucleic acid - Amino acid - Protein - Genetic code - Translate

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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.

Related Topics:
Abiogenesis - Origin of life - Miller-Urey experiment

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The evolutionary process can be exceedingly slow. Fossil evidence indicates that the diversity and complexity of modern life has developed over much of the age of the earth. Geological evidence indicates that the Earth is approximately 4.6 billion years old. (See Timeline of evolution.)

Related Topics:
Geological - 4.6 billion years old - Timeline of evolution

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Studies on guppies http://64.233.161.104/search?q=cache:HYzu8Z9stGoJ:www.nsf.gov/od/lpa/news/press/pr9725.htm+%22We+feel+that+our+work+is+part+of+a+growing+body+of+studies%22&hl=en by the National Science Foundation, however, have shown that evolutionary rates in the wild can proceed 10 thousand to 10 million times faster than what is indicated in the fossil record.

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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.

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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.

Related Topics:
Fossil - Sediment - Fossil record

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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.

Related Topics:
Radiometric dating - Transitional fossil - Paleontologist

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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.

Related Topics:
Phylogeny - Vertebrate - Vestigial organ - Wisdom teeth - Appendix

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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 http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=11170892, and 6.6% from baboonshttp://www.genome.org/cgi/content/full/13/5/813. 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.

Related Topics:
Phylogenetically - Chimpanzee - Gorilla - Baboon

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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 degenerationhttp://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=10833048.

Related Topics:
Pseudogene - Ortholog

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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.

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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.

Related Topics:
Genetic code - Scientific consensus - Archea - Bacteria - Eukaryota - Origin of life - Macromolecule - RNA - Complex system

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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.

Related Topics:
Photosynthesis - Banded iron - Red beds - Aerobic - Cellular respiration - Cambrian explosion - Burgess Shale - Phyla - Hox gene - Plant - Fungi - Arthropod - Ecosystem

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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.

Related Topics:
Genetic material - DNA - RNA - Cell - Cell division - Radiation - Viruses - Meiosis - Hypermutation - Cancer

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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.

Related Topics:
Neutral mutations

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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).

Related Topics:
Trait - Gene - DNA - Mutation

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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.

Related Topics:
Epigenetic inheritance - Prion - Structural inheritance

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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:

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• 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.

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Natural selection can be subdivided into two categories:

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• 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:

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• 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.

Related Topics:
Neutral mutations - Probable mutation effect - Genome degradation

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• 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.

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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.

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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).

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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.

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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.

Related Topics:
Adaptation - Fitness

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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.

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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.

Related Topics:
Speciation - Allopatric speciation - Habitat fragmentation - Sympatric speciation - Ernst Mayr - Peripatric speciation - Punctuated equilibrium

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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.

Related Topics:
Extinction - Gene pool - Geological time

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