Evolution

From Wikipedia, the free encyclopedia

This article is  about evolution in biology. For other uses, see Evolution (disambiguation).

For a non-technical introduction to the topic, please see Introduction to evolution.

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In 1832, while traveling on the Beagle, naturalist Charles Darwin collected giant fossils in South America. On his return, he was informed in 1837 by Richard Owen that fragments of armor were from the gigantic extinct glyptodons, creatures related to the modern armadillos he had seen living nearby. The similarities between these two unusually scaly mammals and  their geographic distribution provided Darwin with a clue that helped him develop his theory of how evolution occurs.^[1]

In biology, evolution refers to the process in which some traits in populations to become more  common over time. This includes pre-existing inherited traits, new traits produced by mutation, and  those brought in by the migration of organisms (or lateral transfer of genes) from one population to another ("gene flow").^[2]

Natural selection is  a key part of this process. Since some traits or  collections of traits allow an  organism to survive and  produce more  offspring than an  organism lacking them, and  genes are  passed on by reproduction, those that increase survival and  reproductive success are  more likely to be passed on in comparison to those genes that do not. Therefore, the number of organisms with these traits will tend to increase with each passing generation, unless conditions change so as to make them no longer favourable.

Other mechanisms of evolutionary change include genetic drift, or  random changes in frequency of traits (most important when the traits are, at that time, reproductively neutral), and  the founder effect, in which a small group of organisms isolated from the main population will have  more of the traits of the founders. This can be observed for many generations after isolation, even when some of the traits are  detrimental.

Given enough time, these passive processes can result in varied adaptations to changing environmental conditions and, eventually, the development of new species from existing ones — a process known as speciation.^[3] According to the theory of evolution, all contemporary organisms on earth are  related to each other through common descent, the products of cumulative evolutionary changes over billions of years. Evolution is  thus the source of the vast diversity of life on Earth, including the many extinct species attested to in the fossil record.^[4][5] With its enormous explanatory and  predictive power, evolution has become the central organizing principle of modern biology.^[6][7][8]

Part of the Biology series on

Evolution

Mechanisms and  processes

Adaptation Genetic drift Gene flow Mutation Selection Speciation

Research and  history

Evidence History Modern synthesis Social effect

Evolutionary biology fields

Ecological genetics Evolutionary development Human evolution Molecular evolution Phylogenetics Population genetics Evo-devo

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[edit] Variation

Evolutionary changes are  the product of evolutionary forces acting on genetic variation. In natural populations, there is  a certain amount of phenotypic variation (e.g., what makes you appear different from your neighbor). This phenotypic variation is  the result of variants in gene sequences among the individuals of a population or  the interaction of a genotype with the environment-phenotypic plasticity There may be one or  more functional variants of a gene or  locus, and  these variants are  called alleles. Most sites in the genome (i.e., complete DNA sequence) of a species are  identical in all individuals in the population; sites with more  than one allele are  called polymorphic or  segregating sites.

All genetic variation begins as a new mutation in a single individual; in subsequent generations the frequency of that variant may fluctuate in the population, becoming more  or less  prevalent relative to other alleles at the site. This change in allele frequency is  the commonly accepted definition of evolution, and  all evolutionary forces act by driving allele frequency in one direction or  another. Variation disappears when it  reaches the point of fixation — when it  either reaches a frequency of zero and  disappears from the population, or  reaches a frequency of one and  replaces the ancestral allele entirely.

[edit] Mechanisms of evolution

Evolution consists of two basic types of processes: those that introduce new genetic variation into a population, and  those that affect the frequencies of existing variation. Paleontologist Stephen J. Gould once phrased this succinctly as "variation proposes and  selection disposes."^[9]

[edit] Mutation

Main article: Mutation

Mutation can occur because of "copy errors" during DNA replication.

Genetic variation arises due to random mutations that occur at a certain rate in the genomes of all organisms. 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; by exposure to radiation, chemicals, or  viruses. In multicellular organisms, mutations can be subdivided into germline mutations that occur in the gametes and  thus can be passed on to progeny, and  somatic mutations that can lead to the malfunction or  death of a cell and  can cause cancer.

Mutations that are  not affected by natural selection are  called neutral mutations. Their frequency in the population is  governed by mutation rate, genetic drift and  selective pressure on linked alleles. it  is understood that most of a species' genome, in the absence of selection, undergoes a steady accumulation of neutral mutations.

Individual genes can be affected by point mutations, also known as SNPs, in which a single base pair is  altered. The substitution of a single base pair may or  may not affect the function of the gene (see mutation) while deletions and  insertions of a single or  several base pairs usually results in a non-functional gene.^[10]

Mobile elements, transposons, make up a major fraction of the genomes of plants and  animals and  appear to have  played a significant role in the evolution of genomes. These mobile insertional elements can jump within a genome and  alter existing genes and  gene networks to produce evolutionary change and  diversity.

On the other hand, gene duplications, which may occur via a number of mechanisms, are  believed to be one major source of raw material for evolving new genes as tens to hundreds of genes are  duplicated in animal genomes every million years.^[11] Most genes belong to larger "families" of genes derived from a common ancestral gene (two genes from a species that are  in the same family are  dubbed "paralogs"). Another mechanism causing gene duplication is  intergenic recombination, particularly 'exon shuffling', i.e., an  aberrant recombination that joins the 'upstream' part of one gene with the 'downstream' part of another. Genome duplications and  chromosome duplications also appear to have  served a significant role in evolution. Genome duplication has been the driving force in the Teleostei genome evolution, where up to four genome duplications are  thought to have  happened, resulting in species with more  than 250 chromosomes.

Large chromosomal rearrangements do not necessarily change gene function, but do generally result in reproductive isolation, and, by definition, speciation (species (in sexual organisms) are  usually defined by the ability to interbreed). an  example of this mechanism is  the fusion of two chromosomes in the homo genus that produced human chromosome 2; this fusion did not occur in the chimp lineage, resulting in two separate chromosomes in extant chimps.

[ and  adaptation">edit] Selection and  adaptation

Main articles: Natural selection and  Adaptation

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A peacock's tail is  the canonical example of sexual selection

Natural selection comes from differences in survival and  reproduction . Differential mortality is  the survival rate of individuals to their reproductive age. Differential fertility is  the total genetic contribution to the next generation. Note that, whereas mutations and  genetic drift are  random, natural selection is  not, as it  preferentially selects for different mutations based on differential fitnesses. For example, rolling dice is  random, but always picking the higher number on two rolled dice is  not random. 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 that survive and  reproduce increase the frequency of their genes in the gene pool over those that do not survive.
• Sexual selection occurs when organisms 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:

• Positive or  directional selection increases the frequency of a beneficial mutation, or  pushes the mean in either direction.
• Purifying or  stabilizing selection maintains a common trait in the population by decreasing the frequency of harmful mutations and  weeding them out of the population. "Living fossils" are  arguably the product of stabilizing selection, as their form and  traits have  remained virtually identical over a long period. it  is argued that stabilizing selection is  the most common form of natural selection.
• Artificial selection refers to purposeful breeding of a species to produce a more  desirable and  “perfect” breed. Humans have  directed artificial selection in the breeding of both animals and  plants, with examples ranging from agriculture (crops and  livestock) to pets and  horticulture. However, because humans are  only part of the environment, the fractions of change in a species due to natural or  artificial means can be difficult to determine. Artificial selection within human populations is  a controversial enterprise known as eugenics.
• Balancing selection maintains variation within a population through a number of mechanisms, including:
  • Heterozygote advantage or  overdominance, 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 rare variants either have  increased fitness or  decreased fitness, because of their rarity.
• Disruptive selection favors both extremes, and  results in a bimodal distribution of gene frequency. The mean may or  may not shift.
• Selective sweeps describe the affect of selection acting on linked alleles. it  comes in two forms:
  • Background selection occurs when a deleterious mutation is  selected against, and  linked mutations are  eliminated along with the deleterious variant, resulting in lower genetic polymorphism in the surrounding region.
  • Genetic hitchhiking occurs when a beneficial allele is  selected for, and  linked alleles, which can be neutral or  beneficial, are  pushed towards fixation along with the beneficial allele.

Through the process of natural selection, organisms 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.

Evolution does not act in a linear direction towards a pre-defined "goal" — it  only responds to various types of adaptationary changes. The belief in a teleological evolution of this sort is  known as orthogenesis, and  is not supported by the scientific understanding of evolution. One example of this misconception is  the erroneous belief humans will evolve more fingers in the future on account of their increased use of machines such as computers. In reality, this would only occur if more  fingers offered a significantly higher rate of reproductive success than those not having them, which seems very unlikely at the current time.

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

[edit] Recombination

Main article: Genetic recombination

In asexual organisms, variants in genes on the same chromosome will always be inherited together — they are  linked, by virtue of being on the same DNA molecule. However, sexual organisms, in the production of gametes, shuffle linked alleles on homologous chromosomes inherited from the parents via meiotic recombination. This shuffling allows independent assortment of alleles (mutations) in genes to be propagated in the population independently. This allows bad mutations to be purged and  beneficial mutations to be retained more  efficiently than in asexual populations.

However, the meitoic recombination rate is  not very high - on the order of one crossover (recombination event between homologous chromosomes) per chromosome arm per generation. Therefore, linked alleles are  not perfectly shuffled away from each other, but tend to be inherited together. This tendency may be measured by comparing the co-occurrence of two alleles, usually quantified as linkage disequilibrium (LD). A set of alleles that are  often co-propagated is  called a haplotype. Strong haplotype blocks can be a product of strong positive selection.

Recombination is  mildly mutagenic, which is  one of the proposed reasons why it  occurs with limited frequency. Recombination also breaks up gene combinations that have  been successful in previous generations, and  hence should be opposed by selection. However, recombination could be favoured by negative frequency-dependent selection (this is  when rare variants increase in frequency) because it  leads to more  individuals with new and  rare gene combinations being produced.

When alleles cannot be separated by recombination (for example in mammalian Y chromosomes), there is  an observable reduction in effective population size, known as the Hill-Robertson effect, and  the successive establishment of bad mutations, known as Muller's ratchet.

[ and  population structure">edit] Gene flow, genetic drift, and  population structure

Main article: Population genetics

Map of the world showing distribution of camelids. Solid black lines indicate possible migration routes.

Gene flow (also called gene admixture or  simply migration) is  the exchange of genetic variation between populations, when geography and  culture are  not obstacles. Ernst Mayr thought that gene flow is  likely to be homogenising, and  therefore counteract selective adaptation. Where there are  obstacles to gene flow, the situation is  termed reproductive isolation and  is considered to be necessary for speciation.

The free movement of alleles through a population may also be impeded by population structure. For example, most real-world populations are  not actually fully interbreeding; geographic proximity has a strong influence on the movement of alleles within the population.

Horizontal gene transfer (HGT) (or Lateral gene transfers) is  any process in which an  organism transfers genetic material (i.e. DNA) to another organism that is  not its offspring. This mechanism allows for the transfer of genetic material between unrelated organisms of the same species or  of different species and  is a form of gene flow.

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A phylogenetic tree of all extant organisms, based on 16S rRNA gene sequence data, showing the evolutionary history of the three domains of life, bacteria, archaea and  eukaryotes. Originally proposed by Carl Woese.

Many mechanisms for horizontal gene transfer have  been observed, such as antigenic shift, reassortment, and  hybridization. Viruses can transfer genes between species via transduction.^[12] Bacteria can incorporate genes from other dead bacteria or  plasmids via transformation, exchange genes with living bacteria via conjugation, and  can have  plasmids "set up residence separate from the host's genome".^[13] Hybridization is  highly significant in plant speciation, and  one out of ten species of birds are  known to hybridize. ^[14] There are  also examples of hybridization in mammals and  insects, however it  most often produces sterile offspring.

HGT has been shown to result in the spread of antibiotic resistance across bacterial populations.^[15] Furthermore, findings indicate that HGT has been a major mechanism for prokaryotic and  eukaryotic evolution.^[16][17]

HGT complicates the inference of the phylogeny of life, as the original metaphor of a tree of life no longer fits. Rather, since genetic information is  passed to other organisms and  other species in addition to being passed from parent to offspring, "biologists [should] use the metaphor of a mosaic to describe the different histories combined in individual genomes and  use [the] metaphor of a net to visualize the rich exchange and  cooperative effects of HGT among microbes."^[18]

Main article: 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. Thus, over time even in the absence of selection upon the alleles, allele frequencies will tend to "drift" upward or  downward, eventually becoming "fixed" - that is, going to 0% or  100% frequency. Thus, fluctuations in allele frequency between successive generations may result in some alleles disappearing from the population due to chance alone. Two separate populations that begin with the same allele frequencies therefore might drift apart by random fluctuation into two divergent populations with different allele sets (for example, alleles present in one population could be absent in the other, or  vice versa).

The consequence of genetic drift depends strongly on the size of the population (generally abbreviated as N): drift is  important in small mating populations (see Founder effect and  Population bottleneck), where chance fluctuations from generation to generation can be large. 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  predominant 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. an  example of the effect of population structure is  the so-called founder effect, resulting from a migration or  population bottleneck, in which a population temporarily has very few individuals, and  therefore loses a lot of genetic variation. In this case, a single, rare allele may suddenly increase very rapidly in frequency within a specific population if it  happened to be prevalent in a small number of "founder" individuals. The frequency of the allele in the resulting population can be much higher than otherwise expected, especially for deleterious, disease-causing alleles. Since population size has a profound effect on the relative strengths of genetic drift and  natural selection, changes in population size can alter the dynamics of these processes considerably.

[ and  extinction">edit] Speciation and  extinction

An Allosaurus skeleton.

Speciation is  the process by which new biological species arise. This may take place by various mechanisms. Allopatric speciation occurs in populations that become isolated geographically, such as by habitat fragmentation or  migration.^[19] Sympatric speciation occurs when new species emerge in the same geographic area^[20][21] 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. an  example of rapid sympatric speciation can be eloquently represented in the triangle of U; where new species of Brassica sp. have  been made by the fusing of separate genomes from related plants.

Extinction is  the disappearance of species (i.e. gene pools). The moment of extinction generally occurs at 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. The Permian-Triassic extinction event was the Earth's most severe extinction event, rendering extinct 90% of all marine species and  70% of terrestrial vertebrate species. In the Cretaceous-Tertiary extinction event many forms of life perished (including approximately 50% of all genera), the most often mentioned among them being the extinction of the non-avian dinosaurs.

See also: Current Research in Evolutionary Biology

[edit] Evidence of evolution

Main article: Evidence of evolution

Tiktaalik in context: one of many species that track the evolutionary development of fish fins into tetrapod limbs.

Evolution has left numerous records that reveal the history of different species. Fossils, together with the comparative anatomy of present-day plants and  animals, constitute the morphological, or  anatomical, record. By comparing the anatomies of both modern and  extinct species, paleontologists can infer the lineages of those species. Important fossil evidence includes the connection of distinct classes of organisms by so-called "transitional" species, such as the Archaeopteryx, which provided early evidence for intermediate species between dinosaurs and  birds,^[22] and  the recently-discovered Tiktaalik, which clarifies the development from fish to animals with four limbs.^[23]

The development of molecular genetics, and  particularly of DNA sequencing, has allowed biologists to study the record of evolution left in organisms' genetic structures. The degrees of similarity and  difference in the DNA sequences of modern species allows geneticists to reconstruct their lineages. it  is from DNA sequence comparisons that figures such as the 95% genotypic similarity between humans and  chimpanzees are  obtained.^[24][25]

Additional evidence of ancestry includes idiosyncratic structures present in certain organisms, such as the panda's "thumb", which indicate how an  organism's evolutionary lineage constrains its adaptive development. Vestigial structures such as the vestigial limbs on pythons or  the degenerate eyes of blind cave-dwelling fish are  also evidences of evolutionary development.

Other evidence used to demonstrate evolutionary lineages includes the geographical distribution of species. For instance, monotremes, such as platypus, and  most marsupials, like kangaroos or  koalas, are  found only in Australia showing that their common ancestor with placental mammals lived before the submerging of the ancient land bridge between Australia and  Asia.

Scientists correlate all of the above evidence, drawn from paleontology, anatomy, genetics, and  geography, with other information about the history of Earth. For instance, paleoclimatology attests to periodic ice ages during which the world's climate was much cooler, and  these are  often found to match up with the spread of species which are  better-equipped to deal with the cold, such as the woolly mammoth.

[edit] Morphological evidence

Letter c in the picture indicates the undeveloped hind legs of a baleen whale, vestigial remnants of its terrestrial ancestors.

Fossils are  critical evidence for estimating when various lineages originated. Since fossilization of an  organism is  an uncommon occurrence, usually requiring hard parts (like teeth, bone or  pollen), the fossil record is  traditionally thought to provide only sparse and  intermittent information about ancestral lineages. Fossilization of organisms without hard body parts is  rare, but happens under unusual circumstances, such as rapid burial, low oxygen environments, or  microbial action.^[26]

The fossil record provides several types of data important to the study of evolution. First, the fossil record contains the earliest known examples of life itself, as well as the earliest occurrences of individual lineages. For example, the first complex animals date from the early Cambrian period, approximately 520 million years ago. Second, the records of individual species yield information regarding the patterns and  rates of evolution, showing for example if species evolve into new species (speciation) gradually and  incrementally, or  in relatively brief intervals of geologic time. Thirdly, the fossil record is  a document of large scale patterns and  events in the history of life, many of which have  influenced the evolutionary history of numerous lineages. For example, mass extinctions frequently resulted in the loss of entire groups of species, such as the non-avian dinosaurs, while leaving others relatively unscathed. Recently, molecular biologists have  used the time since divergence of related lineages to calibrate the rate at which mutations accumulate, and  at which the genomes of different lineages evolve.

Phylogenetics, 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. The appendages on bat wings, for example, are  very structurally similar to human hands, and  may constitute a vestigial structure. Other examples include the presence of hip bones in whales and  snakes. Such structures may exist with little or  no function in a more  current organism, yet have  a clear function in an  ancestral species of the same. Examples of vestigial structures in humans include wisdom teeth, the coccyx and  the vermiform appendix.

[edit] Molecular evidence

Comparison of the DNA sequences allows organisms to be grouped by sequence similarity, and  the resulting phylogenetic trees are  typically congruent with traditional taxonomy, and  are often used to strengthen or  correct taxonomic classifications. Sequence comparison is  considered a measure robust enough to be used to correct erroneous assumptions in the phylogenetic tree in instances where other evidence is  scarce. 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, and  6.6% from baboons.^[27] Genetic sequence evidence thus allows inference and  quantification of genetic relatedness between humans and  other apes.^[γ][28] The sequence of the 16S rRNA gene, a vital gene encoding a part of the ribosome, was used to find the broad phylogenetic relationships between all extant life. The analysis, originally done by Carl Woese, resulted in the three-domain system, arguing for two major splits in the early evolution of life. The first split led to modern Bacteria and  the subsequent split led to modern Archaea and  Eukaryote.

The proteomic evidence also supports the universal ancestry of life. Vital proteins, such as the ribosome, DNA polymerase, and  RNA polymerase are  found in the most primitive bacteria to the most complex mammals. The core part of the protein is  conserved across all lineages of life, serving similar functions. Higher organisms have  evolved additional protein subunits, largely affecting the regulation and  protein-protein interaction of the core. Other overarching similarities between all lineages of extant organisms, such as DNA, RNA, amino acids, and  the lipid bilayer, give support to the theory of common descent. The chirality of DNA, RNA, and  amino acids is  conserved across all known life. As there is  no functional advantage to right or  left handed molecular chirality, the simplest hypothesis is  that the choice was made randomly in the early beginnings of life and  passed on to all extant life through common descent.

There is  also a large body of molecular evidence for a number of different mechanisms for large evolutionary changes, among them genome and  gene duplication, horizontal gene transfer, recombination, and  endosymbiosis. These mechanisms have  been, or  are being incorporated into the Modern Evolutionary Synthesis :

1. Gene and  genome duplication facilitates rapid evolution by providing substantial quantities of genetic material under weak or  no selective constraints.
2. Horizontal gene transfer, the process in which an  organism transfers genetic material (i.e. DNA) to another cell that is  not its offspring, allows for large sudden evolutionary leaps in a species by incorporating beneficial genes evolved in another species.
3. Recombination is  both capable of reassorting large numbers of different alleles, and  establishing reproductive isolation.
4. The Endosymbiotic theory explains the origin of mitochondria and  plastids (e.g. chloroplasts), which are  organelles of eukaryotic cells, as the incorporation of an  ancient prokaryotic cell into ancient eukaryotic cell. Rather than evolving eukaryotic organelles slowly, this theory offers a mechanism for a large evolutionary changes by incorporating the genetic material and  biochemical composition of a separate species. This evolutionary mechanism has been observed. Hatena, a protist, is  an extant organism that is  undergoing endosymbiotic evolution.^[29][30]

Further evidence for reconstructing ancestral lineages comes from junk DNA such as pseudogenes, i.e., 'dead' genes, which steadily accumulate mutations.^[31]

Since metabolic processes do not leave fossils, research into the evolution of the basic cellular processes is  done largely by comparison of existing organisms. Many lineages diverged when new metabolic processes appeared, and  it is  theoretically possible to determine when certain metabolic processes appeared by comparing the traits of the descendants of a common ancestor or  by detecting their physical manifestations. As an  example, the appearance of oxygen in the earth's atmosphere is  linked to the evolution of photosynthesis.

[edit] Theoretical evidence

Mathematical models of evolution, pioneered by the likes of Sewall Wright, Ronald Fisher and  J. B. S. Haldane and  extended via diffusion theory by Motoo Kimura, allow predictions about the genetic structure of evolving populations. Direct examination of the genetic structure of modern populations via DNA sequencing has recently allowed verification of many of these predictions. For example, the Out of Africa theory of human origins, which states that modern humans developed in Africa and  a small sub-population migrated out (undergoing a population bottleneck), implies that modern populations should show the signatures of this migration pattern. Specifically, post-bottleneck populations (Europeans and  Asians) should show lower overall genetic diversity and  a more  uniform distribution of allele frequencies compared to the African population. Both of these predictions are  borne out by actual data from a number of studies. ^{[citation needed]}

[edit] Ancestry of organisms

See also: Common descent

 is  evidence of common descent."> is  evidence of common descent." width="240" height="144" longdesc="../../../h/u/x/Image%7EHuxley_-_Mans_Place_in_Nature.jpg_4481.html" />

Morphologic similarities in the Hominidae family is  evidence of common descent.

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 is  inferred from 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, there is  strong evidence from genetics that all organisms have  a common ancestor. For example, every living cell 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. The universality of these traits strongly suggests common ancestry, because the selection of many of these traits seems arbitrary.

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. However, a great deal of information about the early Earth has been destroyed by geological processes over the course of time.

[edit] History of life

Main article: Timeline of evolution

The chemical evolution (or abiogenesis) from self-catalytic chemical reactions to life (see Origin of life) is  not a part of biological evolution, but it  is unclear at which point such increasingly complex sets of reactions became what we would consider, today, to be living organisms.

Not much is  yet known about the earliest developments in life. However, it  is unmistakably clear that all existing organisms share certain traits, including cellular structure and  the same 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 (Archaea, 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.

Precambrian 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. If true, they would be the earliest known life on earth.

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 the 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.

The evolutionary process may be exceedingly slow. Fossil evidence indicates that the diversity and  complexity of modern life has developed over much of the history of the earth. Geological evidence indicates that the Earth is  approximately 4.57 billion years old. Studies on guppies by David Reznick at the University of California, Riverside, however, have  shown that the rate of evolution through natural selection can proceed 10 thousand to 10 million times faster than what is  indicated in the fossil record.^[32] Such comparative studies however are  invariably biased by disparities in the time scales over which evolutionary change is  measured in the laboratory, field experiments, and  the fossil record.

The ancestry of living organisms has traditionally been reconstructed from morphology, but is  increasingly supplemented with phylogenetic — the reconstruction of phylogenies by the comparison of genetic (usually DNA) sequence.^[33] Biologist Gogarten suggests that "the original metaphor of a tree no longer fits the data from recent genome research", and  that therefore "biologists [should] use the metaphor of a mosaic to describe the different histories combined in individual genomes and  use [the] metaphor of a net to visualize the rich exchange and  cooperative effects of HGT among microbes".^[34]

[edit] History of evolutionary thought

Main article: History of evolutionary thought

Charles Darwin in 1854, five years before publishing The Origin of Species.

The idea of biological evolution has existed since ancient times, notably among Greek philosophers such as Anaximander and  Epicurus and  Indian philosophers such as Patañjali. However, scientific theories of evolution were not proposed until the 18th and  19th centuries, by scientists such as Jean-Baptiste Lamarck and  Charles Darwin.

The transmutation of species was accepted by many scientists before 1859, but Charles Darwin's On the Origin of Species by Means of Natural Selection provided the first convincing exposition^[α] of a mechanism by which evolutionary change could occur: natural selection. After many years of working in private on his theory, Darwin was motivated to publish his work on evolution when he received a letter from Alfred Russel Wallace in which Wallace revealed his own, independent discovery of natural selection. Accordingly, Wallace is  given shared credit for originating the theory.^[35][36]

The publication of Darwin's book sparked a great deal of scientific and  social debate. Although the occurrence of biological evolution of some sort came to be widely accepted by scientists, Darwin's specific ideas about evolution — that it  occurred gradually, through natural selection — were actively attacked and  contested. Additionally, while Darwin was able to observe variation, and  to infer natural selection and  thereby adaptation, he was unable to explain how variation might arise or  be altered over generations.^[37]

Gregor Mendel's work on the inheritance of traits in pea plants laid the foundation for genetics.

Work on plant hybridity by a contemporary of Darwin's, Gregor Mendel, revealed that certain traits in peas occurred in discrete forms (that is, they were either one distinct trait or  another, such as "round" or  "wrinkled") and  were inherited in a well-defined and  predictable manner.^[38] When Mendel's work was "rediscovered" in 1901, it  was initially interpreted as supporting an  anti-Darwinian "jumping", saltationist form of evolution, and  contradicting the biometricians' gradualism.^[β]

However, the simple version of the theory of early Mendelians soon gave way to the classical genetics of Thomas Hunt Morgan and  his school, which thoroughly grounded and  articulated the applications of Mendelian laws to biology. Eventually, it  was shown that a rigorous statistical approach to Mendelism was reconcilable with the data of the biometricians by the work of statistician and  population geneticist R.A. Fisher in the 1930s. Following this, the work of population geneticists and  zoologists in the 1930s and  1940s synthesized Darwinian evolution with genetics, creating the modern evolutionary synthesis.^[38] Genes were then still theoretical entities, and  many paleontologists and  embryologists were inclined to dismiss them as being of no, or  minor, importance, but subsequent advancements have  made genetics a key aspect of evolutionary biology.^[39]

Stephen Jay Gould was a major proponent of punctuated equilibrium.

The most significant recent developments in evolutionary biology have  been the improved understanding of and  advances in genetics.^[40] In the 1940s, following up on Griffith's experiment, Avery, MacLeod and  McCarty definitively identified DNA (deoxyribonucleic acid) as the "transforming principle" responsible for transmitting genetic information. In 1953, Francis Crick and  James D. 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 (see molecular evolution): the mutation of segments of DNA. George C. Williams's 1966 Adaptation and  natural selection: A Critique of some Current Evolutionary Thought and  Richard Dawkins' The Selfish Gene marked a departure from the idea of groups or  organisms as units of selection toward the modern gene-centered view of evolution. In the mid-1970s, Motoo Kimura formulated the neutral theory of molecular evolution, a significant departure from the consensus view of evolution, as it  considered genetic drift, rather than natural selection to be the predominant mode of evolution.

Debates over various aspects of how evolution occurs have  continued. Two prominent debates are  over the theory of punctuated equilibrium, proposed in 1972 by paleontologists Niles Eldredge and  Stephen Jay Gould to explain the paucity of gradual transitions between species in the fossil record, as well as the absence of change or  stasis that is  observed over significant intervals of time; and  also the Neutralist-Selectionist debate.

[edit] Modern synthesis

Main article: Modern evolutionary synthesis

Charles Darwin was able to observe variation, infer natural selection and  thereby adaptation, but didn't know the basis of heritability. He couldn't explain how organisms might change over generations. it  also seemed that when two individuals were crossed, their traits must be blended in the progeny, so that eventually all variation would be lost.

The blending problem was solved when the population geneticists R.A. Fisher, Sewall Wright, and  J. B. S. Haldane, married Darwinian evolutionary theory to population genetics, which was based on Mendelian genetics (genes as discrete units of heredity).

The problem of what the mechanisms might be was solved in principle with the identification of DNA as the genetic material by Oswald Avery and  colleagues, and  the articulation of the double-helical structure of DNA by James Watson and  Francis Crick provided a physical basis for the notion that genes were encoded in DNA.

[edit] Heredity

A section of a model of a DNA molecule.

Gregor Mendel's work provided the first firm basis to the idea that heredity occurred in discrete units. He noticed several traits in peas that occur in only one of two forms (e.g., the peas were either "round" or  "wrinkled"), and  was able to show that the traits were: heritable (passed from parent to offspring); discrete (i.e., if one parent had round peas and  the other wrinkled, the progeny were not intermediate, but either round or  wrinkled); and  were distributed to progeny in a well-defined and  predictable manner (Mendelian inheritance). His research laid the foundation for the concept of discrete heritable traits, known today as genes. After Mendel's work was "rediscovered" in 1900, it  was discovered that the concepts could have  wide applicability, and  that most complex traits were polygenetic and  not controlled by single unit characters.

Later research gave a physical basis to the notion of genes, and  eventually identified DNA as the genetic material, and  identified genes as discrete elements within DNA. DNA is  not perfectly copied, and  rare mistakes (mutations) in genes can affect traits that the genes control (e.g., pea shape).

A gene can have  modifications such as DNA methylation, which do not change the nucleotide sequence of a gene, but do result in the epigenetic inheritance of a change in the expression of that gene in a trait. Another epigenetic mechanism is  via micro RNA's, and  RNA interference which serve regulatory roles in gene transcription and  translation.

Non-DNA based forms of heritable variation exist, such transmission of the secondary structures of prions, and  structural inheritance of patterns in the rows of cilia in protozoans such as Paramecium^[41] and  Tetrahymena. Investigations continue into whether these mechanisms allow for the production of specific beneficial heritable variation in response to environmental signals. If this were 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. However, the processes that produce these variations leave the genetic information intact and  are often reversible, and  are rather rare.

[edit] Study of evolution

Main article: Evolutionary biology

[edit] Academic disciplines

Scholars in a number of academic disciplines continue to document examples of evolution, contributing to a deeper understanding of its underlying mechanisms. Every subdiscipline within biology both informs and  is informed by knowledge of the details of evolution, such as in ecological genetics, human evolution, molecular evolution, and  phylogenetics. Areas of mathematics (such as bioinformatics), physics, chemistry, and  other fields all make important contributions to current understanding of evolutionary mechanisms. Even disciplines as far removed as geology and  sociology play a part, since the process of biological evolution has coincided in time and  space with the development of both the Earth and  human civilization.

Evolutionary biology is  a subdiscipline of biology concerned with the origin and  descent of species, as well as their changes over time. it  was originally an  interdisciplinary field including 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 who use those organisms 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 (informally, evo-devo) is  a field of biology that compares the developmental processes of different animals in an  attempt to determine the ancestral relationship between organisms and  how developmental processes evolved. The discovery of genes regulating development in model organisms allowed for comparisons to be made with genes and  genetic networks of related organisms.

Physical anthropology emerged in the late 19th century as the study of human osteology, and  the fossilized skeletal remains of other hominids. At that time, anthropologists debated whether their evidence supported Darwin's claims, because skeletal remains revealed temporal and  spatial variation among hominids, but Darwin had not offered an  explanation of the specific mechanisms that produce variation. With the recognition of Mendelian genetics and  the rise of the modern synthesis, however, evolution became both the fundamental conceptual framework for, and  the object of study of, physical anthropologists. In addition to studying skeletal remains, they began to study genetic variation among human populations (population genetics); thus, some physical anthropologists began calling themselves biological anthropologists.

[edit] Misunderstandings about modern evolutionary biology

Although the modern synthesis is  a major achievement of modern science, some aspects of it  are often misunderstood. These misunderstandings have  hindered acceptance of the modern synthesis,^[42][43] most notably in the United States.^[44] Some of the most common misunderstandings are  outlined in this section.

[ and  fact">edit] Distinctions between theory and  fact

Main article:  and  fact">Evolution as theory and  fact

Further information: Scientific Theory

See also: Theory vs. Fact

Some critics of evolution claim that it  is merely a theory. This criticism in fact makes two claims: that evolution is  a theory, and  not a fact, and  that theories are  less established than facts. In other words, laypeople usually use the word "theory" to signify "conjecture", "speculation", or  "opinion," and  the word "fact" to refer to parts of the world, or  claims about the world, that are  real or  true regardless of what people think.

In scientific terminology however, a theory is  a model of the world (or some portion of it) from which falsifiable hypotheses can be generated and  tested through controlled experiments involving empirical observation. In this scientific sense, "facts" are  parts of theories. Fact is  not merely something obtained by observation, because there is  no certainty as to what that "something that has been observed" is  or means. In science, for an  observation to be considered a fact it  must be "confirmed to such a degree that it  would be perverse to withhold provisional assent"^[45] through consistent observation or  controlled study. Indeed, "facts" are  not merely phenomena that can be observed, they are  phenomena that are  deemed worthy of notice. it  is "theory" that guides observations and  our understanding of what we observe. In other words, for scientists "theory" and  "fact" do not stand in opposition, but rather exist in a reciprocal relationship.

Moreover, evolution itself is  both a fact and  a theory. When "evolution" is  used to describe a fact, it  refers to the observations that populations of one species of organism do, over time, change into new, or  several new, species. In this sense, evolution occurs whenever a new strain of bacterium evolves that is  resistant to antibodies that had been lethal to prior strains. Another clear case of evolution as fact involves the hawthorn fly, Rhagoletis pomonella. Different populations of hawthorn fly feed on different fruits. A new population spontaneously emerged in North America in the 19th century some time after apples, a non-native species, were introduced. The apple feeding population normally feeds only on apples and  not on the historically preferred fruit of hawthorns. Likewise the current hawthorn feeding population does not normally feed on apples. A current area of scientific research is  the investigation of whether or  not the apple feeding race may further evolve into a new species. Some evidence, such as the fact that six out of thirteen alozyme loci are  different, that hawthorn flies mature later in the season, and  take longer to mature, than apple flies, and  that there is  little evidence of interbreeding (researchers have  documented a 4-6%hybridization rate) suggests that this is  indeed occurring.^[9] (see Berlocher and  Bush 1982, Berlocher and  Feder 2002, Bush 1969, McPheron et. al. 1988, Prokopy et. al. 1988, Smith 1988)

When "evolution" is  used to describe a theory, it  refers to an  explanation for why and  a model of how evolution occurs. an  example of evolution as theory is  the modern synthesis of Darwin and  Wallace's theory of natural selection and  Mendel's principles of genetics. This theory has three major aspects:

1. Common descent of all organisms from a single ancestor or  ancestral gene pool.
2. Manifestation of novel traits in a lineage.
3. Mechanisms that cause some traits to persist while others perish.

When people provide evidence for evolution, in some cases they are  providing evidence that evolution occurs; in other cases they are  providing evidence that the modern synthesis is  the best explanation yet as to why and  how evolution occurs. Heritable variation, natural selection, and  response to selection (e.g. in domesticated plants and  animals) are  "facts", and  the generalization or  extrapolation beyond these phenomena, and  the explanation for them, is  the "theory of evolution".^[46]

Using the scientific meanings of the concepts "fact" and  "theory," it  is common to state that "evolution is  both a fact and  a theory". This is  true for gravity as well:

Exploring this issue, the paleontologist Stephen Jay Gould wrote that "evolution is  a theory. it  is also a fact. and  facts and  theories are  different things, not rungs in a hierarchy of increasing certainty. Facts are  the world's data. Theories are  structures of ideas that explain and  interpret facts. Facts do not go away when scientists debate rival theories to explain them. Einstein's theory of gravitation replaced Newton's, but apples did not suspend themselves in mid-air, pending the outcome. and  humans evolved from ape-like ancestors whether they did so by Darwin's proposed mechanism or  by some other yet to be discovered."^[45]

[ and  devolution">edit] Evolution, complexity, and  devolution

One of the most common misunderstandings of evolution is  that one species can be "more highly evolved" than another; that evolution is  necessarily progressive and/or leads to greater "complexity", or  that its converse is  "devolution".^[47] Evolution is  a non-directional process in the sense that it  does not proceed toward any ultimate goal. Rather, evolution acts to optimize organisms to the conditions of their environment, whatever they might be, through the process of natural selection. Thus, evolution provides no assurance that later generations will be more  complex than earlier generations, a fact that applies equally to human complexity and  intelligence as it  does to all other organisms extant on Earth today and  that have  lived in the past. The claim that evolution results in directional progress is  not part of modern evolutionary theory; it  derives from earlier belief systems which were held around the time Darwin formulated his ideas.

In many cases evolution does proceed in the direction of increasing complexity. The earliest organisms were maximally simple life forms. Evolution caused life to become more  complex, since becoming simpler wasn't advantageous; indeed it  was likely impossible since organisms of any lower complexity would be too simple to carry out basic life functions. Virtually the only avenues open for evolution in the earliest epoch of life’s history was toward more  complex and  specialized forms. Since then, evolved complexity is  a pattern that has occurred again  and again  in numerous lineages since it  produces more  specialized forms that are  capable of exploiting environmental niches more  effectively than competitors. Indeed, specialization through increased complexity allows an  organism to define its own niche and  evolve to it, thus increasing biodiversity as well.

Complexity however is  not an  ultimate goal of evolutionary processes, and  the evidence supporting this abounds. Bacterial forms have  remained comparatively simple over 3.8 billion years of evolution even though numerous forms have  evolved and  specialized to a variety of environments using diverse metabolic pathways. In these cases, complexity was not selected for even though substantial evolutionary change occurred. Simple organisms that rapidly metabolize energy rich nutrients available in the environment are  optimal forms that dominate the biosphere and  the cycling of elements in it.

Among eukarya much greater levels of complexity have  evolved, but it  must be remembered that these organisms represent the smaller portion of the biomass on earth. Given this fact, evolution toward complexity is  the exception and  not the norm. Also, numerous examples of organisms evolving towards simpler forms are  found to occur in this domain.

Vestigial structures constitute one line of evidence of lessening complexity evolving among eukarya. Pythons have  a vestigial pelvis that is  detached from the vertebra and  essentially floats in the abdominal cavity. This structure is  functionless and  degenerate from a functioning pelvis, with legs, in a previous ancestor. The Mexican tetra (Astyanax mexicanus) has evolved from a previous sighted ancestor into a blind form. While the animal retains a rudimentary eye, the nerves and  retina are  degenerate and  functionless.^[48] The human appendix is  a degenerate form of the caecum found in a wide variety of herbivorous vertebrates. In these animals, the caecum is  a significant organ that functions to digest cellulosic plant material. In humans and  hominoid apes, changes in diet have  favored its degeneration. In each instance, natural selection has acted to produce decreasing complexity over time because the maintenance of useless organs represents a drain on the organism’s energy resources and  is thus subject to negative selective pressure.^[49]

Similarly, horses evolved from morphologically more  complex ancestors with five toes on their feet. Over time, the fossil record shows a progression to reduced complexity that has produced the single toe (hoof) observed today. In the case of this animal, a simpler design is  superior for the conditions under which it  must survive. Many parasitic and  symbiotic species have  evolved simpler forms from more  complex to the point that they are  incapable of existing outside the bodies of their hosts or  symbionts. In these cases, reduced complexity offers a selective advantage but comes with the cost of surrendering independence.

There is  no guarantee that any particular organism existing today will become more  complex in the future. In fact, natural selection will only favor increasing complexity if it  increases an  organism’s chance of survival and  ability to produce viable offspring that live long enough to themselves reach sexual maturity. The same mechanism will also favor lower complexity in traits if that confers a selective advantage in the organism's environment.^[50]

[edit] Speciation

Main article: Speciation

 is  evidence of the occurrence of speciation."> is  evidence of the occurrence of speciation." width="250" height="236" longdesc="../../../d/a/r/Image%7EDarwin%27s_finches.jpeg_45d1.html" />

The existence of several different, but related, finches on the Galápagos Islands is  evidence of the occurrence of speciation.

It is  sometimes claimed that speciation — the origin of new species — has never been directly observed, and  thus evolution cannot be called sound science. This is  a misunderstanding of both science and  evolution. First, scientific discovery does not occur solely through reproducible experiments; the principle of uniformitarianism allows natural scientists to infer causes through their empirical effects. Moreover, since the publication of On the Origin of Species scientists have  confirmed Darwin's hypothesis by data gathered from sources that did not exist in his day, such as DNA similarity among species and  new fossil discoveries. Finally, speciation has actually been directly observed.^[51] (See the hawthorn fly example.) Further, there are  a number of examples of speciation in plants,^[52] and  differences in ectodysplasin alleles in stickleback fish speciation has developed as a supermodel for studying gene alterations and  speciation.^[53]

A variation of this assertion, that microevolution has been directly observed and  macroevolution has not, is  subject to the same counterarguments. However, it  is generally accepted that macroevolution uses the same mechanisms of change as those already observed in microevolution.

[ and  life">edit] Entropy and  life

Main articles:  and  life">Entropy and  life and  Self-organization

It is  claimed that evolution, by increasing complexity, violates the second law of thermodynamics. This law posits that in an  idealised isolated system, entropy will tend to increase or  stay the same. Entropy is  a measure of the dispersal of energy in a physical system so that it  is not available to do mechanical work.^[54] The claim ignores the fact that biological systems are  not isolated systems. Life inherently involves open systems, not isolated systems, as all organisms exchange energy and  matter with their environment, and  similarly the Earth receives energy from the Sun and  emits energy back into space. Simple calculations^[55] show that the Sun-Earth-space system does not violate the second law because the enormous increase in entropy due to the Sun and  Earth radiating into space dwarfs the small decrease in entropy caused by the evolution of life.

[edit] Information

Some assert that evolution cannot create information, or  that information can only be created by an  intelligence. Physical information exists regardless of the presence of an  intelligence, and  evolution allows for new information whenever a novel mutation or  gene duplication occurs and  is kept. it  does not need to be beneficial or  visually apparent to be "information." However, even if those were requirements they would be satisfied with the appearance of nylon eating bacteria,^[56] which required new enzymes to efficiently digest a material that never existed until the modern age.^[δ]

Japanese researchers demonstrated that nylon degrading ability can be obtained de novo in laboratory cultures of Pseudomonas aeruginosa strain POA, which initially had no enzymes capable of degrading nylon oligomers. This indicates that the ability of bacteria to digest nylon can evolve if proper artificial selection is  applied.^[57] Recently, the same group found X-ray crystal structure of the newly evolved nylon-digesting enzyme.^[58] Using the structural results, the authors propose "that the amino acid replacements in the catalytic cleft of a preexisting esterase resulted in the evolution of the" nylon-digesting enzyme. This hypothesis still needs to be confirmed by detailed genetic studies.

[ and  religious controversies">edit] Social and  religious controversies

Main articles: Social effect of evolutionary theory and  Creation-evolution controversy

 and  other apes share a common lineage."> and  other apes share a common lineage." width="150" height="190" longdesc="../../../d/a/r/Image%7EDarwin_ape.jpg_5659.html" />

A satirical 1871 image of Charles Darwin as a quadrupedal ape reflects part of the social controversy over whether humans and  other apes share a common lineage.

Ever since the publication of The Origin of Species in 1859, the modern science of evolution has been a source of nearly constant controversy. In general, controversy has centered on the philosophical, cosmological, social, and  religious implications of evolution, not on the science of evolution itself. The proposition that biological evolution occurs through the mechanism of natural selection has been almost completely uncontested within the scientific community for much of the 20th century.^[59]

As Darwin recognized early on, perhaps the most controversial aspect of evolutionary thought is  its applicability to human beings. The idea that all diversity in life, including human beings, arose through natural processes without a need for supernatural intervention poses difficulties for the belief in purpose inherent in most religious faiths — and  especially for the Abrahamic religions. Many religious people are  able to reconcile the science of evolution with their faith, or  see no real conflict;^[60] Judaism and   and  the Roman Catholic Church">Catholicism are  notable as major faith traditions whose adherents generally see no conflict between evolutionary theory and  religious belief.^[61][62] The idea that faith and  evolution are  compatible has been called theistic evolution. Another group of religious people, generally referred to as creationists, consider evolutionary origin beliefs to be incompatible with their faith, their religious texts and  their perception of design in nature, and  so cannot accept what they call "unguided evolution".

One particularly contentious topic evoked by evolution is  the biological status of humanity. Whereas the classical religious view can broadly be characterized as a belief in the great chain of being (in which people are  "above" the animals but slightly "below" the angels), the science of evolution shows that humans are  animals and  share common ancestry with chimpanzees, gibbons, gorillas, and  orangutans, which some people find offensive, for, in their opinion, it  "degrades" humankind. A related conflict arises when critics combine the religious view of people's superior status with the mistaken notion that evolution is  necessarily "progressive". If human beings are  superior to animals yet evolved from them, these critics claim, inferior animals would not still exist. Because animals that are  inferior creatures do demonstrably exist, those criticising evolution sometimes incorrectly take this as supporting their claim that evolution is  false.^[63]

In some countries — notably the United States — these and  other tensions between religion and  science have  fueled what has been called the creation-evolution controversy, which, among other things, has generated struggles over teaching curricula. While many other fields of science, such as cosmology^[64] and  earth science^[65] also conflict with a literal interpretation of many religious texts, evolutionary studies in biology have  borne the brunt of these debates. it  also affects the public school system, where religion is  assumed to have  no effect on the teachings.

Evolution has been used to support philosophical and  ethical choices which most contemporary scientists consider were neither mandated by evolution nor supported by science. For example, the eugenic ideas of Francis Galton were developed into arguments that the human gene pool should be improved by selective breeding policies, including incentives for reproduction for those of "good stock" and  disincentives, such as compulsory sterilization, "euthanasia", and  later, prenatal testing, birth control, and  genetic engineering, for those of "bad stock". Another example of an  extension of evolutionary theory that is  now widely regarded as unwarranted is  "Social Darwinism"; a term given to the 19th century Whig Malthusian theory developed by Herbert Spencer into ideas about "survival of the fittest" in commerce and  human societies as a whole, and  by others into claims that social inequality, racism, and  imperialism were justified.^[66]

[edit] See also

For a more  comprehensive list of topics, see Category:Evolution and  Category:Evolutionary biology

[edit] Notes

[edit] References

[edit] General references

[edit] External links

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• Understanding Evolution from University of California, Berkeley
• Evolution of Life Explained by Carl Sagan
• Natural Selection Explained by Carl Sagan
• National Academies Evolution Resources
• National Academy Press: Teaching About Evolution and  the Nature of Science
• Evolution by Natural Selection, an  introduction to the logic of evolution by natural selection
• Charles Darwin's writings
• A list of evidence for evolution compiled by PZ Myers.
• EvoWiki, a wiki whose goal is  to promote general evolution education, and  provide mainstream scientific responses to the arguments of antievolutionists
• Evolution (provided by PBS)
• Everything you wanted to know about evolution (provided by New Scientist)
• International Journal of Organic Evolution
• New England Complex Systems Institute
• Howstuffworks.com — How Evolution Works
• Synthetic Theory Of Evolution: an  Introduction to Modern Evolutionary Concepts and  Theories
• Evolution News from Genome News Network (GNN)
• Evolution for beginners
• RMCybernetics - AI Evolution can create emergent behavior in a computer program.
• NPR - Science Friday: links to museums, articles and  books.
• "Evolution: Fact and  Theory" by Richard E. Lenski
• Evolution by level Book reviews of books on evolution by knowledge level.
• Understanding Evolution: History, Theory, Evidence, and  Implications Deals heavily with the history of evolutionary thought
• Becoming Human - Journey through the story of human evolution
• Evolution & ID Timeline Focuses on major historical and  recent events in the scientific and  political debate
• Timeline of Evolutionary Thought The life and  work of notable people who have  contributed to evolutionary thought
• Talk.Origins Archive — see also talk.origins

[edit] Evolution simulators

• Isolated species evolves to interact more  efficiently with its environment (java applet)
• Evolution in a predator-prey relationship (java applet)
• Watch small creatures evolve into more  efficient swimmers
• Blind Watchmaker Applet (java)

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