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Evolution

2007 Schools Wikipedia Selection. Related subjects: Evolution and
reproduction

   In 1832, while travelling 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 armour were from the
   gigantic extinct glyptodons, creatures related to the modern armadillos
   he had seen living nearby. The similarities between these two unusually
   scaly animals and their geographic distribution provided Darwin with a
   clue that helped him develop his theory of how evolution occurs.
   Enlarge
   In 1832, while travelling 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 armour were from the
   gigantic extinct glyptodons, creatures related to the modern armadillos
   he had seen living nearby. The similarities between these two unusually
   scaly animals and their geographic distribution provided Darwin with a
   clue that helped him develop his theory of how evolution occurs.

   In biology, evolution is change in the heritable traits of a population
   over successive generations, as determined by shifts in the allele
   frequencies of genes. Over time, this process can result in speciation,
   the development of new species from existing ones. 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.

   The basic mechanisms that produce evolutionary change are natural
   selection (which includes ecological, sexual, and kin selection) and
   genetic drift; these two mechanisms act on the genetic variation
   created by mutation, genetic recombination, and gene flow. Natural
   selection is the process by which individual organisms with favorable
   traits are more likely to survive and reproduce. If those traits are
   heritable, they are passed to the organisms' offspring, with the result
   that beneficial heritable traits become more common in the next
   generation. Given enough time, this passive process can result in
   varied adaptations to changing environmental conditions.

   The modern understanding of evolution is based on the theory of natural
   selection, which was first set out in a joint presentation in 1858 of a
   pair of papers by Charles Darwin and Alfred Russel Wallace and
   popularized in Darwin's 1859 book The Origin of Species. In the 1930s,
   Darwinian natural selection was combined with the theory of Mendelian
   heredity to form the modern evolutionary synthesis, also known as "Neo-
   Darwinism". The modern synthesis describes evolution as a change in the
   frequency of alleles within a population from one generation to the
   next. With its enormous explanatory and predictive power, this theory
   has become the central organizing principle of modern biology, relating
   directly to topics such as the origin of antibiotic resistance in
   bacteria, eusociality in insects, and the staggering biodiversity of
   Earth's ecosystem.

   Although there is overwhelming scientific consensus supporting the
   validity of evolution, it has been at the centre of many social and
   religious controversies since its inception because of its implications
   for the origins of humankind.
   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
   Biology Portal ·

Study of evolution

History of evolutionary thought

   Charles Darwin in 1854, five years before publishing The Origin of
   Species.
   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 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 sometimes given shared
   credit for originating the theory.

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

   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.
   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.
   Stephen Jay Gould was a major proponent of punctuated equilibrium.
   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. 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' 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.

Academic disciplines

   Scholars in a number of academic disciplines continue to document
   examples of the theory 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 foundational contributions to the theory of evolution. 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.

Evidence of evolution

   Tiktaalik in context: one of many species that track the evolutionary
   development of fish fins into tetrapod limbs.
   Enlarge
   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, and the recently-discovered Tiktaalik,
   which clarifies the development from fish to animals with four limbs.

   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.

   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.

Morphological evidence

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

   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.

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. Genetic
   sequence evidence thus allows inference and quantification of genetic
   relatedness between humans and other apes. 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.

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

   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.

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.

Ancestry of organisms

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

History of life

   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.
   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.
   Enlarge
   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.

   Not much is known about the earliest developments in 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 ( Archaea, Bacteria,
   Eukaryota) or the origin of life. Attempts to shed light on the
   earliest history of life generally focus on the behaviour of
   macromolecules, particularly RNA, and the behaviour of complex systems.

   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. 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. 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".

Modern 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 genetic theory, 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.

Heredity

   A section of a model of a DNA molecule.
   Enlarge
   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.

   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 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.

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
   neighbour). This phenotypic variation is the result of variants in gene
   sequences among the individuals of a population. 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.

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."

Mutation

   Mutation occurs because of "copy errors" that occur during DNA
   replication.
   Enlarge
   Mutation occurs because of "copy errors" that occur 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.

   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. 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.

Selection and adaptation

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

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 homomolgous 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.

Gene flow and Population structure

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

   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.

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.

Horizontal gene transfer

   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.
   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.
   Enlarge
   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. 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".

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

   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."

Speciation and extinction

   An Allosaurus skeleton.
   Enlarge
   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. 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. 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.

Misunderstandings about modern evolutionary biology

   Although the modern synthesis is a central theory in science, many
   misunderstandings about ideas of modern synthesis prevail among some
   within the general population. These misunderstandings have hindered
   acceptance of modern synthesis, most notably in the United States. Some
   of the most common misunderstandings are outlined in this section.

Distinctions between theory and fact

   Stephen Jay Gould explained 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."

   The modern synthesis, like its Mendelian and Darwinian antecedents, is
   a scientific theory. A theory is an attempt to identify and describe
   relationships between phenomena or things, and generates falsifiable
   predictions which can be tested through controlled experiments and
   empirical observation. Speculative or conjectural explanations tend to
   be called hypotheses, and well tested explanations, theories. Fact
   tends to mean a datum, an observation, i.e., a fact is obtained by a
   fairly direct observation. However, a fact does not mean absolute
   certainty; in science, fact can only mean "confirmed to such a degree
   that it would be perverse to withhold provisional assent." A theory is
   obtained by inference from a body of facts. A related concept is a
   scientific law. It is common to encounter reference to the "law of
   natural selection" or the "laws of evolution." For example, see the
   article on physical law. Fact and theory denote the epistemological
   status of knowledge: how the knowledge was obtained, what sort of
   knowledge it is.

   In this scientific sense, "facts" are what theories attempt to explain.
   So, for scientists, "theory" and "fact" do not stand in opposition, but
   rather exist in a reciprocal relationship; for example, it is a "fact"
   that apples have fallen to the ground the last billion times they were
   dropped and the "theory" which explains this is the current theory of
   gravitation. In the same way, 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".

Evolution 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". Evolution provides no assurance that
   later generations are more intelligent or complex than earlier
   generations. The claim that evolution results in 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 involve "progression" towards more
   complexity, since the earliest lifeforms were extremely simple compared
   to many of the species existing today, and there was nowhere to go but
   up. However, there is no guarantee that any particular organism
   existing today will become more intelligent, more complex, bigger, or
   stronger in the future. In fact, natural selection will only favour
   this kind of "progression" if it increases chance of survival, i.e. the
   ability to live long enough to raise offspring to sexual maturity. The
   same mechanism can actually favour lower intelligence, lower
   complexity, and so on if those traits become a selective advantage in
   the organism's environment. One way of understanding the apparent
   "progression" of lifeforms over time is to remember that the earliest
   life began as maximally simple forms. Evolution caused life to become
   more complex, since becoming simpler wasn't advantageous. Once
   individual lineages have attained sufficient complexity, however,
   simplifications ( specialization) are as likely as increased
   complexity. This can be seen in many parasite species, for example,
   which have evolved simpler forms from more complex ancestors.

Speciation

   The existence of several different, but related, finches on the
   Galápagos Islands is evidence of the occurrence of speciation.
   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. (See the hawthorn fly example.) Further, there are a number
   of examples of speciation in plants, and differences in ectodysplasin
   alleles in stickleback fish speciation has developed as a supermodel
   for studying gene alterations and speciation.

   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.

Entropy and life

   It is claimed that evolution, by increasing complexity without
   supernatural intervention, 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. 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 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.

   In statistical thermodynamics entropy has been envisioned as a measure
   of the statistical "disorder" at a microstate level, leading to the
   mistaken idea that entropy implies increasing chaos. Misunderstandings
   also arise from the mistaken idea that the second law applies in some
   way to information entropy.

   The flow of matter and energy allows self-organization, enabling an
   increase in complexity without guidance or management. Examples of this
   spontaneous order include mineral crystals, snowflakes, and rain
   droplets. In the world we observe many cases where the natural course
   is increasing order.

   Self assembly is ubiquitous in biological systems and for
   nanostructures under equilibrium and some in non-equilibrium
   conditions. There are numerous examples of entropy driving order and
   self assembly.

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, which required new enzymes to efficiently digest a
   material that never existed until the modern age.

   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. Recently, the
   same group solved the high resolution X-ray crystal structure of the
   newly evolved nylon-digesting enzyme. Using the structural results, the
   authors propose "that the amino acid replacements in the catalytic
   cleft of a preexisting esterase with the beta-lactamase fold resulted
   in the evolution of the" nylon-digesting enzyme. This hypothesis still
   needs to be confirmed by detailed mutagenesis studies.

Social and religious controversies

   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.
   Enlarge
   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.

   Starting with 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.

   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; Judaism and Catholicism are notable as major
   faith traditions whose adherents generally see no conflict between
   evolutionary theory and religious belief. 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.

   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 and earth science also conflict with a
   literal interpretation of many religious texts, evolutionary studies in
   biology have borne the brunt of these debates.

   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.

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