Evolution
In
biology,
evolution is the change in the
heritable traits of a
population over successive generations, as determined by shifts in the
allele frequencies of
genes. Through the course of time, this process results in the origin of new
species from existing ones (
speciation). It is the source of the vast
diversity of extant and
extinct life in the world; all contemporary
organisms are related to each other through
common descent, the products of cumulative evolutionary changes over billions of years.
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 pass them to their 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 1858 paper 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
allele frequency within a population from one generation to the next.
The theory of evolution 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 the living world. The
modern evolutionary synthesis is broadly received as
scientific consensus and has replaced earlier explanations for the origin of species, including
Lamarckism, and is currently the most
powerful theory explaining biology.
Because of its potential implications for the origins of humankind, evolutionary theory has been at the center of many
social and religious controversies since its inception.
History of evolutionary thought
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. Scientific theories of evolution were proposed in the 18th and 19th centuries, by scientists such as
Jean-Baptiste Lamarck and
Charles Darwin.
Classical Darwinian theory
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
[In the years after Darwin's publication and fame, numerous "predecessors" to natural selection were discovered, such as William Charles Wells and Patrick Matthew, who had published unelaborated and undeveloped versions of similar theories earlier to little or no attention. Historians acknowledge that Darwin was the first to develop the theory rigorously and developed it independently. On Matthew, one historian of evolution has written that he "did suggest a basic idea of selection, but he did nothing to develop it; and he published it in the appendix to a book on the raising of trees for shipbuilding. No one took him seriously, and he played no role in the emergence of Darwinism. Simple priority is not enough to earn a thinker a place in the history of science: one has to develop the idea and convince others of its value to make a real contribution. Darwin's notebooks confirm that he drew no inspiration from Matthew or any of the other alleged precursors." ] of a mechanism by which evolutionary change could occur:
natural selection. Darwin worked in private for many years, developing comprehensive justification for his theory, then brought forward publication of his work on evolution after receiving 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. Darwin's work relied on many different fields of scientific inquiry for its evidence, and as a consequence debates over the theory took place in many different arenas. The book also was very popular among the literate public, and was soon translated into many languages.
Darwin was able to observe variation, and infer natural selection and thereby adaptation, but the basis of heritability wasn't known, so he couldn't explain
how variation might arise, or be altered over generations, and Darwin's proposal of a
hereditary mechanism (
pangenesis) was not compelling to biologists. Although the occurrence of 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. From the end of the 19th century through the early 20th century, forms of neo-Lamarckism, "progressive" evolution (
orthogenesis), and an evolution which worked by "jumps" (
saltationism, as opposed to
gradualism) became popular, although a form of neo-Darwinism, led by
August Weismann, also enjoyed some minor success. The biometric school of evolutionary theory, resulting from the work of Darwin's cousin,
Francis Galton, emerged as well, using statistical approaches to biology which emphasized gradualism and some aspects of natural selection.
Modern synthesis
Darwin's lack of a hereditary mechanism is often seen today as a major stumbling block in the historical acceptance of his theory, but in his time it was not a pressing issue as questions of the development of an organism were seen as more important than questions of the transmission of hereditary traits; Darwin and other biologists of his day thought that the answers to heredity would be found in
embryology rather than in breeding experiments. Work on plant hybridity by a contemporary of Darwin's, an obscure
Augustinian monk in
Bohemia named
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. Mendel's
Law of Segregation and
Law of Independent Assortment would eventually become key theories in the development of
genetics, but in Darwin's time their significance was not seen (even by Mendel himself).
When Mendel's work was "rediscovered" in 1901, it was initially interpreted as supporting an anti-Darwinian "jumping" form of evolution. The convinced Mendelians, such as
William Bateson and
Charles Benedict Davenport, and biometricians, such as
Walter Frank Raphael Weldon and
Karl Pearson, became embroiled in a bitter debate, with Mendelians charging that the biometricians did not understand biology, and biometricians arguing that most biological traits exhibited continuous variation rather than the "jumps" expected by the early Mendelian theory (We now know that the Mendelians were investigating
Mendelian traits (
i.e., those where existing variation is controlled by one gene, and therefore is discrete, and the biometricians were investigating
complex traits (
i.e., those controlled by multiple genes, where the variation is therefore continuous)). 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 —notably
Sewall Wright and
J. B. S. Haldane — 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.
[Resynthesizing evolutionary and developmental biology. Gilbert SF, Opitz JM, Raff RA. Developmental Biology 1996 Feb 1;173(2):357-72]Debates over various aspects of how evolution occurs have continued. One prominent debate was 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.
Molecular genetics
The most significant recent developments in
evolutionary biology have been the improved understanding of and advances in
genetics.
[ According to the BBC, Colin Norman, news editor of Science, said "[S]cientists tend to take for granted that evolution underpins modern biology [...] Evolution is not just something that scientists study as an esoteric enterprise. It has very important implications for public health and for our understanding of who we are" and Dr. Mike Ritchie, of the school of biology at the University of St Andrews, UK said "The big recent development in evolutionary biology has obviously been the improved resolution in our understanding of genetics. Where people have found a gene they think is involved in speciation, I can now go and look how it has evolved in 12 different species of fly, because we've got the genomes of all these species available on the web."] 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 marked a departure from the idea of
group selection towards the modern notion of the gene as the
unit of selection. In the mid-1970s,
Motoo Kimura formulated the
neutral theory of molecular evolution, firmly establishing the importance of
genetic drift as a mechanism of evolution.
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.
Evolution has left numerous records which 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 the link 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 the organisms' genetic structures. The degree 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% similarity between humans and chimpanzees come.
Other evidence used to demonstrate evolutionary lineages includes the geographical distribution of species. For instance,
monotremes and most
marsupials 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. |
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.
[Two sources: 'Genomic divergences between humans and other hominoids and the effective population size of the common ancestor of humans and chimpanzees'. and 'Quantitative Estimates of Sequence Divergence for Comparative Analyses of Mammalian Genomes' "[1] [2]"] Genetic sequence evidence thus allows inference and quantification of genetic relatedness between humans and other apes.
[The picture labeled "Human Chromosome 2 and its analogs in the apes" in the article Comparison of the Human and Great Ape Chromosomes as Evidence for Common Ancestry is literally a picture of a link in humans that links two separate chromosomes in the nonhuman apes creating a single chromosome in humans. It is considered a missing link, and the ape-human connection is of particular interest. Also, while the term originally referred to fossil evidence, this too is a trace from the past corresponding to some living beings which when alive were the physical embodiment of this link.][The New York Times report Still Evolving, Human Genes Tell New Story, based on A Map of Recent Positive Selection in the Human Genome, states the International HapMap Project is "providing the strongest evidence yet that humans are still evolving" and details some of that evidence.] 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.
Molecular evidence also offers a mechanism for large evolutionary leaps and
macroevolution.
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. 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 sudden evolutionary leap by incorporating the genetic material and biochemical composition of a separate species. This evolutionary mechanism has been observed.
Heneta, 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.
[Pseudogene evolution and natural selection for a compact genome. "[3]"]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.
Evidence from studies of complex iteration
"It has taken more than five decades, but the electronic computer is now powerful enough to simulate evolution" assisting
bioinformatics in its attempt to solve biological problems.
[Simulated evolution gets complex] Computer science allows the
iteration of self changing
complex systems to be studied, allowing a mathematically exact understanding of the nature of the processes behind evolution and providing evidence for the hidden causes of known evolutionary events. The evolution of specific cellular mechanisms like
spliceosomes that can turn the cell's genome into a vast workshop of billions of interchangeable parts can be studied for the first time in an exact way.
Christoph Adami et al., for example, make this point in
Evolution of biological complexity:
}}
David J. Earl and Michael W. Deem also make this point in
Evolvability is a selectable trait:
}}
"Computer simulations of the evolution of linear sequences have demonstrated the importance of recombination of blocks of sequence rather than point mutagenesis alone. Repeated cycles of point mutagenesis, recombination, and selection should allow in vitro molecular evolution of complex sequences, such as proteins."
Evolutionary molecular engineering, also called "directed evolution" or "in vitro molecular evolution", involves the iterated cycle of mutation, multiplication with recombination, and selection of the fittest of individual molecules (proteins, DNA and RNA). The process of natural evolution can be reconstructed, showing possible paths from catalytic cycles based on proteins to ones based on RNA to ones based on DNA.
[scripps.edubio.kaist.ac.kr free-tutorial pubmedcentral.nih.gov] |
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 may be found in traits shared between all living organisms. In Darwin's day, the evidence of shared traits was based solely on visible observation of morphologic similarities, such as the fact that all birds; even those which do not fly; have wings. Today, 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 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 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 behavior of
macromolecules, particularly
RNA, and the behavior 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
4.6 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.
[Evaluation of the Rate of Evolution in Natural Populations of Guppies (Poecilia reticulata) "[5]"]. 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.
[Oklahoma State - Horizontal Gene Transfer: "Sequence comparisons suggest recent horizontal transfer of many genes among diverse species including across the boundaries of phylogenetic 'domains'. Thus determining the phylogenetic history of a species can not be done conclusively by determining evolutionary trees for single genes."] 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".
[esalenctr.org]Charles Darwin was able to observe variation, and infer natural selection and thereby adaptation, but didn't know the basis of heritability, and therefore 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).
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. |
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[BEISSON, J. & SONNEBORN, T. M. (1965). Cytoplasmic inheritance of the organization of the cell cortex of Paramecium aurelia. Proc. natn. Acad Sci. U.S.A. 53, 275-282 ] 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 neighbor). This phenopyic 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."
These mechanisms of evolution have all been observed in the present and in evidence of their existence in the past. Their study is being used to guide the development of new medicines and other health aids such as the current effort to prevent a
H5N1 (i.e. bird flu) pandemic.
[The use of evolutionary principles to guide disease diagnosis and drug development with respect to bird flu (i.e. H5N1 virus) is shown here at CDC. Here is the "tree of life" showing the evolution by reassortment of H5N1 that created the Z genotype in 2002 and here is evolution by antigenic drift that created dozens of highly pathogenic varieties of the Z genotype of avian flu virus H5N1, some of which are increasingly adopted to mammals. Evolution. Right before our eyes. ]Mutation
 |
Mutation occurs because of "copy errors" that occur during DNA replication. |
Natural genetic variation arises as random mutations that inevitably occur at a certain rate in genes. 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 often 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 a species' genome, in the absence of selection, undergoes a steady accumulation of neutral mutations.
Not all mutations are created equal; simple point mutations (substitutions) or
SNPs (Single Nucleotide Polymorphisms), which comprise a major class of genetic variation, and insertions and deletions (
indels) usually can only alter the function or regulation (spatial and temporal expression; levels of expression) of existing genes.
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; 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 for is intergenic recombination, particularly '
exon shuffling',
i.e., an abberant recombination that joins the 'upstream' part of one gene with the 'downstream' part of another.
Finally, large chromosomal rearrangements (like the fusion of two chromosomes in the chimp/human common ancestor that produced human chromosome 2) do not necessarily change gene function, but do generally result in reproductive isolation, and, by definition, speciation (since "species" (in sexual organisms) are usually defined by the ability to interbreed).
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, 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), we see a 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
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 Mayer 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, allele frequencies will tend to "drift" upward or downward, eventually becoming "fixed" - that is, going to 0% or 100% frequency. Fluctuations in allele frequency between successive generations may result in some alleles disappearing from the population. Two separate populations that begin with the same allele frequencies therefore might drift 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).
Many aspects of genetic drift depend on the size of the population (generally abbreviated as N). This is especially important in small mating populations (see
Founder effect), 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 'more efficient' in large populations, or equivalently, genetic drift is stronger in small populations. Finally, the time for an allele to become fixed in the population by genetic drift (that is, for all individuals in the population to carry that allele) depends on population size, with smaller populations requiring a shorter time to fixation.
Horizontal gene transfer
One source of genetic variation is horizontal
gene transfer, the movement of genetic material across species boundaries, which can include
horizontal gene transfer,
antigenic shift,
reassortment, and
hybridization. Viruses can transfer genes between species via
transduction,
[enmicro.pdf]. 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"
[Pennisi_2003.pdf].
Selection and adaptation
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 of time. 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, species become better adapted to their environments.
Adaptation is any evolutionary process that increases the
fitness of the individual, or sometimes the trait that confers increased fitness, e.g. a stronger prehensile tail or greater visual acuity. Note that adaptation is context-sensitive; a trait that increases fitness in one environment may decrease it in another.
Evolution does not act in a linear direction towards a pre-defined "goal" — it only responds to various types of adaptionary changes. The belief in a
telelogical 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.
Speciation and extinction
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.
Evolution is still an active field of research in the scientific community. Improvements in sequencing methods have resulted in a large increase of sequenced genomes, allowing for the testing and refining of the theory of evolution with respect to whole genome data. Advances in computational hardware and software have allowed for the testing and extrapolation of increasingly advanced evolutionary models. Discoveries in biotechnology have produced methods for the ‘'de novo'' synthesis of proteins and, potentially, entire genomes, driving evolutionary studies at the molecular level.
Though the modern synthesis is almost universally accepted within the
scientific community, people often find that it introduces concepts which go against their perception of
design, purpose, directive principle, or finality in nature. As
Louis Menand has pointed out, "Darwin wanted to establish... that the species — including human beings — were created by, and evolve according to, processes that are entirely natural, chance-generated, and blind."
[(Menand 2001: 121)] People can feel that such a theory robs life and the universe of any transcendental meaning.
In the resulting
controversy, publicity is given to
creationist arguments against evolution and natural selection, which generally involve misunderstandings or misconceptions about evolution or about science in general.
[15 Answers to Creationist Nonsense Scientific American] Some of the most common arguments are examined in this section. More are considered at
An Index to Creationist Claims.
Distinctions between theory and fact
Further information: Theory
The modern synthesis, like its Mendelian and Darwinian antecedents, is a scientific
theory.
When speaking casually, people use the word "theory" to signify "conjecture", "speculation", or "opinion", and the word "fact" to signify true, or verifiably true, statements. [[6]] In this sense, "theories" can be opposed to "facts". In a more strict sense, though, fact
and theory
denote the epistemological status of knowledge; how the knowledge was obtained, what sort of knowledge it is. In science, fact
tends to mean a datum
, an observation
, i.e.
, a fact is obtained by a fairly direct observation. In contrast, a theory
is obtained by inference from a body of facts
.
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, or empirical observation. Speculative or conjectural explanations tend to be called hypotheses, and well tested explanations, theories
.
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 an apple will fall to the ground if it becomes dislodged from a branch 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 is a Fact and a Theory]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".
[talkorigins Claim CB932: Evolution of degenerate forms] 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 devised his theory of evolution.
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 favor 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 favor 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.
[Scientific American; Biology: Is the human race evolving or devolving?]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. A variation of this assertion is that "microevolution" has been observed and "macroevolution" has not been observed. Some creationists redefine
macroevolution as a change from one "kind" to another (see
Created kind), though it is unclear what a "kind" in this context is intended to refer to. 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, above.)
Self-organization and entropy
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 amount of energy in a physical system which cannot be used to do mechanical work, and in
statistical thermodynamics it is envisioned as a measure of the statistical "disorder" at a
microstate level.
The claim ignores the fact that biological systems are not
isolated systems. The
Sun provides a large amount of energy to the
Earth, and this flow of heat results in huge increases in entropy, when compared with decreases associated with decreasing the disorder of biological systems.
In fact, the flow of matter and energy through
open systems allows
self-organization enabling an increase in complexity without guidance or management. Examples include mineral crystals and snowflakes. 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.
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.
[It wasn't a highly competent design because the bacteria weren't extracting a lot of energy from the process, just enough to get by. And it was based on a simply frame shift reading of a gene that had other uses. But with a simple frame shift of a gene that was already there, it could now "eat" nylon. Future mutations, perhaps point mutations inside that gene, could conceivably heighten the energy gain of the nylon decomp process, and allow the bacteria to truly feast and reproduce faster and more plentifully on just nylon, thus leading perhaps in time to an irreducibly complex arrangement between bacteria who live solely on nylon and a man-made fiber produced only by man. Darwinism or Directed Mutations?]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.
|
A satirical 1871 image of Charles Darwin as an ape reflects part of the social controversy over whether humans and 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.
[An overview of the philosophical, religious, and cosmological controversies by a philosopher who strongly supports evolution is: Daniel Dennett, Darwin's Dangerous Idea: Evolution and the Meanings of Life (New York: Simon & Schuster, 1995). On the scientific and social reception of evolution in the 19th and early 20th centuries, see: Peter J. Bowler, Evolution: The History of an Idea, 3rd. rev. edn. (Berkeley: University of California Press, 2003).]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
[[7]];
Judaism is notable as a major faith tradition whose adherents generally see no conflict between evolutionary theory and religious belief.
[The Rabbinical Council of America notes that significant Jewish authorities have maintained that evolutionary theory, properly understood, is not incompatible with belief in a Divine Creator, nor with the first 2 chapters of Genesis. [8]] [The High Council of B'nei Noah a body of non-Jews guided by the Beit Din of B'nei Noah a sub-court of the developing Sanhedrin: Science and Religion: A proper perspective through an understanding of Hebrew sources ] [Aish HaTorah According to a possible reading of ancient commentators' description of God and nature, the world may be simultaneously young and old.] 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 be broadly 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 is clear both that humans are animals and that they share common ancestry with
chimpanzees,
gibbons,
gorillas, and
orangutans. Some people find the idea of common ancestry repellent, as, 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 (in their view) "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 the teaching curriculum. While many other fields of science, such as
cosmology and
earth science, also conflict with a literal interpretation of many religious texts, evolutionary studies have borne the brunt of these debates.
Evolution has been used to support philosophical and ethical choices which most modern scientists argue are 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". Another example of an extension of evolutionary theory that is 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.
[On the history of eugenics and evolution, see Daniel Kevles, In the Name of Eugenics: Genetics and the Uses of Human Heredity (New York: Knopf, 1985).]
*
Sean B. Carroll, 2005,
Endless Forms Most Beautiful: The New Science of Evo Devo and the Making of the Animal Kingdom, W. W. Norton & Company. ISBN 0393060160
* |issue=6194}}
*Williams, G.C. (1966). Adaptation and Natural Selection: A Critique of some Current Evolutionary Thought. Princeton, N.J.: Princeton University Press.
*Zimmer, Carl.
Evolution: The Triumph of an Idea. Perennial (October 1, 2002). ISBN 0060958502
*Garcia-Fernà ndez, Jordi.
Amphioxus: a peaceful anchovy fillet to illuminate Chordate Evolution. Int J Biol Sci (May, 2006).
*
Talk.Origins Archive â€" see also
talk.origins*
Understanding Evolution from
University of California, Berkeley*
National Academies Evolution Resources*
EvoWiki â€" A wiki whose goal is to promote general evolution education, and provide mainstream scientific responses to the arguments of antievolutionists.
*
Evolution by Natural Selection â€" An introduction to the logic of evolution by natural selection
*
Evolution â€" Provided by
PBS.
*
Everything you wanted to know about evolution â€" Provided by
New Scientist.
*
International Journal of Organic Evolution*
International Journal of Biological Sciences*
New England Complex Systems Institute*
Howstuffworks.com â€" How Evolution Works*
Synthetic Theory Of Evolution: An Introduction to Modern Evolutionary Concepts and Theories*
Charles Darwin's writings*
Evolution News from Genome News Network (GNN)*
National Academy Press: Teaching About Evolution and the Nature of Science*
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
;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)For a more comprehensive list of topics, see :Category:Evolution and :Category:Evolutionary biology
