| This article is about biological evolution. For other possible meanings, see
Evolution (disambiguation).
Generally, evolution is any process of change over time. In the context of
life science, evolution is a change in gene frequencies in a population. Since the emergence of modern genetics
in the 1940s, evolution has been defined more specifically as a change in the frequency
of alleles from one generation to the next.
Because the word evolution can be used in many different contexts, even within biological circles, it is useful to correctly
identify some of the key terms. Evolution, strictly speaking, is the change in frequency of genetic occurrences within a given
gene pool over time. The theory of evolution is the scientific model that describes the descent of all living organisms from a
common ancestor. Natural Selection and punctuated equilibrium are two of the mechanisms used to
describe how evolution has occurred. In common parlance the word "evolution" is often used as a shorthand for both the modern
theory that all extant species share a common ancestor as well as the mechanisms
through which evolution acts such as Charles Darwin's theory of
natural selection and Gregor Mendel's theory of genetics.
As the theory of evolution by natural selection and genetics has become widely accepted in the mainstream scientific
community, it has replaced other explanations including creationism and
Lamarckism. Skeptics – often creationists – sometimes criticize the
presentation of evolution as proven fact rather than scientific theory; defenders object to
these criticisms, maintaining that presenting it as "just a theory" constitutes an attempt to characterize it as an arbitrary
choice and degrade its claims to truth. Such debates often relate to the scientifically accepted use of the word "theory" to mean a falsifiable and well-supported hypothesis.
Scientific theory
The prevailing formulation of the theory of evolution is the modern
synthesis (sometimes called neo-Darwinism), which brings together Darwin's theory of evolution by natural selection and
Gregor Mendel's theory of inherited characteristics, now called genes. In the modern synthesis,
"evolution" means a change in the frequency of an allele within a gene pool. This change may be caused by a number of different mechanisms: natural selection, genetic drift or changes in population structure (gene
flow).
Modern synthesis theory has three major aspects:
- The common descent of all organisms from a single ancestor.
- The origin of novel traits in a lineage.
- The mechanisms that cause some traits to persist while others perish.
Ancestry of organisms
- Main article: Common descent
A group of organisms is said to have common descent if they have a
common ancestor. In biology, the theory of universal common descent proposes that all
organisms on Earth are descended from a common ancestor or ancestral gene pool.
Evidence for common descent may be found in traits shared between all living organisms. In Darwin's day, the evidence of
shared traits was based solely on visible observation of morphologic similarities, such as the fact that all birds – even those which do not fly –
have wings. Today, the theory of evolution has been strongly confirmed by the science of DNA
genetics. For example, every living thing makes use of nucleic acids as its
genetic material, and uses the same twenty amino acids as the building blocks
for proteins. All organisms use the same genetic code (with some extremely rare and minor deviations) to translate nucleic acid sequences into proteins. Because the selection of these traits is
somewhat arbitrary, their universality strongly suggests common ancestry.
In addition, abiogenesis–the generation of life from non-living
matter–has never been observed, indicating that the origin of
life from non-life is either extremely rare or only happens under conditions very unlike those of modern Earth. However, the
1953 Miller-Urey experiment does suggest that
abiogenesis is possible.
Since abiogenesis is rare or impossible under modern conditions and the evolutionary process is exceedingly slow, the
diversity and complexity of modern life requires that the Earth be very old, on the order of billions of years. This is
compatible with geological evidence that the Earth is approximately 4.6 billion years old. (See Timeline of evolution.)
Information about the early development of life includes input from the fields of geology and planetary science. These sciences provide information about the history
of the Earth and the changes produced by life. A great deal of information about the early Earth has been destroyed by geological
processes over the course of time.
Morphological evidence
Fossils are important for estimating when various lineages developed. As
fossilization is an uncommon occurrence, usually requiring hard parts (like bone) and death near a site where sediments are being deposited, the fossil record only provides sparse and intermittent information about the evolution of life. Fossil evidence
of early life is sparse before the evolution of organisms with hard body parts, such as shell, bone, and teeth, but exists in the
form of ancient microfossils and the fossilization of ancient burrows and a few soft-bodied organisms.
Nevertheless, fossil evidence of prehistoric organisms has been found all over the Earth. The age of fossils—even their
absolute age, determined through radiometric dating of
rocks—can often be deduced based upon the geologic context in which they are found. Some fossils bear a resemblance to
organisms alive today, while others are radically different. Fossils have been used to determine at what time a lineage
developed, and can be used to demonstrate the continuity between two different lineages through transitional fossils. Paleontologists investigate evolution largely through analysis of fossils.
Phylogeny, the study of the ancestry of species, has revealed that structures
with similar internal organisation may perform divergent functions. Vertebrate
limbs are a favorite example of such homologous structures. Other vestigial structures may exist with little or no purpose in one organism, though they have a clear purpose
in others. The human wisdom teeth and appendix are common examples.
Genetic sequence evidence
Comparison of the genetic sequence of organisms reveals that organisms that are phylogenetically close have a higher degree of sequence similarity than organisms that are phylogenetically
distant. For example, neutral human DNA sequences are approximately 1.2% divergent (based on substitutions) from those of their
nearest genetic relative, the chimpanzee, 1.6% from gorillas [4]
(http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=11170892),
and 6.6% from baboons[5] (http://www.genome.org/cgi/content/full/13/5/813). Sequence comparison is considered such a
robust measure that it is sometimes used to correct mistakes in the phylogenetic tree, in instances where other evidence is
scarce.
Further evidence for common descent comes from genetic detritus such as pseudogenes, regions of DNA which are orthologous to a gene in a
related organism, but are no longer active and appear to be undergoing a steady process of degeneration[6]
(http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=10833048).
Since metabolic processes do not leave fossils, research into the evolution
of the basic cellular processes is also done largely by comparison of existing organisms. Many lineages diverged at different
stages of development, so it is theoretically possible to determine when certain metabolic processes appeared by comparing the
traits of the descendants of a common ancestor.
Origin of life
- Main article: Origin of life
Not much is known about the earliest development of life. However, all existing organisms share certain traits, including the
cellular structure, and the genetic code. Most scientists interpret this to
mean all existing organisms share a common ancestor, which had already developed the most fundamental cellular processes, but
there is no scientific consensus on the relationship of the
three domains of life (Archea, Bacteria, Eukaryota) or the origin of life. Attempts to shed light on the earliest history of life generally focus on the behavior of
macromolecules, particularly RNA, and
the behavior of complex systems.
Though the origins of life are murky, other milestones in the evolutionary history of life are well-known. The emergence of
oxygenic photosynthesis (c. 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 c. 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 (phyla) of modern animals. 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 emergence of novel traits
Mechanisms of inheritance
In Darwin's time, scientists did not share broad agreement on how traits were
inherited. Today most inherited traits are traced to discrete, persistent entities called genes, encoded in linear molecules called DNA. Though by and large
faithfully maintained, DNA is both variable across individuals and subject to a process of change or mutation (described below).
However, other non-DNA based forms of heritable variation exist. The processes that produce these variations leave the genetic
information intact and are often reversible. This is called epigenetic inheritance and may include phenomena such as DNA methylation, prions, and structural inheritance.
Investigations continue into whether these mechanisms allow for the production of specific beneficial heritable variation in
response to environmental signals. If this is shown to be the case, then some instances of evolution would lie outside of the
framework that Darwin established, which avoided any connection between environmental signals and the production of heritable
variation.
Mutation
- Main article: Mutation
Mutations are permanent, transmissible changes to the genetic
material (usually DNA or RNA) of a cell. Mutations can be caused by copying errors in the genetic material
during cell division and by exposure to radiation, chemicals, or viruses, or can occur
deliberately under cellular control during the processes such as meiosis or hypermutation. In multicellular organisms, mutations can be subdivided into
germline mutations, which can be passed on to progeny and somatic mutations, which (when accidental) often lead to
the malfunction or death of a cell and can cause cancer.
Mutations are considered the driving force of evolution, since they introduce new genetic variation, without which evolution
cannot proceed. Neutral
mutations do not affect the organism's chances of survival in its natural environment and can accumulate over time, which
might result in what is known as punctuated
equilibrium, the modern interpretation of classic evolutionary theory.
Most biologists believe that adaptation occurs through the accumulation of
many mutations of small effect. However, macromutation is an alternative
process for adaption which involves a single, very large scale mutation.
Differential survival of traits
While mutation can create new alleles, other factors influence the frequency of
existing alleles. These factors mean that some characteristics will become more frequent while others diminish or are lost
entirely. There are three known processes that affect the survival of a characteristic; or, more specifically, the frequency of
an allele:
Natural selection
Natural selection is based on differential survival and reproduction rates as a result of the environment. Differential
mortality is the survival rate of individuals before their reproductive age. If they survive, they are then selected further by
differential fertility — that is, their total genetic contribution to the next generation.
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 that are more attractive
to the opposite sex because of their features reproduce more and increase the frequency of those features in the gene pool.
Natural selection also operates on mutations in several different ways:
- Purifying or
background
selection eliminates deleterious mutations from a population.
- Positive selection
increases the frequency of a beneficial mutation.
- Balancing selection maintains variation within a
population through a number of mechanisms, including:
The central role of natural selection in evolutionary theory has given rise to a strong connection between that field and the
study of ecology.
Mutations that are not affected by natural selection are called neutral mutations. Their frequency in the population is governed entirely by genetic
drift and gene flow. It is understood that an organism's DNA sequence, in the absence of selection, undergoes a steady
accumulation of neutral mutations. The probable mutation effect is the proposition that a gene that is not under selection will be
destroyed by accumulated mutations. This is an aspect of genome degradation.
- Baldwinian evolution refers to the way human beings, as
cultured animals capable of symbolic (extrasomatic) learning, can change their
environment, or the environment of any species, in such a way as to result in new selective forces.
Genetic drift
Genetic drift describes changes in allele frequency that cannot be
ascribed to selective pressures, but are due instead to events that are unrelated to inherited traits. This is especially
important in small mating populations, where chance fluctuations from generation to generation can be large. Such fluctuations in
allele frequency between successive generations may result in some alleles disappearing from the population. Two separate
populations that begin with the same allele frequency might, therefore, "drift" by random fluctuation into two divergent
populations with different allele sets (for example, allele that are present in one have been lost in the other). Rare sporadic
events (volcanic explosion, meteor impact,
etc.) might contribute to genetic drift by altering the allele frequency outside of "normal" selective pressures.
Many aspects of genetic drift depend on the size of the population (generally abbreviated as N). In small populations, genetic
drift can cause large changes in allele frequencies from one generation to the next, whereas in large populations, changes in
allele frequencies in each generation are usually very small. The relative importance of natural selection and genetic drift in
determining the fate of new mutation 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.
Gene flow
Gene flow (or gene admixture) is the only mechanism whereby populations can
become closer genetically while building larger gene pools. Migration of one population into another area occupied by a second
population can result in gene flow. Gene flow operates when geography and culture are not obstacles.
Micro- and macro- evolution
Microevolution consists of small-scale changes in gene frequencies
in a population over the course of a few generations. These changes may be due to a number of processes: mutation, gene flow,
genetic drift, as well as natural selection. Population
genetics is the branch of biology that provides the mathematical structure for the study of the process of
microevolution.
Macroevolution works through large-scale changes in gene-frequencies
in a population over a long period of time, and is usually taken to refer to events that result in speciation, the evolution of a new species. An absolute
distinction between macroevolution and microevolution isn't normally drawn by biologists for a number of reasons, including no
definition of what constitutes a 'macroevolutionary' change. Mutations to existing species resulting in entirely new species have
been observed in the laboratory and in the field.
The relation between microevolution and macroevolution can be summed up as such: macroevolution is the long-term result of
many microevolutions that, over time, result in two populations of organisms so different that speciation can be said to have
occurred.
Speciation and extinction
Speciation is the creation of two or more species from one. There are
various mechanisms by which this may take place. Allopatric
speciation begins when subpopulations of a species become isolated geographically, for example by habitat fragmentation or migration. Sympatric speciation occurs when new species emerge in the same
geographic area. Ernst Mayr's peripatric speciation is a type of speciation that exists in between the extremes of allopatry and
sympatry. Peripatric speciation is a critical underpinning of the theory of punctuated equilibrium.
Extinction is the disappearance of species (i.e. gene pools). The moment of extinction is generally considered to be the death of the last individual of
that species. Extinction is not an unusual event in geological
time—species are created by speciation, and disappear through extinction.
Evolutionary biology
- Main article: Evolutionary biology
Evolutionary biology is a subfield of biology concerned with the origin and
descent of species, as well as their change over time. Evolutionary biology is a kind
of meta field because it includes scientists from many traditional taxonomically-oriented disciplines. For example, it generally includes scientists who may have a specialist
training in particular organisms such as mammalogy, ornithology, or herpetology but use those organisms as systems to answer general questions in evolution.
Evolutionary biology as an academic discipline in its own
right emerged as a result of the modern
evolutionary synthesis in the 1930s and 1940s. It was not until the 1970s and 1980s, however, that a significant number of universities had departments that specifically included the term
evolutionary biology in their titles.
History of evolutionary thought
- Main article: History of
evolutionary thought
The idea of biological evolution has existed since ancient times, but the modern theory wasn't established until the 18th and
19th centuries, with scientists such as Jean-Baptiste
Lamarck and Charles Darwin. Darwin greatly emphasized the
difference between his two main points: establishing the fact of evolution, and proposing the theory of natural selection to explain the mechanism of evolution.
While transmutation of species was accepted by a sizeable number of scientists before 1859, it was the publication of Charles Darwin's The Origin of Species which provided the first cogent mechanism by which evolutionary change
could persist: his theory of natural selection. Darwin was motivated to publish his work on evolution after receiving a letter
from Alfred Russel Wallace, in which Wallace revealed his
own discovery of natural selection. As such, Wallace is sometimes given shared credit for the theory of evolution. However,
Wallace himself backed away from claiming too much credit, admitting that Darwin's formulation of the theory and his work on
evolution went far beyond Wallace's conjectures in scope and explanatory power (he would later, to Darwin's great disappointment,
back away completely from the idea that humans were evolved by natural means as he began to turn towards spiritualism).
Darwin's theory, though it succeeded in profoundly shaking scientific opinion regarding the development of life (and indeed
resulted in a small social revolution), could not explain several critical components of the evolutionary process. Namely, he was
unable to explain the source of variation in traits within a species, and he could not provide a mechanism whereby traits were
passed faithfully from one generation to the next. Darwin's theory of pangenesis, while relying in part on the inheritance of acquired characteristics, proved to be useful for
statistical models of evolution developed by his cousin Francis Galton
and the "biometric" school of evolutionary thought. It was, however, found to be of little use to biologists.
These questions were not settled until the end of the 19th century,
beginning with the work of an Austrian monk named Gregor Mendel, who
outlined, through a series of ingeniously devised experiments, a model for inheritance of traits based on the fundamental unit of
the gene. Mendel's work was unappreciated at the time and largely ignored by the biological
community. When it was "rediscovered" in 1900, it led to a storm of conflict between Mendelians and biometricians
(Walter Frank Raphael Weldon and Karl Pearson), who insisted that the great majority of traits important to
evolution must show continuous variation that was not explainable by Mendelian analysis.
Eventually, the two models were reconciled and merged, primarily through the work of the biologist and statistician R.A. Fisher. This combined approach, applying a rigorous statistical model to
Mendel's theories of inheritance via genes, became known in the 1930s and 1940s as the modern synthesis of Darwin's theory.
In the 1940s, following up on Griffith's experiment,
Avery, McCleod and McCarty definitively
identified deoxyribonucleic acid (DNA) as the "transforming principle" responsible for
transmitting genetic information. In 1953, Francis Crick and James Watson published their famous
paper on the structure of DNA, based on the research of Rosalind
Franklin and Maurice Wilkins. These developments ignited the era
of molecular biology and transformed the understanding of
evolution into a molecular process: the mutation of segments of DNA.
In the mid-1970s, Motoo Kimura formulated the neutral theory of molecular
evolution, firmly establishing the importance of genetic drift as a
major mechanism of evolution.
Debates have continued within the field. One of the most prominent outstanding debates is over the theory of punctuated equilibrium, a theory propounded by Niles Eldredge and Stephen Jay Gould to explain the paucity of transitional forms between phyla.
Social effect of evolutionary theory
- Main article: Social
effect of evolutionary theory
As the scientific explanation of life's diversity has developed, it has often displaced alternative, sometimes very widely
held, explanations. Because the theory of evolution includes an explanation of humanity's origins, it has had a profound impact
on human societies. Some have vigorously opposed acceptance of the scientific
explanation due to its perceived religious implications (e.g. its implied rejection of the special creation of humans described
in the Bible). This has led to a vigorous conflict between creation and evolution in
public education.
The theory of evolution by natural selection has also been adopted as a foundation for various ethical and social systems,
such as social Darwinism, an idea popular in the 19th century which holds that "the survival of the fittest" explains and justifies
differences in wealth and success among societies and people. A similar interpretation was one created by Darwin's cousin,
Francis Galton, known as eugenics, which claimed that human civilization was subverting natural selection by allowing the "less fit" to
survive and "out-breed" the "more fit." Later advocates of this theory would suggest radical and often coercive social measures
to attempt to "correct" this imbalance. Stephen Jay Gould and
others have argued that social Darwinism is based on misconceptions of evolutionary theory, and many ethicists regard it as a
case of the is-ought problem. After the atrocities of the Holocaust became linked with eugenics, it greatly fell out of favor with public and
scientific opinion (though it was never universally accepted by either).
The notion that humans share ancestors with other animals has also affected how some people view the relationship between
humans and other species. Many proponents of animal rights hold that if
animals and humans are of the same nature, then rights cannot be distinct to humans.
The theory has also been incorporated into other fields of knowledge, creating hybrids such as evolutionary psychology and sociobiology.
Evolution and religion
Before Darwin's argument and presentation of the evidence for evolution, Western religions almost unanimously discounted or condemned any claims that life is the result of an evolutionary
process, as did nearly all scientists. Literal or authoritative interpretation of Scripture holds that a supreme being directly created
humans and other animals as separate species. This view is commonly referred to as creationism, and continues to be defended by some religious groups, particularly among American Protestants.
However, in response to the wide scientific acceptance of the theory of evolution, some religions have formally or informally
synthesized the scientific and religious viewpoints. Sometimes claiming that life shows evidence of intelligent design, some conclude that God has provided a divine spark to ignite the process of
evolution, and possibly guided evolution in one way or another; or that Darwinian evolution is essentially God's default method
of creation, perhaps with critical reservations, such as stipulating that human souls are
created directly by God. These views fall under the umbrella of "evolutionary creationism."
In countries where the majority of people hold strong religious beliefs, creationism has a much broader appeal than in
countries where the majority of people hold secular beliefs. Among Western
countries, the United States of America is the only
country where creationist ideas are widely seriously considered for being taught as acceptable theories in schools. A series of
polls in the United States in 1999 suggested that over half of American voters supported the teaching of creationism in public schools alongside evolution [7] (http://1stam.umn.edu/main/pubop/creationism.htm).
Some of those who reject the scientific theory of evolution have proffered what they believe to be physical proof of the
impossibility of macroevolution in particular; this viewpoint does not
bar the idea of microevolution.
Evolution and the Roman Catholic Church
The Roman Catholic Church, beginning in 1950 with Pope Pius XII's encyclical Humani Generis, took up a neutral position with regard to evolution.
"The Church does not forbid that… research and discussions, on the part of men experienced in both fields, take place
with regard to the doctrine of evolution, in as far as it inquires into the origin of the human body as coming from pre-existent
and living matter." [8]
(http://www.vatican.va/holy_father/pius_xii/encyclicals/documents/hf_p-xii_enc_12081950_humani-generis_en.html)
In an October 22, 1996, address to the
Pontifical Academy of Science, Pope John Paul II updated the Church's position:
- "In his encyclical Humani Generis, my predecessor Pius XII has already affirmed that there is no conflict between
evolution and the doctrine of the faith regarding man and his vocation… Today, more than a half-century after the
appearance of that encyclical, some new findings lead us toward the recognition of evolution as more than an hypothesis. In fact
it is remarkable that this theory has had progressively greater influence on the spirit of researchers, following a series of
discoveries in different scholarly disciplines… The convergence in the results of these independent studies—which was
neither planned nor sought—constitutes in itself a significant argument in favor of the theory." [9] (http://www.ewtn.com/library/PAPALDOC/JP961022.HTM)
Reference
- Darwin, Charles November 24, 1859.
On the Origin of Species by means of Natural Selection.
London: John Murray, Albemarle Street. 502 pages. Reprinted: Gramercy (May 22, 1995). ISBN 0517123207
- Zimmer, Carl. Evolution: The Triumph of an Idea. Perennial (October 1, 2002). ISBN 0060958502
- Larson, Edward J. Evolution: The Remarkable History of a Scientific Theory (Modern Library Chronicles). Modern Library
(May 4, 2004). ISBN 0679642889
- Mayr, Ernst. What Evolution Is. Basic Books (October, 2002). ISBN 0465044263
- Gigerenzer, Gerd, et al., The empire of chance: how probability changed science and everyday life (New York: Cambridge
University Press, 1989).
External links
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