- This article is about biological evolution. For other possible meanings, see Evolution (disambiguation).
Evolution generally refers to any process of change over time. In the context of life science, evolution is a change in the genetic makeup of a population of interbreeding individuals within a species. 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.
The word "evolution" is often used as a shorthand for the modern theory of evolution of species based upon Charles Darwin's theory of natural selection, which states that modern species are the products of an extensive process that began over three billion years ago with simple single-celled organisms, and Gregor Mendel's theory of genetics.
As the theory of evolution by natural selection and genetics has become nearly universally accepted in the scientific community, it has replaced other explanations including creationism and Lamarckism. Skeptics, often creationists, sometimes deride evolution as "just a theory" in an attempt to characterize it as an arbitrary choice and degrade its claims to reality. Such criticism overlooks the scientifically-accepted use of the word "theory" to mean a falsifiable and well-supported hypothesis.
The prevailing formulation of the theory of evolution is the modern synthesis, 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
in the Siyeh Formation, Glacier National Park
. In 2002, William Schopf of UCLA
published a controversial paper in the journal Nature
arguing that formations such as this possess 3.5 billion year old fossilized algae
microbes.  (http://www.abc.net.au/science/news/space/SpaceRepublish_497964.htm)
If true, they would be the earliest known life on earth.
- Main article: Common descent
A central assumption of evolutionary theory is that life on Earth had a single point of origin; all subsequent life-forms are descendants of this progenitor organism. This is called the theory of common descent.
Evidence for common descent may be found in traits shared between all living organisms. 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 under controlled conditions, indicating that the origin of life from non-life is either very rare or only happens under conditions very unlike those of modern Earth. However, 1953's 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.
A phylogenetic tree
of all living things
, based on rRNA gene
data, showing the separation of the three domains bacteria
, and eukaryotes
as described initially by Carl Woese
. Trees constructed with other genes are generally similar, although they may place some early-branching groups very differently, presumably owing to rapid rRNA evolution. The exact relationships of the three domains are still being debated.
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, thanks to 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 without purpose in one organism, though they have a clear purpose in others. The human wisdom teeth and appendix are common examples.
Genetic sequence evidence
Genetic testing has shown that humans and chimpanzees have most of their DNA
in common. In a study of 90,000 base pairs
, Wayne State University
's Morris Goodman found humans and Chimpanzes share 99.4% of their DNA.  (http://www.freep.com/news/nw/chimp20_20030520.htm)  (http://www.reasons.org/resources/apologetics/humans_chimps_same_genus.shtml)
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, (http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=11170892) and 6.6% from baboons. (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. (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
However, not even comparative biology can shed much light on the earliest development of life since 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.
This is a NASA
recreation of the famous Miller-Urey experiment. In 1953
, Stanley Miller
and Harold Urey
sealed the chemical precursors to life in a closed environment, and subjected them to conditions similar to primordial earth. The results of the experiment suggest that the chemicals necessary for life did tend to arise under those circumstances, supporting the theories of Abiogenesis
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, there was no widely accepted mechanism for the inheritance of traits. Today most inherited variation is traced to discrete, persistent entities called genes, encoded in linear molecules called DNA. Though by and large faithfully maintained, DNA is 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.
Different types of 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:
Genetic drift describes changes in gene 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, which simply cannot have enough offspring to maintain the same gene distribution as the parental generation. Such fluctuations in gene frequency between successive generations may result in some genes disappearing from the population. Two separate populations that begin with the same gene frequency might, therefore, "drift" by random fluctuation into two divergent populations with different gene sets (for example, genes 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 gene frequency outside of "normal" selective pressures.
Unlike natural selection, genetic drift is the random fluctuation of gene frequencies from generation to generation in a small, relatively isolated population. Its chief mechanism of operation is chance within small populations. The term small population is relative, however. Thus, genetic drift occurs when N <= 0.5s, N <= 0.5µ, N <= 0.5m where N is the population numbered in the hundreds, s is the selective value of the allele s, µ is mutation pressure, and m is 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 genetic admixture. Gene flow operates when geography and culture are not obstacles.
Natural selection is based on three principles: (a) there is variation within a species and this variation is heritable; (b) parents have more offspring than can survive; and (c) surviving offspring have favorable traits. The mechanism by which it operates is termed survival of the fitter, meaning differential mortality and fertility (see fitness). 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.
Selection of organisms for desirable characteristics, when done by humans, e.g. for agriculture or as pets, is called artificial selection.
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 field.
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.
- 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
The 1859 edition of On the Origin of Species
- 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 a correspondence with Alfred Russel Wallace, who had independently come to the same conclusions as Darwin. As such, Wallace is sometimes credited the co-discover of evolution. However, Wallace himself backed away from sharing 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.
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.
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 a biological community that was baffled by the mathematical nature of his theories. When it finally gained widespread acknowledgement, it led to a storm of conflict between Mendelians and biometricians, who insisted that the great majority of traits important to evolution must show continuous variation that was not explainable by Mendelian analysis.
Eventually, the Mendelians won out, and a series of papers in the 1930s and 1940s led to the development of the modern synthesis, which brought together Darwin's theories of natural selection with Mendel's theories of inheritance via genes.
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. 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 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 displaced alternative, often 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 social conservatives 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 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. 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.
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, 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 most scripture implies that a supreme, presumably supernatural, 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 and a small group of iconoclast scientists. Some of those who reject the scientific theory of evolution have profered what they believe to be physical proof of the impossibility of macroevolution in particular; this viewpoint does not bar the idea of microevolution.
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. A series of polls in the U.S. in 1999 suggested that over half of American voters supported the teaching of creationism in public schools alongside evolution. (http://1stam.umn.edu/main/pubop/creationism.htm).
However, in response to the arguments, evidence, and wide scientific acceptance for the theory of evolution, some religions have formally synthesized the scientific and religious viewpoints. 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."
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."  (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, recognizing that Evolution is "more than a hypothesis" - "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."  (http://www.ewtn.com/library/PAPALDOC/JP961022.HTM)
- Darwin, Charles. Origin of Species. 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
- EvoWiki (http://www.evowiki.org/index.php/Main_Page) - A wiki dedicated to Evolution
- Evolution (http://www.pbs.org/wgbh/evolution/index.html) - Provided by PBS.
- Howstuffworks.com - How Evolution Works (http://science.howstuffworks.com/evolution.htm/printable)
- Talk.Origins Archive (http://www.talkorigins.org) - see also talk.origins
- Evolution by Natural Selection (an introduction) (http://www.thinking-aloud.org/evolution-by-natural-selection/current.htm)
- Charles Darwin Books (http://charles-darwin.classic-literature.co.uk/)
- Evolution News from Genome News Network (GNN) (http://www.genomenewsnetwork.org/categories/index/genome/evolution.php)
- National Academy Press: Teaching About Evolution and the Nature of Science (http://www.nap.edu/books/0309063647/html/)