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Evolution

In biology, evolution is the process by which populations of organisms acquire and pass on novel traits from generation to generation, affecting the overall makeup of the population and even leading to the emergence of new species. The terms organic evolution or biological evolution are often used to distinguish this meaning from other usages.

The development of the modern theory of evolution began with the introduction of the concept of natural selection in a joint 1858 paper by Charles Darwin and Alfred Russel Wallace. This theory achieved a wider readership in Darwin's 1859 book, The Origin of Species. Darwin and Wallace proposed that evolution occurs because a heritable trait that increases an individual's chance of successfully reproducing will become more common, by inheritance, from one generation to the next, and likewise a heritable trait that decreases an individual's chance of reproducing will become rarer. This work was groundbreaking, and overturned other evolutionary theories, such as that advanced by Jean Baptiste Lamarck. Because of its potential implications for the origins of humankind, the theory has been at the center of many social and religious controversies since its first inception.

In the 1930s, scientists combined Darwinian natural selection with the re-discovered theory of Mendelian heredity to create the modern synthesis, now one of the fundamental scientific theories of biology. In the modern synthesis, "evolution" is defined as a change in the frequency of alleles within a population from one generation to the next. The basic mechanisms that produce these changes are natural selection, genetic drift, and genetic variation. The primary sources of genetic variation are mutation, sex, and gene flow.

The process of evolution has left behind numerous records which reveal the history of species. While the best-known of these are the fossils, fossils are only a small part of the overall physical record of evolution. Fossils, taken together with the comparative anatomy of present-day plants and animals, constitute the morphological record. By comparing the anatomies of both modern and extinct species, biologists can reconstruct the lineages of those species with some accuracy. Using fossil evidence, for instance, the connection between dinosaurs and birds has been established by way of so-called "transitional" species such as Archaeopteryx.

The development of genetics has allowed biologists to study the genetic record of evolution as well. Although we cannot obtain the DNA sequences of most extinct species, the degree of similarity and difference among modern species allows geneticists to reconstruct lineages with greater accuracy. It is from genetic comparisons that claims such as the 98-99% similarity between humans and chimpanzees come from, for instance.

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 the earth. For instance, paleoclimatology attests to periodic ice ages during which the climate was much cooler; and these are found to match up with the spread of species such as the woolly mammoth which are better-equipped to deal with cold.

Fossils are important for estimating when various lineages developed. As fossilization on an organism 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 organisms without hard body parts, such as shell, bone, and teeth, is sparse but exists in the form of ancient microfossils and the fossilization of ancient burrows and a few soft-bodied organisms.

Fossil evidence of prehistoric organisms has been found all over the Earth. The age of fossils can often be deduced from the geologic context in which they are found; and their absolute age can be verified with radiometric dating. 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 transitional fossils can be used to demonstrate continuity between two different lineages. Paleontologists investigate evolution largely through analysis of fossils.

Phylogeny, 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. Bat wings, for example, are very similar to hands. A vestigial organ or structure may exist with little or no purpose in one organism, though they have a clear purpose in other species. The human wisdom teeth and appendix are common examples.

The idea of biological evolution has existed since ancient times, notably among Hellenists such as Epicurus and Anaximander, but the modern theory was not established until the 18th and 19th centuries, by scientists such as Jean-Baptiste Lamarck and Charles Darwin. 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 by Means of Natural Selection which provided the first cogent mechanism by which evolutionary change could occur: 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.

Darwin's theory, though it succeeded in profoundly shaking scientific opinion regarding the development of life, could not explain the source of variation in traits within a species, and Darwin's proposal of a hereditary mechanism (pangenesis) was not compelling to most biologists. It was not until the late 19th and early 20th centuries that these mechanisms were established.

There are a number of common misconceptions concerning evolutionary biology. Some are based on former or colloquial uses of the term "evolution". Others seem to be due to misunderstandings of related scientific concepts. Still others are rooted in earlier scientific hypotheses which have been disproven.

One of the most common misconceptions is that one species can be "more highly evolved" than another, or that evolution is necessarily progressive. Evolution provides no assurance that later generations are more intelligent, complex, or morally worthy than earlier generations. This claim was often made in pursuit of Social Darwinism, a 19th-20th century political ideology which held that the subjection of the poor and minority groups was favored by evolution.

Creationists often make the claim that evolution has never been observed in action. This was true when Darwin first hypothesized evolution by natural selection, but it is no longer true. Speciation – the origin of new species – has been observed; so have novel behavior and adaptations within a species. Moreover, Darwin's hypothesis has been confirmed by new data gathered from sources that did not exist in his day, such as DNA similarity among species and new fossil discoveries. A variation of this assertion is that "macroevolution" has never been observed. The problem is creationists redefine macroevolution as a change from one "kind" to another; and while it is true (depending on the definition of kind) it is also irrelevant as such cumulative large changes take millions of years to occur naturally. (although artificial selection could do it in a much shorter time)

A misconception based on thermodynamics is the claim that evolution violates the second law of thermodynamics. The second law holds that a closed system will become less ordered, or will tend to a state of thermal equilibrium. The misconception is that this tendency would rule out evolution, which is taken to mean an increase in order. However, the system in which evolution operates – life on Earth – is not a closed system in the thermodynamic sense, since it receives an energy input from the sun. If evolution were impossible under thermodynamics, then so would be the growth of plants; however, it is not.

Evolutionary biology is frequently mischaracterized as claiming that species come into existence at random, or that evolution proceeds by chance. It is true that randomness is part of evolution, but it is not the whole process. Random processes such as mutation and recombination are involved in the formation of each new organism, and produce small new variations upon existing forms. Natural selection – a nonrandom process – eliminates those variations which are not viable.

* Matter cannot organize itself.

Crystals such as diamonds and snowflakes can and do self-organize; likewise proteins fold in very specific ways based on their chemical makeup. Amino acids, the building blocks of proteins; have been shown to assemble naturally.

* Evolution cannot create information, which is a manifestation of 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 nor visually apparent to be "information". However, even if those were requirements they could be satisfied with the appearance of nylon eating bacteria, which required new enzymes to digest a material that never existed until the modern age.

 

This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "evolution".