"Evolution" Summary

Reference: Wikipedia, Evolution

Evolution is the change in the inherited traits of a population of organisms through successive generations.[1] After a population splits into smaller groups, these groups evolve independently and may eventually diversify into new species. Ultimately, life is descended from a common ancestry through a long series of these speciation events, stretching back in a tree of life that has grown over the 3.5 billion years of life on Earth. This is visible in anatomical, genetic and other likenesses between groups of organisms, geographical distribution of related species, the fossil record and the recorded genetic changes in living organisms over many generations. To distinguish from other uses of the word evolution, it is sometimes termed biological evolution, genetic evolution or organic evolution.[2][3][4]

Evolution is the product of two opposing forces: processes that constantly introduce variation in traits, and processes that make particular variants become more common or rare. A trait is a particular characteristic, such as eye color, height, or a behavior, that is expressed when an organism's genes interact with its environment. Genes vary within populations, so organisms show heritable differences (variation) in their traits. The main cause of variation is mutation, which changes the sequence of a gene. Altered genes, or alleles, are then inherited by offspring. There can sometimes also be transfer of genes between species.

Two main processes cause variants to become more common or rare in a population. One is natural selection, through which traits that aid survival and reproduction become more common, while traits that hinder survival and reproduction become more rare.[1][5] Natural selection occurs because only a few individuals in each generation will survive, since resources are limited and organisms produce many more offspring than their environment can support. Over many generations, mutations produce successive, small, random changes in traits, which are then filtered by natural selection and the beneficial changes retained. This adjusts traits so they become suited to an organism's environment: these adjustments are called adaptations.[6] Not every trait, however, is an adaptation. Another cause of evolution is genetic drift, which produces entirely random changes in how common traits are in a population. Genetic drift comes from the role that chance plays in whether a trait will be passed on to the next generation.

Evolutionary biologists document the fact that evolution occurs, and also develop and test theories that explain its causes. The study of evolutionary biology began in the mid-nineteenth century, when research into the fossil record and the diversity of living organisms convinced most scientists that species changed over time.[7][8] The mechanism driving these changes remained unclear until the theories of natural selection were independently proposed by Charles Darwin and Alfred Wallace. In 1859, Darwin's seminal work On the Origin of Species brought the new theories of evolution by natural selection to a wide audience,[9] leading to the overwhelming acceptance of evolution among scientists.[10][11][12][13] In the 1930s, Darwinian natural selection became understood in combination with Mendelian inheritance, forming the modern evolutionary synthesis,[14] which connected the units of evolution (genes) and the mechanism of evolution (natural selection). This powerful explanatory and predictive theory has become the central organizing principle of modern biology, directing research and providing a unifying explanation for the history and diversity of life on Earth.[11][12][15] Evolution is therefore applied and studied in fields as diverse as agriculture, anthropology, conservation biology, ecology, medicine, paleontology, philosophy, and psychology along with other specific topics in the previous listed fields.

Contents
1 History of evolutionary thought
2 Heredity
3 Variation
3.1 Mutation
3.2 Sex and recombination
3.3 Population genetics
3.4 Gene flow
4 Mechanisms
4.1 Natural selection
4.2 Genetic drift
5 Outcomes
5.1 Adaptation
5.2 Co-evolution
5.3 Co-operation
5.4 Speciation
5.5 Extinction
6 Evolutionary history of life
6.1 Origin of life
6.2 Common descent
6.3 Evolution of life
7 Applications
8 Social and cultural responses
9 See also
10 References
11 Further reading
12 External links

To be continued
18:38 　2010/08/19