Evolution (also known as biological, genetic or organic evolution) is the change in the inherited traits of a population of organisms through successive generations. This change results from interactions between processes which introduce variation into a population, and other processes which remove it. As a result, variants with particular traits become more, or less, common. A trait is a particular characteristic, anatomical, biochemical or behavioural, that is the result of gene–environment interaction.

The main source of variation is mutation, which changes the base pair sequences of genes. These altered genes can be passed on through reproduction, and give rise to alternative varieties, or alleles, of traits in organisms. Another source of variation is genetic recombination which shuffles the genes into new combinations that can result in organisms exhibiting different traits. Under certain circumstances, variation can also be increased by the transfer of genes between species, and by the extremely rare, but significant, wholesale incorporation of genomes through endosymbiosis.

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. Natural selection occurs because only a small proportion of individuals in each generation will survive and reproduce, since resources are limited and organisms produce many more offspring than their environment can support. Over many generations, heritable variation in traits is filtered by natural selection and the beneficial changes are successively retained through differential survival and reproduction. This iterative process adjusts traits so they become better suited to an organism's environment: these adjustments are called adaptations.

However, not all change is adaptive. Another cause of evolution is genetic drift, which leads to random changes in how common traits are in a population. Genetic drift is most important when traits do not strongly influence survival, particularly so in small populations where chance plays a disproportionate role in the frequency of traits passed on to the next generation. Genetic drift is important in the neutral theory of molecular evolution, and plays a role in the molecular clocks that are used in phylogenetic studies.

A key process in evolution is speciation, in which a single ancestral species splits and diversifies into multiple new species, and there are several modes through which this occurs. Ultimately, all living (and extinct) species are descended from a common ancestor via a long series of speciation events. These events stretch back in a diverse "tree of life" that has grown over the 3.5 billion years in which life has existed on Earth. This is visible in anatomical, genetic and other similarities between groups of organisms, geographical distribution of related species, the fossil record and the recorded genetic changes in living organisms over many generations.

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. The mechanism driving these changes remained unclear until the theory of natural selection was independently proposed by Charles Darwin and Alfred Wallace. In 1859, Darwin's seminal work On the Origin of Species brought the new theory of evolution by natural selection to a wide audience, leading to the overwhelming acceptance of evolution among scientists. In the 1930s, Darwinian natural selection became understood in combination with Mendelian inheritance, forming the modern evolutionary synthesis, which connected the substrate of evolution (inherited genetics) 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. Evolution is 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.

Evolution is change in populations of organisms over generations. Offspring differ from their parents in various ways. When these differences are helpful, the offspring have a greater chance of surviving and reproducing, making the differences more common in the next generation. In this way, differences can accumulate over time, leading to major changes in a population.

The scientific theory of evolution—the explanation for how evolution occurs—states that all living things are descended, at some point in the distant past, from a single common ancestor. This is called common descent. Since the beginning of life, evolution has transformed the first species into more and more different species as life has found a variety of ways to survive and flourish. This has resulted in the many diverse forms of life that exist today.

Sourced

• The antagonism between science and religion, about which we hear so much, appears to me to be purely factitious–fabricated, on the one hand, by short-sighted religious people who confound a certain branch of science, theology, with religion; and, on the other, by equally short-sighted scientific people who forget that science takes for its province only that which is susceptible of clear intellectual comprehension; and that, outside the boundaries of that province, they must be content with imagination, with hope, and with ignorance.
  • Thomas Huxley, The interpreters of Genesis and the interpreters of Nature (1885)

• Anthropological, biological, and genetic evidence all put the origin of modern humans at between 200,000 and 100,000 years ago, probably in Africa. There is also much data that show an outburst of cultural behavior occurring around 50,000-40,000 years ago in Europe. That's when archaeologists date the oldest evidence of burial ceremonies, body ornaments, and cave paintings.
  • William J. Cromie, Facing up to Modern Man; Harvard Gazette

From Wikiquote under the GNU Free Documentation License.
Sat Jul 31 07:50:22 2010