Evolution

Evolutionary ideas such as common descent and the transmutation of species have existed since at least the 6th century BCE, when they were expounded by the Greek philosopher Anaximander. Such views rose to prominence in the 18th century, but the first convincing exposition of a mechanism by which evolutionary change could occur was not proposed until 1858, when Charles Darwin and Alfred Russel Wallace jointly presented the theory of evolution by natural selection to the Linnean Society of London in separate papers. Shortly after, the publication of Darwin's The Origin of Species popularized and provided detailed support for the theory.

Although the occurrence of evolution came to be widely accepted, Darwin's specific ideas about it, such as gradualism and natural selection, were strongly contested at first. Lamarckists argued that parents passed on adaptations which they acquired during their lifetimes. Giraffes, for example, were thought to have acquired long necks by continuously stretching their necks until they lengthened, rather than through the selective process of taller giraffes out-competing shorter ones. Eventually, when experiments failed to support it, this popular rival theory was abandoned in favor of Darwinism.

However, Darwin could not account for how traits were passed down from generation to generation. A mechanism was provided in 1865 by Gregory Mendel, whose research revealed that distinct traits were inherited in a well-defined and predictable manner. When Mendel's work was rediscovered in 1900, disagreements over the rate of evolution seemingly predicted by early geneticists and biometricians led to a rift between the Mendelian and Darwinian models of evolution. This contradiction was reconciled in the 1930s through the efforts of biologists such as Ronald Fisher. The end result was a combination of Darwinian natural selection with Mendelian inheritance, the modern evolutionary synthesis, or "Neo-Darwinism".

In the 1940s, the identification of DNA as the genetic material by Oswald Avery and colleagues, and the articulation of the double-helical structure of DNA by James Watson and Francis Crick, provided a physical basis for the notion that genes were encoded in DNA. Since then, the role of genetics in evolutionary biology has become increasingly central.

Evolution consists of two basic types of processes: those that introduce new genetic variation into a population, and those that affect the frequencies of existing gene variants, or alleles. Mutations in genetic material, migration between populations (gene flow), and the reshuffling of genes during sexual reproduction (genetic recombination) create variation in organisms. In some organisms, like bacteria and plants, variation is also produced by the mixing of genetic material between different species in horizontal gene transfer and hybridization. Genetic drift and natural selection act on this variation by increasing or decreasing the frequency of traits: genetic drift does so randomly, while natural selection does so based on whether a trait increases fitness (reproductive success).

Genetic variation is often the result of a new mutation in a single individual (usually point mutations, insertions, or deletions); in subsequent generations, the frequency of that variant may fluctuate in the population, becoming more or less prevalent relative to other alleles at the site. All evolutionary forces act by driving this change in allele frequency in one direction or another. Variation disappears when an allele reaches the point of fixation вЂ" when it either reaches a frequency of zero and disappears from the population, or reaches a frequency of one and replaces the ancestral allele entirely. Most sites in the complete DNA sequence, or genome, of a species are identical in all individuals in the population. Consequently, relatively small genotypic changes can lead to dramatic phenotypic ones. Sites with more than one allele are called polymorphic, or segregating, sites. Polymorphism leads to distinct groups of traits arising within the same species, such as different hair colors or sexes. Interactions between a genotype and the environment may also affect the phenotype, as reflected in developmental and phenotypic plasticity.

Migration into or out of a population may be responsible for a marked change in allele frequencies; that is, the number of individual members carrying a particular variant of a gene can change because of migration. Immigration may result in the addition of new genetic material to the established gene pool of a particular species or population, and conversely emigration may result in the removal of genetic material. As reproductive isolation is a necessary condition for speciation, gene flow within a species may delay speciation by partially homogenizing two otherwise diverging populations.

Depending on how far two species have diverged since their last common ancestor, it may still be possible for them to mate and produce viable offspring. For example, horses and donkeys can be mated to produce mules or hinnies. Such hybrids are generally infertile, failing to reproduce due to mispairings of chromosomes during meiosis. Closely-related species may, in some cases, regularly interbreed, with natural selection strongly discriminating against the hybrids and keeping the populations distinct. This has been noted in toads, butterflies, clams, mussels, and other species. Selection against hybrids may be accompanied by reinforcement (the emergence of traits that increase reluctance to mate outside the species) and/or character displacement. In rare cases, hybrids may be well adapted to a zone between the extremes favored by the two parents, and may fill that zone.

Natural selection, one of the processes that drive evolution, results from the difference in reproductive success between individuals in a population. It has often been called a "self-evident" mechanism because it necessarily follows from the following facts:

Natural, heritable variation exists within populations and among species

Organisms are super fecund (produce more offspring than can possibly survive)

Organisms in a population vary in their ability to survive and reproduce

In any generation, successful reproducers necessarily pass their heritable traits to the next generation, while unsuccessful reproducers do not.

If these traits increase the evolutionary fitness of the individuals that carry them, then those individuals will be more likely to survive and reproduce than other organisms in the population, thus passing more copies of those heritable traits on to the next generation. The corresponding decrease in fitness for deleterious