Evolution

Darwin did not use the term in the first edition of On the Origin of Species, referring instead to descent with modification. In the introduction to the Origin Darwin suggests that a naturalist reflecting on affinities, embryology, geographical distribution, and geological succession might conclude that species had not been independently created but had descended from other species.

Evolutionary thought did not begin with Darwin. In 1801 the French naturalist Jean Baptiste Pierre Antoine de Monet, Chevalier de Lamarck, took a great conceptual step and proposed a full-blown theory of evolution. Rejected by some on religious grounds and shunned by scientists for lack of deductive rigor, Lamarck died in 1829 in poverty and obscurity, but the notion of evolution did not die with him.

Charles Darwin spent five years aboard HMS Beagle, and his collections from the voyage include a Megatherium skull and Toxodon specimens. A visit to the Galapagos Islands in 1835 helped him formulate his ideas on natural selection; he found several species of finch adapted to different environmental niches. In his garden he bred pigeons, crossing birds with different characteristics to generate different offspring and gathering valuable evidence for his theory of evolution by natural selection.

In the mid-1800s Darwin and the British biologist Alfred Russel Wallace independently conceived of a natural, even observable way for life to change — a process Darwin called natural selection. The theory was proposed by Charles Darwin and Alfred Russel Wallace, independently, in the 1850s. When Darwin and Wallace read Malthus on population, it occurred to both that animals and plants should also be experiencing the same population pressure.

Wallace was a leading Victorian naturalist with wide-ranging interests from biogeography and evolutionary theory to spiritualism and politics; he was born in 1823 in Usk, a small town in south-east Wales. Between 1854 and 1862 he travelled some 14,000 miles across different islands and collected around 125,000 specimens, especially butterflies and birds, many unknown to European science.

Wallace's essay On the Tendency of Varieties to depart indefinitely from the Original Type was written at Ternate in February 1858 and sent to Darwin with the wish that it be forwarded to Sir Charles Lyell. On 18 June 1858 Darwin had received from Wallace a letter containing an essay that formulated the same hypothesis Darwin had worked on for the last twenty years but had never published. Charles Lyell and Joseph Dalton Hooker arranged for the Darwin and Wallace theories to be presented to a meeting of the Linnaean Society in 1858. The Darwin and Wallace paper was read before the Linnean Society on 1 July 1858 and published as On the tendency of species to form varieties; and on the perpetuation of varieties and species by natural means of selection. In his presidential address Thomas Bell reported that the year had not been marked by any of those striking discoveries which at once revolutionise the department of science on which they bear.

The following year Darwin's On the Origin of Species was published. The key publication was On the Origin of Species by Means of Natural Selection, London, 1859. The theory is also known as evolution by natural selection, or Darwinism.

On the death of Charles Darwin on April 19 in 1882 the president of the Royal Society asked that he be buried in Westminster Abbey, in the place where the kings and queens of England had long been crowned and interred. Twenty years after Darwin's death his theory lay on its own deathbed; only a handful of scientists believed natural selection to be the driving force of evolution. Astronomer Sir John Herschel described natural selection as the 'law of higgledy-piggledy', voicing a widespread feeling that so important a process as the development of life on earth could not be explained as an endless sequence of unpredictable migrations.

Natural selection is the core mechanism of evolution, acting on heritable variation within populations. He called the principle by which each slight variation, if useful, is preserved the term Natural Selection, to mark its relation to man's power of selection.

In each generation some individuals may, just by chance, leave behind a few more descendants than others, and the next generation's genes are those of the lucky individuals.

By sequencing ancient DNA researchers reconstructed the entire Neanderthal genome and found that ancient humans and Neanderthals interbred, introducing new genetic variation to the human population.

Other mechanisms include mutation, migration (gene flow), genetic drift, and sexual selection. Barriers to gene flow that may contribute to speciation result from natural selection, sexual selection, or even genetic drift.

Sickle-cell anemia illustrates selection in the wild: one copy of the allele gives some protection against malaria, and natural selection finds a balance between that advantage and the disadvantage of two copies.

The case for evolution rests on several independent streams of evidence: the fossil record, comparative anatomy and homologies, biogeography, embryology, and DNA. The National Academy of Sciences describes the discovery and understanding of the processes of evolution as one of the most powerful achievements in the history of science and the foundation for modern biology.

Comparative anatomy supplies some of the most direct evidence, through homologous characters — characters in different organisms that are similar because they were inherited from a common ancestor that also had that character. Birds, bats, mice, and crocodiles all have four limbs inherited from the ancestor of tetrapods, so the presence of four limbs is a homology. As forelimbs, bird and bat wings are homologous: birds and bats did not inherit wings from a common ancestor with wings, but they did inherit forelimbs from a common ancestor with forelimbs. The wings themselves are analogous, with separate evolutionary origins, and are superficially similar because natural selection shaped both for flight — an example of convergent evolution.

The fossil record displays a consistent pattern: fossils that more closely resemble today's life forms are found in younger rock layers, while many fossils that only distantly resemble life today occur in older layers. The record is also predictive. Using principles of evolution, scientists predicted what type of creature existed and where it would be found, and in 2004 a team found the fossil Tiktaalik with features of fish and of land-dwellers. For any two species living today, their evolutionary lines can be traced back in time until the two lines intersect in a common ancestor.

Genetic evidence independently corroborates the fossil pattern: species that appear more distantly related in the fossil record show correspondingly greater differences in their DNA than species that appear more closely related. Dating the events these comparisons reveal relies on radiometric dating, which depends on the half-life decay of radioactive elements; on stratigraphy, which gives a sequence of events; and on molecular clocks, which use genetic divergence to estimate dates. Life began 3.8 billion years ago, and insects diversified 290 million years ago.

The scientific standing of evolution is not in dispute among researchers; the debate in the scientific community is focused on the details of how evolution occurs, not whether it occurs, and nearly all scientists agree that biological evolution is the most sound theory to explain the diversity of life. In science, a theory is not a guess but a well-supported explanatory framework, and evolution holds that status. A 1998 National Academy of Sciences guidebook reported that many public school students receive little or no exposure to the theory of evolution, the most important concept in understanding biology.

The physical basis of heredity was clarified when Watson joined Francis Crick at Cambridge to analyze x-ray data collected by Rosalind Franklin and others, and in 1953 they built a model of DNA. That model described the molecule as a double helix. The discovery of DNA gave scientists a means of comparing genetic codes, working down to the deepest branches of life dating back billions of years.

Molecular evolution acquired a distinctive theoretical framework when Kimura introduced the neutral theory in 1968. The theory posited that differences between species are due to neutral substitutions rather than adaptive evolution, and that polymorphisms within species have dynamics dominated by mutation-drift equilibrium. Kimura in 1968, together with King and Jukes in 1969, played important roles in the field's acceptance of neutral mutations at all. In a review published fifty years after the theory's introduction, the authors argue that in light of modern genome-scale data its universality can be firmly rejected.

The ubiquity of adaptive variation both within and between species means that a more comprehensive theory of molecular evolution must be sought.

Dobzhansky analyzed the genes of wild fruit flies from Canada to Mexico and found that different populations of Drosophila pseudoobscura did not have identical sets of genes. In 1937 he published the landmark book Genetics and the Origin of Species, sketching out an explanation for how species come into existence.

During the 1930s and 1940s, the Modern Synthesis unified genetics with Darwinian selection, drawing genetics, paleontology, systematics, and many other sciences into a single powerful explanation of evolution.

The theoretical population geneticists Ronald A. Haldane, and Sewall Wright laid the foundation of the Modern Synthesis. Their work demonstrated that Mendelian inheritance could produce the continuous variation required by Darwinian natural selection, and they built sophisticated mathematical models of evolution. Wright introduced the 'adaptive landscape' metaphor, imagining the varying fitness of gene combinations as a hilly landscape whose valleys are less-fit combinations and whose peaks are the fitter ones.

The Extended Evolutionary Synthesis remains an ongoing debate over developmental bias, epigenetic inheritance, plasticity, niche construction, and non-genetic forms of inheritance. It has yet to crystallize into a fully integrated theoretical model. Advocates argue for incorporating these factors, while critics contend that the Modern Synthesis is flexible enough to accommodate them.

Human evolution is the lengthy process of change by which people originated from apelike ancestors, and the traits shared by all people evolved over a period of approximately six million years. The human and chimpanzee lineages diverged only five million years ago.

Humans and the great apes of Africa share a common ancestor that lived between 8 and 6 million years ago. The fossils of early humans who lived between 6 and 2 million years ago come entirely from Africa.

One of the earliest defining human traits, bipedalism, evolved over 4 million years ago. Other characteristics, such as a large and complex brain and the capacity for language, developed more recently.

A species is often defined as a group of individuals that actually or potentially interbreed in nature, forming the biggest gene pool possible under natural conditions. This interbreeding definition is difficult to apply to bacteria that reproduce mainly asexually.

Hybridization complicates the picture further, since many plants and some animals form hybrids in nature. In some areas hooded crows and carrion crows hybridize, although they largely mate within their own groups.

An external barrier such as a river or mountain range may start speciation, but the evolution of genetically based internal barriers to gene flow is necessary for the process to be complete.