On the 24th of November 1859, On the Origin of Species, by the esteemed British naturalist Charles Darwin FRS (1809-1882), was published in London. It revealed for the reading public the scientific explanation that the origin of species occurs through evolution. Scientists and thinkers before Darwin, including his grandfather, Erasmus Darwin (1731-1802), had broadly speculated as much, but Darwin presented a much, much more comprehensive and evidence-based explanation, including one of evolution's most fundamental components, what he termed the process of "natural selection," a concept that was also being explored by a contemporary of Darwin's, Alfred Russel Wallace FRS (1823-1913). Evolution through natural selection is sometimes referred to simply as "Darwinism." (Photo, Charles Darwin in 1854.)
Summarized extremely simplistically, evolution by natural selection is an offspring's biological inheritance of traits that improve the individual's chances of survival, and an accumulation of such traits over generations to the extent that a new species has emerged.
"The world becomes full of organisms that have what it takes to become ancestors. That, in a sentence, is Darwinism." - Richard Dawkins FRS (1941-); evolutionary biologist and author.
Along with other great scientific achievements such as the Theories of Special and General Relativity, the Germ Theory of Disease, and Newtonian Physics, Darwin's is one of the greatest in history. Today, evolution is the bedrock of all of the life sciences. The decades since Darwin have witnessed new discoveries and observations in various fields by which our understanding of evolution has been refined and expanded in ways sometimes complex, sometimes elementary, often startlingly elegant, and occasionally simply startling.
"Nothing in Biology Makes Sense Except in the Light of Evolution." - Theodosius Dobzhansky (1900-1975); geneticist and evolutionary biologist.
Origin's overwhelming evidence for evolution drew in large parts from the geological record, from observing anatomical similarities among different species across the planet, including fossilized remnants of extinct species, and from studies of what Darwin called "artificial selection"--the processes by which humans breed species (e.g., pigeons, dogs) in order to produce preferred traits among offspring. Darwin wrote in Origin that "light will be thrown on the origin of man and his history," a topic which he dealt with in a later publication.
Darwin's discovery of evolution by natural selection required that he take the concept of the great age of the Earth--millions and millions of years, sometimes referred to as "geological time"--that James Hutton MD (1726-1797), Sir Charles Lyell FRS (1797-1875), and others had written about before Darwin--and consider his biological observations in light it. Darwin realized that the greater or lesser biological affinity between species was something that could be understood in light of species' proximity to a shared ancestry; species had not just anatomical similarities, but also greater or lesser temporal proximity to one another, too.
This idea might be expressed as a diagram of a "Tree of Life," with species placed in relation to each other not only based on shared traits--of form or function or both; some obvious, some perhaps less so--but also based on the species' evolutionary history. A complete tree of life metaphor would also somehow indicate the extinctions of species, too, since the evolution of species via the inheritance of favorable traits conversely means that there is also the condition of individuals possessing less favored traits, even to the extent that it can lead to the disappearance of a species. Here Darwin's thinking was informed in part by writings of The Revd. Dr. Thomas Robert Malthus FRS (1766-1834) concerning how the limited nature of resources' abundance will act to curtail the growth of human populations.
Darwin wrote in Origin:
The affinities of all the beings of the same class have sometimes been represented by a great tree.... As buds give rise by growth to fresh buds, and these if vigorous, branch out and overtop on all sides many a feebler branch, so by generation I believe it has been with the great Tree of Life, which fills with its dead and broken branches the crust of the earth, and covers the surface with its ever branching and beautiful ramifications.
(Photo: Charles Darwin's 1839 entry beginning with the words "I think," in his "B notebook" that he kept while naturalist aboard the second voyage of the HMS Beagle, is an early visual representation of the idea of evolutionary relationships between species.)
Yet, nowhere in Orgin did Darwin successfully explain an actual mechanism for biological inheritance; he never discovered exactly how a trait favorable to survival was passed from parent to offspring. Decades later, with the discovery that offspring inherit traits by way of units of inheritance that we call genes, the picture of evolution became much more complete, emerging as what Sir Julian Huxley FRS (1887-1975)--evolutionary biologist and grandson of the champion of evolutionary biology in Darwin's day, Thomas Henry Huxley FRS (1825-1895)--termed the "modern evolutionary synthesis."
Through the field of genetics Darwin's ideas have been and continue to be time and again resounding confirmed and expanded, revealing that all known life--every species of plant, animal, and bacteria--is based on genes, that all species are related to one another at a genetic level, and that the same genes carry the same instructions pertaining to the same or similar functions in the genetic codes of species as different as chimpanzees and slugs, jellyfish and human beings. All of life shares common biological ancestory, and is indeed like a tree--some prefer the metaphor of a web or a shrub--on which all species have a place.
Darwin wrote as the final sentence in On the Origin Of Species (Chapter 14) -
"There is grandeur in this view of life, with its several powers, having been originally breathed into a few forms or into one; and that, whilst this planet has gone cycling on according to the fixed law of gravity, from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved."
You can learn more about evolution on the Understanding Evolution website.
Also, consider viewing the 1991 Royal Institution Christmas Lecture for Children, Growing Up In The Universe, delivered by Richard Dawkins when he was a Reader in Zoology at Oxford. He later was named to the Charles Simony Chair for the Public Understanding of Science at Oxford University. (The Institution's annual Christmas Lectures were founded by Michael Faraday FRS (1791-1867) in 1825, with himself as the inaugural lecturer.)
Dawkins was also interviewed about evolution on the 24th of November 2009 by Brian Lehrer (WNYC radio).
Some quotations about evolution's place in science:
[Evolution] is a general postulate to which all theories, all hypotheses, all systems must henceforward bow and which they must satisfy in order to be thinkable and true. Evolution is a light which illuminates all facts, a trajectory which all lines of thought must follow — this is what evolution is. - Pierre Teilhard de Chardin, SJ (1881-1955); paleontologist, geologist.
“Today, the theory of evolution is an accepted fact for everyone but a fundamentalist minority, whose objections are based not on reasoning but on doctrinaire adherence to religious principles” - James Watson (1928-); molecular biologist, co-discoverer of DNA structure, co-recipient 1962 Nobel Prize in Physiology or Medicine.
The theory of evolution by cumulative natural selection is the only theory we know of that is in principle capable of explaining the existence of organized complexity.... [Evolution is] a theory in a special philosophical sense of science, but in terms of ordinary laymen's use of language, it's a fact. Evolution is a fact in the same sense that it's a fact that the Earth is round and not flat, [that] the Earth goes round the Sun.” - Richard Dawkins FRS.