article : More Know about Evolution Topics

Pre-Darwinian Evolutionary Theory: John Lamarck, George Cuvier, James Hutton and Charles Lyell

You may believe Darwin to be the alpha and omega on evolutionary thought, but the proponents of Larmackianism, Catastrophism and Uniformitarianism paved the way.

Charles Darwin & Evolution: Natural Selection as the Mechanism of Biological Change

Darwin was not the first to understand that organisms evolve or change over time. His crucial contribution was identifying the mechanism of change, natural selection.

Evolution as Theory & Fact: The Certainty of Evolutionary Change & Theories of Its Mechanism

"It is important to understand that the current questions about how life evolves in no way imply any disagreement over the fact of evolution." - Neil A. Campbell. This article explores the misunderstanding of the work theory as it applies to evolution.

What Is Natural Selection?

Science Prof Online has an entire web page of links and articles on natural selection, the driving force of evolutionary change, first proposed by Charles Darwin. Select the link above to learn more!

DNA Gene Mutations & Evolution: Genetic Mistakes, Natural Selection and Biological Change 5/09

A mutation is an alteration of a gene's DNA sequence. Mutations are usually bad news, but those rare changes that benefit an organism are the raw material of evolution.

See the Evolution Online Bookstore for recommended readings and direct links to purchase books on evolution.

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Cell Endomembrane Illustration : Mariana Ruiz

Other image credits to be updated.

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article : Huge fossil of new dinosaur found in Patagonia

• Rory Carroll, Latin America correspondent
• The Guardian,
• Tuesday October 16 2007
• Article history

Brazilian and Argentinean palaeontologists present a replica of a skeleton of what could be a new dinosaur species. Photograph: Antonio Lacerda/EPA

It was four storeys tall, had a neck 10 times longer than a giraffe's and was possibly history's biggest vegetarian: meet Futalognkosaurus dukei, "chief of the lizards". Brazilian and Argentinian paleontologists yesterday said they had discovered a fossil of a new species of giant dinosaur which lived in what is now Patagonia, 80m years ago, during the late Cretaceous period.

A largely complete fossil of the herbivore shows it measured between 32 to 34 metres (105 to 112ft) from head to tail, and possessed an extraordinary neck which, at 17 metres, was a match for today's articulated bendy buses.

"It's a new species, a new group," an Argentinian paleontologist, Juan Porfiri, told a news conference in Rio de Janeiro. "Its neck was very big in diameter, strong and huge." Jorge Calvo, the director of the paleontology centre of the National University of Comahue, Argentina, said: "This is one of the biggest in the world and one of the most complete of these giants that exist."The fossil, which is believed to belong to a previously unknown species of Titanosaur, is 70% preserved.

The name Futalognkosaurus dukei was derived from the indigenous Mapuche language, meaning "giant chief of the lizards", and from the US company Duke Energy Corp, which funded much of the excavation in Argentina.

Dinosaur experts not involved in the discovery marvelled at the scale and said that what was found on the banks of Lake Barreales in the Argentine province of Neuquen did appear to represent a new species. The tail measures around 15 metres and the excavated spinal column alone weighs about eight tonnes.

Patagonia, a landscape of windswept plain which once was forested and humid, has yielded the other two largest dinosaur skeletons, Argentinosaurus, at around 35 metres long, and Puertasaurus reuili, between 35 and 40 metres long.

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article : Sea scorpion fossil belonged to biggest bug ever: scientists

A giant fossilized claw discovered in Germany belonged to an ancient sea scorpion that was much bigger than the average man, an international team of geologists and archaeologists reported Tuesday.

A computer-generated image shows a size comparison between a human and an ancient sea scorpion.
(University of Bristol/Associated Press)

The 46-centimetre-long claw was discovered by report co-author Markus Poschmann, from Germany, in a quarry near Pruem, a city about 200 kilometres east of Frankfurt.

The researchers said the scorpion lived between 460 and 255 million years ago and would have been among the top predators in its environment, feeding on early vertebrates and smaller arthropods. The report said the creature likely only lived in the water because with the construction of its body, "it is hardly imaginable how such a huge arthropod could effectively walk on land."

In a report in the Royal Society's journal Biology Letters, the team said the claw indicates that sea scorpion Jaekelopterus rhenania was almost 2.5 metres long, making it the largest arthropod — an animal with a segmented body, jointed limbs and a hard exoskeleton — ever found. In the report, the authors said the scorpion exceeds previous size records for arthropods by almost half a metre.

The fossil, found in a 390-million-year-old rock, suggests that spiders, insects, crabs and similar creatures were much larger in the past than previously thought, the researchers said.

This claw belonged to a giant sea scorpion, estimated to be 2.5 metres long.
(Markus Poschmann)

"This is an amazing discovery. We have known for some time that the fossil record yields monster millipedes, super-sized scorpions, colossal cockroaches, and jumbo dragonflies, but we never realized, until now, just how big some of these ancient creepy-crawlies were," said co-author Dr. Simon Braddy from the University of Bristol.

Some geologists believe that the giant arthropod evolved due to high oxygen levels, while others argue that they evolved in an "arms race" alongside their prey, the early armoured fish.

http://www.cbc.ca

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article : Fossil is new family of dinosaur

The specimen was left untouched for 113 years

A fossilised bone dug up near Hastings 113 years ago has been recognised as a completely new family of dinosaur.

The animal belongs to a general type of dinosaur called a sauropod - which was characterised by a large body, a long neck and a small head.

A PhD student from the University of Portsmouth stumbled upon the specimen while browsing through the shelves of London's Natural History Museum.

The work is to appear in the academic journal Palaeontology.

The fossil represents the dorsal vertebra (back bone) of a new family, genus and species of dinosaur now named Xenoposeidon proneneukus.

It lived about 140 million years ago, was about the size of an elephant and weighed 7.5 tonnes.

"It leapt out at me as being different," said Mike Taylor, a computer programmer who is studying sauropod vertebrae as part of his PhD at Portsmouth.

"It was unmistakably a dorsal vertebra from a sauropod, but it didn't look like any dorsal I'd ever seen before."

The bone has lain in the Natural History Museum since its discovery in the early 1890s in Ecclesbourne Glen, near Hastings, by fossil collector Philip James Rufford.

Odd one out

It was briefly described by the British palaeontologist Richard Lydekker but was then left untouched for the next 113 years.

Dr Paul Barrett, a researcher in palaeontology at the museum, said: "Dinosaur bones are being constantly reassessed and our collections still offer us lots of surprises."

Mr Taylor and fellow palaeontologist Dr Darren Naish know the preserved bone came from near the hip area of the dinosaur.

From this they made an informed guess about the size and shape of the animal and were able to establish why Xenoposeidon is not only a new genus and species, but probably a new family of dinosaur.

"The difference between this specimen and other sauropod vertebrae is sufficiently great that I concluded that it could not be placed in any existing species or genus," said Mr Taylor.

"In fact it can't be placed in any existing sauropod family."

http://news.bbc.co.uk

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article : Fossil Called Missing Link From Sea to Land Animals (2)

Published: April 6, 2006

One creationist site on the Web (emporium.turnpike.net/C/cs /evid1.htm) declares that "there are no transitional forms," adding: "For example, not a single fossil with part fins, part feet has been found. And this is true between every major plant and animal kind."

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Dr. Novacek responded: "We've got Archaeopteryx, an early whale that lived on land, and now this animal showing the transition from fish to tetrapod. What more do we need from the fossil record to show that the creationists are flatly wrong?"

Duane T. Gish, a retired official of the Institute for Creation Research in San Diego, said, "This alleged transitional fish will have to be evaluated carefully." But he added that he still found evolution "questionable because paleontologists have yet to discover any transitional fossils between complex invertebrates and fish, and this destroys the whole evolutionary story."

Dr. Shubin and Dr. Daeschler began their search on Ellesmere Island in 1999. They were attracted by a map in a geology textbook showing an abundance of Devonian rocks exposed and relatively easy to explore. At that time, the land had a warm climate: it was part of a supercontinent straddling the Equator.

It was not until July 2004, Dr. Shubin said, that "we hit the jackpot." They found several of the fishes in a quarry, their skeletons largely intact and in three dimensions. The large skull had the sharp teeth of a predator. It was attached to a neck, which allowed the fish the unfishlike ability to swivel its head.

If the animal spent any time out of water, said Dr. Jenkins, of Harvard, it needed a true neck that allowed the head to move independently on the body.

Embedded in the pectoral fins were bones that compare to the upper arm, forearm and primitive parts of the hand of land-living animals. The joints of the fins appeared to be capable of functioning for movement on land, a case of a fish improvising with its evolved anatomy. In all likelihood, the scientists said, Tiktaalik flexed its proto-limbs mainly on the floor of streams and might have pulled itself up on the shore for brief stretches.

In their report, the scientists concluded that Tiktaalik was an intermediate between the fishes Eusthenopteron and Panderichthys, which lived 385 million years ago, and early tetrapods. The known early tetrapods are Acanthostega and Ichthyostega, about 365 million years ago.

Tiktaalik, Dr. Shubin said, is "both fish and tetrapod, which we sometimes call a fishapod."

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article : Fossil Called Missing Link From Sea to Land Animals (1)

By JOHN NOBLE WILFORD

Published: April 6, 2006

Scientists have discovered fossils of a 375-million-year-old fish, a large scaly creature not seen before, that they say is a long-sought missing link in the evolution of some fishes from water to a life walking on four limbs on land.

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A model of Tiktaalik roseae, a fish in transition to becoming a land animal.

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In two reports today in the journal Nature, a team of scientists led by Neil H. Shubin of the University of Chicago say they have uncovered several well-preserved skeletons of the fossil fish in sediments of former streambeds in the Canadian Arctic, 600 miles from the North Pole.

The skeletons have the fins, scales and other attributes of a giant fish, four to nine feet long. But on closer examination, the scientists found telling anatomical traits of a transitional creature, a fish that is still a fish but has changes that anticipate the emergence of land animals — and is thus a predecessor of amphibians, reptiles and dinosaurs, mammals and eventually humans.

In the fishes' forward fins, the scientists found evidence of limbs in the making. There are the beginnings of digits, proto-wrists, elbows and shoulders. The fish also had a flat skull resembling a crocodile's, a neck, ribs and other parts that were similar to four-legged land animals known as tetrapods.

Other scientists said that in addition to confirming elements of a major transition in evolution, the fossils were a powerful rebuttal to religious creationists, who have long argued that the absence of such transitional creatures are a serious weakness in Darwin's theory.

The discovery team called the fossils the most compelling examples yet of an animal that was at the cusp of the fish-tetrapod transition. The fish has been named Tiktaalik roseae, at the suggestion of elders of Canada's Nunavut Territory. Tiktaalik (pronounced tic-TAH-lick) means "large shallow water fish."

"The origin of limbs," Dr. Shubin's team wrote, "probably involved the elaboration and proliferation of features already present in the fins of fish such as Tiktaalik."

In an interview, Dr. Shubin, an evolutionary biologist, let himself go. "It's a really amazing, remarkable intermediate fossil," he said. "It's like, holy cow."

Two other paleontologists, commenting on the find in a separate article in the journal, said that a few other transitional fish had been previously discovered from approximately the same Late Devonian time period, 385 million to 359 million years ago. But Tiktaalik is so clearly an intermediate "link between fishes and land vertebrates," they said, that it "might in time become as much an evolutionary icon as the proto-bird Archaeopteryx," which bridged the gap between reptiles (probably dinosaurs) and today's birds.

The writers, Erik Ahlberg of Uppsala University in Sweden and Jennifer A. Clack of the University of Cambridge in England, are often viewed as rivals to Dr. Shubin's team in the search for intermediate species in the evolution from fish to the first animals to colonize land.

H. Richard Lane, director of paleobiology at the National Science Foundation, said in a statement, "These exciting discoveries are providing fossil 'Rosetta Stones' for a deeper understanding of this evolutionary milestone — fish to land-roaming tetrapods."

The science foundation and the National Geographic Society were among the financial supporters of the research. Besides Dr. Shubin, the principal discoverers were Edward B. Daeschler of the Academy of Natural Sciences in Philadelphia and Farish A. Jenkins Jr., a Harvard evolutionary biologist. Casts of the fossils will be on view at the Science Museum of London.

Michael J. Novacek, a paleontologist at the American Museum of Natural History in Manhattan, who was not involved in the research, said: "Based on what we already know, we have a very strong reason to think tetrapods evolved from lineages of fishes. This may be a critical phase in that transition that we haven't had before. A good fossil cuts through a lot of scientific argument."

Dr. Shubin's team played down the fossil's significance in the raging debate over Darwinian theory, which is opposed mainly by some conservative Christians in this country, but other scientists were not so reticent. They said this should undercut the argument that there is no evidence in the fossil record of one kind of creature becoming another kind.

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article : Ancient ancestor reveals skeletal stamina - new fossil found in South Africa - Brief Article

Science News, Dec 19, 1998 by B. Bower

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A new fossil find in South Africa represents the most complete skeleton to date attributed to the australopithecines, an extinct line of two-legged, small-brained creatures who were early members of the human evolutionary family.

Anthropologists last week announced their discovery of the largely intact skull and lower body, which they found in an underground cave. A brief description of the skeleton appears in the Dec. 10 NATURE. The October SOUTH AFRICAN JOURNAL OF SCIENCE contains a more detailed account by Ronald J. Clarke of the University of the Witwatersrand in Johannesburg.

Clarke supervised two colleagues who found the australopithecine specimen after descending about 45 feet into the Silberberg Grotto of the Sterkfontein caves. Prior excavations elsewhere at Sterkfontein yielded fragmentary remains of Australopithecus africanus, a human ancestor of uncertain evolutionary status dating approximately to between 3.2 million and 2.4 million years ago.

"This is a remarkable find," says Witwatersrand's Phillip V. Tobias, who directs all Sterkfontein excavations. "We don't know its species or sex yet, but it looks like an adult australopithecine."

Clarke found foot bones from the skeleton while working at Silberberg Grotto in 1994. He discovered more parts of the same foot stored in boxes from prior excavations there (SN: 7/29/95, p. 71).

Clarke then organized another exploration of the site. Investigators identified many of the individual fossil's limb, hip, and back bones, as well as a nearly complete skull, protruding from limestone in the cave. The skull's jaws bear full sets of teeth.

It will take at least a year to remove the entire skeleton from its rocky resting place, Clarke says. He plans to examine the limb joints to test the theory--so far, based only on the foot bones--that this creature combined upright walking with considerable tree climbing.

Analyses of magnetic properties of limestone, taken from below and above the fossil, place it between 3.2 million and 3.6 million years old.

Until now, the most complete australopithecine skeleton was that of Lucy, a 3.2-million-year-old find assigned to Australopithecus afarensis.

"This new skeleton contains important information about the size and shape of the australopithecine body that's been hard to come by," remarks anthropologist Bernard Wood of George Washington University in Washington, D.C.

Wood suspects that the specimen belongs to A. africanus, which in his view lessens its potential for generating evolutionary insights. Lower-body remains from at least one other australopithecine species at Sterkfontein from the same time, he asserts, would allow for the identification of species-specific limb features.

COPYRIGHT 1998 Science Service, Inc.
COPYRIGHT 2000 Gale Group

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article : What is some evidence for proof of evolution?

In his pivotal opus Origin of Species (1859) Darwin presented various evidences for proof of evolution. Among these he sited domestic breeding, anatomical similarities among species (“homology”), the sequential order of fossils, the presence of “vestigial” organs, and the natural phenomenon which he dubbed “natural selection.”

In the century-and-a-half since Darwin published his work, advances in science have made some of these various evidences for evolution dubious. For example, in Darwin’s day it was believed that there were dozens of vestigial organs in the human body. Estimates have ranged from 80 to 200. Scientists at the time did not know what purpose these organs served so they assumed that they were useless vestiges from our evolutionary past. One-hundred fifty years later, only a handful of so-called vestigial organs remain. Scientists have discovered biological functions for the rest. Moreover, critics of Darwin’s theory point out that if vestigial organs are truly useless, the progression is towards a loss of function, not new function. Darwinian evolution requires biological innovation.

Advances in genetics have also shown new light upon the dynamics of homology (anatomical similarities among species) and domestic breeding (the ability of breeders to produce dramatic changes in domestic animal populations by selecting individuals to breed, thereby suppressing and emphasizing traits gradually over time). It is now known that structural similarities do not necessary equal genetic relationship and there appear to be genetic limits to the potential for biological change. A bird can adapt to its environment to a certain degree but it is doubtful that it could cross genetic boundaries to evolve into a reptile, for example.

Advocates for Darwinian evolution believe that genetics have provided a new mechanism for biological innovation in the form of genetic mutation. The incorporation of genetics into Darwinian evolution has produced what is now known as the Neo-Darwinian Synthesis. Nevertheless, the debate rages on whether or not mutations simply destroy existing genetic structure or whether they can provide new genetic information, which Darwinian evolution requires. While Darwinian evolution remains the dominant biological paradigm, there is a growing minority of scientists who “are skeptical of claims for the ability of random mutation and natural selection to account for the complexity of life. Careful examination of the evidence for Darwinian theory should be encouraged.” (From A Scientific Dissent From Darwinism, signed by over 680 Ph.D. scientists.)

Evidence for Evolution - Learn More!

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article : Evolution, scientific acceptance and extension to other disciplines

he theory of evolution makes statements about three different, though related, issues: (1) the fact of evolution—that is, that organisms are related by common descent; (2) evolutionary history—the details of when lineages split from one another and of the changes that occurred in each lineage; and (3) the mechanisms or processes by which evolutionary change occurs.

The first issue is the most fundamental and the one established with utmost certainty. Darwin gathered much evidence in its support, but evidence has accumulated continuously ever since, derived from all biological disciplines. The evolutionary origin of organisms is today a scientific conclusion established with the kind of certainty attributable to such scientific concepts as the roundness of Earth, the motions of the planets, and the molecular composition of matter. This degree of certainty beyond reasonable doubt is what is implied when biologists say that evolution is a “fact”; the evolutionary origin of organisms is accepted by virtually every biologist.

But the theory of evolution goes far beyond the general affirmation that organisms evolve. The second and third issues—seeking to ascertain evolutionary relationships between particular organisms and the events of evolutionary history, as well as to explain how and why evolution takes place—are matters of active scientific investigation. Some conclusions are well established. One, for example, is that the chimpanzee and the gorilla are more closely related to humans than is any of those three species to the baboon or other monkeys. Another conclusion is that natural selection, the process postulated by Darwin, explains the configuration of such adaptive features as the human eye and the wings of birds. Many matters are less certain, others are conjectural, and still others—such as the characteristics of the first living things and when they came about—remain completely unknown.

Since Darwin, the theory of evolution has gradually extended its influence to other biological disciplines, from physiology to ecology and from biochemistry to systematics. All biological knowledge now includes the phenomenon of evolution. In the words of Theodosius Dobzhansky, “Nothing in biology makes sense except in the light of evolution.”

The term evolution and the general concept of change through time also have penetrated into scientific language well beyond biology and even into common language. Astrophysicists speak of the evolution of the solar system or of the universe; geologists, of the evolution of Earth's interior; psychologists, of the evolution of the mind; anthropologists, of the evolution of cultures; art historians, of the evolution of architectural styles; and couturiers, of the evolution of fashion. These and other disciplines use the word with only the slightest commonality of meaning—the notion of gradual, and perhaps directional, change over the course of time.

Toward the end of the 20th century, specific concepts and processes borrowed from biological evolution and living systems were incorporated into computational research, beginning with the work of the American mathematician John Holland and others. One outcome of this endeavour was the development of methods for automatically generating computer-based systems that are proficient at given tasks. These systems have a wide variety of potential uses, such as solving practical computational problems, providing machines with the ability to learn from experience, and modeling processes in fields as diverse as ecology, immunology, economics, and even biological evolution itself.

To generate computer programs that represent proficient solutions to a problem under study, the computer scientist creates a set of step-by-step procedures, called a genetic algorithm or, more broadly, an evolutionary algorithm, that incorporates analogies of genetic processes—for instance, heredity, mutation, and recombination—as well as of evolutionary processes such as natural selection in the presence of specified environments. The algorithm is designed typically to simulate the biological evolution of a population of individual computer programs through successive generations to improve their “fitness” for carrying out a designated task. Each program in an initial population receives a fitness score that measures how well it performs in a specific “environment”—for example, how efficiently it sorts a list of numbers or allocates the floor space in a new factory design. Only those with the highest scores are selected to “reproduce,” to contribute “hereditary” material—i.e., computer code—to the following generation of programs. The rules of reproduction may involve such elements as recombination (strings of code from the best programs are shuffled and combined into the programs of the next generation) and mutation (bits of code in a few of the new programs are changed at random). The evolutionary algorithm then evaluates each program in the new generation for fitness, winnows out the poorer performers, and allows reproduction to take place once again, with the cycle repeating itself as often as desired. Evolutionary algorithms are simplistic compared with biological evolution, but they have provided robust and powerful mechanisms for finding solutions to all sorts of problems in economics, industrial production, and the distribution of goods and services. (See also artificial intelligence: Evolutionary computing.)

Darwin's notion of natural selection also has been extended to areas of human discourse outside the scientific setting, particularly in the fields of sociopolitical theory and economics. The extension can be only metaphoric, because in Darwin's intended meaning natural selection applies only to hereditary variations in entities endowed with biological reproduction—that is, to living organisms. That natural selection is a natural process in the living world has been taken by some as a justification for ruthless competition and for “survival of the fittest” in the struggle for economic advantage or for political hegemony. Social Darwinism was an influential social philosophy in some circles through the late 19th and early 20th centuries, when it was used as a rationalization for racism, colonialism, and social stratification. At the other end of the political spectrum, Marxist theorists have resorted to evolution by natural selection as an explanation for humankind's political history.

Darwinism understood as a process that favours the strong and successful and eliminates the weak and failing has been used to justify alternative and, in some respects, quite diametric economic theories (see economics). These theories share in common the premise that the valuation of all market products depends on a Darwinian process. Specific market commodities are evaluated in terms of the degree to which they conform to specific valuations emanating from the consumers. On the one hand, some of these economic theories are consistent with theories of evolutionary psychology that see preferences as determined largely genetically; as such, they hold that the reactions of markets can be predicted in terms of largely fixed human attributes. The dominant neo-Keynesian (see economics: Keynesian economics) and monetarist schools of economics make predictions of the macroscopic behaviour of economies (see macroeconomics) based the interrelationship of a few variables; money supply, rate of inflation, and rate of unemployment jointly determine the rate of economic growth. On the other hand, some minority economists, such as the 20th-century Austrian-born British theorist F.A. Hayek and his followers, predicate the Darwinian process on individual preferences that are mostly underdetermined and change in erratic or unpredictable ways. According to them, old ways of producing goods and services are continuously replaced by new inventions and behaviours. These theorists affirm that what drives the economy is the ingenuity of individuals and corporations and their ability to bring new and better products to the market.

http://www.britannica.com

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article : Evolution, molecular biology and earth sciences

he most important line of investigation after 1950 was the application of molecular biology to evolutionary studies. In 1953 the American geneticist James Watson and the British biophysicist Francis Crick deduced the molecular structure of DNA (deoxyribonucleic acid), the hereditary material contained in the chromosomes of every cell's nucleus. The genetic information is encoded within the sequence of nucleotides that make up the chainlike DNA molecules. This information determines the sequence of amino acid building blocks of protein molecules, which include, among others, structural proteins such as collagen, respiratory proteins such as hemoglobin, and numerous enzymes responsible for the organism's fundamental life processes. Genetic information contained in the DNA can thus be investigated by examining the sequences of amino acids in the proteins.

n the mid-1960s laboratory techniques such as electrophoresis and selective assay of enzymes became available for the rapid and inexpensive study of differences among enzymes and other proteins. The application of these techniques to evolutionary problems made possible the pursuit of issues that earlier could not be investigated—for example, exploring the extent of genetic variation in natural populations (which sets bounds on their evolutionary potential) and determining the amount of genetic change that occurs during the formation of new species.

Comparisons of the amino acid sequences of corresponding proteins in different species provided quantitatively precise measures of the divergence among species evolved from common ancestors, a considerable improvement over the typically qualitative evaluations obtained by comparative anatomy and other evolutionary subdisciplines. In 1968 the Japanese geneticist Motoo Kimura proposed the neutrality theory of molecular evolution, which assumes that, at the level of the sequences of nucleotides in DNA and of amino acids in proteins, many changes are adaptively neutral; they have little or no effect on the molecule's function and thus on an organism's fitness within its environment. If the neutrality theory is correct, there should be a “molecular clock” of evolution; that is, the degree to which amino acid or nucleotide sequences diverge between species should provide a reliable estimate of the time since the species diverged. This would make it possible to reconstruct an evolutionary history that would reveal the order of branching of different lineages, such as those leading to humans, chimpanzees, and orangutans, as well as the time in the past when the lineages split from one another. During the 1970s and '80s it gradually became clear that the molecular clock is not exact; nevertheless, into the early 21st century it continued to provide the most reliable evidence for reconstructing evolutionary history. (See below The molecular clock of evolution and The neutrality theory of molecular evolution.)

The laboratory techniques of DNA cloning and sequencing have provided a new and powerful means of investigating evolution at the molecular level. The fruits of this technology began to accumulate during the 1980s following the development of automated DNA-sequencing machines and the invention of the polymerase chain reaction (PCR), a simple and inexpensive technique that obtains, in a few hours, billions or trillions of copies of a specific DNA sequence or gene. Major research efforts such as the Human Genome Project further improved the technology for obtaining long DNA sequences rapidly and inexpensively. By the first few years of the 21st century, the full DNA sequence—i.e., the full genetic complement, or genome—had been obtained for more than 20 higher organisms, including human beings, the house mouse (Mus musculus), the rat Rattus norvegicus, the vinegar fly Drosophila melanogaster, the mosquito Anopheles gambiae, the nematode worm Caenorhabditis elegans, the malaria parasite Plasmodium falciparum, the mustard weed Arabidopsis thaliana, and the yeast Saccharomyces cerevisiae, as well as for numerous microorganisms.

The changing Earth through geologic time, from the late Cambrian (c. 500 million years ago) to …
Adapted from C.R. Scotese, The University of Texas at Arlington

The Earth sciences also experienced, in the second half of the 20th century, a conceptual revolution with considerable consequence to the study of evolution. The theory of plate tectonics, which was formulated in the late 1960s, revealed that the configuration and position of the continents and oceans are dynamic, rather than static, features of Earth. Oceans grow and shrink, while continents break into fragments or coalesce into larger masses. The continents move across Earth's surface at rates of a few centimetres a year, and over millions of years of geologic history this movement profoundly alters the face of the planet, causing major climatic changes along the way. These previously unsuspected massive modifications of Earth's past environments are, of necessity, reflected in the evolutionary history of life. Biogeography, the evolutionary study of plant and animal distribution, has been revolutionized by the knowledge, for example, that Africa and South America were part of a single landmass some 200 million years ago and that the Indian subcontinent was not connected with Asia until geologically recent times.

Ecology, the study of the interactions of organisms with their environments, has evolved from descriptive studies—“natural history”—into a vigorous biological discipline with a strong mathematical component, both in the development of theoretical models and in the collection and analysis of quantitative data. Evolutionary ecology (see community ecology) is an active field of evolutionary biology; another is evolutionary ethology, the study of the evolution of animal behaviour. Sociobiology, the evolutionary study of social behaviour, is perhaps the most active subfield of ethology. It is also the most controversial, because of its extension to human societies.

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article : Evolution, The Shintetic theory

The rediscovery in 1900 of Mendel's theory of heredity, by the Dutch botanist and geneticist Hugo de Vries and others, led to an emphasis on the role of heredity in evolution. De Vries proposed a new theory of evolution known as mutationism, which essentially did away with natural selection as a major evolutionary process. According to de Vries (who was joined by other geneticists such as William Bateson in England), two kinds of variation take place in organisms. One is the “ordinary” variability observed among individuals of a species, which is of no lasting consequence in evolution because, according to de Vries, it could not “lead to a transgression of the species border [i.e., to establishment of new species] even under conditions of the most stringent and continued selection.” The other consists of the changes brought about by mutations, spontaneous alterations of genes that result in large modifications of the organism and give rise to new species: “The new species thus originates suddenly, it is produced by the existing one without any visible preparation and without transition.”

Mutationism was opposed by many naturalists and in particular by the so-called biometricians, led by the English statistician Karl Pearson, who defended Darwinian natural selection as the major cause of evolution through the cumulative effects of small, continuous, individual variations (which the biometricians assumed passed from one generation to the next without being limited by Mendel's laws of inheritance [see Mendelism]).

The controversy between mutationists (also referred to at the time as Mendelians) and biometricians approached a resolution in the 1920s and '30s through the theoretical work of geneticists. These scientists used mathematical arguments to show, first, that continuous variation (in such characteristics as body size, number of eggs laid, and the like) could be explained by Mendel's laws and, second, that natural selection acting cumulatively on small variations could yield major evolutionary changes in form and function. Distinguished members of this group of theoretical geneticists were R.A. Fisher and J.B.S. Haldane in Britain and Sewall Wright in the United States. Their work contributed to the downfall of mutationism and, most important, provided a theoretical framework for the integration of genetics into Darwin's theory of natural selection. Yet their work had a limited impact on contemporary biologists for several reasons—it was formulated in a mathematical language that most biologists could not understand; it was almost exclusively theoretical, with little empirical corroboration; and it was limited in scope, largely omitting many issues, such as speciation (the process by which new species are formed), that were of great importance to evolutionists.

A major breakthrough came in 1937 with the publication of Genetics and the Origin of Species by Theodosius Dobzhansky, a Russian-born American naturalist and experimental geneticist. Dobzhansky's book advanced a reasonably comprehensive account of the evolutionary process in genetic terms, laced with experimental evidence supporting the theoretical argument. Genetics and the Origin of Species may be considered the most important landmark in the formulation of what came to be known as the synthetic theory of evolution, effectively combining Darwinian natural selection and Mendelian genetics. It had an enormous impact on naturalists and experimental biologists, who rapidly embraced the new understanding of the evolutionary process as one of genetic change in populations. Interest in evolutionary studies was greatly stimulated, and contributions to the theory soon began to follow, extending the synthesis of genetics and natural selection to a variety of biological fields.

The main writers who, together with Dobzhansky, may be considered the architects of the synthetic theory were the German-born American zoologist Ernst Mayr, the English zoologist Julian Huxley, the American paleontologist George Gaylord Simpson, and the American botanist George Ledyard Stebbins. These researchers contributed to a burst of evolutionary studies in the traditional biological disciplines and in some emerging ones—notably population genetics and, later, evolutionary ecology (see community ecology). By 1950 acceptance of Darwin's theory of evolution by natural selection was universal among biologists, and the synthetic theory had become widely adopted.

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article : The Darwinian aftermath

The publication of the Origin of Species produced considerable public excitement. Scientists, politicians, clergymen, and notables of all kinds read and discussed the book, defending or deriding Darwin's ideas. The most visible actor in the controversies immediately following publication was the English biologist T.H. Huxley, known as “Darwin's bulldog,” who defended the theory of evolution with articulate and sometimes mordant words on public occasions as well as in numerous writings. Evolution by natural selection was indeed a favourite topic in society salons during the 1860s and beyond. But serious scientific controversies also arose, first in Britain and then on the Continent and in the United States.

Alfred Russel Wallace, detail of a painting over a photograph; in the National Portrait Gallery, …
Courtesy of the National Portrait Gallery, London

One occasional participant in the discussion was the British naturalist Alfred Russel Wallace, who had hit upon the idea of natural selection independently and had sent a short manuscript about it to Darwin from the Malay Archipelago, where he was collecting specimens and writing. On July 1, 1858, one year before the publication of the Origin, a paper jointly authored by Wallace and Darwin was presented, in the absence of both, to the Linnean Society in London—with apparently little notice. Greater credit is duly given to Darwin than to Wallace for the idea of evolution by natural selection; Darwin developed the theory in considerably more detail, provided far more evidence for it, and was primarily responsible for its acceptance. Wallace's views differed from Darwin's in several ways, most importantly in that Wallace did not think natural selection sufficient to account for the origin of human beings, which in his view required direct divine intervention.

A younger English contemporary of Darwin, with considerable influence during the latter part of the 19th and in the early 20th century, was Herbert Spencer. A philosopher rather than a biologist, he became an energetic proponent of evolutionary ideas, popularized a number of slogans, such as “survival of the fittest” (which was taken up by Darwin in later editions of the Origin), and engaged in social and metaphysical speculations. His ideas considerably damaged proper understanding and acceptance of the theory of evolution by natural selection. Darwin wrote of Spencer's speculations:

His deductive manner of treating any subject is wholly opposed to my frame of mind.…His fundamental generalizations (which have been compared in importance by some persons with Newton's laws!) which I dare say may be very valuable under a philosophical point of view, are of such a nature that they do not seem to me to be of any strictly scientific use.

Most pernicious was the crude extension by Spencer and others of the notion of the “struggle for existence” to human economic and social life that became known as social Darwinism (see below Scientific acceptance and extension to other disciplines).

The most serious difficulty facing Darwin's evolutionary theory was the lack of an adequate theory of inheritance that would account for the preservation through the generations of the variations on which natural selection was supposed to act. Contemporary theories of “blending inheritance” proposed that offspring merely struck an average between the characteristics of their parents. But as Darwin became aware, blending inheritance (including his own theory of “pangenesis,” in which each organ and tissue of an organism throws off tiny contributions of itself that are collected in the sex organs and determine the configuration of the offspring) could not account for the conservation of variations, because differences between variant offspring would be halved each generation, rapidly reducing the original variation to the average of the preexisting characteristics.

The missing link in Darwin's argument was provided by Mendelian genetics. About the time the Origin of Species was published, the Augustinian monk Gregor Mendel was starting a long series of experiments with peas in the garden of his monastery in Brünn, Austria-Hungary (now Brno, Czech Republic). These experiments and the analysis of their results are by any standard an example of masterly scientific method. Mendel's paper, published in 1866 in the Proceedings of the Natural Science Society of Brünn, formulated the fundamental principles of the theory of heredity that is still current. His theory accounts for biological inheritance through particulate factors (now known as genes) inherited one from each parent, which do not mix or blend but segregate in the formation of the sex cells, or gametes.

Mendel's discoveries remained unknown to Darwin, however, and, indeed, they did not become generally known until 1900, when they were simultaneously rediscovered by a number of scientists on the Continent. In the meantime, Darwinism in the latter part of the 19th century faced an alternative evolutionary theory known as neo-Lamarckism. This hypothesis shared with Lamarck's the importance of use and disuse in the development and obliteration of organs, and it added the notion that the environment acts directly on organic structures, which explained their adaptation to the way of life and environment of the organism. Adherents of this theory discarded natural selection as an explanation for adaptation to the environment.

August Weismann, German biologist and one of the founders of the science of genetics.
The Bettmann Archive

Prominent among the defenders of natural selection was the German biologist August Weismann, who in the 1880s published his germ plasm theory. He distinguished two substances that make up an organism: the soma, which comprises most body parts and organs, and the germ plasm, which contains the cells that give rise to the gametes and hence to progeny. Early in the development of an egg, the germ plasm becomes segregated from the somatic cells that give rise to the rest of the body. This notion of a radical separation between germ plasm and soma—that is, between the reproductive tissues and all other body tissues—prompted Weismann to assert that inheritance of acquired characteristics was impossible, and it opened the way for his championship of natural selection as the only major process that would account for biological evolution. Weismann's ideas became known after 1896 as neo-Darwinism.

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article : Evolution, Charles Darwin

Charles Darwin, oil over a photograph, c. 1855.
The Granger Collection, New York

The founder of the modern theory of evolution was Charles Darwin. The son and grandson of physicians, he enrolled as a medical student at the University of Edinburgh. After two years, however, he left to study at the University of Cambridge and prepare to become a clergyman. He was not an exceptional student, but he was deeply interested in natural history. On December 27, 1831, a few months after his graduation from Cambridge, he sailed as a naturalist aboard the HMS Beagle on a round-the-world trip that lasted until October 1836. Darwin was often able to disembark for extended trips ashore to collect natural specimens.

Title page of the 1859 edition of Charles Darwin's On the Origin of Species by …
Library of Congress, Washington, D.C.

The discovery of fossil bones from large extinct mammals in Argentina and the observation of numerous species of finches in the Galapagos Islands were among the events credited with stimulating Darwin's interest in how species originate. In 1859 he published On the Origin of Species by Means of Natural Selection, a treatise establishing the theory of evolution and, most important, the role of natural selection in determining its course. He published many other books as well, notably The Descent of Man and Selection in Relation to Sex (1871), which extends the theory of natural selection to human evolution.

Darwin must be seen as a great intellectual revolutionary who inaugurated a new era in the cultural history of humankind, an era that was the second and final stage of the Copernican revolution that had begun in the 16th and 17th centuries under the leadership of men such as Nicolaus Copernicus, Galileo, and Isaac Newton. The Copernican revolution marked the beginnings of modern science. Discoveries in astronomy and physics overturned traditional conceptions of the universe. Earth no longer was seen as the centre of the universe but was seen as a small planet revolving around one of myriad stars; the seasons and the rains that make crops grow, as well as destructive storms and other vagaries of weather, became understood as aspects of natural processes; the revolutions of the planets were now explained by simple laws that also accounted for the motion of projectiles on Earth.

The significance of these and other discoveries was that they led to a conception of the universe as a system of matter in motion governed by laws of nature. The workings of the universe no longer needed to be attributed to the ineffable will of a divine Creator; rather, they were brought into the realm of science—an explanation of phenomena through natural laws. Physical phenomena such as tides, eclipses, and positions of the planets could now be predicted whenever the causes were adequately known. Darwin accumulated evidence showing that evolution had occurred, that diverse organisms share common ancestors, and that living beings have changed drastically over the course of Earth's history. More important, however, he extended to the living world the idea of nature as a system of matter in motion governed by natural laws.

Before Darwin, the origin of Earth's living things, with their marvelous contrivances for adaptation, had been attributed to the design of an omniscient God. He had created the fish in the waters, the birds in the air, and all sorts of animals and plants on the land. God had endowed these creatures with gills for breathing, wings for flying, and eyes for seeing, and he had coloured birds and flowers so that human beings could enjoy them and recognize God's wisdom. Christian theologians, from Aquinas on, had argued that the presence of design, so evident in living beings, demonstrates the existence of a supreme Creator; the argument from design was Aquinas's “fifth way” for proving the existence of God. In 19th-century England the eight Bridgewater Treatises were commissioned so that eminent scientists and philosophers would expand on the marvels of the natural world and thereby set forth “the Power, wisdom, and goodness of God as manifested in the Creation.”

The British theologian William Paley in his Natural Theology (1802) used natural history, physiology, and other contemporary knowledge to elaborate the argument from design. If a person should find a watch, even in an uninhabited desert, Paley contended, the harmony of its many parts would force him to conclude that it had been created by a skilled watchmaker; and, Paley went on, how much more intricate and perfect in design is the human eye, with its transparent lens, its retina placed at the precise distance for forming a distinct image, and its large nerve transmitting signals to the brain.

The argument from design seems to be forceful. A ladder is made for climbing, a knife for cutting, and a watch for telling time; their functional design leads to the conclusion that they have been fashioned by a carpenter, a smith, or a watchmaker. Similarly, the obvious functional design of animals and plants seems to denote the work of a Creator. It was Darwin's genius that he provided a natural explanation for the organization and functional design of living beings. (For additional discussion of the argument from design and its revival in the 1990s, see below Intelligent design and its critics.)

Darwin accepted the facts of adaptation—hands are for grasping, eyes for seeing, lungs for breathing. But he showed that the multiplicity of plants and animals, with their exquisite and varied adaptations, could be explained by a process of natural selection, without recourse to a Creator or any designer agent. This achievement would prove to have intellectual and cultural implications more profound and lasting than his multipronged evidence that convinced contemporaries of the fact of evolution.

Darwin's theory of natural selection is summarized in the Origin of Species as follows:

As many more individuals are produced than can possibly survive, there must in every case be a struggle for existence, either one individual with another of the same species, or with the individuals of distinct species, or with the physical conditions of life.…Can it, then, be thought improbable, seeing that variations useful to man have undoubtedly occurred, that other variations useful in some way to each being in the great and complex battle of life, should sometimes occur in the course of thousands of generations? If such do occur, can we doubt (remembering that many more individuals are born than can possibly survive) that individuals having any advantage, however slight, over others, would have the best chance of surviving and of procreating their kind? On the other hand, we may feel sure that any variation in the least degree injurious would be rigidly destroyed. This preservation of favourable variations and the rejection of injurious variations, I call Natural Selection.

Natural selection was proposed by Darwin primarily to account for the adaptive organization of living beings; it is a process that promotes or maintains adaptation. Evolutionary change through time and evolutionary diversification (multiplication of species) are not directly promoted by natural selection, but they often ensue as by-products of natural selection as it fosters adaptation to different environments.

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article : Evolution, Early Ideas

All human cultures have developed their own explanations for the origin of the world and of human beings and other creatures. Traditional Judaism and Christianity explain the origin of living beings and their adaptations to their environments—wings, gills, hands, flowers—as the handiwork of an omniscient God. The philosophers of ancient Greece had their own creation myths. Anaximander proposed that animals could be transformed from one kind into another, and Empedocles speculated that they were made up of various combinations of preexisting parts. Closer to modern evolutionary ideas were the proposals of early Church Fathers such as Gregory of Nazianzus and Augustine, both of whom maintained that not all species of plants and animals were created by God; rather, some had developed in historical times from God's creations. Their motivation was not biological but religious—it would have been impossible to hold representatives of all species in a single vessel such as Noah's Ark; hence, some species must have come into existence only after the Flood.

The notion that organisms may change by natural processes was not investigated as a biological subject by Christian theologians of the Middle Ages, but it was, usually incidentally, considered as a possibility by many, including Albertus Magnus and his student Thomas Aquinas. Aquinas concluded, after detailed discussion, that the development of living creatures such as maggots and flies from nonliving matter such as decaying meat was not incompatible with Christian faith or philosophy. But he left it to others to determine whether this actually happened.

The idea of progress, particularly the belief in unbounded human progress, was central to the Enlightenment of the 18th century, particularly in France among such philosophers as the marquis de Condorcet and Denis Diderot and such scientists as Georges-Louis Leclerc, comte de Buffon. But belief in progress did not necessarily lead to the development of a theory of evolution. Pierre-Louis Moreau de Maupertuis proposed the spontaneous generation and extinction of organisms as part of his theory of origins, but he advanced no theory of evolution—i.e., the transformation of one species into another through knowable, natural causes. Buffon, one of the greatest naturalists of the time, explicitly considered—and rejected—the possible descent of several species from a common ancestor. He postulated that organisms arise from organic molecules by spontaneous generation, so that there could be as many kinds of animals and plants as there are viable combinations of organic molecules.

The English physician Erasmus Darwin, grandfather of Charles Darwin, offered in his Zoonomia; or, The Laws of Organic Life (1794–96) some evolutionary speculations, but they were not further developed and had no real influence on subsequent theories. The Swedish botanist Carolus Linnaeus devised the hierarchical system of plant and animal classification that is still in use in a modernized form. Although he insisted on the fixity of species, his classification system eventually contributed much to the acceptance of the concept of common descent.

The great French naturalist Jean-Baptiste de Monet, chevalier de Lamarck, held the enlightened view of his age that living organisms represent a progression, with humans as the highest form. From this idea he proposed, in the early years of the 19th century, the first broad theory of evolution. Organisms evolve through eons of time from lower to higher forms, a process still going on, always culminating in human beings. As organisms become adapted to their environments through their habits, modifications occur. Use of an organ or structure reinforces it; disuse leads to obliteration. The characteristics acquired by use and disuse, according to this theory, would be inherited. This assumption, later called the inheritance of acquired characteristics (or Lamarckism), was thoroughly disproved in the 20th century. Although his theory did not stand up in the light of later knowledge, Lamarck made important contributions to the gradual acceptance of biological evolution and stimulated countless later studies.

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