Biology: Evolutionary Theory

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Biology: Evolutionary Theory


In The Origin of Species (1859), Charles Darwin (1809–1882) introduced the idea of natural selection, theorizing that organisms evolve to adapt to their environment; those that do so successfully ensure their survival and that of their offspring. His work eventually became the dominant paradigm of evolutionary theory. Until the advent of modern genetics, however, the biological basis behind natural selection was unknown; at the turn of the twentieth century, in fact, there were rival evolutionary models. The history of evolutionary theory is thus the rise of Darwinism, challenges to its dominance, and the genetics revolution that firmly established evolution as one of the key ideas in modern science.

Historical Background and Scientific Foundations

Evolutionary Predecessors

Darwin was not the first to propose the idea of evolution. The transmutation of species was first postulated by the pre-Socratics, ancient Greek philosophers in the fifth and sixth centuries BC. Aristotle (384–322 BC) rejected their ideas in favor of a “chain of being” in which species were fixed and immutable, a view later adopted by the early Christian Church.

In the eighteenth century, Enlightenment thinkers revisited the idea of transmutation. Charles Darwin's grandfather Erasmus (1731–1802), in fact, helped develop its first comprehensive theories in his books Zoonomia or the Laws of Organic Life (1794) and The Temple of Nature or the Origin of Society (1803). Early paleontologists such as Georges Cuvier (1760–1832), geologists like Charles Lyell (1797–1875), and biologist Jean-Baptiste Lamarck (1744–1829) also gathered anatomical and fossil evidence that suggested species both evolved and became extinct.

In the early 1800s Lamarck's theory of acquired characteristics, which proposed that organisms could pass on traits they had acquired while adapting to their environment, was widely discussed by radical thinkers but was rejected by the conservative scientific community. Most scientists and theologians at that time, including clergyman William Paley (1743–1805), subscribed to natural theology, which claimed that species had been fixed in their characteristics at the time of creation. Natural theologians also believed that the power of God could be revealed by studying the material universe, and that the existence of rational laws of physics and the precise adaptation of species to their environment indicated a divine intelligence.

Natural selection was eventually accepted, largely because Robert Chambers' (1802–1871) Vestiges of the Natural History of Creation (1844) had done valuable advance work, as Darwin himself said, “in removing prejudice, and in thus preparing the ground for the reception of analogous views.” Chambers believed that evolution was an inherently progressive process in which God's plan of creation unfolded through geological time. Chambers also claimed that social progress, evidenced by the Industrial Revolution and the flourishing British Empire, showed the continuation of natural progress.

Charles Darwin

Charles Darwin was born to a wealthy English family of physicians and philosophers; his mother was an heiress to the Wedgwood pottery fortune. Darwin was originally supposed to become a physician like his father and grandfather, and he duly studied medicine at the University of Edinburgh. He did not last long, however, as he could not stand the sight of blood. Too ashamed at first to tell his father he had stopped going

to lectures, the truth eventually came out, and his father proposed that he study theology at Cambridge to become a vicar (clergyman), a respectable profession appropriate to someone of his family's social and economic status.

While at Cambridge, Charles became intensely interested in natural history, botany, and geology; the founder of the Cambridge Botanical Gardens, John Stevens Henslow (1796–1861), became his mentor. In a letter to English botanist Joseph Dalton Hooker (1817–1911) in 1861, Darwin said of Henslow, “I fully believe a better man never walked this earth.” When in 1831 the position of naturalist on the HMS Beagle, a survey ship destined for Central and South America, became available, Henslow recommended young Darwin for the post. Henslow wrote, “I have stated that I considered you to be the best qualified person I know of … not on the supposition of your being a finished Naturalist, but as amply qualified for collecting, observing, & noting any thing worthy to be noted in natural history.”

Darwin's keen powers of empiricism converted him to evolutionism during the Beagle voyages (1831–1836), several years before he discovered the principle of natural selection. He had brought Charles Lyell's Principles of Geology (1830) on the voyage with him; the book delineated the theory of uniformitarianism, a hypothesis stating that most changes in the physical world were the result of gradual and ordinary geological processes, like erosion, that can still be observed today. By implication, to allow for this gradual change, the earth must be very old, much older than the 6,000 years claimed by natural theologians. Taking Lyell's old Earth as his starting point, Darwin began to realize that both organisms and the earth's surface could change over time, setting the stage for evolutionary theory; Darwin later credited Lyell with giving him the “gift of time.”

Darwin sent a huge number of specimens back to Henslow, and he also took notes on both the fossils and living creatures he encountered. The overwhelming numbers of fossils he encountered caused him to conclude that living organisms were outnumbered by extinct forms.

As he traveled from north to south in South America, Darwin also observed that species were replaced by similar, but slightly different varieties. He encountered the same phenomena of local variations among the giant tortoises in the Galapagos Islands, each island possessing a tortoise with a slightly different shell shape and pattern. One tortoise had a ridged shell that let it more easily stretch its neck to feed on overhanging vegetation, and another's pattern gave it camouflage. In his account of the Beagle voyages, he noted “It was evidence that such facts could only be explained on the supposition that species gradually became modified, and the subject haunted me.”

Upon his return to England in 1836, Darwin became convinced of the idea of “descent with modification”—the idea that all living creatures are modified descendents of previously existing species. Over the next 20 years, he compiled evidence for his theory in his landmark 1859 book The Origin of Species.

Although Darwin's observations convinced him that evolution was taking place, it was difficult to identify what drove it. This was a source of frustration for Darwin, until he was inspired by the work of animal breeders, who via artificial selection promoted the expression of particular hereditary traits in their livestock. Darwin then realized that nature selected individuals on the basis of their fitness for the environment, and that this natural selection was the key to evolution.

But how did natural selection work in the wild? In 1838 Darwin read Thomas Malthus's (1766–1834) An Essay on the Principle of Population as It Affects the Future Improvement of Society, his 1798 book about human populations, which gave him the solution:

In October 1838, that is fifteen months after I had begun my systematic enquiry, I happened to read for amusement Malthus' Population, and being well prepared to appreciate the struggle for existence [a phrase used by Malthus] which everywhere goes on from long-continued observation of animals and plants, it at once struck me that under these circumstances favourable variations would tend to be preserved and unfavourable ones to be destroyed. The result of this would be a new species. Here then I had at last got hold of a theory by which to work.

Charles Darwin

web books publishing. “the autobiogr aphy of charles darwin.” (accessed november 28, 2007).

Malthus observed that most of the offspring of any organism do not survive. Even when there are plentiful resources, population size increases geometrically until the population outstrips its food supply. Malthus thought that disease, famine, and poverty were inevitable, leading to what he called the “struggle for existence.” Human, animal, and plant populations were thus reduced periodically by starvation and disease, but some individuals still managed to survive—those whose characteristics best suited them to live long enough to reproduce in the face of environmental challenges. Darwin adapted this philosophy to form his principle of natural selection, which he believed was not “progress” or the process of becoming more advanced or intelligent, but merely individuals adapting more (or less) successfully to their environment.

Public Reaction

Conventional wisdom claims that despite his vast accumulation of evidence, Darwin took 20 years to publish On the Origin of Species because he feared the social consequences of being called a transmutationist. Another school of thought suggests that because his wife Emma was a devout Christian, he didn't want to distress her with his ideas that nature, and not God, was behind the development of species.

Darwin's correspondence, however, makes it clear that close friends were not put off by his unorthodox views. Further, it seems that his work on Origin and the question of transmutation simply took a backseat to the publication of the Beagle voyage's scientific results, including his work on barnacles. Not until this was done did Darwin intend to work on evolutionary theory. Nor did Emma seem distressed by the nature of her husband's work, writing to Darwin in 1839, “Your mind and time are full of the most interesting subjects and thought of the most absorbing kind, viz. following up your own discoveries.”

By the 1850s Darwin began to prepare Origin for publication, spurred on by a letter and paper he received from Alfred Russel Wallace (1823–1913) that also proposed evolutionary theory and natural selection. Wallace was a naturalist who traveled to America and Borneo to collect exotic specimens and study indigenous

flora and fauna. In 1858 Wallace sent Darwin an article he'd written outlining his ideas that new species emerge from adaptation to the struggle for existence. Struck by the similarities to his own theory, Darwin arranged, through Lyell and Hooker, for both Wallace's paper and an excerpt from his own work to be presented to the Linnean Society in 1858. The two were published jointly that same year; Origin was published the following year.

Wallace went on to create the field of biogeography, in which he demonstrated that natural selection is intimately tied to geography and the ecological niche that an organism inhabits. In his Geographical Distribution of Animals (1876), Wallace demonstrated that geographical barriers, like mountains and rivers, often marked the range of species boundaries. Although Darwin would get priority for the discovery of evolution, the relationship between the two men was one of mutual professional respect.

Origin was a best seller, selling out its first print run of 1,250 copies on the first day of publication. Darwin's colleague and friend, British biologist Thomas Henry Huxley (1825–1895), a lecturer at the Royal College of Mines, wrote a witty review of it for the Times, which created even more demand for the book. There also was public uproar, epitomized in a debate and scientific meeting held at the Oxford Museum of Natural History on June 30, 1860. The Bishop of Oxford, Samuel Wilberforce, also known as “Soapy Sam” due to his slippery rhetorical techniques, was presenting a speech against Darwin's views. Wilberforce was well known to have publicly humiliated the transmutationist writer Robert Chambers (1802–1871) 13 years before. Although Darwin himself did not attend, his close friend Huxley, nicknamed “Darwin's Bulldog” for his public support of evolutionary theory, offered to debate in Darwin's stead.

At the end of his speech, Wilberforce asked Huxley “Is it on your grandmother's or grandfather's side that you are descended from an ape?” After presenting the evidence for evolution, Huxley replied to the Bishop:

If … the question is put to me would I rather have a miserable ape for a grandfather or a man highly endowed by nature and possessed of great means of influence and yet who employs those faculties and that influence for the mere purpose of introducing ridicule into a grave scientific discussion—I unhesitatingly affirm my preference for the ape.

The crowd responded with partisan enthusiasm and uproar, and caricatures of Darwin as an “ape-man” soon appeared in popular periodicals such as Punch. Such caricatures became even more prevalent when in 1871, Darwin published his Descent of Man and Selection in Relation to Sex in which he argued that humans evolved from a common ancestor shared with apes.

Scientific Reaction to Darwinism until 1900

Despite the initial brouhaha, Darwin's concepts of evolution and descent of species from common ancestors were adopted by the scientific community. One of the initial objections to the theory prompted the question: if species had descended from other forms via fine gradations, why we do not see innumerable transition forms in the fossil record? Only one year after Origin was published, there was a key fossil find of Archaeopteryx, which demonstrated both bird and reptilian characteristics; it had a mouth full of teeth like a reptile and feathers like a bird. This fossil seemed to suggest an evolutionary process from a more generalized ancestor—just as Darwin had predicted.

But Darwin was less successful in convincing his colleagues that natural selection plays a central role in evolutionary change—he simply did not have a well developed theory of heredity. At the same time that Origin was published, an Austrian monk named Gregor Mendel (1822–1884) was doing experiments with pea plants that enabled him to link an organism's phenotype (its external appearance) to the inheritance of its genotype (its genetic expression). Unfortunately, Mendel's isolation kept him on the margins of the scientific community. As a result, his work was generally ignored until 1900.

The lack of a genetic explanation for natural selection led to an “eclipse of Darwinism,” that lasted from the time of Darwin's death in 1882 until the 1930s. Biologists during this time accepted non-Darwinian theories of evolution that were perceived to remedy its “defects.” Some thinkers combined natural theology with Darwinism into a theory called “theistic evolution” in which natural selection was a process directed by a wise and benevolent Creator rather than resulting from an organism's adaptation to the environment. In this theory, supported by English physician William Carpenter (1824–1885) and prominent American botanist Asa Gray (1810–1888) among others, God directed the evolutionary processes, and the diversity of life was a reflection of His munificence. Bright plumage, for instance, was not explained by the bird's desire to attract mates or adapt to its environment, but as divine concern for beauty.

However, theistic evolution could not explain vestigial organs like the appendix, which seemed to have no purpose, much less a divine one. Other scientists such as Saint George Jackson Mivart (1827–1900), who wrote Genesis of Species (1871), thought that natural selection was false precisely because there were characteristics that species possessed that seemed to have no purpose or adaptive value.

Other scientists revived the work of Lamarck, who proposed that acquired characteristics could be inherited. For instance, via this model, the giraffe's long neck is a product of thousands of generations of animals stretching up to meet the leaves of trees. What was once a short-necked animal could become a longer-necked animal as the ability to stretch increased from generation to generation. For neo-Lamarckians such as British sociologist Herbert Spencer (1820–1903), this solved what was perceived as a problem with Darwininan theory—that natural selection was only a negative force that eliminated unfit species (giraffes survived because they were long-necked and could easily reach an available food source while shorter-necked animals trying to reach the same food source died out) but did not seem to provide a mechanism to create new characteristics. Lamarck's work not only provided for the acquisition of new characteristics, but seemed to indicate that evolution could proceed more rapidly than the slow trial-and-error manner that natural selection suggested. Lamarck's acquired characteristics also implied that humans had free will in shaping their own evolution and were not subject to the whims of natural selection.

Spencer combined the idea of natural selection as elimination of the unfit and the mechanism of acquired characteristics in his theory of social Darwinism. Although Darwin purposely avoided discussing humans or human society in the context of “natural selection,” other theorists like Spencer used Darwinism to justify the extremely hierarchical nature of Victorian England. It was Spencer, in fact, not Darwin, who coined the phrase “survival of the fittest.”

To Spencer, an ideal human society should be modeled on natural processes, which select only the fittest human beings for survival into the next generation. He considered handouts to the poor, state schooling, and systematized health care dangerous; helping the weak survive damaged the human drive for self-improvement and evolutionary progress. Spencer strongly believed in individual liberty and considered social programs an infringement on individuals' rights to exercise their own faculties.

The work of German zoologist Ernst Haeckel (1834–1919) in the late 1800s also seemed to lend credence to the Lamarckian idea that evolution was progress toward higher forms. Darwin showed that embryos from both lower and higher life forms exhibited striking similarities in development, leading some scientists to ask if embryonic development provided a record of the evolutionary history of that species. Human embryos, for instance display gill slits in early developmental phases, and tunicates or sea squirts develop the same sort of notochord that vertebrates form in their back as embryos. Haeckel's biogenetic law famously (and erroneously) proposes that “ontogeny recapitulates phylogeny” that is, an embryo's development displays the evolutionary progress of its species. Haeckel believed that new species emerged as evolution added new stages to embryonic development; this, he postulated, is how humans evolved. Although Haeckel did not realize it at the time, however, embryonic development is actually regulated by individual genes that control the speed and direction of development, and these can mutate to produce new species.

Evolution and the Rediscovery of Mendel

For all its attractions, neo-Lamarckism eventually fell to the wayside as there was little experimental evidence to show that acquired characteristics were passed down from generation to generation. Instead, genetics was determined to be one of the driving forces of evolutionary change.

In 1856 a shy Austrian monk had began to experiment with the hybridization of pea plants. Mendel was interested in testing Lamarck's idea of acquired characteristics. Upon finding an atypical variety of an ornamental plant in the monastery gardens, he uprooted it and planted it adjacent to the typical variety. Mendel then grew their offspring side by side to see if their proximity alone (the result of environment) would result in any approximation of the traits to be passed on to the next generation. The plant's offspring, however, retained the traits of the parents, showing characteristics were genetically inherited—and that Lamarck was wrong.

Mendel then began to work on inheritance, using pea plants because they germinate rapidly and have readily identifiable phenotypes (external characteristics) of flower color, height, and seed textures. His careful observation and analysis allowed him to formulate the laws of heredity. He also showed genetic factors were inherited in mathematical ratios, and that there were dominant and recessive alleles.

Mendel's work was rediscovered independently in 1900 by German botanist Carl Correns (1864–1933), Dutch botanist Hugo de Vries (1848–1935), and Austrian agronomist Erich von Tschermak-Seysenegg (1871–1962). They realized that Mendel's laws supported natural selection—adaptive characteristics were transmitted by heredity. De Vries speculated that if genes were altered or mutated, new characteristics or species could be affected in a single jump or saltation. His conclusion seemed to contradict Darwin's idea that natural selection resulted in a gradual evolution of species over a large span of time.

American geneticist and embryologist Thomas Hunt Morgan (1866–1945) devoted his professional life to studying the nature of these mutations. His experimental model was Drosophila, the fruit fly. He subjected the flies to various environmental factors such as x rays and acids to induce mutation. He eventually succeeded when a white-eyed fly appeared, a mutant of the normal wild red-eyed type. Breeding white-eyed flies with wild types produced both types in Mendelian ratios. Morgan's work illustrated that mutations were genetic, but unlike de Vries's postulate, they did not instantly create new species.

Morgan's work suggested that an accrual of small mutations could be behind evolutionary change, a conclusion shared by scientists such as English statistician Ronald Fisher (1890–1962), British geneticist and evolutionary biologist J.B.S. Haldane (1892–1964), and Sewell Wright (1889–1988), who created the discipline of population genetics in the 1920s and 1930s.

Population genetics uses mathematical models of evolution to examine how mutations arise and how, via natural selection, they spread through a population. Wright introduced the metaphor of the adaptive landscape to explain natural selection, in which “valleys” represent more poorly adapted species and “peaks” more fit species. Because the environment is always shifting, the peaks and valleys also change, and the populations follow the dictates of environmental pressures. Wright also demonstrated how population size affected the spread of a given mutation.

Morgan's colleague Ukranian-American geneticist Theodosius Dobzhansky (1900–1975) brought population genetics into the laboratory. He demonstrated that wild populations of Drosophila exhibit the same types of genetic variations—distinctive markers in their chromosomes—mutations that could be induced

artificially in the laboratory. He also realized that while some mutations are harmful to survival, others are fairly neutral, yet their existence creates tremendous genetic variability, and this was what prompted the emergence of new species. Dobzhansky's studies also indicated that genetic variations determined which species were sexually compatible (could reproduce). His results, summarized in his 1937 book Genetics and the Origin of Species, synthesized genetic mutation and natural selection into a single driving force for evolution.

The Discovery of DNA, Evolution, and Molecular Genetics

The discovery of DNA, deoxyribonucleic acid, was extremely significant to the scientific exploration of evolution. Although biologists had identified several chemical components of cells, they did not know which particular molecule contained and copied genetic information. For instance, chromosomes contain a good deal of protein, and proteins were originally considered good candidates for genetic material as they were made of up to 20 different amino acids. By the mid–1940s however, DNA rather than protein was identified as the genetic carrier, and the race to determine its biochemical structure was on.

On February 28, 1953 American geneticist James Watson (1928–) and British biophysicist Francis Crick (1916–2004) announced to the lunchtime regulars at the Eagle Pub in Cambridge, England, that they had discovered the chemical structure of DNA: a double helix of two sugar-phosphate backbones on the outside, with paired nucleic acids adenine, guanine, cytosine, and thymine held together by hydrogen bonding on the inside. Each gene is a string of base pairs, which instruct organelles called ribosomes to make particular proteins responsible for cellular structure and development.

Evolutionary biologists soon ascertained that mutations were caused by changes in the sequence of base pairs, the proteins of which could change or be duplicated. It also meant that if two species were evolutionarily related, they have similar gene sequences and proteins. This similarity, called molecular homology, implies that the origins of the human species could be dated not only from fossils but from genetic mutations accumulated since they diverged from a common ancestor. The data can be used to create a phylogenetic tree of ancestry.

At the University of California Berkeley in the 1970s New Zealand biologist Allan Wilson (1934–1991) and his American anthropology student Vincent Sarich (1934–) analyzed the DNA proteins of humans and great apes. They realized that humans, gorillas, and chimps had a common ancestor as little as 5 million years ago, a date at great variance with anthropologists who believed fossil evidence argued for a date of 25 million years. Darwin's argument in Descent of Man was thus correct, and Wilson and Sarich's data would be vindicated in later analyses of DNA.

At the same time Wilson and Sarich were doing their work, American ornithologist and molecular biologist Charles Sibley (1917–1998) used a technique called DNA-DNA hybridization to compare the sequence of base pairs between different species of birds; similar sequences indicate a shared evolutionary history.

In this process, DNA molecules can be heated to separate the strands, which can be reattached when they cool. DNA from separate species is heated to separate the strands. After separating the strands, restriction enzymes are used to cut strands in the same areas (of specific chromosomes) into similar fragments. The fragments of DNA of the two species are then mixed. The greater the similarity between the DNA of the species being compared, the more strand fragments will fit the gaps and thus bind chromosomes back together. Because tightly bound DNA denatures (falls apart) at higher temperatures, the tightness measured after the fragmentation and recombination gives scientists a method to measure genetic similarities (and infer commonality of ancestry). Using this technique, scientists found, for example, that chimp and human DNA are very closely related.

Since the 1980s PCR (polymerase chain reaction) has made it possible to sequence DNA quickly, allowing closer examination of molecular homologies between species. Such molecular genetics techniques permit evolutionary biologists to recreate the deepest branches of the tree of life (a discipline called cladistics), seeing where species converge and diverge over time.

One of the most striking discoveries in evolutionary theory resulting from molecular genetics was endo-symbiosis, an interdependent relationship in which one organism lives inside another to create a new entity. In the 1960s, American biologist Lynn Margulis (1938–) noted that the organelles of mitochondria (which make energy [adenosine triphosphate (ATP)] for the cell via aerobic respiration) were remarkably similar in their structure to bacteria. Chloroplasts (the site of photosynthesis) in plant cells also resemble cyanobacteria, which live in both salt and fresh water.

In The Origin of Eukaryotic Cells (1970) Margulis argued this meant that in the evolutionary past, bacteria merged with what now are eukaryotic cells (those that have nuclei); the bacteria's energy-harnessing abilities using oxygen gave the eukaryotes distinct evolutionary advantages. (Many earlier life forms were restricted to anaerobic respiration—making energy without oxygen—a less-efficient process.) Later genetic analysis confirmed that DNA in chloroplasts was indeed originally from cyanobacteria, and mitochondrial DNA resembles that within a group of bacteria that cause typhus. These bacteria may have originally infected a host cell and eventually developed a symbiotic relationship with the host, evolving into cellular organelles.

Modern Cultural Connections

The “evolutionary synthesis” that integrates genetics with field observation has, since the 1940s, led to greater understanding of speciation, phylogenetic trees, and ancient symbiotic relationships. Arguments still continue about the overall nature of evolution, particularly whether it is a gradual process over a long period of time as Darwin argued, or if there are evolutionary jumps that create new organisms more quickly.

The latter hypothesis, termed “punctuated equilibrium,” was advanced by American paleontologist and science writer Stephen Jay Gould (1941–2002) and his colleague, American evolutionary biologist Niles Eldridge (1943–). Examining the fossil record in subsequent geological layers or strata, they realized that there seemed to be long intervals in which there was little change in species, followed by revolutionary transitions of extinction and creation of new forms of life.

Their 1972 paper posited that in the large and central populations of living beings, genetic mutations tend to cancel each other out and are diluted in the sheer bulk of the population, leading to stable homogeny. But smaller population groups that are more isolated and live on the edge of the species boundaries tend to be subject to more pressure from the environment, and any favorable genetic mutations conferring survival advantages will tend to spread quickly. It is in these peripheral populations that the revolutions or “jumps” in punctuated equilibrium occur; Gould called them a “laboratory of evolutionary change.”

The theory of punctuated equilibrium has also been applied to the social sciences by Frank Baumgartner and Bryan Jones, who claim that most social policies tend to be fairly stable until outside radical groups foment change and build momentum, leading to social revolutions. Such a model has also been used by American science historian Thomas S. Kuhn (1922–1996) to explain the process of scientific revolution.

Outside the scientific community, evolutionary theory continues to face challenges. Creationists and others who believe the creation of life was the result of “intelligent design” or divine intervention argue, among other things, that because evolution is not an observable process it is not a valid theory. Lawsuits have tried unsuccessfully to mandate that evolution be replaced or countered in public school curricula.

Recent work, however, by Rosemary and Peter Grant on finches in the Galapagos Islands indicates that observable evolution of species takes place constantly. The finches they have studied since 1973 live in a challenging and rapidly changing environment with strong selective pressures, such as droughts and floods, which affect food supply. These are birds living on the edge, and therefore susceptible to the environmental pressures. Using measurements of population numbers, size of individual birds, and the length and strength of their beaks (which indicates their ability to access particular types of food), the Grants have demonstrated that the size of birds and their beaks adapt to local conditions within a generation via the twin forces of natural selection and sexual selection.

Looking closely at the bird population, generation by generation, year by year, the changes in the finches are quite striking. But when taking the long view of 30 years of data, the Grants have discovered that evolutionary changes and mutations tend to cancel each other out. This would explain why evolution does not appear to be happening, but on closer examination proves to be an underlying process of nature. The origin of species is not an extraordinary event, but happens in real time simply due to the selection pressures of the environment, which initiates and perpetuates changes in organisms.

Primary Source Connection

In the early 1980s, Arkansas adopted the “Balanced Treatment for Creation-Science and Evolution-Science Act7” (Act 590), mandating that creationism and evolution be given equal attention in Arkansas public school science classrooms. The controversy resulted in the court case of McLean v. Arkansas Board of Education.

Many in the scientific community rallied to provide expert testimony in the McLean case. Stephen Jay Gould, then Professor of Geology at Harvard University, provided testimony on how scientific paleontology and evaluation of the fossil record refutes creationist claims.

In his ruling, Judge William Overton concluded that Act 590 was unconstitutional because it violated the principle of separation of church and state. He stated that Act 590 was “a religious crusade, coupled with a desire to conceal…fact” and that “creation-science” was “simply not science.” Judge Overton's ruling was only binding on schools within the Eastern District of Arkansas. The issue of teaching creationism in public school science classes, or the closely-related Intelligent Design, did not reach the U.S. Supreme Court until the 1987 case of Edwards v. Aguillard. Though the Court again struck down a law mandating the teaching of creationism in public classrooms, the issue remains controversial.


MR. NOVIK: …Professor Gould, I'm showing you a copy of Act 590. Have you had an opportunity to read that act?

MR. GOULD: Yes, I have.

Q     Have you read Act 590's definition of creation-science as it relates specifically to geology?

A     Yes. As it relates specifically to geology, point number 5 proclaims that the earth's geology should be explained by catastrophism, including the occurrence of a world wide flood.

Q     Have you read the creation science literature relative to geology?

A     I have indeed. Let me say just for the record, though, I'll use the term ‘creation science’ because it's so enjoined by the Act, but in my view there is no such item and creation science is not science. I would prefer to refer to it as creationism.

But yes, I have read the creation science literature, so called.

Q     Is the statutory definition of creation science as it relates to geology consistent with that creation science literature?

A     Yes. The creation science literature attempts to interpret, in most of that literature, the entire geological column as the product of Noah's Flood and its …consequences, and it is certainly consistent with point number 5 of the Act….

Q     Have you read Act 590's definition of evolution as it relates specifically to geology?

A     Yes. I would say that that primarily is the point that uniformitarianism is—

Q     And the Act defines it as—

A     Oh, yes. An explanation of the earth's geology by catastrophism. Or it says that evolution is the explanation of the earth's geology and evolutionary sequence by uniformitarianism.

Q     What does uniformitarianism mean?

A     As creation science defines it, it refers to the theory that I would call the notion of gradualism, namely, that the phenomena of the earth and geological record were produced by slow, steady, imperceptible change, and the bar scale events were produced by this slow accumulation of imperceptible change.


Evolutionary theory is the cornerstone of modern biology and unites all the fields of biology under one theoretical umbrella. Although the exact mechanisms are often hotly debated, within the scientific community the acceptance of evolutionary theory is essentially universal. Despite being well tested, fundamentalist and often extremist religious opposition (in the industrialized world mostly based in the United States) cloaked in various forms such as “Intelligent Design” still rises to challenge the teaching of evolutionary theory in schools.

Q     And it is in that sense that uniformitarianism is used in the Act?

A     In the Act, yes.

Q     Are you familiar with scientific literature in the field of geology?

A     Yes, I [am]. In fact, I have authored several articles on the meaning of uniformitarianism.

Q     Is Act 590's definition of evolution in respect to uniformitarianism consistent with the scientific literature?

A     Certainly not. It may be true that Charles Lyell, a great nineteenth century geologist, had a fairly extreme view of gradualism, but that's been entirely abandoned by geologists today.

Geologists have been quite comfortable with the explanations that some events have been the accumulation of small changes, and others as the result of, at least, local catastrophes.

Q     So modern geologists believe in both; is that correct?

A     Yes.

Q     Is the Act's definition of evolution in terms of uniformitarianism creation consistent with the creation science literature?

A     Oh, yes. The creation science literature continues to use the term “uniformitarianism” only to refer to the notion of extreme gradualism. For example, they argue that because fossils are generally only formed when sediments accumulate very rapidly, that, therefore, there is evidence for catastrophe, and somehow that confutes uniformitarianism.

In fact, paleontologists do not deny that fossils that are preserved are generally buried by at least locally catastrophic events, storms or rapid accumulations of sediments. And indeed, that's why we believe the fossil record is so imperfect and most fossils never get a chance to be preserved, because the rate of sedimentation is usually slow and most fossils decay before they can be buried.

Q     Is there any sense in which modern geologists do believe in uniformitarianism?

A     Indeed, but in a totally different meaning. The term ‘uniformitarianism’ has two very distinct meanings that are utterly separate. First is the methodological claim that the laws of nature are unvaried, but natural laws can be used to explain the past as well as the present.

That's a methodological claim that we assert in order to do science.

The second meaning which we've been discussing, the substantiative claim of falsifiable, the claim is often false, about actual rates of change. Namely, the rates of change are constant….

Q     Does the creation science literature accurately reflect these two different meanings of uniformitarianism?

A     No, it doesn't. It continually confuses the two, arguing that because we can't refute constancy of rates, in many cases which indeed we can, that, therefore, somehow the principle of the uniformity of law, or the constancy of natural law, is also thrown into question. And they are totally separate issues….

Q     You testified that at its core the flood theory is a supernatural, relies on a supernatural process; is that correct?

A     Yes.

Q     Are there any predictions based on flood geology that can be tested?

A     Yes, they do make certain testable predictions. They have been tested and falsified long ago.

Q     Could you give an example, please?

A     Yes. The creation science literature assumes that since God created all forms of life in six days of twenty-four hours, that, therefore, all animals lived simultaneously together. One would, therefore, assume, at first thought, that the geological strata or the earth would mix together all the forms of life, and yet that is outstandingly not so.

And the outstanding fact of the fossil record which must be admitted by everybody, creationists and evolutionists alike, of course, is that rather than mixing together all the animals, that the geological record is very well ordered; that is, we have sequence of strata, and different kinds of animals and plants characterize different layers of those strata.

For example, in a rather old strata, we get certain kinds of invertebrate, such as trilobites that are never found in higher strata. In strata of the middle age we find dinosaurs, but never trilobites. They're gone. Never large mammals. In upper strata we find large mammals but never any dinosaurs. There is a definite sequence that occurs in the same manner throughout the world and that would seem to contradict the expectation that all forms of life lived simultaneously should not so order themselves.

And therefore, creation scientists, in order to get around this dilemma and to invoke another aspect of the Genesis story, call upon Noah's flood and say that all the animals and plants were mixed up together in this gigantic flood and that the ordering in the strata of the earth records the way in which these creatures settled out in the strata after the flood or as the result of the flood.

Q     Have creation scientists advanced any specific arguments or claims for why a worldwide flood would sort out the fossils in this unvarying sequence?

A     Yes. As I read the literature, there are three primary explanations that they invoke. First, what might be called the principle of hydrodynamic sorting. That when the flood was over, those creatures that were denser or more streamlined would fall first to the bottom and should end up in the lower strata.

The second principle you might call the principle of ecological zonation, namely, things living in the bottom of the ocean end up in the lowest strata, where those that lived in mountaintops, for example, would probably end up in the uppermost strata. And the third principle that they use is what I might call differential intelligence of mobility. That smarter animals or animals that can move and avoid the flood waters might end up in higher strata because they would have escaped the rising flood waters longer than others.

Q     Are those three claims or hypotheses consistent with the observable facts?

A     Certainly not.

Q     In your opinion, have they been falsified by the observable facts?

A     Yes, they have.

Q     Could you give an example, please?

A     Yes. If you look at the history of any invertebrate group, for example, our record is very good. We have thousands upon thousands of species in those groups, and each species is confined to strata at a certain point in the geological column. They are recognizable species that only occur in a small part of the geological column and in the same order everywhere. And yet we find that throughout the history of invertebrates, we get species each occurring at a separate level, but they do not differ in any of those properties.

For example, in the history of clams, clams arose five or six hundred million years ago. Initially almost all clams were shallow burrowers, in that they burrowed into the sediment. Now, it's true that in the history of clams there have been some additions to that repertoire, some clams like the scallops now swim, others are attached to the top, but in fact, a large majority, large number of species of clams still live in the same way.

So there is no difference in the hydrodynamic principles among those clams throughout time; there is no difference in ecological life-style, they are all shallow water burrowers; they are not different in terms of intelligence or mobility, indeed, clams can't even have heads. So they cannot be intelligent creatures.

And yet, as I stated, each species of clam lives in a definite part of the stratigraphic column and only there. There are large-scale extinctions of certain kinds; you never see them again, yet they do not differ in any of the ways that the creation scientists have invoked to explain the order in the strata as the results of the single flood….

Q     Do geologists and paleontologists have natural law explanations for the universal sequences found in the fossil record?

A     Yes. The earth is very ancient, and those animals that were alive at any given time occur in the rocks deposited at that time. They then become extinct or evolve into something else, and that's why they're never found in younger rocks deposited on top of those.

Q     Is it possible to determine at least relative dates for the different strata in the stratigraphic record?

A     Yes, indeed, just by noting which fossils invariably occur in strata on top of others, and, therefore, we assume deposited later and, therefore, younger.

Q     In assigning relative dates to the stratigraphic record, is it necessary to rely at all on any theory of evolution or any assumption of evolution?

A     Certainly not. It's merely a question of observation, to see what fossils occur in what sequences. It's the same way throughout the earth; there is no assumptionary process at all involved in that.

Q     Do creation scientists claim that evolutionary theory does play a role in the relative dating of the geologic column?

A     Yes. One of the most persistent claims is that the whole geological column is probably invalid, because it's involved in a circular argument, namely, that since you need to assume evolution in order to establish the sequence of fossils, but then use that sequence to demonstrate evolution, that the whole subject is tautological….

Stephen Jay Gould

gould, stephen jay. “expert testimony of dr. stephen jay gould, professor of geology, harvard university (direct examination).” mclean v. arkansas board of education, 529 f. supp. 1255 (d. ark. 1982): court transcript, 514–18.

Primary Source Connection

Richard Dawkins holds the Charles Simonyi chair for the public understanding of science at Oxford. His most recent book is The God Delusion.


I had expected to be as irritated by Michael Behe's second book as by his first. I had not expected to feel sorry for him. The first—“Darwin's Black Box” (1996), which purported to make the scientific case for “intelligent design”—was enlivened by a spark of conviction, however misguided. The second is the book of a man who has given up. Trapped along a false path of his own rather unintelligent design, Behe has left himself no escape. Poster boy of creationists everywhere, he has cut himself adrift from the world of real science. And real science, in the shape of his own department of biological sciences at Lehigh University, has publicly disowned him, via a remarkable disclaimer on its Web site: “While we respect Prof. Behe's right to express his views, they are his alone and are in no way endorsed by the department. It is our collective position that intelligent design has no basis in science, has not been tested experimentally and should not be regarded as scientific.” As the Chicago geneticist Jerry Coyne wrote recently, in a devastating review of Behe's work in The New Republic, it would be hard to find a precedent.

For a while, Behe built a nice little career on being a maverick. His colleagues might have disowned him, but they didn't receive flattering invitations to speak all over the country and to write for The New York Times. Behe's name, and not theirs, crackled triumphantly around the memosphere. But things went wrong, especially at the famous 2005 trial where Judge John E. Jones III immortally summed up as “breathtaking inanity” the effort to introduce intelligent design into the school curriculum in Dover, Pa. After his humiliation in court, Behe—the star witness for the creationist side—might have wished to re-establish his scientific credentials and start over. Unfortunately, he had dug himself in too deep. He had to soldier on. “The Edge of Evolution” is the messy result, and it doesn't make for attractive reading.

We now hear less about “irreducible complexity,” with good reason. In “Darwin's Black Box,” Behe simply asserted without justification that particular biological structures (like the bacterial flagellum, the tiny propeller by which bacteria swim) needed all their parts to be in place before they would work, and therefore could not have evolved incrementally. This style of argument remains as unconvincing as when Darwin himself anticipated it. It commits the logical error of arguing by default. Two rival theories, A and B, are set up. Theory A explains loads of facts and is supported by mountains of evidence. Theory B has no supporting evidence, nor is any attempt made to find any. Now a single little fact is discovered, which A allegedly can't explain. Without even asking whether B can explain it, the default conclusion is fallaciously drawn: B must be correct. Incidentally, further research usually reveals that A can explain the phenomenon after all: thus the biologist Kenneth R. Miller (a believing Christian who testified for the other side in the Dover trial) beautifully showed how the bacterial flagellar motor could evolve via known functional intermediates.

Behe correctly dissects the Darwinian theory into three parts: descent with modification, natural selection and mutation. Descent with modification gives him no problems, nor does natural selection. They are “trivial” and “modest” notions, respectively. Do his creationist fans know that Behe accepts as “trivial” the fact that we are African apes, cousins of monkeys, descended from fish?

The crucial passage in “The Edge of Evolution” is this: “By far the most critical aspect of Darwin's multifaceted theory is the role of random mutation. Almost all of what is novel and important in Darwinian thought is concentrated in this third concept.”

What a bizarre thing to say! Leave aside the history: unacquainted with genetics, Darwin set no store by randomness. New variants might arise at random, or they might be acquired characteristics induced by food, for all Darwin knew. Far more important for Darwin was the nonrandom process whereby some survived but others perished. Natural selection is arguably the most momentous idea ever to occur to a human mind, because it—alone as far as we know—explains the elegant illusion of design that pervades the living kingdoms and explains, in passing, us. Whatever else it is, natural selection is not a “modest” idea, nor is descent with modification.

But let's follow Behe down his solitary garden path and see where his overrating of random mutation leads him. He thinks there are not enough mutations to allow the full range of evolution we observe. There is an “edge,” beyond which God must step in to help. Selection of random mutation may explain the malarial parasite's resistance to chloroquine, but only because such microorganisms have huge populations and short life cycles. A fortiori, for Behe, evolution of large, complex creatures with smaller populations and longer generations will fail, starved of mutational raw materials.

If mutation, rather than selection, really limited evolutionary change, this should be true for artificial no less than natural selection. Domestic breeding relies upon exactly the same pool of mutational variation as natural selection. Now, if you sought an experimental test of Behe's theory, what would you do? You'd take a wild species, say a wolf that hunts caribou by long pursuit, and apply selection experimentally to see if you could breed, say, a dogged little wolf that chivies rabbits underground: let's call it a Jack Russell terrier. Or how about an adorable, fluffy pet wolf called, for the sake of argument, a Pekingese? Or a heavyset, thick-coated wolf, strong enough to carry a cask of brandy, that thrives in Alpine passes and might be named after one of them, the St. Bernard? Behe has to predict that you'd wait till hell freezes over, but the necessary mutations would not be forthcoming. Your wolves would stubbornly remain unchanged. Dogs are a mathematical impossibility.

Don't evade the point by protesting that dog breeding is a form of intelligent design. It is (kind of), but Behe, having lost the argument over irreducible complexity, is now in his desperation making a completely different claim: that mutations are too rare to permit significant evolutionary change anyway. From Newfies to Yorkies, from Weimaraners to water spaniels, from Dalmatians to dachshunds, as I incredulously close this book I seem to hear mocking barks and deep, baying howls of derision from 500 breeds of dogs—every one descended from a timber wolf within a time frame so short as to seem, by geological standards, instantaneous.

If correct, Behe's calculations would at a stroke confound generations of mathematical geneticists, who have repeatedly shown that evolutionary rates are not limited by mutation. Single-handedly, Behe is taking on Ronald Fisher, Sewall Wright, J. B. S. Haldane, Theodosius Dobzhansky, Richard Lewontin, John Maynard Smith and hundreds of their talented co-workers and intellectual descendants. Notwithstanding the inconvenient existence of dogs, cabbages and pouter pigeons, the entire corpus of mathematical genetics, from 1930 to today, is flat wrong. Michael Behe, the disowned biochemist of Lehigh University, is the only one who has done his sums right. You think?

The best way to find out is for Behe to submit a mathematical paper to The Journal of Theoretical Biology, say, or The American Naturalist, whose editors would send it to qualified referees. They might liken Behe's error to the belief that you can't win a game of cards unless you have a perfect hand. But, not to second-guess the referees, my point is that Behe, as is normal at the grotesquely ill-named Discovery Institute (a tax-free charity, would you believe?), where he is a senior fellow, has bypassed the peer-review procedure altogether, gone over the heads of the scientists he once aspired to number among his peers, and appealed directly to a public that—as he and his publisher know—is not qualified to rumble him.

Richard Dawkins

dawkins, richard. “inferior design.” review of the edge of evolution: the search for the limits of darwinism, by michael j. behe. new york times (july 1, 2007).

See Also Biology: Botany; Biology: Classification Systems; Biology: Comparative Morphology: Studies of Structure and Function; Biology: Concepts of Heredity and Change Prior to the Rise of Evolutionary Theory; Biology: Developmental Biology; Biology: Marine Biology; Biology: Miller-Urey Experiment; Biology: Ontogeny and Phylogeny; Biology: Paleontology; Biology: Zoology.



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Web Sites

University of California at Berkeley. “Understanding Evolution: Your One-Stop Source for Information on Evolution.” (accessed October 2, 2007).

University of Cambridge. “Darwin Correspondence Project.” (accessed October 2, 2007).


Gould, Stephen Jay. “Expert Testimony of Dr. Stephen Jay Gould, Professor of Geology, Harvard University (Direct Examination).” McLean v. Arkansas Board of Education, 529 F. Supp. 1255 (D. Ark. 1982): Court Transcript, 514–18.

Anna Marie Eleanor Roos

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