Evolution

B

Linnaeus and Scientific Classification

Many centuries later, the idea of a perfect and unchanging natural world—the product of divine creation—was predominant not only in religion and philosophy, but in science. Gradually, however, as knowledge accumulated from seemingly disparate areas, the beginnings of modern evolutionary theory began to take shape. A key figure in this regard was the Swedish naturalist Carolus Linnaeus, who became known as the father of modern taxonomy, the science of classifying organisms.

In his major work Systema Naturae (The System of Nature), first published in 1735, Linnaeus devised a system of classification of organisms that is still in use today. This system places living things within increasingly specific categories based on common attributes—from a general grouping (kingdom) down to the specific individual (species). Using this system, Linnaeus named nearly 10,000 plant and animal species in his lifetime. Not an evolutionist by any means, Linnaeus believed that each species was created by God and was incapable of change. Nevertheless, his orderly groupings of living things provided important insights for later theorists.

C

19th-Century Foundations

Perhaps the most prominent of those who embraced the idea of progressive change in the living world was the early 19th-century French biologist Jean-Baptiste Lamarck. Lamarck’s theory, now known as Lamarckism and based in part on his study of the fossils of marine invertebrates, was that species do change over time. He believed, furthermore, that animals evolve because unfavorable conditions produce needs that animals try to satisfy. For example, short-necked ancestors of the modern giraffe voluntarily stretched their necks to reach leaves high in trees during times when food was scarce. Proponents of Lamarckism thought this voluntary use slightly changed the hereditary characteristics controlling neck growth; the giraffe then transmitted these alterations to its offspring as what Lamarck called acquired characteristics. Modern scientists know that adaptation and natural selection are far more complicated than Lamarck supposed, having nothing to do with an animal’s voluntary efforts. Nevertheless, the idea of acquired characteristics, with Lamarck as its most famous proponent, persisted for many years.

French naturalist and paleontologist Georges Cuvier feuded with Lamarck. Unearthing the fossils of mastodons and other vanished species, Cuvier produced proof of long-extinct life forms on Earth. Unlike Lamarck, however, Cuvier did not believe in evolution. Instead, Cuvier believed that floods and other cataclysms destroyed such ancient species. He suggested that after each cataclysmic event, God created a new set of organisms.

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At around the same time that Cuvier and Lamarck were squabbling, British economist Thomas Robert Malthus proposed ideas extremely influential in evolutionary theory. In his 1798 work An Essay on the Principle of Population, Malthus theorized that the human population would increase at a much greater rate than its food sources. This theory introduced the key idea of competition for limited resources—that is, there is not enough food, water, and living space to go around, and organisms must somehow compete with each other to obtain resources necessary for survival. Another key idea came from Scottish geologist Charles Lyell, who supplied a deeper understanding of Earth’s history. In his book Principles of Geology (1830), Lyell set forth his case that the Earth was millions of years old rather than only a few thousand years old, as was maintained by those who accepted the biblical story of divine creation as fact.

D

Darwin and Natural Selection

In 1831 Charles Darwin, who was intending to become a country minister, had an opportunity to sail as ship’s naturalist aboard the HMS Beagle on a five-year, round-the-world mapmaking voyage. During the journey, as the ship anchored off South America and other distant shores, Darwin had the opportunity to travel inland and make observations of the natural world. In the Galápagos Islands, he noted how species on the various islands were similar but distinct from one another. He also observed fossils and other geological evidence of the Earth’s great age. The observations Darwin made on that voyage seemed to suggest the evolution, rather than the creation, of the many local forms of life.

In 1837, shortly after returning to England, Darwin began a notebook of his observations and thoughts on evolution. Although Darwin had developed the major components of his theory of evolution by natural selection in an 1842 unpublished paper circulated among his friends, he was unwilling to publish the results until he could present as complete a case as possible. He labored for almost 20 additional years on his theory of evolution and on its primary mechanism, natural selection. In 1858 he received a letter from British naturalist Alfred Russel Wallace, a professional collector of wildlife specimens. Much to Darwin’s surprise, Wallace had independently hit upon the idea of natural selection to explain how species are modified by adapting to different conditions. Not wanting Darwin to be unfairly deprived of his share of the credit for the theory, some of Darwin’s scientific colleagues presented extracts of Darwin’s work along with Wallace’s paper at a meeting of the Linnean Society, a London-based science organization, in June 1858. Wallace’s paper stimulated Darwin to finish his work and get it into print. Darwin published On the Origin of Species by Means of Natural Selection on November 24, 1859. All 1,250 copies of the first printing were sold on that day.

Darwin’s book and the theory it popularized—evolution through natural selection—set off a storm of controversy. Some of the protest came from the clergy and other religious thinkers. Other objections came from scientists. Many scientists continued to believe in Lamarckism, the idea that living things could consciously strive to accumulate modifications during a lifetime and could pass these traits on to their offspring. Other scientists objected to the seemingly random quality of natural selection. If natural selection depended upon random combinations of traits and variations, critics asked, how could it account for such refined and complex structures as the human eye? Perhaps the most serious question—one for which Wallace and Darwin had no answer—concerned the inheritance of traits. How exactly were traits passed along to offspring?

E

Mendel and Early Genetics

Darwin did not know it, but the answer was at hand—although it would not be acknowledged in his lifetime. In the Augustinian monastery at Brünn (now Brno in the Czech Republic), Austrian monk Gregor Mendel experimented with the breeding of garden peas, observing how their traits were passed down through generations. In crossbreeding pea plants to produce different combinations of traits—color, height, smoothness, and other characteristics—Mendel noted that although a given trait might not appear in every generation, the trait did not disappear. Mendel discovered that the expression of traits hinged on whether the traits were dominant or recessive, and on how these dominant and recessive traits combined. He learned that contrary to what most scientists believed at the time, the mixing of traits in sexual reproduction did not result in a random blending. Traits were passed along in discrete units. These units are now known as genes. Mendel performed hundreds of experiments and produced precise statistical models and principles of heredity, now known as Mendel’s Laws, showing how dominant and recessive traits are expressed over generations. However, no one appreciated the significance of Mendel’s work until after his death. But his work ultimately gave birth to the modern field of genetics.

In 1900, Dutch botanist Hugo Marie de Vries and others independently discovered Mendel’s laws. The following year, de Vries’s book The Mutation Theory challenged Darwin’s concept of gradual changes over long periods by proposing that evolution occurred in abrupt, radical steps. Having observed new varieties of the evening primrose plant coming into existence in a single generation, de Vries had subsequently determined that sudden change, or mutation, in the genetic material was responsible. As the debate over evolution continued in the early 20th century, some scientists came to believe that mutation, and not natural selection, was the driving force in evolution. In the face of these mutationists, Darwin’s central theory threatened to fall out of favor.

F

Population Genetics and the Modern Synthesis

As the science of genetics advanced during the 1920s and 1930s, several key scientists forged a link between Mendel’s laws of inheritance and the theory of natural selection proposed by Darwin and Wallace. British mathematician Sir Ronald Fisher, British geneticist J.B.S. Haldane, and American geneticist Sewall Wright pioneered the field of population genetics. By mathematically analyzing the genetic variation in entire populations, these scientists demonstrated that natural selection, and not just mutation, could result in evolutionary change.

Further investigation into population genetics and such fields as paleontology, taxonomy, biogeography, and the biochemistry of genes eventually led to what is called the modern synthesis. This modern view of evolution integrated discoveries and ideas from many different disciplines. In so doing, this view reconciled the many disparate ideas about evolution into the all-encompassing evolutionary science studied today. The modern synthesis was advanced in such books as Genetics and the Origin of Species, published in 1937 by Russian-born American geneticist Theodosius Dobzhansky; Evolution: The Modern Synthesis (1942) by British biologist Sir Julian Huxley; and Systematics and the Origin of Species (1942) by German-born American evolutionary biologist Ernst Mayr. In 1942, American paleontologist George Gaylord Simpson demonstrated from the fossil record that rates and modes of evolution are correlated: New kinds of organisms arise when their ancestors invade a new niche, and evolve rapidly to best exploit the conditions in the new environment. In the late 1940s American botanist G. Ledyard Stebbins showed that plants display evolutionary patterns similar to those of animals, and especially that plant evolution has demonstrated diverse adaptive responses to environmental pressures and opportunities.

In addition, biologists reviewed a broad range of genetic, ecological, and anatomical evidence to show that observation and experimental evidence strongly supported the modern synthesis. The theory has formed the basis of evolutionary science since the 1950s. It has also led to an effort to classify organisms according to their evolutionary history, as well as their physical similarities. Modern scientists use the principles of genetics and molecular biology to study relationships first proposed by Carolus Linnaeus more than 200 years ago.

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