History of the Theory of Evolution
Evolution implies a change in one or more characteristics in a population of organisms over a period of time. The concept of evolution is as ancient as Greek writings, where philosophers speculated that all living things are related to one another, although remotely. The Greek philosopher Aristotle perceived a "ladder of life" where simple organisms gradually change to more elaborate forms. Opponents of this concept were led by several theologians who pointed to the biblical account of creation as set forth in the Book of Genesis. One prelate, James Ussher, calculated that creation had taken place on October 26, 4004 B.C., at 9 a.m.
Opponents of the creationist argument were encouraged by geologists who postulated that the earth is far older than 4,004 years. In 1785, James Hutton postulated that the earth was formed by an ancient progression of natural events, including erosion, disruption, and uplift. In the early 1800s, Georges Cuvier suggested that the earth was 6,000 years old, based on his calculations. In 1830, Charles Lyell published evidence pushing the age of the earth back several million years.
Amid the controversy over geology and the age of the earth, French zoologist Jean Baptiste de Lamarck suggested a theory for evolution based on the development of new traits in response to a changing environment. For example, the neck of the giraffe stretched as it reached for food. Lamarck's theory of "use and disuse" gained favor, and his concept of "acquired characteristics" was accepted until the time of Charles Darwin, many years later.
Charles Darwin was the son of an English physician. As a naturalist on the ship H.M.S. Beagle, Darwin traveled to remote regions of South America. His observations on this trip led him to develop his own theory of evolution. Darwin was particularly interested in the finches and tortoises of the Galapagos Islands. He pondered how different species of animals could have developed on this remote set of islands 200 miles west of Ecuador.
Darwin returned to England from South America in 1838 and continued to ponder the theory of evolution. He was influenced by Thomas Malthus's Essay on the Principle of Population. In his book, Malthus pointed out the human population's continual struggle for survival. Darwin applied this principle to animals and plants, and his theory of evolution began to develop.
In 1858, another English naturalist, Alfred Russell Wallace, developed a concept of evolution similar to Darwin's. Wallace wrote a paper on the subject and corresponded with Darwin. The two men decided to simultaneously present papers on evolution to London's scientific community in 1858. The next year, 1859, Darwin published his famous book, On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life. The book has become known simply as The Origin of Species.
The Theory of Evolution
In his book The Origin of Species, Darwin presents evidence in a sober manner for his "descent by modification" theory, which has come down to us as the theory of evolution, although Darwin avoided the term "evolution." Essentially, Darwin suggested that random variations take place in living things and that some external agent in the environment selects those individuals better able to survive. The method of selecting individuals is known as natural selection. The selected individuals pass on their traits to their offspring, and the population continues to evolve.
Two essential points underlie natural selection. First, the genetic variations that take place in living things are random variations. Second, the genetic variations are small and cause little effect relative to a given population. Over time, these small genetic variations lead to the gradual development of a species rather than the sudden development of a species. Darwin proposed that variations appear without direction and without design. He assumed that among inherited traits, some traits were better than others. If an inherited trait provided an advantage over another, it would provide a reproductive advantage to the bearer of the trait. Thus, if long-necked giraffes could reach food better than short-necked giraffes, the long-necked giraffes would survive, reproduce, and yield a population consisting solely of long-necked giraffes.
As the central concept of Darwin's theory of evolution, natural selection implies that the fittest survive and spread their traits through a population. This concept is referred to as the survival of the fittest. The fitness implied is reproductive fitness, that is, the ability to survive in the environment and propagate the species. Natural selection serves as a sieve to remove the unfit from a population and allow the fittest to reproduce and continue the population. Today, scientists know that other factors also influence evolution.
Evidence for Evolution
In his book, Darwin offered several pieces of evidence that favored evolution. In a subdued manner, he attempted to convince the scientific community of the validity of his theory.
One piece of evidence offered by Darwin is found in the science of paleontology. Paleontology deals with locating, cataloging, and interpreting the life forms that existed in past millennia. It is the study of fossils—the bones, shells, teeth, and other remains of organisms, or evidence of ancient organisms, that have survived over eons of time.
Paleontology supports the theory of evolution because it shows a descent of modern organisms from common ancestors. Paleontology indicates that fewer kinds of organisms existed in past eras, and the organisms were probably less complex. As paleontologists descend deeper and deeper into layers of rock, the variety and complexity of fossils decreases. The fossils from the uppermost rock layers are most like current forms. Fossils from the deeper layers are the ancestors of modern forms.
More evidence for evolution is offered by comparative anatomy (Figure 1 ). As Darwin pointed out, the forelimbs of such animals as humans, whales, bats, and other creatures are strikingly similar, even though the forelimbs are used for different purposes (that is, lifting, swimming, and flying). Darwin proposed that similar forelimbs have similar origins, and he used this evidence to point to a common ancestor for modern forms. He suggested that various modifications are nothing more than adaptations to the special needs of modern organisms.
Darwin also observed that animals have structures they do not use. Often these structures degenerate and become undersized compared with similar organs in other organisms. The useless organs are called vestigial organs. In humans, they include the appendix, the fused tail vertebrae, the wisdom teeth, and muscles that move the ears and nose. Darwin maintained that vestigial organs may represent structures that have not quite disappeared. Perhaps an environmental change made the organ unnecessary for survival, and the organ gradually became nonfunctional and reduced in size. For example, the appendix in human ancestors may have been an organ for digesting certain foods, and the coccyx at the tip of the vertebral column may be the remnants of a tail possessed by an ancient ancestor.
Darwin noted the striking similarity among embryos of complex animals such as humans, chickens, frogs, reptiles, and fish. He wrote that the uniformity is evidence for evolution. He pointed out that human embryos pass through a number of embryonic stages inherited from their ancestors because they have inherited the developmental mechanisms from a common ancestor. These mechanisms are modified in a way that is unique to an organism's way of life.
The similarities in comparative embryology are also evident in the early stages of development. For example, fish, bird, rabbit, and human embryos are similar in appearance in the early stages. They all have gill slits, a two-chambered heart, and a tail with muscles to move it. Later on, as the embryos grow and develop, they become less and less similar.
Although the biochemistry of organisms was not well known in Darwin's time, modern biochemistry indicates there is a biochemical similarity in all living things. For example, the same mechanisms for trapping and transforming energy and for building proteins from amino acids are nearly identical in almost all living systems. DNA and RNA are the mechanisms for inheritance and gene activity in all living organisms. The structure of the genetic code is almost identical in all living things. This uniformity in biochemical organization underlies the diversity of living things and points to evolutionary relationships.
From observing the domestic breeding experiments of animal and plant scientists, Darwin developed an idea about how evolution takes place. Domestic breeding brings about new forms that differ from ancestral stock. For example, pigeon fanciers have developed many races of pigeons through domestic breeding experiments. In effect, evolution has taken place under the guidance of human hands. The development of new agricultural crops by farmers and botanists provides more evidence for directed evolution.
Darwin was particularly interested in the life forms of the Galapagos Islands. He noticed how many of the birds and other animals on the islands were found only there. The finches were particularly puzzling because Darwin found 13 species of finches not found anywhere else in the world, as far as he knew. He concluded that the finches had evolved from a common ancestor that probably reached the island many generations earlier. In the isolation of the Galapagos Islands, the original finches had probably evolved into the 13 species.
Other geographic distributions also help to explain evolution. For instance, alligators are located only in certain regions of the world, presumably because they have evolved in those regions. The islands of Australia and New Zealand have populations of animals found nowhere else in the world because of their isolated environments.
Mechanism of Evolution
Evolution does not occur in individuals but in populations. A population is an interbreeding group of individuals of one species in a given geographic area. A population evolves because the population contains the collection of genes called the gene pool. As changes in the gene pool occur, a population evolves.
Mutation, a driving force of evolution, is a random change in a population's gene pool. It is a change in the nature of the DNA in one or more chromosomes. Mutations give rise to new alleles; therefore, they are the source of variation in a population.
Mutations may be harmful, but they may also be beneficial. For example, a mutation may permit organisms in a population to produce enzymes that will allow them to use certain food materials. Over time, these types of individuals survive, while those not having the mutations perish. Therefore, natural selection tends to remove the less-fit individuals, allowing more-fit individuals to survive and form a population of fit individuals.
Another mechanism of evolution may occur during the migration of individuals from one group to another. When the migrating individuals interbreed with the new population, they contribute their genes to the gene pool of the local population. This establishes gene flow in the population. Gene flow occurs, for example, when wind carries seeds far beyond the bounds of the parent plant population. As another example, animals may be driven off from a herd. This forces them to migrate to a new population, thereby bringing new genes to a gene pool. Gene flow tends to increase the similarity between remaining populations of the same species because it makes gene pools more similar to one another.
Another mechanism for evolution is genetic drift. Genetic drift occurs when a small group of individuals leaves a population and establishes a new one in a geographically isolated region. For example, when a small population of fish is placed in a lake, the fish population will evolve into one that is different from the original. Fitness of a population is not considered in genetic drift, nor does genetic drift occur in a very large population.
Clearly, the most important influence on evolution is natural selection, which occurs when an organism is subject to its environment. The fittest survive and contribute their genes to their offspring, producing a population that is better adapted to the environment. The genes of less-fit individuals are eventually lost. The important selective force in natural selection is the environment.
Environmental fitness may be expressed in several ways. For example, it may involve an individual's ability to avoid predators, it may imply a greater resistance to disease, it may enhance ability to obtain food, or it may mean resistance to drought. Fitness may also be measured as enhanced reproductive ability, such as in the ability to attract a mate. Better-adapted individuals produce relatively more offspring and pass on their genes more efficiently than less-adapted individuals.
Several types of natural selection appear to act in populations. One type, stabilizing selection, occurs when the environment continually eliminates individuals at extremes of a population. Another type of natural selection is disruptive selection. Here, the environment favors extreme types in a population at the expense of intermediate forms, thereby splitting the population into two or more populations. A third type of natural selection is directional selection. In this case, the environment acts for or against an extreme characteristic, and the likely result is the replacement of one gene group with another gene group. The development of antibiotic-resistant bacteria in the modern era is an example of directional selection.
A species is a group of individuals that share a number of features and are able to interbreed with one another. (When individuals of one species mate with individuals of a different species, any offspring are usually sterile.) A species is also defined as a population whose members share a common gene pool.
The evolution of a species is speciation, which can occur when a population is isolated by geographic barriers, such as occurred in the isolation of Australia, New Zealand, and the Galapagos Islands. The variety of life forms found in Australia but nowhere else is the characteristic result of speciation by geographic barriers.
Speciation can also occur when reproductive barriers develop. For example, when members of a population develop anatomical barriers that make mating with other members of the population difficult, a new species can develop. The timing of sexual activity is another example of a reproductive barrier. Spatial difference, such as one species inhabiting treetops while another species occurs at ground level, is another reason why species develop.
Gradual versus rapid change
Darwin's theory included the fact that evolutionary changes take place slowly. In many cases, the fossil record shows that a species changed gradually over time. The theory that evolution occurs gradually is known as gradualism.
In contrast to gradualism is the theory of punctuated equilibrium, which is a point of discussion among scientists. According to the theory of punctuated equilibrium, some species have long, stable periods of existence interrupted by relatively brief periods of rapid change.
Both groups of scientists agree that natural selection is the single most important factor in evolutionary changes in species. Whether the change is slow and gradual, or punctuated and rapid, one thing is certain: Organisms have evolved over time.