Evolutionary Theory: Causes and Evidence of Evolution

Introduction to Evolutionary Theory

The term "evolution" evokes varied connotations. In simple and direct terms, it is a process of change or development over a long period of time. Defined so broadly, evolution can refer to any phenomenon—the evolution of the universe, the evolution of human culture—that changes over time. However, in common parlance, evolution refers to biological or organic evolution: the formation and development of life on Earth. Biological evolution is "an unpredictable and natural process of temporal descent with genetic modification that is affected by natural selection, chance, historical contingencies, and changing environments" (NABT, 1997). It is, fundamentally, the change in the genetics of a population over time.

The principal idea behind the theory of evolution is that all living things evolved from simple organisms and developed through the years to produce millions of species. This generally accepted scientific account of the development of life has three major aspects:

1. The ancestral lineage between organisms—living and extinct.
2. The appearance of new traits in a lineage.
3. The reasons and processes that cause some traits to endure while others disappear.

Historical Development of the Theory

Evolutionary theory proposes that all species probably evolved from a single form of life that existed about three and a half billion years ago. Over the ages, that original basic life form is considered to have evolved into more species, which in turn gave rise to many others. This "speciation" process led to the development of the more than ten million species that live on Earth today. The idea of speciation leads naturally to another essential concept: common ancestry. Because all species evolved from one basic life form, any two species once shared a common ancestor. Closely related species, such as human beings and gorillas, share a more recent common ancestor, whereas dissimilar species, such as human beings and reptiles, share an ancestor that inhabited the Earth in the very distant past. Evolutionary changes occur only over long spans of time—ranging from decades to millions of years—and it is widely held that evolution continues to occur today at rates comparable to those of the past.

The first scientific studies on evolution were conducted in the 1700s by two French biologists—Comte de Buffon and Baron Cuvier—who studied fossils and their anatomy and concluded that life on Earth had undergone a series of changes. Chevalier de Lamarck, the French naturalist, proposed the first formal theory of evolution in 1809, suggesting that an animal's body part could change depending on the extent to which it is used, and that such acquired traits became hereditary. Although his theory of the inheritance of acquired characteristics interested many scientists, it was not until 1858 that Charles R. Darwin, the English biologist, presented his theory of evolution and the subject began receiving adequate scientific consideration. Since then, scientific advancement and investigation have led to many refinements of the theory, though its principal ideas remain largely unchanged.

There is a tendency to reject evolutionary theory on the grounds that it is "only a theory." However, the theory of evolution is supported by a wide range of evidence from diverse scientific disciplines, and an evidentially corroborated theory becomes accepted as scientific fact. As Campbell (1990) observed: "Today, nearly all biologists acknowledge that evolution is a fact. The term theory is no longer appropriate except when referring to the various models that attempt to explain how life evolves… It is important to understand that the current questions about how life evolves in no way implies any disagreement over the fact of evolution" (p. 434). Today, the theory of evolution is considered the most important foundational concept in biological studies (Dobzhansky, 1973).

Evolutionary theory and its various versions essentially attribute the cause of evolutionary change to the interaction of three processes: mutation, natural selection, and genetic drift.

Causes of Evolution

Mutation is a permanent change in the hereditary component of an organism. Mutation produces random or chance variation in the genetic or inherited features of an organism. To understand the process of mutation, it is essential to understand how characteristics are inherited. Hereditary or genetic characteristics are carried by chromosomes, which in turn carry a large number of genes in every cell. Genes consist of DNA—deoxyribonucleic acid—which contains the coded information that establishes hereditary characteristics. Mutation of a hereditary characteristic occurs when the DNA in genes is altered, either through exposure to environmental factors such as radiation and chemicals, or through errors in the copying of DNA during cell division. After a gene is altered, it duplicates itself in the altered form. The presence of such mutated genes in the egg or sperm cells of an organism leads to the alteration of inherited characteristics or the introduction of new ones. Mutations are thus the building blocks of evolutionary development and help explain the emergence of new species.

Natural selection differentiates the random changes brought about by mutation and selects those changes that enhance an individual's reproduction and survival. Natural selection ensures that only those variations making a species better adapted to its environment are passed on to future generations. Conversely, it eliminates changes that limit a species' ability to reproduce and survive. Natural selection involves such characteristics as appearance, physiology, body chemistry, and behavior. The process is said to occur only when two biological conditions are met:

First, individuals within a population must vary in their hereditary or genetic characteristics. Such variation includes differences in physiological appearance—height, weight, skin color, hair color, and other features—as well as intrinsic features such as bone thickness and blood characteristics. Second, certain inherited characteristics must affect the chances of reproduction and survival. Only when this condition holds can the fittest individuals pass on more copies of their genes to future generations than other individuals. Over time, the species accumulates genes that increase its ability to reproduce and survive in its environment.

There are essentially three types of natural selection:

Directional selection: This process produces new characteristics or features that enable a species to adapt to its environment.
Stabilizing selection: The most common type, stabilizing selection occurs when a species is already well adapted to its environment.
Sexual selection: This selection occurs primarily in animals and results from preferences for sexual partners with certain specific characteristics.

Genetic drift is the third process that explains evolutionary change, particularly with reference to the evolution of populations. This mechanism has not received as much acceptance as natural selection theory. Genetic drift involves a random change in the occurrence of genes within a population. It is caused by the random manner in which sperm and egg cells receive certain chromosomes from each parent as they form. Because reproductive cells contain only half the respective sets of chromosomes, only half of a parent's genes are present in any given egg or sperm. If the parents produce a limited number of offspring, some of their genes may not be passed on at all. Unlike natural selection, genetic drift does not help species adapt to their living environment, since it causes only random changes in the frequency of characteristics.

However, over time genetic drift modifies the genetic makeup of a population. It is worth noting that biologists and naturalists today remain uncertain about the relative importance of genetic drift versus natural selection in explaining evolutionary change. Many species possess characteristics that have no obvious benefit for adapting to their particular environment. It is not always possible to determine whether certain features—such as changes in color or shape of a genetic trait—affect an organism's reproduction and survival, or whether they are simply random variations caused by genetic drift. This limitation has generated substantial criticism of Darwin's theory of evolution (Smith, 1989, p. 180).

Conclusion

Though the natural process of evolution is normally not visible or observable—being very slow—in certain cases evolution occurs rapidly in living species, when an organism undergoes a genetic change in response to a disturbance of its environment by human beings. The rapid development of immunity in insects to chemical pesticides is one such example, and it further suggests that organisms evolve while adapting to their environment. Together, the mechanisms of mutation, natural selection, and genetic drift, supported by converging lines of physical and biological evidence, establish evolution as the cornerstone of modern biological science.