Scientists began to understand the mechanics of heredity and how evolution works in populations long before molecular
Biologists identified the genetic basis of evolutionary change. With the discovery of DNA (deoxyribonucleic acid) molecules in 1953, scientists came to understand how genetic information is stored in the chromosomes of a cell. Genes, specific portions of DNA molecules, direct the synthesis of the protein molecules upon which all living organisms depend. Through the process of biological reproduction, each of us inherits a combination of genes from our biological parents that creates a unique new individual.
What Is Evolution?
Although all living creatures ultimately share a common ancestry, they have come to differ from one another through the process of evolution. Biological evolution refers to genetic change over successive generations. The process of change is characterized by descent with modification, as descendant populations diverge from ancestral ones. As a population’s genetic variation changes from one generation to another, genetic change is reflected in visible differences between organisms. With sufficient genetic change, a new species can appear. Thus, the process of evolution provides a mechanism to account for the diversity of life on earth.
What Are the Forces Responsible for Evolution?
Four evolutionary forces—mutation, genetic drift, gene flow, and natural selection—account for change in the genetic composition of populations. Random mutations introduce new genetic variation into individual organisms. Gene flow (the introduction of new gene variants from other populations), genetic drift (random changes in frequencies of gene variants in a population), and natural selection shape genetic variation at the population level. Natural selection is the mechanism of evolution that results in adaptive change, favoring individuals with genetic variants relatively better adapted to the conditions of local environments.
The mythology of most peoples includes a story explaining the appearance of humans on earth. The account of creation recorded in Genesis in the Bible, for example, explains human origins. A vastly different example, serving the same function, is the traditional belief of the Nez Perce, American Indians native to eastern Oregon and Idaho. For the Nez Perce, humanity is the creation of Coyote, a trickster-transformer inhabiting the earth before humans. Coyote chased the giant beaver monster Wishpoosh over the earth, leaving a trail to form the Columbia River. When Coyote caught Wishpoosh, he killed him, dragged his body to the riverbank, and cut it into pieces, each body part transforming into one of the various peoples of this region. The Nez Perce were made from Wishpoosh’s head, thus conferring on them great intelligence and horsemanship.1
Creation stories depict the relationship between humans and the rest of the natural world, sometimes reflecting a deep connection among people, other animals, and the earth. In the traditional Nez Perce creation story, groups of people derive from specific body parts—each possessing a special talent and relationship with a particular animal. By contrast, the story of creation in the Book of Genesis emphasizes human uniqueness and the concept of time. Creation is depicted as a series of actions occurring over the course of six days. God’s final act of creation is to fashion the first human from the earth in his own image before the seventh day of rest.
This linear creation story from Genesis—shared by Jews, Christians, and Muslims—differs from the cyclical creation stories characteristic of Hinduism, which emphasize reincarnation and the cycle of life, including creation and destruction. For Hindus, the diversity of life on earth comes from three gods—Lord Brahma, the creator; Lord Vishnu, the preserver; and Lord Shiva, the destroyer and re-creator—all of whom are part of the Supreme One. When Lord Brahma sleeps the world is destroyed, then re-created again when he awakes. Similarly, according to the Intelligent Design movement—proposed by a conservative think tank called the Discovery Institute in Seattle, Washington—creation is the result of some sort of supreme intelligent being.
Like creation stories, evolution, the major organizing principle of the biological sciences, accounts for the diversity of life on earth. Theories of evolution provide 15 explanations for how it works and for how the variety of organisms, both in the past and today, came into being. However, evolution differs from creation stories in that it explains the diversity of life in consistent scientific language, using testable ideas (hypotheses). Contemporary scientists make comparisons among living organisms to test hypotheses drawn from evolutionary theory. Through their research, scientists have deciphered the molecular basis of evolution and the mechanisms through which evolutionary forces work on populations of organisms. Though scientific theories of evolution treat humans as biological organisms, at the same time historical and cultural processes also shape evolutionary theory and our understanding of it.
World History
