The Genetic Foundation of Natural Selection

Overview

By 1900 scientists had been trying to explain how natural selection worked for 40 years. The idea proposed by Charles Darwin (1809-1882) was accepted, but the mechanism was unclear. When they turned to microscopic investigation, identifying structures and processes within cells, the mechanism of natural selection became clear. Scientists studied specific traits, followed populations in the wild, and applied chemical and statistical techniques and principles to the problem. By 1950 natural selection was supported by the new field of genetics, and DNA was about to be deciphered. The focus of genetics then turned to the structural makeup of the gene. By mid-century this would revolutionize medicine as well as genetics. It would lead to manipulation of genes, gene therapies, identification of individuals by their DNA, replacement of defective genes, and identification and location of specific genes.

Background

Darwin's theory of evolution of living species was published in 1859, suggesting that living species survive and change by natural selection. The offspring of organisms that possess favorable characteristics in a changing habitat will survive, leave the most progeny, and pass favorable traits to their descendants. Scientists greeted this idea with fascination and questions, though many in the religious community believed that God controlled creation and change in all species. Darwin specified no mechanism for the change that made evolution and natural selection work. Biologists, botanists and other scientists set to work trying to find it. For 40 years scientists tried to locate and define a mechanism to explain the phenomenon.

A number of theories were formulated, including the idea that mutations arise spontaneously and are passed on to offspring. Another was orthogenesis, or the idea that life evolved naturally from small to large organisms. A third proposal was saltation, or changes made in great sudden leaps, and a fourth was pangenesis, or the idea that small particles mingle to create new characteristics. Some scientists tried to confirm the reality of natural selection by doing field studies, while others used comparative anatomy or embryology, the study of the development of the embryo of an organism.

During this time, they were unaware that a large body of work on the subject of genes and the passing on of traits had already been done by Gregor Mendel (1822-1884) and had been published in an obscure journal in 1866. In 1900 this work was discovered by three men working on the problem independently. Mendel had experimented with sweet peas and formulated laws that governed the passing of traits to offspring. He recognized and named genes as the agent that passed the traits and attributed the results to the dominant or recessive character of each trait.

The two threads came together in 1900. Evolutionary change by way of natural selection was accepted by scientists, although they were still unable to explain the mechanism. Cells and their internal structures had been described by two German scientists in the mid-1850s. Chromosomes, thread-like substances on which genes are carried, were believed to pass traits to offspring as they split during cell division. This was confirmed in the 1930s. Researchers turned to microscopic, internal structures of the reproductive system and began to focus on the inheritance of specific traits. By 1910 it was known that species changed over time, that Mendel's laws were generally correct, and that more work was needed to understand how the mechanism worked.

In the first decades of the twentieth century a number of studies were undertaken in the United States, Great Britain, Germany, France, and Russia that led to the new field of biology called genetics. The study of the passing of traits of living organisms from one generation to the next was a new field of study. Originally called Mendelism, it was created by the rediscovery and republication of Mendel's work on transmission and inheritance of characteristics from parent to offspring. The field earned its modern name by the 1920s.

Many scientists had set the framework for this infant science. Early researchers approached it from several different angles. According to Darwin, natural selection is the conversion of variations among individuals into variations between groups. Some geneticists tried to reconcile the contrasting realities that organisms resemble their parents and differ from their parents at the same time. One early researcher, Thomas Hunt Morgan (1866-1945), used a small vinegar or fruit fly (Drosophila melanogaster) in a series of lab experiments. Since this insect is found everywhere, it was used in many labs. Low in maintenance, it needs little space, has a short life, and a measurable generation time, all advantageous characteristics in the study of genetics. Evolutionary change occurs in mammals over periods too long to study in a lab.

In 1908 Morgan began to raise Drosophila and to introduce specific mutations in succeeding generations. For this work, Morgan took on many assistants. Working in a lab at Columbia that came to be known as "the fly room," they produced the first genetic map and combined Mendelian and cytological, or cell, theories. This group developed their findings into a chromosome theory of heredity—that is, that genes are carried on the chromosomes within generative cells and a parent passes his genes to his offspring in predictable ratios. The idea spread to other researchers, and by 1930 a revolution in genetics was underway. It recognized genes as the mechanism of natural selection.

Many scientists were instrumental in creating the mature science of genetics. J. B. S. Haldane (1892-1964), working at Oxford University in England in the 1920s, developed a mathematical procedure for dealing with evolution on a larger scale (i.e. in whole populations). Such a study, he said, could follow dominant or recessive genes in a population and explain its changes. Haldane was a chemist interested in respiration, thermodynamics, and enzymes. He discussed the extent of linkage of characteristics to observed bands or intervals on a chromosome. He formulated laws in 1922 that explained that cross-breeding of animal species produces sterile offspring, he mapped the x chromosome, and he showed a genetic linkage between hemophilia and color blindness. His 1932 book Causes of Evolution, reinforced the conservative Darwinian theory that natural selection rather than mutation was the driving force of evolution. Haldane brought together ideas from many fields. His work was largely theoretical and remained unproven for years, but his ideas pointed in new directions and were a milestone in understanding the mechanism of evolution and using mathematics to study biology.

Two other researchers were inspired by the new genetics and, working along similar lines, introduced new ideas into the process. Ronald A. Fisher (1890-1962) had graduated from Cambridge with degrees in math and physics. Soon he became interested in biometrics, or the application of statistics to variations in populations. Combining theory and practice, Fisher used mathematics to show that Mendel's laws do actually lead to the conclusions he drew from them and that Mendel and Darwin are compatible. His statistical techniques led to major advances in the design of experiments and the use of samples. His methods were adopted and used wherever statistical analysis was applicable.

American geneticist Sewall Wright (1889-1988) studied inbreeding and cross breeding of animals. He developed a mathematical scheme to describe evolutionary development. He also formulated the idea of genetic drift—that genes could be lost and new species appear without natural selection. This is meaningful in small populations. Also in the 1930s, Barbara McClintock (1902-1992) demonstrated that a physical exchange of material occurs between chromosomes.

In the 1930s and 1940s Wright and Fisher, along with American Ernst Mayr (1904- ), created what is called the synthetic theory of evolution. It is called "synthetic" because it is a combination, or synthesis, of ideas of natural selection and principles of genetics and related sciences on both observable and microscopic levels. Between 1900 and 1930 Russians were also making headway in genetics. Sergei Chetverikov (1880-1959) made the first systematic studies of genetics in wild populations. Along with Fisher, Haldane, and Wright, he is considered one of the founders of the study of population genetics and a pioneer in modern evolution theory. Chetverikov used statistical, biometric techniques in biological studies and set the agenda for a new synthesis of ideas.

Theodosius Dobzhansky (1900-1975), another Russian, had emigrated to the United states in 1927 in order to join Morgan's group at Columbia University. He later wrote a book that brought genetics and mathematics together and created a single theory of the processes of evolution. He said natural mutation, aided by variation, can lead to change when acted upon by natural selection. One example is the ability of an insect to become resistant to pesticides. He also believed new species could not arise from single mutations and must be isolated from others of its species by time, geography, habitat, or breeding season.

Impact

By the mid-twentieth century genetics was a mature science with three major areas of concentration. Molecular genetics focuses on the regulatory processes that cause genes to work. Transmission genetics, often called classical genetics, analyzes the patterns of inheritance and how traits are transmitted over generations. Population genetics investigates how mutations and other processes of natural selection work among a population over time. Modern genetics is the union of several systems of knowledge and techniques of investigation. The triumph of the field is that genetics can explain the constancy of inheritance from the visible individual to the component molecules. In the 1940s DNA was identified as the basic substance within the gene. Its structure was deduced and modeled in the 1950s, and its role in evolutionary change became clear. This led to an explosion of new ideas that would affect all life on Earth. Medicine, experimental biology, law enforcement, ethics, and government control would all be affected. Gene mapping, locating each gene on a chromosome and identifying each trait, took ten years of work by many scientists. DNA identification is routinely used in legal cases. Gene therapy, gene manipulation, replacing defective genes with non-defective ones, cloning, and enhancing traits by gene replacement are ethical questions still being debated. While biological and medical research go on, these questions are the focus of a great deal of controversy.

LYNDALL B. LANDAUER