Evolution

G

New Techniques in Molecular Biology

In 1953, American biochemist James Watson and British biophysicist Francis Crick described the three-dimensional shape of DNA, the molecule that contains hereditary information in nearly all living organisms. In the following decade, geneticists developed techniques to rapidly compare DNA and proteins from different organisms. In one such procedure, electrophoresis, geneticists evaluate different specimens of DNA or proteins by observing how they behave in the presence of a slight electric charge. Such techniques opened up entirely new ways to study evolution. For the first time geneticists could quantitatively determine, for example, the genetic change that occurs during the formation of new species.

Electrophoresis and other biochemical techniques also demonstrated to geneticists that populations varied extensively at the molecular level. They learned that much of population variation at the molecular or biochemical level has no apparent benefit. In 1968 Japanese geneticist Motoo Kimura proposed that much of the variation at the molecular level results not from the forces of natural selection, but from chance mutations that do not affect an organism’s fitness. Not all scientists agree with the neutral gene theory.

H

Sociobiology

In recent decades, another branch of evolutionary theory has appeared, as researchers have explored the possibility that not only physical traits, but behavior itself, might be inherited. Behavioral geneticists have studied how genes influence behavior, and more recently, the role of biology in social behavior has been explored. This field of investigation, known as sociobiology, was inaugurated in 1975 with the publication of the book Sociobiology: The New Synthesis by American evolutionary biologist Edward O. Wilson. In this book, Wilson proposed that genes influence much of animal and human behavior, and that these characteristics are also subject to natural selection.

Sociobiologists examine animal behaviors that are called altruistic—that is, unselfish, or demonstrating concern for the welfare of others. When birds feed on the ground, for example, one individual may notice a predator and sound an alarm. In so doing, the bird also calls the predator’s attention to itself. What can account for the behavior of such a sentry, who would seem to derive no evolutionary benefit from its unselfish behavior and so seem to defy the laws of natural selection?

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Darwin was aware of altruistic social behavior in animals, and of how this phenomenon challenged his theory of natural selection. Among the different types of bees in a colony, for example, worker bees are responsible for collecting food, defending the colony, and caring for the nest and the young, but they are sterile and create no offspring. Only the queen bees reproduce. If natural selection rewards those who have the highest reproductive success, how could sterile worker bees come about by natural selection when worker bees devote themselves to others and do not reproduce?

Scientists now recognize that among social insects, such as bees, wasps, and ants, the sterile workers are actually more closely related genetically to one another and to their fertile sisters, the queens, than brothers and sisters are among other organisms. By helping to protect or nurture their sisters, the sterile worker bees preserve their own genes—more so than if they actually reproduced themselves. Thus, the altruistic behavior evolved by natural selection.

I

Punctuated Equilibria

Evolutionary theory has undergone many further refinements in recent years. One such theory challenges the central idea that evolution proceeds by gradual change. In 1972 American paleontologists Stephen Jay Gould and Niles Eldredge proposed the theory of punctuated equilibria. According to this theory, trends in the fossil record cannot be attributed to gradual transformation within a lineage, but rather result from quick bursts of rapid evolutionary change. In Darwinian theory, new species arise by gradual, but not necessarily uniform, accumulation of many small genetic changes over long periods of geologic time. In the fossil record, however, new species generally appear suddenly after long periods of stasis—that is, no change. Gould and Eldredge recognized that speciation more likely occurs in small, isolated, peripheral populations than in the main population of the species, and that the unchanging nature of large populations contributes to the stasis of most fossil species over millions of years. Occasionally, when conditions are right, the equilibrium state becomes “punctuated” by one or more speciation events. While these events probably require thousands or tens of thousands of years to establish effective reproductive isolation and distinctive characteristics, this is but an instant in geologic time compared with an average life span of more than ten million years for most fossil species. Proponents of this theory envision a trend in evolutionary development to be more like climbing a flight of stairs (punctuations followed by stasis) than rolling up an inclined plane (Darwinian gradualism).

J

Role of Extinction

In the last several decades, scientists have questioned the role of extinction in evolution. Of the millions of species that have existed on this planet, more than 99 percent are extinct. Historically, biologists regarded extinction as a natural outcome of competition between newly evolved, adaptively superior species and their older, more primitive ancestors. Recently, however, paleontologists have discovered that many different, unrelated species living in large ecosystems tend to become extinct at nearly the same time. The cause is always some sort of climate change or catastrophic event that produces conditions too severe for most organisms to endure. Moreover, new species evolve after the wave of extinction removes many of the species that previously occupied a region for millions of years. Thus extinction does not result from evolution, but actually causes it.

Scientists have identified several instances of mass extinction, when species apparently died out on a huge scale. The greatest of these episodes occurred during the end of the Permian Period, some 245 million years ago. At that time, according to estimates, more than 95 percent of species—nearly all life on the planet—died out. Another extensively studied extinction took place at the boundary of the Cretaceous Period and the Tertiary Period, roughly 65 million years ago, when the dinosaurs disappeared. In all, more than 20 global mass extinctions have been identified. Some scientists theorize that such events may even be cyclical, occurring at regular intervals.

In the view of many scientists, mass extinctions can be explained by changes in climate—episodes of global warming or cooling that destroy sensitive ecosystems, such as tropical or marine habitats. Other theories have centered on abrupt changes in the levels of the world’s oceans, for example, or on the effect of changing salinity on early sea life. Another theory blames catastrophic events for mass extinction. Strong evidence, for example, supports the theory that a meteorite some 10 km (6 mi) in diameter struck the Earth 65 million years ago. The dust cloud from the collision, according to this impact theory, shrouded Earth for months, blocking the sunlight that plants need to survive. Without plants to eat, the dinosaurs and many other species of land animals were wiped out.

Extinction as a cause of evolution rather than the result of it is perhaps best demonstrated in terms of our own ancestors—ancient mammals. During the time of the dinosaurs, mammals constituted only a small percentage of the animals that roamed the planet. The demise of dinosaurs provided an opportunity for mammals to expand their numbers and ultimately to become the dominant land animal. Without the catastrophe that took place 65 million years ago, mammals may have remained in the shadow of the dinosaurs.

IX

Human Impact

Extinction is not exclusively a natural phenomenon. For thousands of years, as the human species has grown in number and technological sophistication, we have demonstrated our power to cause extinction and to upset the world’s ecological balance. In North America alone, for example, about 40 species of birds and more than 35 species of mammals have become extinct in the last few hundred years—mostly as a result of human activity. Humans drive plants and animals to extinction by relentlessly hunting or harvesting them, by destroying and replacing their habitat with farms and other forms of development, by introducing foreign species that hunt or compete with local species, and by poisoning them with chemicals and other pollutants.

The rain forests of South America and other tropical regions offer a particularly troubling scenario. Upwards of 50 million acres of rain forest disappear every year as humans raze trees to make room for agriculture and livestock. Given that a single acre of rain forest may contain thousands of irreplaceable species of plant and animal life, the threat to biodiversity is severe. The conservation of wildlife is now an international concern—as evidenced by treaties and agreements enacted at the 1992 Earth Summit in Rio De Janeiro, Brazil. In the United States, federal laws protect endangered species. But the problem of dwindling biodiversity seems certain to worsen as the human population continues to expand, and no one knows for sure how it will affect evolution.

Advances in medical technology may also affect natural selection. The study from the mid-20th century showing that babies of medium birth weights were more likely to survive than their heavier or lighter counterparts would be difficult to reproduce today. Advances in neonatal medical technology have made it possible for small or premature babies to survive in much higher numbers.

Recent genetic analysis shows the human population contains harmful mutations in unprecedented levels. Researchers attribute this to genetic drift acting on small human populations throughout history. They also expect that improved medical technology may exacerbate the problem. Better medicine enables more people to survive to reproductive age, even if they carry mutations that in past generations would have caused their early death. The genetic repercussions of this are still unknown, but biologists speculate that many minor problems, such as poor eyesight, headaches, and stomach upsets may be attributable to our collection of harmful mutations.

Humans have also developed the potential to affect evolution at the most basic level—the genes. The techniques of genetic engineering have become commonplace. Scientists can extract genes from living things, alter them by combining them with another segment of DNA, and then place this recombinant DNA back inside the organism. Genetic engineering has produced pest-resistant crops as well as larger cows and other livestock. To an increasing extent, genetic engineers fight human disease, such as cancer and heart disease. The investigation of gene therapy, in which scientists substitute functioning copies of a given gene for a defective gene, is an active field of research. The way this tinkering with genetic material will affect evolution remains to be determined.