Evolution

I

Introduction

Evolution, in biology, complex process by which the characteristics of living organisms change over many generations as traits are passed from one generation to the next. The science of evolution seeks to understand the biological forces that caused ancient organisms to develop into the tremendous and ever-changing variety of life seen on Earth today. It addresses how, over the course of time, various plant and animal species branch off to become entirely new species, and how different species are related through complicated family trees that span millions of years.

Evolution provides an essential framework for studying the ongoing history of life on Earth. A central, and historically controversial, component of evolutionary theory is that all living organisms, from microscopic bacteria to plants, insects, birds, and mammals, share a common ancestor. Species that are closely related share a recent common ancestor, while distantly related species have a common ancestor further in the past. The animal most closely related to humans, for example, is the chimpanzee. The common ancestor of humans and chimpanzees is believed to have lived approximately 6 million to 7 million years ago (see Human Evolution). On the other hand, an ancestor common to humans and reptiles lived some 300 million years ago. And the common ancestor to even more distantly related forms lived even further in the past. Evolutionary biologists attempt to determine the history of lineages as they diverge and how differences in characteristics developed over time.

Throughout history, philosophers, religious thinkers, and scientists have attempted to explain the history and variety of life on Earth. During the rise of modern science in western Europe in the 17th and 18th centuries, a predominant view held that God created every organism on Earth more or less as it now exists. But in that time of burgeoning interest in the study of fossils and natural history, the beginnings of a modern evolutionary theory began to take shape. Early evolutionary theorists proposed that all of life on Earth evolved gradually from simple organisms. Their knowledge of science was incomplete, however, and their theories left too many questions unanswered. Most prominent scientists of the day remained convinced that the variety of life on Earth could only result from an act of divine creation.

In the mid-19th century a modern theory of evolution took hold, thanks to British naturalist Charles Darwin. In his book On the Origin of Species by Means of Natural Selection, published in 1859, Darwin described the evolution of life as a process of natural selection. Life, he suggested, is a competitive struggle to survive, often in the face of limited resources. Living things must compete for food and space. They must evade the ravages of predators and disease while dealing with unpredictable shifts in their environment, such as changes in climate. Darwin offered that, within a given population in a given environment, certain individuals possess characteristics that make them more likely to survive and reproduce. These individuals will pass these critical characteristics on to their offspring. The number of organisms with these traits increases as each generation passes on the advantageous combination of traits. Outmatched, individuals lacking the beneficial traits gradually decrease in number. Slowly, Darwin argued, natural selection tips the balance in a population toward those with the combination of traits, or adaptations, best suited to their environment.

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While On the Origin of Species was an instant sensation and best-seller, Darwin’s theories faced hostile reception by critics who railed against his blasphemous ideas. Other critics pointed to questions left unresolved by Darwin’s careful arguments. For instance, Darwin could not explain the mechanism that caused life forms to change from generation to generation.

Hostility gave way to acclaim as scientists vigorously debated, explored, and built on Darwin’s theory of natural selection. As the 20th century unfolded, scientific advances revealed the detailed mechanisms missing from Darwin’s theory. Study of the complex chemistry of all organisms unveiled the structure of genes as well as how they are duplicated, altered, and passed from generation to generation. New statistical methods helped explain how genes in specific populations change over generations. These new methods provided insight into how populations remain adaptable to changing environmental circumstances and broadened our understanding of the genetic structure of populations. Advances in techniques used to determine the age of fossils provided clues about when extinct organisms existed and details about the circumstances surrounding their extinction. And new molecular biology techniques compare the genetic structures of different species, enabling scientists to determine heretofore undetectable evolutionary relationships between species. Today, evolution is recognized as the cornerstone of modern biology. Uniting such diverse scientific fields as cell biology, genetics, paleontology, and even geology and statistics, the study of evolution reveals an exquisitely complex interaction of the forces that act upon every life form on Earth.

II

Genetic Basis of Evolution

Natural selection is tied to traits that organisms pass from one generation to the next (see Heredity). In humans, these traits include hundreds of features such as eye color, blood type, and height. Nature offers countless other examples of traits in living things, such as the pattern on a butterfly’s wings, the markings on a snail’s shell, the shape of a bird’s beak, or the color of a flower’s petals.

Such traits are controlled by specific bits of biochemical instructions known as genes. Genes are composed of individual segments of the long, coiled molecule called deoxyribonucleic acid (DNA). They direct the synthesis of proteins, molecular laborers that serve as building blocks of cells, control chemical reactions, and transport materials to and from cells. Proteins are themselves composed of long chains of amino acids, and the biochemical instructions found in DNA determine the arrangement of amino acids in a chain. The specific sequence of amino acids dictates the structure and resulting function of each protein.

All genetic traits result from different combinations of gene pairs, one gene inherited from the mother and one from the father. Each trait is thus represented by two genes, often in different forms. Different forms of the same gene are called alleles. Traits depend on very precise rules governing how genetic units are expressed through generations. For example, some people have the ability to roll their tongue into a U-shape, while others can only curve their tongue slightly. A single gene with two alleles controls this heritable trait. If a child inherits the allele for tongue rolling from one parent and the allele for no tongue rolling from the other parent, she will be able to roll her tongue. The allele for tongue rolling dominates the gene pair, and so its trait is expressed. According to the laws governing heredity, when a dominant allele (in this case, tongue rolling) and a recessive allele (no tongue rolling) combine, the trait will always be dictated by the dominant allele. The no tongue rolling trait, or any other recessive trait, will only occur in an individual who inherits the two recessive alleles.

A

Genetic Variation in Populations

Evolutionary change takes place in populations over the course of many generations. Since individual organisms cannot evolve in a single lifetime, evolutionary science focuses on a population of interbreeding individuals. All populations contain some variations in traits. In humans, for example, some people are tall, some are short, and some are of medium height (see Population Biology).

In interbreeding populations, genes are randomly shuffled among members of the population through sexual reproduction, the process that produces genetically unique offspring. Individuals of different sexes develop specialized sex cells called gametes. In humans and other vertebrates (animals with backbones), these gametes are sperm in males and eggs in females. When males and females mate, these sex cells join in fertilization. A series of cell divisions creates individuals with a unique assembly of genes. No individual members of any population (except identical twins, which develop from a single egg) are exactly alike in their genetic makeup. This diversity, referred to as genetic diversity or variation, is essential to evolution. The greater a population’s genetic diversity, the more likely it is to evolve specific traits that enable it to adapt to new environmental pressures, such as climate change or disease. In contrast, such pressures might drive a population with a low degree of genetic diversity to extinction.

Sexual reproduction ensures that the genes in a population are rearranged in each generation, a process termed recombination. Although the combinations of genes in individuals change with each new generation, the gene frequency, or ratio of different alleles in the entire population, remains relatively constant if no evolutionary forces act on the population. One such force is the introduction of new genes into the genetic material of the population, or gene pool.

A

1

Gene Flow

When individuals move between one population and another, new genes may be introduced to populations. This phenomenon, known as gene flow, results from chance dispersal as well as intentional migration. Take, for example, two populations of related wildflowers, one red and one white, separated by a large tract of land. Under normal circumstances, the two groups do not interbreed because the wind does not blow hard enough to carry pollen between the populations so that pollination can occur. If one day an unusually strong wind carries pollen from the red wildflower population to the white wildflower population, the gene for red flowers may be introduced to the white population’s gene pool.

In many animals, gene flow results when individuals from one population migrate to another population. The Lake Erie water snake provides an excellent example. Although skilled swimmers, these snakes spend much of their day basking in the sun on overhanging vegetation or rocks. In Lake Erie, water snakes form distinct populations—snakes that live on the rock islands in the lake, and others that live in the vegetation close to the shore. Shore populations have gray bodies with black bands, a coloration that helps them evade hungry seagulls by blending with the shoreline vegetation. Island snakes are primarily light gray with no banding, coloring that helps them to blend in to their rocky surroundings. An easy target for seagulls, banded island snakes rarely survive to reproductive age. Yet every year, biologists count banded newborns among the island litters. What at first appeared to be a mystery was later revealed to be gene flow at work. Although the populations do not regularly interbreed, once in a while shore snakes, carried by currents or winds, migrate to the islands. Once there, they mate with island snakes, reintroducing the gene for banding into the island population as they do.

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