Introduction to Evolution
Evolution, as defined in biology, the continual process by which one form of life changes, or evolves, into another form. Some religious groups deny that evolution exists, but most scientists accept it as fact. The theory of evolution suggests that all living things descended from one or several kinds of simple organisms. It also explains why there are so many different kinds of organisms. The inherited characteristics of nearly all living things change from generation to generation. Eventually the accumulated changes may become so great that the descendant bears little likeness to its remote ancestor and may belong to a different species. Evolution is of two types: anagenesis and cladogenesis. Anagenesis is gradual change in a single species. Cladogenesis involves the branching of the species into two or more species over generations. It took a single species more than 3 million years to evolve into the current 10 million species on Earth today. This implies that all species on the planet today had a common ancestor.
During evolution, natural selection works in a way that ensures that species that are more capable of adapting to their environment live and evolve further, while species that are unsuited to their environment die out.
How Evolution Takes Place
The mechanisms through which evolution occurs are discussed below.
Natural SelectionThere are more living things produced than can possibly survive, because there is not enough space and food to support them. The result is competition among organisms in the struggle for survival. The individuals in each species are not all exactly alike. They have varying characteristics, and these variations may either help or hinder them in their struggle to live.
Organisms become extinct (die out), if they do not have enough of the kind of characteristics that enable them to get sufficient food and other necessities, to withstand the climate, or to compete successfully with other forms of life. The ones that survive and reproduce pass on to the next generation at least some of the characteristics that made them better adapted to their environment. In this way the successive generations improve their internal efficiency and their adjustment to their environment. Natural selection causes a gradual change in the characteristics of the species.
In some species, females prefer males with different traits. This leads to evolution of a species where males have traits that are markedly different from that of females.
Directional selection is the evolution of characteristics that assist the species in adapting to their habitat. A species that needs longer limbs in order to run fast and catch its prey will evolve longer limbs.
Stabilizing selection is the continuance of the characteristics of a species that already has all the suitable traits needed to survive in its environment. If the average population of a species has all the characteristics it needs to survive, then individuals born with different traits do not survive long enough to pass on their traits through reproduction.
Diversifying selection ensures that the population has two different characteristics. A gene causes a disorder of blood cells within the human population, which is fatal if inherited from both parents. But when inherited from only one parent, it provides carriers of the gene resistance to malaria.
MutationTo explain how plants and animals develop variations, biologists turn to genetics, the science of heredity. Genetics shows that characteristics are passed from parents to offspring by means of genes—segments of large nucleic acid molecules contained in chromosomes. Offspring inherit these genes from their parents during sexual reproduction.
When a gene is altered in any way, the offspring acquires a new characteristic not possessed by either parent. The gene is passed on to the offspring in the mutated form. This change in the gene is called a mutation. Mutations are responsible for genetic variability(the random occurrence of variations) in a given species. When a mutation introduces a harmful characteristic, the organism probably will not survive unless the harmful effect is balanced by beneficial effects. Helpful characteristics—those which enhance the chances of survival—acquired by mutation are likely to be passed on to future generations.
Mutations can be produced in laboratories by the action of X rays, other forms of radiation, or chemicals. Mutations occur frequently in nature, often from random errors in chromosome replication. Radiation and chemicals present in the environment can also cause the genes to mutate.
Sexual RecombinationThe combining of genes from two parents is called sexual recombination and controls the rate at which variations occur. Sexual recombination permits enough variation to allow progressive evolution but not so much variation that the changes would be chaotic. Most organisms reproduce through sexual recombination. Related to sexual recombination is genetic drift, where many of the parents' genes are not passed on to the offspring. During sexual recombination, only half of the chromosomes from each parent are passed on to the progeny. The lesser the number of offspring, the more the chance of some genes of the parents not being passed on.
DivergenceWhen different populations of the same species become isolated from each other (as by a physical barrier such as a mountain, desert, or ocean) each population adapts to its own environment. When a physical barrier separates two populations of the same species, it can give rise to a new species. When the isolated population is subjected to genetic drift or other mechanisms of natural selection, it causes the evolution of a new species. In time, the isolated populations may diverge into different species. A physical barrier is not always necessary for populations of a species to diverge. For example, different populations sometimes eat different foods in the same general area. Such populations will have little interaction with each other and may eventually diverge into different species.
The various species of finches, known as Darwin's finches, that live on the Galápagos Islands provide an example of both forms of divergence. It is thought that these finches evolved from members of a single species blown by the wind from the mainland of South America, some 600 miles (965 km) away. The expanse of ocean isolated the displaced birds from the rest of their species. Different groups of finches found different types of food on the islands. Each group developed adaptations (such as a particular beak shape) for efficiently eating its own type of food; eventually, the groups diverged into more than a dozen species.
ConvergenceWhen unrelated species adapt to the same kind of environment, they independently develop similar characteristics—that is, they converge. For example, though whales are mammals and penguins are birds, they each share certain characteristics with fish (such as finlike appendages and streamlined bodies).
Evolutionary LineageThe evolutionary lineage of a species is the traits individuals of the species share that helps them survive and reproduce in their environment. For the species to continue, it must maintain its evolutionary lineage. Therefore, nature has certain barriers that prevents the species from producing offspring without the traits that are part of the evolutionary lineage. These barriers include unique courtship rituals that prevent mating between species, or inability of the offspring produced through inter-species mating to produce their own offspring.
Sometimes, the barriers to reproduction between species or populations of the same species are physical, such as creation of a new mountain range or river that happen over millions of years. This leads to divergence.
Other Evolutionary PrinciplesOther evolutionary principles include the following:
1. The pace of evolution differs from species to species. Some species of brachiopods, for example, have not changed in hundreds of millions of years. Gradual evolution occurs over a long period.On the other hand, several species of horses have appeared and become extinct within a few hundred thousand years. In addition, the pace of evolution may vary throughout the history of a particular species. The fossil record indicates that many species have had short periods of rapid change followed by long periods of virtually no change, a process called punctuated equilibrium.
2. Highly complex species evolve from relatively simple ones. Simple species, however, may evolve from complex ones—a process called degeneration—if a simpler structure helps a species to adapt better to its environment.
3. When different species interact closely, as in predator-prey or parasite-host relationships, they coevolve—that is, they evolve together. For example, through evolution, rabbits have developed keen hearing, which helps them escape predators, such as owls. Owls, on the other hand, have developed feathers that permit virtually silent flight.
Evidence of Evolution
Direct evidence of evolution is obtained when animal or plant breeders raise new varieties by cross-breeding. Further evidence is provided by laboratory experiments with bacteria and other primitive forms of life. Since these organisms produce new generations within minutes or hours, scientists can observe evolutionary changes that would take thousands of years in more complex forms. Another type of direct evidence comes from paleontology, the study of fossils, in which scientists trace the history of life on Earth.
There is also a great deal of indirect evidence of evolution. For example, before birth, all animals with backbones—including humans—pass through a stage of development in which they have gill clefts. This fact suggests that these animals are descended from organisms that once lived in water. Another example is the fact that all organisms are similar in certain essential ways. By comparing the traits of different species, we can find certain similarities that indicate that they evolved from a common ancestor. All living cells, whether in a bacterial colony or an elephant, in a tree or a human, undergo similar processes of metabolism, growth, and reproduction.
Some species have organs that are not useful to them. These vestigial organs have been passed down to them from an ancestor who once needed these organs. The human appendix is an example of a vestigial organ. Many species with dark caves as their habitat have vestigial eyes.
Evidence of evolution can also be found in certain genes, such as those for the proteins cytochrome cand hemoglobin, which occur in many species. The subunits, or nucleotides, that make up such genes vary from species to species. However, these nucleotides have been found to be most similar among species whose classification indicates a close relationship. For example, the gene for hemoglobin in humans is much more similar to the corresponding gene in monkeys than to that in kangaroos.
Artificial selection parallels natural selection in many ways, and provides further evidence of evolution. Crop breeders breed plants with traits that are useful, and this results in offspring with new traits. In a few generations, the traits of the species would be different from their ancestors. Wild animals were selectively bred and domesticated this way by our human ancestors.
History of the Theory of Evolution
Many ancient myths were attempts to explain how the various plants and animals were created. Thales suggested that all life came from water. Aristotle believed that living things could arise out of nonliving matter, and that more complex forms of life probably arose from simpler forms.
With the spread of Christianity, people throughout the Western world accepted the story of Creation as told in Genesis, the first book of the Bible. For hundreds of years no further scientific thought was devoted to the origin of life.
By the 18th century scientists had again begun to investigate the development of living things. Georges Louis Buffon thought that environment acted directly to change the structure of organisms. Erasmus Darwin and Jean Baptiste Lamarck each advanced the now discredited theory that organisms inherit characteristics acquired during the lifetime of their parents.
The basis for the modern theory of evolution was formulated independently by Charles Darwin, grandson of Erasmus Darwin, and by Alfred Russel Wallace. They introduced their ideas about natural selection in joint papers read to a London learned society in 1858. Darwin's Origin of Species (1859) made the theory famous. The Darwin-Wallace theory was incomplete, however, because the science of genetics was unknown at the time. Gregor Mendel discovered the laws of heredity in the 1860's, but his findings were neglected until 1900.
In 1937 the theories of natural selection and genetic variability were fused by Theodosius Dobzhansky, a Russian-born American biologist, in Genetics and the Origin of Species. Dobzhansky's theory, called the Synthetic Theory (because it synthesizes, or combines, previously existing ideas), states that evolution is a two-step process. In the first step, genetic variations occur through mutations or by a chance combination of inherited genes. In the second step, desirable variations—those that produce traits that help the organism survive—are passed on to the next generation and become permanently established in the species.
When the theory of evolution first came to public attention in 1859 it was met with a storm of protest and ridicule. Some people, misinterpreting the theory, thought it claimed that humans were descended from apes. (Actually, the theory states that humans and apes had a common ancestor that was neither human nor ape.) One religious objection to evolution was that it conflicted with the Biblical story of Creation.
In the United States some states passed laws making it illegal to teach evolution in public schools. In 1925 John Scopes, a teacher in Dayton, Tennessee, was brought to trial for violating such a law. Scopes was defended by Clarence Darrow, and William Jennings Bryan was an attorney for the prosecution. Although Scopes was found guilty, the trial helped to end much opposition to the theory of evolution, and the various antievolution state laws were eventually repealed.
In most countries, the theory of evolution achieved acceptance by most religious groups. Opposition to the theory remained greatest in the United States, where many groups adhere to creationism.
Almost all scientists agree on the fact of evolution, but there have been several disputes over the theory. In the 1960's, the principal dispute was between scientists of the neutralist school and those of the selectionist school. The neutralists rejected traditional Darwinism and contended that the genetic characteristics of an organism had no effect on the chances of its survival. The neutralist position lost support when most additional research found little evidence in its favor.
The theory of punctuated equilibrium was proposed in 1972 by the American paleontologists Niles Eldredge and Steven Jay Gould. A dispute arose between scientists who held this theory and those who believed that the evolution of a species remains at a slow pace. An increasing amount of fossil evidence has supported the theory of punctuated equilibrium.
In the late 20th century, sophisticated tools have allowed people to study the process of evolution at the genetic level. Studies of genes of various species have shown that there is a great variation in the gene pools of animals. The genes shared by members of a species are not same. By comparing the DNA of various species, scientists can find out how closely two species are related, and whether they share a recent common ancestor.
Evolutionary scientists who work with DNA analysis not just compare the DNA of living organisms, but also conduct analysis on the DNA of fossil animals and plants to find out more about their evolutionary history. DNA analysis of fossil remains of our human ancestors has established the theory of a common ancestor of all humans.