Evolution

Evolution is the process by which gradual genetic change occurs over time to a group of living things. As one of the major theories in the life sciences, it can be applied in a narrow sense to the development of a species that adapts to its environment over many generations. In a broader sense, evolution can mean that all life on Earth has progressed from simple to more complex organisms. Evolution is one of the great explanatory theories of science because it describes how biological change occurs over time. However, since it also applies to the human species, it has philosophical and even religious implications and can be considered controversial.

DARWIN PROPOSES THEORY

The theory of evolution was first presented in 1858 by the English naturalist Charles Darwin (1809–1882). Its basic idea is that species undergo genetic change (changes in inherited characteristics) over time and appear different from their long-dead ancestors (although not noticeably different from other living members of their species). Darwin stated that these changes are brought about by the organism's response to the environment. He believed that organisms that are best adapted to their environment have a greater chance of passing on those "fit" traits to their offspring. Nature therefore "selects" the best adapted (fittest) and "selects out" those organisms whose genes contain traits that are less fit.

Darwin's theory further stated that evolution has given rise to all the different forms of life that have ever existed. This means that all species came from a single form of life or a common ancestor that may have first appeared about 3,500,000,000 years ago. The branching out from this first life form that eventually resulted in the enormous variety of living things is called "speciation." Darwin also theorized that the process of evolution is extremely slow. This notion of "gradualism" means that many generations must pass before any changes are noticeable. While Darwin's theory of evolution is not accepted by some who believe in the biblical account of creation, his theory is supported by a vast amount of factual evidence and has withstood all serious attempts to disprove it.

NATURAL SELECTION

The main idea behind evolution is "natural selection," because this concept explains how evolution occurs. Natural selection favors the passing on of beneficial or "fit" genes to the offspring and discourages the transmittal of less valuable genes. As the "engine that drives" evolution, natural selection does not work on the individual unit of life but at the population level. The term population refers to all members of the same species, which are slightly different from one another and live together in a particular place. For example, some deer have larger antlers than others, others are larger and stronger, and others may have a slightly different coloring. Therefore among any population or group of organisms that breed with one another, there will necessarily be a large genetic pool of slightly different traits. Nature, or the population's local environmental conditions, selects which individuals are best adapted. Those individuals with traits that best fit their habitat will get more food, be better prepared to escape a predator, or better able to resist disease than those not so well adapted. Those with favorable traits get to survive and to reproduce—passing these traits on to their young. Individuals with less favored traits do not reproduce as much, and eventually, an entire population will have the traits (in their genes) of the favored group.

Understanding natural selection allows us to understand the basics of evolution. Often the term "survival of the fittest" is used to mean that only the strongest survive or get to live a very long time. Actually, however, the term means that organisms poorly suited or badly "fitted" to their environment are not able to reproduce as much as the "fittest" among them. Consequently, the "fittest" get to survive longer and produce the most offspring. This process assures that it is their genes that get spread the most throughout the population. The fittest organism does not have to be the strongest (or even smartest) individual. What determines fitness is based entirely on the population's environmental conditions. This means that it is essentially a random process. Nature has no master plan that it uses to make species change. Rather, a variety of traits exist in a given population, and whichever one gives an individual an edge over others in the population is the trait that is favored. In many ways, natural selection appears to be a matter of chance. An example of this is the peppered moth of England. Before the Industrial Revolution of the eighteenth century took place and created the steam-powered factories of London, this particular moth was a beige color that enabled it to blend in almost perfectly against local tree bark. Over time, however, as its environment became more and more sooty from the burning of coal, the moth became a much darker color. Its environment change meant that those slightly darker moths had a greater chance of surviving than the lighter ones who were easily seen (and eaten by birds) against a blacker environment.

GENETIC VARIETY

The idea of natural selection is always connected to that of reproduction. It does no good for an organism to live a very long time if it does not reproduce. The fittest not only survive—they reproduce. Of the many factors that influence natural selection, the factor called "genetic variety" is probably the most important. Genetic variety means that the individuals making up a population must have basic genetic differences between them—that is, they have inherited characteristics that make them slightly different from one another. This becomes obvious when we realize that if each individual were identical genetically to all the others, it would not matter which one reproduced. When genetic differences exist, however, who gets to pass on which traits makes a big difference.

Mutations. Genetic variety occurs in two ways: by mutations, which are accidental changes in a gene, and by genetic recombination, which is the mixing of genes that happens to organisms that reproduce sexually. The word mutation often implies something bad, but such is not always the case. Mutations at the genetic level occur during the reshuffling that happens when meiosis takes place. Meiosis (may-OH-sihs) is the type of cell division that makes sex cells. During this division process, genes are sometimes accidentally altered, simply by chance, or by a change in the shape or number of chromosomes, which are the structures that contain the cell's deoxyribonucleic acid (DNA). When a mutation occurs, it means that a particular trait has been changed. Sometimes the mutation is unfavorable since it may eliminate a characteristic that proved beneficial, but it can also be advantageous since it may give a new and even better trait to an individual. If a mutation is favored by natural selection, it gets passed on from one generation to the next. In many ways, pure chance allows mutations to happen and contributes to evolution.

Genetic Recombination. Sexual reproduction is also a source of genetic variety. Sex cells are unlike cells in the rest of the body in that they have only a single set of genes (or only one copy of each gene). It is only after fertilization occurs (when the egg and sperm unite) that the cell gets two copies of each set of genes. During sexual reproduction (the joining of egg and sperm), genes are combined in new ways that result in an individual having a unique combination of genes. This mixing of genes is called genetic recombination, and it accounts for most of the variety needed for natural selection to occur.

ADAPTATION

The end result of natural selection is a process called adaptation. Through natural selection, which favors organisms that "fit" their environment best and weeds out those badly "fitted," living things become better suited, or "adapted," to their habitat or local environment. Darwin called this process of "molding" organisms to their environment over many generations "adaptation." It has come to mean any trait or characteristic that allows an organism to fit better in its environment. Furthermore, it includes an organism's behavior as well as its anatomy (structure) and physiology (internal processes).

THE RATE OF EVOLUTION

Ever since Darwin, the pace or rate at which evolution occurs has been thought to be a gradual one, taking place over a long period of time. However, in recent times two biologists, Stephen Jay Gould of Harvard University and Niles Eldredge of the American Museum of Natural History, have proposed the idea of "punctuated equilibrium." This new hypothesis states that a large number of new species can appear in sudden spurts rather than very gradually over extremely long periods. Gould and Eldredge support their argument by referring to the fossil record, which often shows how some species remained unchanged for millions of years. If change was slow but ever-present, it should appear that way in that fossil record. However, Gould and Eldredge point out cases where new species seem to appear out of nowhere, and use these examples to argue for evolution by sudden bursts.

While there may still be controversy over the rate at which evolution occurs, there is little argument about its major points: living things are constantly competing with one another for the necessities of life; the characteristics of each vary a great deal and are inherited; some of these variations make certain individuals better adapted to their environment; the process of natural selection decides which are more "fit"; more "fit" individuals are more likely to survive and pass on their genes; and all of today's life is the result of evolution progressing from the simple to the complex.

[See alsoAdaptation; Evolution, Evidence of; Human Evolution; Evolutionary Theory; Natural Selection ]