Evolution of life

The term evolution refers to the physical changes that take place in all of the earth's species over time. From a one-celled ancestor, many billions of species have evolved over the course of the planet's history. Evolution is the basis of all life's diversity.

Inheritance and Natural Selection

Evolution comes from the Latin word meaning “to unroll.” In a general sense, it refers to any change through time, but it is often restricted to biological change. For most of human history, the universe was thought to be unchanging. Then, in the eighteenth century, expeditions to new continents and the discovery of extinct fossil animals convinced many people that the biological world was not as unchanging as had been thought. However, there could be no proof for this hypothesis until an explanation of how such change occurred could be found. In 1809, French naturalist Jean-Baptiste Lamarck became the first to propose an explanation. His theory was based on the inheritance of acquired traits. According to this theory, giraffes, for example, obtained long necks because individual giraffes stretched their neck muscles more and more to reach ever higher leaves, and the longer necks were passed on to the offspring. This idea was quickly shown to be false by experiment. Traits acquired during an individual's lifetime, such as larger muscles acquired through weightlifting, are not passed onto offspring.

It was 1859 when English naturalist Charles Darwin proposed what is now known to be the actual process by which evolution occurs: natural selection. Natural selection occurs because individuals in a species vary in their traits, and some individuals have more offspring than others, depending on how advantageous their particular traits are. Those whose traits make them better suited to their environment will survive longer and therefore have more offspring. As a result, advantageous traits will increase in a species, while disadvantageous traits will be lost through time and as the environment changes. For example, climatic change may cause a forested environment to become snowy tundra. Creatures with dark coats are better able to avoid predators in a forest setting because of the concealment provided by the dark, shadowy environment. As the lighter, snowy environment becomes dominant, however, the dark-colored animals will be easier to spot, while the lighter-colored animals will have an advantage because they will be more easily concealed. Over a long period, enough changes will accumulate in a group that an observer might say that a new species had been created. This process has often been called “survival of the fittest,” but the “fittest” organisms are not always the fastest, fiercest, or even most competitive. For example, animals that cooperate with other animals, or are more timid and conceal themselves more readily, may survive more often and produce more offspring.

When the whole species changes at once, non-branching evolution occurs. Many species, however, have wide ranges and occur in many different geographic areas, so often only some populations of a species are subjected to environmental changes. This is an important point because it explains how so many species can be created from one ancestral species. Branching evolution is especially common when one of the populations becomes cut off from the others by a barrier of some kind. For example, a new river may form that divides a population in two. If the animals are unable to cross the river, the two halves of the population will be unable to interbreed, enabling each new population to form its own pool of traits. In time, differences between the two environments will cause the two populations to become so distinct that they form two different species.

Species have previously been described as groups that are visibly distinct enough to be distinguishable from one another. However, there is a much more objective definition of species, based on the criterion of interbreeding. To a biologist, members of a species can produce fertile offspring only when they breed with other members. Therefore, a new species has evolved not when its appearance is sufficiently different from its ancestors or neighboring populations, but rather when it can no longer successfully interbreed with them. This definition of reproductive isolation is important because many closely related species look quite similar but cannot successfully interbreed.

Evolution by natural selection explains not only how species have changed, but also why they are so well adapted to their surroundings. The best-adapted individuals have the most offspring. Further, it explains some crucial aspects of basic anatomy, such as why vestigial, or remnant, organs exist: they are in the process of being lost. For example, the now-useless human appendix was once an important part of the human digestive tract. Also, it explains why many organisms have similar organs that are used for different purposes, such as five-fingered hands on humans and five-digit organs on bat wings. Such homologous organs have been modified from a common ancestor. This modification also explains why many organisms pass through similar embryonic stages; human embryos, for example, have tails and gills like a fish.

Laws of Heredity

After Darwin proposed natural selection as the process of evolution, his theory was readily accepted, and most scientists have accepted it ever since. The explanation was incomplete, however, in one major area: Darwin could not explain how variation was produced or passed on. The laws of heredity were discovered by Gregor Mendel in 1866 while Darwin was wrestling with this problem. Mendel's work lay unnoticed until the early twentieth century, when other scientists independently discovered the gene as the basic unit of heredity. Genes are now known to be molecular “blueprints” that are repeatedly copied within each cell. They contain instructions on how to build the organism and how to maintain it.

For most multicellular organisms, genes are passed on to the offspring when a sperm and an egg cell unite. The resulting fertilized egg consists of one cell that contains all the genes on strands, or chromosomes, in the cell nucleus. The chromosomes occur in pairs such that one member of each pair is from the father, and one is from the mother. As growth occurs, certain genes in each cell will be biochemically “read” and will give instructions on what happens next. Genes are composed of the molecule deoxyribonucleic acid (DNA), which is shaped like a twisted ladder (a double helix) and is copied when the double helix splits in half at the middle of the “rungs.” Once the instructions are copied, they are carried outside the cell nucleus by messenger molecules, which proceed to build proteins (such as enzymes and muscle tissue) using the rungs as a blueprint.

With this added knowledge of genes as the units of heredity, evolutionists could see that natural selection acting on individuals selects not only traits, but also the genes that serve as blueprints for those traits. Therefore, as well as being a change in a species’ traits through time, evolution is also often defined as a change in the gene pool of a species. The gene pool is the total of all the genes contained in a species. Individual variation in a gene pool originally arises via mutations, errors made in the DNA copying process. Usually, mutation involves a change in the DNA sequence that causes a change in the genetic instructions.

Most mutations have little effect, which is fortunate because those that are expressed usually kill or handicap the offspring. This occurs because any multicellular organism is a highly integrated, complex system, and any major alterations to it are therefore likely to disrupt it. Nevertheless, rare improvements do occur, and it is these that are passed on and become part of the breeding gene pool.

Although mutations provide the ultimate source of variation, the sexual recombination of genes provides the more immediate source. Each organism has a unique combination of genes, and it is the fitness of this combination that determines how well those genes survive and are passed on. Although brothers and sisters have the same parents, they are not alike because genes are constantly shuffled and reshuffled in the production of each sperm and egg cell.

Rates and Patterns of Evolution

A major area of debate is how fast evolution occurs. Some scientists believe that most evolution occurs in short, rapid bursts, interspersed with long periods of little to no alteration. This view has been called punctuated equilibrium. Another group argues that evolution is more often gradual, as Darwin originally proposed. To some extent, this disagreement is a matter of different perspectives. A geneticist working with flies in a laboratory would see the evolution of a new species in ten thousand years as very slow. In contrast, to a paleontologist, who often deals with fossil species lasting millions of years, ten thousand years is brief indeed. Nevertheless, there is more to the debate than perspective alone. Punctuationists argue not only that evolution is rapid but also that species have such tightly integrated gene pools that virtually no change at all occurs during most of a species’ existence. In contrast, gradualists view species as being much less integrated, so that change can be a continuous process.

The fossil record at first glance seems to support the punctuated view. Most species show very little change for long spans of time and then either disappear or rapidly give rise to another species. The fossil record is very incomplete, however, being full of gaps where no fossils were deposited. As a result, it is often impossible to tell if the (relatively) rapid change in species evident in the fossil record is real, or if it merely represents a gap in what was a gradual sequence. Also, fossils preserve only part of the original organism—usually only the hard parts, such as shells, bones, or teeth. Therefore, any changes in soft anatomy, such as tissues or biochemistry, are lost, making it impossible to say with certainty that no change occurred. Whatever the outcome of the debate, all scientists agree that evolutionary rates vary.

In addition to hypotheses related to the rate of evolution, much has been written about the patterns produced by evolution since life arose about 3.5 million years ago. Evolutionary trends are directional changes seen in a group. The most common trend, found in many groups, is an increase in size. Another trend, seen mainly in mammals, is an increase in brain size. Life as a whole has shown an increase in total diversity and complexity. These trends, however, are only statistical tendencies. They are not inevitable laws, as many have misinterpreted them in the past. Often, groups do not follow these trends, and in those that do, the change is not constant and may reverse itself at times. Finally, trends are often interrupted by mass extinctions. At least five times in the past 600 million years, more than 50 percent of all the species on Earth have been wiped out by catastrophes of different kinds, from temperature changes to impacts of huge meteorites.

Study of Fossils

Fossils, the remains of former life, provide the only record of most evolution because more than 99 percent of all species that have ever existed are now extinct. Paleontology is the study of fossils. Such study begins with identification of the remains—usually hard parts, such as bones—and ends with measurement of fossil size, shape, and abundance. The extreme incompleteness of the fossil record is a major obstacle to this method, since only some parts are preserved, and these are from strictly limited periods of the evolutionary past. Nevertheless, many evolutionary lineages can be traced through time. Indeed, refined measurements of rate and direction of anatomical change are often possible when used in conjunction with dating techniques.

The study of living organisms permits observation of the complete organism. Comparative anatomy reveals similarities among related species and shows how evolution has modified them since they separated from their common ancestor. For example, humans and chimpanzees are extremely similar in their organ and muscle anatomy. This method is not limited to a comparison of adults but includes earlier stages of development as well. Comparative embryology often shows anatomical similarities, such as those between humans and other vertebrates. For example, the human embryo goes through a stage with gills and a tail, resembling stages of an amphibian embryo. Comparative biochemistry is also very useful, revealing similarities in proteins and many other molecules. Such comparisons are based on differences in molecular sequences, such as amino acids. Molecular “clocks” are sometimes calculated in this manner. More distantly related species are thought to have more differences. The accuracy of such clocks, however, is hotly debated.

A major technique for researching evolution is DNA sequencing, whereby the exact genetic information is read directly from the gene. This method will greatly add to knowledge of evolutionary relationships, although it is expensive and time-consuming. Biogeography, or the distribution of organisms in nature, is a method that Darwin used which remains relevant in the twenty-first century. This technique often reveals populations within a species’ overall range that differ from one another because they inhabit slightly different geographic areas. These populations give the scientist a “snapshot” of evolution in progress. Given more time, many of them would eventually become different species.

Artificial breeding is a method of directly manipulating evolution. The most widely used experimental organism for this purpose is the fruit fly, which is used in part because its exceptionally large chromosomes make its genes easy to identify. A common experiment is to subject the flies to radiation or chemicals that cause mutations and then to analyze the effects. The gene pool is then subjected to extreme artificial selection as the experimenter allows only certain individuals to breed. For example, only those with a gene for a certain kind of wing may reproduce. Although such experiments have often altered the organisms’ gene pools and created new varieties within the species, no truly new species has ever been created in the laboratory. Apparently, more time is needed to produce a new species.

Outside the laboratory, artificial breeding has been done for thousands of years. Food plants and domesticated animals have had much of their evolution controlled by humans. Analysis of the effect of this breeding on the organisms’ gene pools is the most complete and direct method of studying evolution.

Significance

The study of fossils has been a major tool in understanding Earth's history. This understanding has allowed more efficient exploitation of the planet's resources. For example, petroleum and coal provide the major energy resources today and were both formed by organisms of the past. Petroleum comes from the biochemicals of marine organisms, and coal comes from fossilized plants. Most paleontologists are employed in the costly search for these fossil fuels, and knowing the evolutionary history of these groups helps determine the most productive places to search. Fossils also form non-energy resources. Limestone is used in many processes, from making cement to making steel. Most limestone is composed of the fossilized remains of seashells and other marine skeletons. Phosphate minerals, essential for fertilizers in almost all forms of agriculture, come from marine fossil deposits as well.

Darwin's theory of natural selection caused a violent reaction throughout much of the world when it was applied in social contexts as Social Darwinism—an application with which Darwin himself disagreed. Still, the notion that humans evolved from other life-forms such as apes was truly revolutionary; instead of creatures of a divine plan, humans were now seen as products of natural, sometimes random, processes. The impact of this realization on ethics, the arts, and society in general maintains relevance in the twenty-first century. Evolution, however, does not necessarily conflict with religion, as is often thought. Science seeks to discover only how things happen, not the ultimate reasons why they happen. Therefore, most major religions have reconciled their tenets with the fact of evolution by viewing natural selection as simply a mechanism that enacts divine will.

Principal Terms

evolutionary trends: statistical directions of evolutionary changes; major trends have been toward increased body size, brain size, and complexity

gene: the basic unit of heredity; composed of deoxyribonucleic acid (DNA), genes are located in the cell nucleus on chromosomes

gradual evolution: the theory that evolution occurs throughout much of a species’ existence, mostly at slow rates

mutation: a spontaneous change in a gene; the ultimate source of variation on which natural selection acts

natural selection: the main process of biological evolution; the production of the most offspring by individuals with the most adaptive traits

punctuated evolution: the theory that evolution occurs mainly in rapid “spurts”

species: a group of individuals that can successfully interbreed only among themselves

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