Evolution

The remains of stadiums, temples, and aqueducts indicate as clearly as any ancient document that the Roman Empire once existed. Likewise, fossils speak eloquently of a time when dinosaurs and not humans dominated Earth. Even without ancient ruins, similarities in appearance, language, customs, and genetic makeup show that the Italians, Spanish, English, and French all came from the same ancestral culture. Likewise, similarities in structure and genetic makeup persuade humankind that algae and plants, insects and crus­taceans, chimpanzees and humans came from the same ancestral species.

Evolution, which can be defined as the natural change in the inherited characteristics of groups of organisms, is as well established as the Roman Empire or any other event that is accepted as fact. Unfortunately, the com­mon phrase “theory of evolution” has misled many people into believing that evolution is “only” a theory. To biologists, “theory of evolution” refers to a proposal about how evolution occurs, not whether it occurs. There are, in fact, several theories of evolution. Like evolution itself, some of these the­ories are well supported by observations and experiments.

Development of Evolutionary Theory

Evolution is generally associated with Charles Darwin (1809-1882), but by the time he wrote about it in 1858, it had already been suggested by many people. In fact, Charles Darwin’s grandfather, Erasmus Darwin (1731-1802), was one of many who suggested that living species had de­scended from different species that had lived in the past. His theory of how evolution occurred was similar to that of French biologist Jean-Baptiste Lamarck (1744-1829) and was based on the belief that characteristics that develop in an adult can be passed on to its offspring. Thus, for example, gi­raffes could have evolved because their short-necked ancestors stretched their necks to reach higher leaves and therefore had offspring with longer necks. Both Lamarck and Erasmus Darwin were ignored, scorned, and ridiculed for this idea.

Charles Darwin was well aware of the controversy over evolution. As a theology student at Cambridge University with a passion for biology, he heard his professors dismiss evolution as nonsense, and he saw no reason to doubt them. Between 1831 and 1836, however, while serving as naturalist on an around-the-world voyage of The Beagle, young Darwin made obser­vations that convinced him that evolution had, in fact, occurred. He saw that the fossil animals in parts of South America were different from, but similar to, the animals still living there. This gave Darwin the idea that liv­ing organisms were descendants of extinct ones that had lived in the same place in the past.

Darwin also observed that regions isolated from each other often had different but similar species. He noted, for example, that each of the Gala­pagos Islands had distinct species of mockingbirds. This suggested that all were descendants of the same ancestral species, and each had taken its own evolutionary path after being separated from the others. Darwin was also influenced by reading Principles of Geology by Charles Lyell (1797-1875). Lyell argued convincingly that geological changes were not caused by sud­den global catastrophes, as most geologists then thought, but by gradual processes like erosion. This made Darwin realize that evolution must also have been gradual, otherwise organisms could not have remained adapted to their changing environments.

A cactus finch in the Galapagos Islands, where Charles Darwin began to formulate his theory of evolution. Darwin observed that regions isolated from each other often had different but similar species.

Darwin eventually returned to England convinced of the reality of evo­lution. He knew, however, that no one else would believe it unless he could find a better theory to explain it than his grandfather and Lamarck had pro­posed. Since some of his relatives owned estates on which they had suc­cessfully altered domesticated animals by selective breeding, it occurred to Darwin that something like this artificial selection might explain evolution. But how could unconscious nature select which individuals would breed and which would not? Darwin studied agricultural journals, conducted breeding experiments, and pondered the question for months. Then one day in 1838 he decided to read (“for amusement,” he says in his autobiography) the fa­mous piece Essay on the Principle of Population (1798) by Thomas Malthus (1766-1834).

Natural Selection

The essential idea of this essay is now called the Malthusian Principle. It proposes that human population has a tendency to increase much faster than the food supply. Consequently, there will always be competition be­tween those who can get food and those who cannot. Darwin saw in a flash that the same principle applies to all organisms. Virtually all species have the natural ability to produce many more offspring than can survive with the available resources. Within any species there will be some indi­viduals that are better able to compete for food, mates, and other re­sources, and they will be more likely than others to produce more offspring. Scientists would now say that they have a greater fitness. To the extent that their fitness is hereditary, their offspring will also be bet­ter able to compete, and so on, generation after generation. In this way the fitter individuals become increasingly numerous, and the species grad­ually evolves. Darwin gave his theoretical mechanism of evolution the name “natural selection.”

Natural selection may be the simplest yet most powerful theory in science. With it one can immediately see that evolution is not only pos­sible but, given enough time, inescapable. All that is required is that there be competition among individuals of the same species, and that individ­ual organisms have inherited traits that make some better able than oth­ers to compete. Darwin must have realized the importance of his theory. Rather than risk his budding reputation with a hasty report to a scien­tific journal, however, he began to accumulate supporting evidence for a book. Twenty years later he was still at work on his book when a re­markable coincidence forced him to publish. In 1858 he received a man­uscript from an English collector in the East Indies, Alfred Russel Wallace (1823-1913).

As Darwin read the manuscript he was stunned to see that Wallace had hit upon the same theory of natural selection that he had been laboring over for two decades. Darwin reluctantly agreed to publish an outline of his ideas along with Wallace’s paper. (It was discovered later that the basic concept of evolution by natural selection had already been proposed almost thirty years earlier by a little-known Scotsman named Patrick Matthew [1790-1874]. Matthew had also been ignored.) Ultimately, what finally made the words “evolution” and “Darwinism” well known was Darwin’s book, On the Origin of Species by Means of Natural Selection, which was published in 1859. Its vast documentation and powerful arguments soon convinced the majority of biologists that evolution is a fact, and natural selection is one of the reasons why it occurs.

Since the publication of On the Origin of Species, few biologists have doubted that evolution occurs. By the early twentieth century, however, nat­ural selection appeared to be heading toward extinction. One criticism of natural selection was that any adaptation that made an individual only slightly more fit would be diluted when the individual mated. For example, if a giraffe ancestor with a slightly longer neck mated with a normal mem­ber of its species, their offspring would have necks with lengths between that of the two parents. This reduction in neck length would continue with each generation. Thus any adaptation would be blended out of the species before natural selection would have a chance to favor it. In addition, be­ginning in 1900, genetic mutation seemed to provide an alternative theory that was better than natural selection. The discovery of the work of Gregor Mendel and further research on genetics suggested that new species resulted from large mutations occurring within a single generation instead of small mutations being selected over many generations.

Neo-Darwinism

By the middle of the twentieth century, however, biologists saw that Dar­win’s theory of natural selection was not really in conflict with genetics. They synthesized the two views, resulting in what is now called the neo- Darwinian or Synthetic Theory of Evolution. The neo-Darwinian theory was aided by a shift in thinking about the scale of evolution. Rather than conceiving of evolution as something that happened to entire species, bi­ologists began to think of it as occurring within smaller groups of inter­breeding organisms, called populations. Most species comprise many populations.

The neo-Darwinian Theory was also made possible by a mathematical proof called the Hardy-Weinberg equilibrium. The Hardy-Weinberg equi­librium showed that adaptations would not be blended out of populations, and it also showed that natural selection was indeed a possible cause of evo­lution. This proof, which was proposed in 1908 independently by English mathematician G. H. Hardy (1877-1947) and German physician Wilhelm Weinberg (1862-1937), shows that under certain conditions even rare mu­tations persist indefinitely. In modern terms, scientists would say that the Hardy-Weinberg equilibrium shows that the gene frequency—the propor­tion of a particular type of gene in a population—will remain constant if certain conditions occur. These conditions are as follows:

1-The size of the population is practically infinite

2-Individuals in the population mate at random

3-All individuals in the population have the same fitness, regardless of their genes

4-There is no gain or loss of genes due to immigration into or emigration out of the population

5-There is no new mutation in the population

Violating any one of these conditions can lead to a change in gene fre­quency. This is important because changes in gene frequency can result in evolution. In fact, many biologists now define evolution as any change in gene frequency. As an example, suppose a genetic mutation had caused an ancestor of giraffes to have a slightly longer neck. A departure from the Hardy-Weinberg conditions could continually increase the frequency of that mutated gene in the population. Gradually the entire population would have longer necks. This process repeated over thousands of generations could cause that population to evolve into the giraffe. The Hardy-Weinberg equi­librium therefore amounts to a list of conditions that, if absent, can cause evolution. The potential causes of evolution include small population size, nonrandom mating, natural selection, immigration and emigration, and mu­tation.

Small Population Size. A change in gene frequency due to small popula­tion size is called genetic drift. Genetic drift is now recognized as one of the major causes of evolution, although its results are usually random rather than adaptive. Chance events operating in small populations can have huge effects on gene frequency. Imagine, for instance, an isolated population of a very rare, endangered species of mountain sheep, whose males have horns that are either curved or straight. If a severe snowstorm happened to kill the few sheep with genes for curved horns, the proportion of sheep with straight horns would increase greatly in future generations.

A related phenomenon, called a population bottleneck, occurs when a large population is decimated by disease, predation, or habitat destruction. The few surviving members constitute the “bottleneck” through which the species passes. The genes of those few members dominate the gene pool of future generations. Similarly, a population of organisms could differ from others simply because the few founders of the population happened to have a gene frequency different from that of the species as a whole. This is called the founder effect. The wide differences in blood group frequencies between the Old Order Amish of Pennsylvania and other U.S. populations of Euro­pean ancestry are due to the founder effect operating in the Amish popula­tion. The role of genetic drift in species formation is an important area of research in evolution.

Nonrandom Mating. A second potential cause of evolution is nonrandom mating. Nonrandom mating usually occurs when individuals choose their mates. Animals often select mates on the basis of fitness, and the results of such sexual selection are indistinguishable from natural selection. On the other hand, mate selection can be based on characteristics that have noth­ing to do with fitness. For example, the tail feathers of the peacock or the bright coloration of the male pheasant are not thought to confer selective advantage in any arena other than mate selection. But because females choose the showier bird, the trait is selected for in males. This is called sex­ual selection.

Natural Selection. Natural selection, which is due to hereditary differ­ences in fitness, is a third potential cause of evolution, as Charles Darwin argued. Natural selection is now considered to be the main, if not the only, cause of the evolution of adaptations that increase fitness. For example, the speed of the gazelle and the cheetah that chases it are both due to natural selection.

Immigration and Emigration. Immigration and emigration can bring in or remove particular genes. The global travel of human beings has increased the importance of these forces not only in human populations, but in many other species that travel with humans, such as Africanized honey bees. The so-called killer bees from Africa are currently changing the gene frequen­cies of bee populations in the southern United States.

Mutation. Finally, mutation can obviously change the frequency of a gene. Mutation can be especially potent when combined with genetic drift in small populations.

Mutation

As noted earlier, many biologists once thought that mutation by itself was the major cause of evolution. In the 1920s, however, British biologist J. B. S. Haldane (1892-1964), British statistician Ronald A. Fisher (1890-1962), and American geneticist Sewall Wright (1889-1988) published three differ­ent mathematical proofs showing that mutation by itself is insufficient. They showed that a rate of mutation fast enough to cause evolution would also be fast enough to undo any evolution that had happened in the past. Sci­entists now know that mutations are too rare (about one per billion nucleotides per human lifetime) to account for most evolutionary change without the help of natural selection. Also, contrary to what Erasmus Dar­win and Lamarck thought, scientists know of no way that the efforts or ex­perience of an organism can induce specific, adaptive mutations in its offspring.

A petri dish culture of antibiotic-resistant Staphylococcus aureus. Resistance to antibiotics evolves when antibiotics are used improperly, allowing the survival of a few bacteria with mutated genes that confer resistance.

For a time, many biologists thought that natural selection was so rig­orous that it would eliminate most mutations since most mutations were presumed to be harmful. Starting in the 1950s, however, it was found that genetic variations resulting from past mutations are quite abundant in most species. Most mutations have little effect on fitness, and they can accumu­late generation after generation with little selection against them. With in­creased competition or some change in the environment, however, some of these mutations may result in differences in fitness. Natural selection can then bring about evolution by increasing the frequency of the beneficial mu­tations. Natural selection therefore seldom has to sit and wait for just the right mutation to come along and make an individual more fit. The muta­tions are usually already present in most populations.

Microevolution and Macroevolution

Changes in gene frequency that occur within a population without produc­ing a new species are called microevolution. As microevolution continues, a population may become so different that it is no longer able to reproduce with members of other populations. At that point, the population becomes a new species. As the new species continues to evolve, biologists might even­tually consider it to be a new genus, order, family, or higher level of classi­fication. Such evolution at the level of species or higher is called macroevolution.

Microevolution can occur very quickly; indeed, it is probably always oc­curring. For example, in less than half a century after the discovery of an­tibiotics, many bacteria evolved resistance to them. Resistance to antibiotics evolves when antibiotics are used improperly, allowing the survival of a few bacteria with mutated genes that confer resistance. Natural selection then leads to the evolution of antibiotic-resistant strains. Pesticide-resistant in­sects and herbicide-resistant weeds are additional examples of rapid mi­croevolution.

Macroevolution occurs over much longer periods and is seldom ob­served within the human life span. Occasionally, however, scientists do see evidence that new species have recently evolved. There are species of par­asitic insects, for example, that are unable to reproduce except in domes­ticated plants that did not even exist a few centuries ago. The pace of evolution can be quite variable, with long periods in which there is little change being punctuated by relatively brief periods of tens of thousands of years in which most changes occur. This idea that the pace of evolution is not always slow and constant is referred to as punctuated equilibrium. It was first proposed by paleontologists Niles Eldredge and Stephen Jay Gould in 1979, and it is one of many examples of how scientists’ views of evolu­tion are continually changing.

Several possible mechanisms exist for rapid evolution. Chromosomal aberrations, such as breakages and rejoining of chromosomal parts, can in­troduce large changes in genes and the sequences that regulate them. This may lead to changes much larger than that brought about by simple point mutations.

Environmental catastrophes can set the stage for rapid evolution as well. It is thought that the extinction of the dinosaurs was triggered by a large comet impact. This rapid loss of the dominant fauna in many ecosystems opened up many new niches for mammals, which at the time were a small group of fairly unimportant creatures. The sudden appearance of many new opportunities led to rapid and widespread speciation, in a process called adaptive radiation.

Other areas of biology are also continually changing under the influ­ence of evolution. For example, as Charles Darwin predicted in The Origin of Species, classification has become more than simply the grouping of or­ganisms into species, genera, families, and so on based on how physically similar they are. Classification now aims to group species according to their evolutionary history. Thus two species that diverged recently from the same ancestor should be in the same genus, whereas species that shared a more distant common ancestor might be in different genera or higher taxonomic levels.

Until the 1980s, evolutionary history, or phylogeny, of organisms could only be inferred from anatomical similarities. Since that time, however, it has been possible to determine phylogeny from comparisons of molecules. Often this molecular phylogeny agrees with the phylogeny based on anatomy. For example, about 99 percent of the sequence of bases in the de­oxyribonucleic acid (DNA) of chimpanzees and humans is identical. This finding confirms the conclusion from anatomy that chimpanzees and hu­mans evolved from the same ancestor only a few million years ago. Such agreement between anatomical and molecular phylogeny would not be ex­pected if each species were a totally different creation unrelated to other species, but it makes sense in light of evolution. It is one of many examples of the famous saying by the geneticist Theodosius Dobzhansky (1900-1975): “Nothing in biology makes sense except in light of evolution.”

References

Dawkins, Richard, The Selfish Gene. Oxford: Oxford University Press, 1990.

Freeman, Scott, and Jon C. Herron. Evolutionary Analysis, 2nd ed. Upper Saddle River, NJ: Prentice Hall, 2001.

Futuyma, Douglas J. Evolutionary Biology, 3rd ed. Sunderland, MA: Sinauer Associ­ates, 1998.

Stanhope, Judith. Hardy-Weinberg Equilibrium. <www.accessexcellence.org/AE/ AEPC/WWC/1994/hwintro.html>.

References