Genetics and Evolution
Genetics and Evolution
The essence of Darwinism lies in a single phrase: natural selection is the major creative force of evolutionary change. No one denies that natural selection will play a negative role in eliminating the unfit. Darwinian theories require that it create the fit as well.
—Stephen Jay Gould, "The Return of Hopeful Monsters" (Natural History, June-July 1977)
The term evolution has multiple meanings; it is most generally used to describe the theory that all organisms are linked via descent to a common ancestor. Evolution also refers to the gradual process during which change occurs. In biology it is the theory that groups of organisms, such as species, change or develop over long periods of time so that their descendants differ from their ancestors morphologically (in form, structure, and physiology) in terms of their life processes, activities, and functions. (Species are the smallest groups into which most living things that share common characteristics are divided. Among the key characteristics that define a species is that its members can breed within the group but not outside it.)
It is important to understand that not all change is considered evolution; evolution encompasses only those changes that are inheritable and may be passed on to the next generation. For example, evolution does not explain why humans are taller and bigger today than they were a century ago. This phenotypic (observable) change is attributable to changes in the environment—that is, improvements in nutrition and medicine—and is not inherited. Similarly, it should also be noted that while evolution leads to increasing complexity, it does not necessarily signify progress because an adaptation, trait, or strategy that is successful at one time may be unsuccessful at another.
In genetic terms evolution can be defined as any change in the gene pool of a population over time or changes in the frequency of alleles in populations of organisms from generation to generation. Evolution requires genetic variation, and the incremental and often uneven changes described by the process of evolution arise in response to an organism's or species' genetic response to environmental influences.
Evidence of evolution has been derived from fossil records, genetics study, and changes observed among organisms over time. The process produces the transformations that generate new species only able to survive if they can respond quickly and favorably enough to environmental changes. Population genetics is the discipline that considers variation and changing ratios of genetic types within populations to explain how populations evolve. Such changes within a population are called microevolution. In contrast, macroevolution describes larger-scale changes that produce entirely new species. Although some researchers speculate that the two processes are different, many scientists believe that macro-evolutionary change is simply the final outcome of the collected effects of microevolution.
Molecular evolution is the term used to describe the period before cellular life developed on Earth. Scientists speculate that specific chemical reactions occurred that created information-containing molecules that contributed to the origin of life on this planet. Theories about molecular evolution presume that these early information-containing molecules were precursors to genetic structures capable of replication (duplication of deoxyribonucleic acid [DNA] by copying specific nucleic acid sequences) and mutation (change in DNA sequence).
Natural selection is a mechanism of evolution. The principles of organic evolution by means of natural selection were described by the English naturalist Charles Darwin. Much of his early research focused on geology, and he developed theories about the origin of different land formations when he went on a five-year expedition around the world aboard the HMS Beagle. During his travels he developed an interest in population diversity.
When Darwin identified twelve different species of finches in the islands of the Galápagos chain off the coast of Ecuador, he speculated that the birds must have descended from a common ancestor even though they differed in terms of beak shape and overall size. The birds became known as "Darwin's finches" and are examples of a process called adaptive radiation, in which species from a common ancestor successfully adapt to their environment via natural selection. Darwin suspected that the finches had become geographically isolated from one another and after years of adapting to their distinctive environments had developed and gradually evolved into separate species incapable of interbreeding.
To explain this occurrence, Darwin relied on his own observations of the existence of variation in and between species, his knowledge of animal breeding, and the results of zoological research conducted by French naturalist Jean Baptiste Lamarck. Lamarck suggested four laws to explain how animal life might change:
- The life force tends to increase the volume of the body and to enlarge its parts.
- New organs can be produced in a body to satisfy a new need.
- Organs develop in proportion to their use.
- Changes that occur in the organs of an animal are transmitted to that animal's progeny.
Darwin famously took issue with this last point, Lamarck's theory of acquired traits, particularly his suggestion that giraffes that make their necks longer by stretching to reach the uppermost leaves on tall trees would then pass on longer necks to their offspring. However, even though Darwin discredited the specifics of Lamarck's theories concerning evolution, he agreed with Lamarck's idea that species changed over time, and he acknowledged Lamarck as an important forerunner and influence on his own work.
Darwin's ideas were also influenced by a 1798 pamphlet, written by Thomas Robert Malthus, titled An Essay on the Principle of Population, as it Effects the Future Improvement of Society. In this work Malthus put forth his hypothesis that unchecked population growth always exceeds the growth of the means of subsistence (the food supply needed to sustain it). In other words, if there were no outside factors stopping population growth, there would inevitably be more people than food. According to Malthus, actual population growth is kept in line with food supply growth by "positive checks," such as starvation and disease, which increase the death rate, and "preventive checks," such as postponement of marriage, which reduce the birthrate. Malthus's hypothesis suggested that actual population always tended to rise above the food supply, but that historically overpopulation had been prevented by wars, famine, and epidemics of disease.
Darwin also knew that farmers had been able to modify species of domestic animals for hundreds of years. Cattle breeders produced breeds that yielded exceptional milk production by mating their best milk producers. Superior egg-laying hens had been bred using the same technique. Because it was possible for farmers to modify a species by artificially selecting those members permitted to reproduce, Darwin hypothesized that nature might have a comparable mechanism for determining which characteristics might be passed on to future generations.
He also realized that while individual organisms in every species had the potential to produce many offspring, the natural population of any species remains relatively constant over time. Darwin concluded that the natural environment acts as a natural selector, determining over long periods of time which variations are best suited to survive and, by virtue of their survival, reproduce and pass on traits and adaptations that improve health and longevity.
Applying the principles of natural selection to the question of giraffes' neck lengths provides an explanation that is different from the one proposed by Lamarck. Less able to obtain food, short-necked giraffes faced starvation. As such, the genes linked to the potential to develop long necks were more likely to be passed to the next generation than the genes for short necks. Over time, the process of natural selection resulted in a population of giraffes with long necks.
Laboratory research and observation also refuted the theory of inheritance of acquired characteristics. When white mice had their tails cut off and were permitted to reproduce, each new generation was born with tails. Children of parents who had suffered amputations or disfiguring accidents did not share their parents' disabilities. Darwin's belief in evolution by natural selection was based on four premises:
- Individuals within a species are variable.
- Some of these variations are passed on to offspring.
- In every generation more offspring are produced than can survive.
- The survival and reproduction of individuals are not random. The individuals who survive and reproduce or reproduce the most are those with the most favorable variations. They are naturally selected.
As support for the theory of acquired inheritance diminished, appreciation of the underlying assumptions for the role of natural selection in evolution grew.
Attacks on Darwin's Theories
Darwin's theories were met with criticism from scientists and members of the clergy. Even some scientists who subscribed to evolutionary theory took issue with the concept of natural selection. Followers of Lamarck, known as Lamarckians, were among the most outspoken opponents of Darwin's theories. This was especially ironic because it was Lamarck's work that had inspired Darwin.
Other objections raised by scientists were related to how poorly inheritance was understood at that time. The notion of blending inheritance was popular. This is the idea that an organism blends together the traits it inherits from its parents. Those who endorsed it observed that, according to Darwin's assumptions, any new variation would mix with existing traits and would no longer exist after several generations. Although Gregor Mendel published a paper in 1866 proposing particulate as opposed to blended inheritance, his theory was not widely accepted until 1900, when it was revisited and confirmed by scientists.
The other objection to Darwin's theory was the argument that variation within species was limited and that, once the existing variation was exhausted, natural selection would cease abruptly. In 1907 the American biologist Thomas Hunt Morgan and his colleagues effectively dispelled this objection. Their experiments with fruit flies demonstrated that new hereditary variation occurs in every generation and in every trait of an organism.
The clergy were even more vociferous adversaries. Darwin's major works, On the Origin of Species by Means of Natural Selection (1859) and The Descent of Man, and Selection in Relation to Sex (1871), were published during a period of heightened religious fervor in England. Many religious leaders were aghast at Darwin's assertion that all life had not been created by God in one fell swoop. Moral outrage and opposition to Darwinian theory persisted into the twentieth century. Although society grew more tolerant and many religions accepted and incorporated evolutionary theory into their beliefs, in the early twenty-first century the debate was revived, as many fundamentalist Christian denominations in the United States became more vocal about their creationist beliefs.
In some instances opponents protested the teaching of evolution in schools and continued to defend creationist theory. In July 1925 a science teacher named John T. Scopes was tried in a Tennessee court for teaching his high school class Darwin's theory of evolution. Scopes had violated the Butler Act, which prohibited teaching evolution theory in public schools in Tennessee. Dubbed the "Monkey Trial" because of the simplified interpretation of Darwin's idea that humans evolved from apes, the courtroom drama pitted the defense attorney Clarence S. Darrow against the prosecutor William Jennings Bryan in a debate that began over the teaching of evolution but became a conflict of deeply held social, intellectual, and religious values. In his acerbic account of the trial proceedings, the American social critic H. L. Mencken wrote:
The Scopes trial, from the start, has been carried on in a manner exactly fitted to the anti-evolution law and the simian imbecility under it. There hasn't been the slightest pretense to decorum. The rustic judge, a candidate for re-election, has postured the yokels like a clown in a ten-cent side show, and almost every word he has uttered has been an undisguised appeal to their prejudices and superstitions…. Darrow has lost this case. It was lost long before he came to Dayton. But it seems to me that he has nevertheless performed a great public service by fighting it to a finish and in a perfectly serious way. Let no one mistake it for comedy, farcical though it may be in all its details. It serves notice on the country that Neanderthal man is organizing in these forlorn backwaters of the land, led by a fanatic, rid of sense and devoid of conscience. Tennessee, challenging him too timorously and too late, now sees its courts converted into camp meetings and its Bill of Rights made a mock of by its sworn officers of the law. There are other States that had better look to their arsenals before the Hun is at their gates.
—"'The Monkey Trial': A Reporter's Account," http://www.law.umkc.edu/faculty/projects/ftrials/scopes/menk.htm
At the end of deliberations, Darrow requested a guilty verdict so that the case could be heard before the Tennessee Supreme Court on appeal. The jury complied, and the presiding judge fined Scopes $100. A year later, however, the Tennessee Supreme Court overturned the verdict on a technicality. Wanting to close the case once and for all, the court dismissed it altogether.
Misuse of Darwin's Theories
After Darwin's theories became well known, some made use of his terminology and concepts to argue that certain groups of human beings were naturally superior to others. The term social Darwinism is used to refer to these ideas, but it is important to note that Darwin himself did not believe in social Darwinism.
One example of social Darwinist thinking would be arguing that the rich and successful members of society are fitter, superior, or in some way more highly evolved than the poor. Social Darwinism has also been used to justify racism and colonialism, as in the nineteenth and early twentieth centuries, when many white Europeans and Americans asserted that they were naturally superior to Africans and Asians and used this claim to justify taking control of their land and resources. Some argued further that it was not just a right but an obligation—the "white man's burden"—for Europeans and Americans to rule over and "civilize" people in less industrialized parts of the world.
None of these social Darwinist theories are scientific in nature, and all are false. Modern genetics has shown that there is no group of human beings that is more evolved or otherwise better than the rest of humanity. Despite having been discredited by scientific research, Social Darwinism continues to be used in attempts to justify various prejudices and inequalities.
MODERN EVOLUTION-CREATION DEBATE
Arguments over the accuracy and importance of Darwin's theories have continued to the present day. Creationists believe the biblical account of the Earth's creation as it appears in the book of Genesis. Some acknowledge microevolution—changes in a species over time in response to natural selection—but they generally do not believe in speciation—that one species can beget or become another over time. There are various gradations of creationist beliefs, but all reject evolution and its argument that the interaction of natural selection and environmental factors explains the diversity of life on Earth.
At the core of the conflict is the observation that evolution threatens the view that human beings have a special place in the universe. Many creationists find it disturbing to contemplate the idea that human existence is a random occurrence, or that universal order is a chance occurrence rather than a response to a divine decree or plan.
The issue remains prominent in present-day society. In the November 2004 issue of National Geographic (http://magma.nationalgeographic.com/ngm/0411/feature1/fulltext.html), David Quammen asks, "Was Darwin Wrong?" Quammen, an award-winning science writer, reports that despite seemingly overwhelming evidence, many Americans believe evolution is simply an unproven speculation rather than an explanatory statement that fits the evidence. Although observation and experiment support evolutionary theory, its apparent contradiction with many religious tenets renders it unacceptable to those Americans who choose to believe that God alone, and not evolution, produced human life on Earth.
Nearly One-Half of Americans Believe Humans Did Not Evolve
Nearly a century and a half after Darwin's publication of On the Origin of Species by Means of Natural Selection, his theory remains highly controversial. Although scientists assert that evolution is well established by scientific evidence, Gallup Organization surveys repeatedly reveal that a substantial portion of Americans do not believe that the theory of evolution best explains the origins of human life.
For example, a 2006 Gallup Poll reveals that 46%—nearly half of the U.S. population—rejects evolution in favor of the belief that humans were created by God approximately 10,000 years ago. (See Figure 3.1.) In another study, "Science Communication: Public Acceptance of Evolution" (Science, August 2006), by Jon D. Miller, Eugenie C. Scott, and Shinji Okamoto, people in the United States and in thirty-two European countries were asked whether they considered the statement, "Human beings, as we know them, developed from earlier species of animals," to be true or false. Of the nations surveyed, the United States had the second-highest percentage of adults who said the statement was false and the second-lowest percentage who said the statement was true. Miller, Scott, and Okamoto conclude that "[t]he acceptance of evolution is lower in the United States than in Japan or Europe, largely because of widespread fundamentalism and the politicization of science in the United States."
Frank Newport of the Gallup Poll reports in "Almost Half of Americans Believe Humans Did Not Evolve" (June 5, 2006) and "American Beliefs: Evolution vs. Bible's Explanation of Human Origins" (March 8, 2006) that analysis of aggregate poll data from 2001 to 2005 characterizes those most likely to believe the biblical explanation of the origin of humans as those with lower levels of education, those who attend church regularly, people aged sixty-five and older, and those who identify with the Republican Party. Figure 3.2 shows that the belief that God created humans in their current form steadily decreases with advancing education. Figure 3.3 reveals that nearly twice as many Americans who attend church regularly believe the biblical explanation as do those who attend church less frequently. Figure 3.4 shows that although the proportion of Americans that accepts the biblical explanation is relatively unchanged among young and middle-aged adults, it rises sharply among adults aged sixty-five and older.
BELIEFS ABOUT EVOLUTION AMONG TEENS
A 2005 Gallup Poll asked teenagers to choose one of three statements as most consistent with their own beliefs about the origins of human life on Earth:
- Humans developed over millions of years, but God guided the process.
- Humans developed over millions of years, but God had no part in the process.
- God created humans pretty much in their present form within the last 10,000 years or so.
As Figure 3.5 shows, 43% of teens believed that God guided human evolution over millions of years; more than a third of teens (38%) thought that God created humans in more or less their present form; and just 18% believed in evolutionary theory without divine guidance.
The Gallup pollsters also asked teens whether or not they believe evolutionary theory is supported by scientific evidence. More than a third (37%) thought evolution is a theory well supported by evidence; 30% believed it is not well supported by evidence; and 33% conceded that they did not know enough to say. (See Figure 3.6.)
Some States Move to Teach Creationism in Public Schools
Because almost 50% of Americans say they do not believe in evolution, it is not surprising that the decision about which theory—evolution or creationism—should be taught in public schools has been hotly contested. The American Institute of Biological Sciences (http://www.aibs.org/public-policy/evolution_state_news.html), a not-for-profit organization dedicated to advancing biological research and education, reports that during 2006 there were school board battles over the issue in twenty-seven states.
Some opponents of evolution propose an alternative known as intelligent design (ID), an explanation that credits intelligence, rather than an undirected process such as natural selection, as the source of life on Earth. Proponents of ID disagree with a basic tenet of evolutionary theory: Darwin's claim that the complex design of biological systems resulted by chance. They contend that direction from an intelligent designer—a supernatural being—is necessary to explain adequately the origins and complexity of life on Earth, particularly human life.
Throughout the United States, school boards have considered whether they want to teach students Darwin's theory, creationism, or alternatives such as ID. In January 2005, eighty years after the Scopes trial, the school board in rural Dover, Pennsylvania, became the first in the nation to require that students be taught that an alternative to evolution exists. A 2005 Gallup Poll found Americans divided about which explanation of the origin of human life should be taught in public school science classes. Table 3.1 shows that 61% believed evolution should be taught, 54% thought creationism should be taught, and 43% wanted ID taught in public school science classes. As of November 2006, no state school board had mandated the teaching of ID exclusively; however, many school boards have moved to include it in the science curricula.
|Public opinion about which explanation of the origin of human life should be taught in public school science classes, 2005|
|ON A DIFFERENT SUBJECT, DO YOU THINK EACH OF THE FOLLOWING EXPLANATIONS ABOUT THE ORIGIN AND DEVELOPMENT OF LIFE ON EARTH SHOULD OR SHOULD NOT BE TAUGHT IN PUBLIC SCHOOL SCIENCE CLASSES, OR ARE YOU UNSURE?|
|2005 Aug 8-11 (sorted by "yes, should")||Yes, should||No, should not||Unsure||No answer|
|Source: "On a different subject, do you think each of the following explanations about the origin and development of life on earth should or should not be taught in public school science classes, or are you unsure? How about—[RANDOM ORDER]?" in Education, The Gallup Organization, October 2005, http://www.galluppoll.com/content/?ci=1612&pg=1 (accessed October 16, 2006). Copyright © 2006 by The Gallup Organization. Reproduced by permission of The Gallup Organization.|
Even some staunch advocates of Darwin's theory do not necessarily wish to exclude the teaching of creationism or ID—they would simply prefer that these alternative explanations be taught in the context of religion classes rather than in science classes. There are even those who advocate both theories. The Vatican has stated that it does not consider evolution to be in conflict with Christian faith. Francis Collins, a committed Christian and director of the Human Genome Institute at the National Institutes of Health, has repeatedly expressed his view that God created the universe and chose the remarkable mechanism of evolution to create plants, animals, and humans.
VARIATION AND ADAPTATION
Effective adaptations and variations are perpetuated in a species and tend to be incorporated into the normal or predominant phenotype for most individuals in the species. Variation persists, but it ranges around an evolutionarily determined norm. This is called adaptive radiation. For example, over time Darwin's finches have developed beaks best suited to their functions. The finches that eat grubs have long, thin beaks to enter holes in the ground and pull out the grubs. Finches that eat buds and fruit have clawlike beaks to grind their food, giving them a survival advantage in environs where buds are the only available food source. In another example of adaptive radiation, present-day giraffes have necks of varying lengths, but most tend to be long.
Ancient humans underwent many evolutionary changes. An example of adaptive radiation in humans is the development and refinement of the upper limbs to perform fine motor skills necessary to make and use complex tools. Another adaptation is the quantity of the pigment melanin present in the skin. People from areas near the equator have more melanin, an adaptation that darkens their skin and protects them from the sun. Anatomical structure also appears to have responded to the environment. Those who thrive in colder climates produce offspring that are shorter and broader than those who live in warmer climates. This adaptive stature, with its relatively low surface area, enables those people to conserve rather than lose body heat. For example, native Alaskans, who are generally of short stature, are well suited to their cold climate.
Natural selection does not produce uniformity or perfection. Instead, it generates variability that persists when it helps a species to adapt to, and thrive in, its environment. It acts on outward appearance (phenotypes), not on internal coding (genotypes), and it is not a process that always discards individual genes in favor of others that might produce traits better suited for survival. For traits attributable to multiple genes, many different combinations of gene pairs may produce the same or comparable phenotypes. Multiple phenotypes may be neutral or even beneficial in terms of survival in a given environment, and there is no reason for such variation to be eliminated by natural selection.
Furthermore, natural selection does not completely or rapidly eliminate genes that produce traits unsuited for adaptation or survival. Even though some individuals with harmful traits die young or do not reproduce, some do reproduce and pass their genes and traits to the next generation. Culling out these genes may require several generations. In other instances, seemingly harmful genes may be retained in the gene pool because there may be circumstances or environments in which their presence would improve survival. For example, the recessive allele that causes sickle-cell diseases (sickle-cell anemia and sickle B-thalassemia, in which the red blood cells contain abnormal hemoglobin) may have had a role in survival in some parts of the world. Although people who are pure recessive for this trait become ill and die prematurely, those who are hybrid for the trait may have retained a survival advantage in areas where malaria is present because people who have the sickle-cell trait or anemia are immune to the effects of malaria. This phenomenon is called balanced polymorphism and is yet another example of the seemingly counterintuitive actions of natural selection. Even though the sickle-cell trait is not beneficial on its own, historically it was advantageous in areas where malaria was a greater threat to survival than sickle-cell diseases.
By definition, natural selection is an unending, continuous process. The popular understanding of natural selection as "survival of the fittest" is somewhat misleading because organisms and species with phenotypes most suited to survive in their environments are not necessarily the "fittest." Present-day examples of natural selection include the evolution of bacteria that are antibiotic resistant and insects that resist extermination with pesticides.
Increasing and Decreasing Genetic Variation
A gene pool comprises the alleles for all the genes in a population. Gene pools in natural populations contain considerable variation, and for evolution to proceed there must be mechanisms to create and increase genetic variation. Mutation, a change in a gene, serves to create or increase genetic variation. Recombination, a process that creates new alleles and new combinations of alleles, also increases genetic variation. Another way for new alleles to enter a gene pool is by migrating from another population. In closely related species new organisms can enter a population, mate within it, and produce fertile hybrids. This action is known as gene flow.
The tendency toward increased genetic variation within a population is balanced by other mechanisms that act to decrease it. For example, some variations have the effect of limiting an organism's ability to reproduce. Any such variations, as well as those incidentally paired with them, will be nonadaptive because they will have a lower probability of being passed to the next generation. Under such circumstances, the process of natural selection serves to reduce genetic variation. In fact, differences in reproductive capability are often called natural selection. Whenever natural selection weeds out nonadaptive alleles by limiting an organism's reproductive capability, it is depleting genetic variation within the population.
Other evolutionary mechanisms contribute to genetic variation. Genetic drift is random change in the genetic composition of a population. It may occur when two groups of a species are separated and as a result cannot reproduce with each other. The gene pool of these groups will naturally differ over time. When the two groups are reunited and reproduce, gene migration occurs as their genetic differences combine, serving to increase genetic diversity. If they remain separate, their genetic differences may become so great that they develop into two separate species.
In small populations genetic drift can cause relatively rapid change because each individual's alleles constitute a large proportion of the gene pool and when an individual does not reproduce, the results are felt more acutely in smaller, rather than larger, populations. When small populations are affected by genetic drift, they may suffer a loss of valuable diversity. For this reason scientists and others involved in conservation, such as zoo curators of endangered species, make every effort to ensure that populations are large enough to withstand the effects of genetic drift.
Variation is the essence of life, and mutations are the source of all genetic variation. A staggering number and variety of alterations, rearrangements, and duplications of genetic material have occurred since the first living cells, in which there is an incredible range of life forms, from amoebas and fruit flies to giant dinosaurs and humans. These dramatically different life forms were all produced using genetic material that was present and reproduced from the first living cells. They resulted from the process of mutation, an alteration in genetic material.
Perhaps because of their negative depiction in science fiction and horror films, there is a widespread common misperception that all mutations are harmful, dramatic, and deleterious. Most mutations are not harmful, and the same limited number of mutations has probably been recurring in each species for millions of years. Many helpful mutations have already been incorporated into the normal genotype through natural selection. Harmful mutations do occur, and while they tend to be eliminated through natural selection, they do recur randomly. It is a mistake, however, to consider mutation as sudden or exclusively harmful. Mutation cannot generate sudden, drastic changes—it requires many generations to generalize throughout a species population. Furthermore, if the changes caused by mutation did not favor survival, natural selection would work against their generalization throughout the species population. In the absence of mutation, there would have been no development of life and no evolution would have occurred.
The impact of mutation varies greatly. Though mutations are changes in genetic material, they do not necessarily affect an individual organism's phenotype. When phenotype is affected, it is because the code for protein synthesis has been changed. Whether mutation will affect phenotype and the extent to which it will be influenced depends on how protein manufacture is affected, when and where the mutation occurs, and the complexity of the genetic controls governing the selected trait.
When a trait is governed by the interaction of many genes, with each exerting about the same influence, the impact of a mutation might be negligible. In traits controlled by a single pair of genes, mutation is likely to exert a much greater influence. For traits governed by the interaction between a major controlling gene pair and several other less influential gene pairs, the location of the mutation will determine its impact. Other considerations such as whether the mutated gene is dominant or recessive also determine whether it will directly act on an individual's phenotype—a mutated recessive gene might not appear in the phenotype for several generations.
Mutation can occur in any cell in an organism's body, but only germinal mutations (those that affect the cells that give rise to sperm or eggs) are passed to the next generation. When mutation occurs in somatic (other than sperm and egg) cells in the body such as muscle, liver, or brain cells, only cells that derive from mitotic division of the affected cell will contain the mutation. Although mutations in somatic cells can cause disease, most do not have a significant impact, because if the mutated gene is not involved in the specialized function of the affected cell, these "silent mutations" will not be detected. Furthermore, mutations that appear only in somatic cells disappear when the organism dies; they are not passed on to subsequent generations and do not enter the gene pool that is the source of genetic variation for the species.
Another reason that many mutations are not expressed in the phenotype is that they affect only one copy of the gene, leaving diploid organisms with an intact copy of the gene. These types of mutations have a recessive inheritance pattern and do not affect phenotype unless an individual inherits two copies of the mutation. There is also the question of the probability of a mutation in a gamete affecting offspring. The mutation may occur in a single gamete and so may only be passed on if that particular gamete is involved in conception. For example, human semen contains more than 50 million sperm per ejaculate, so it is unlikely that a mutation carried by a single sperm will be passed on. When mutation occurs during embryonic (before birth) development and all gametes are affected, there is a greater chance that it might influence the phenotype of future generations. Alternatively, if, like many mutations, the gene defect occurs with advancing age, and the affected individuals are beyond their reproductive years, then it will have no impact on future generations.
How Different Types of Genetic Mutations Occur
Mutation is a normal and fairly frequent occurrence, and the opportunity for a mutation to take place exists every time a cell replicates. In general the cells that divide many times throughout the course of an organism's life—such as skin cells, bone marrow cells, and the cells that line the intestines—are at greater risk for mutation than those that divide less frequently, such as adult brain and muscle cells.
Although DNA nearly always reproduces itself accurately, even a minor alteration produces a mutation that may alter a protein, prevent its production, or have no effect at all. There are five broad classes, and within them many varieties, of mutations, and each is named for the error or action that causes it:
- Point mutations are substitutions, deletions, or insertions in the sequence of DNA bases in a gene. The most common point mutation in mammals is called a base substitution and occurs when an A-T pair replaces a G-C pair. Base substitutions are further classified as either transitions or transversions. Transitions occur when one pyrimidine (C or T) is substituted for the other and one purine (A or G) is substituted on the other strand of DNA. Transversions occur when a purine replaces a pyrimidine. Sickle-cell anemia results from a transversion in which T replaces A in the gene for a component of hemoglobin.
- Structural chromosomal aberrations occur when the DNA in chromosomes is broken. The broken ends may remain loose or join those occurring at another break to form new combinations of genes. When movement of a chromosome section from one chromosome to another takes place, it is called translocation. Translocation between human chromosomes 8 and 21 has been implicated in the development of a specific type of leukemia (cancer of the white blood cells). It has also been shown to cause infertility (inability to sexually reproduce) by hindering the distribution of chromosomes during meiosis.
- Numerical chromosomal aberrations are changes in the number of chromosomes. In a duplication mutation genes are copied, so the new chromosome contains all of its original genes plus the duplicated one. Polyploidy is a numerical chromosomal aberration in which the entire genome has been duplicated and an individual who is normally diploid (having two of each chromosome) becomes tetraploid (containing four of each chromosome). Polyploidy is responsible for the creation of thousands of new species, acting to increase genetic diversity and produce species that are bigger, stronger, and more able to resist disease.
- Aneuploidy refers to occasions when just one or a few chromosomes are involved and describes the loss of a chromosome. Examples of aneuploidy are Down syndrome (multiple, characteristic physical and cognitive disabilities), in which there is an extra chromosome 21—usually caused by an error in cell division called nondisjunction—and Turner's syndrome, in which there is only one X chromosome. Common characteristics of Turner's syndrome include short stature and lack of ovarian development as well as increased risk of cardiovascular problems, kidney and thyroid problems, skeletal disorders such as scoliosis (curvature of the spine) or dislocated hips, and hearing and ear disturbances.
- Transposon-induced mutations involve sections of DNA that copy and insert themselves into new locations on the genome. Transposons usually disrupt and inactivate gene function. In humans selected types of hemophilia have been linked to transposon-induced mutations.
Frequency and Causes of Mutation
In the absence of external environmental influences, mutations occur rarely and are seldom expressed because many forms of mutation are expressed by a recessive allele. The most common naturally occurring mutations arise simply as accidents. Susceptibility to mutation varies during the life cycle of an organism. For example, among humans mutation of egg cells increases with advancing age—the older the mother the more likely she is to carry gametes with mutations. Susceptibility also varies among members of a species such as humans based on their geographic location and ethnic origin.
The mutation rate is the frequency of new mutations per generation in an organism or a species. Mutation rates vary widely from one gene to another within an organism and between organisms. The mutation rate for bacteria is 1 per 100 million genes per generation. Despite this relatively low rate, the enormous number of bacteria—there are more than twenty billion produced in the human intestines each day—translates into millions of new mutations to the bacteria population every day. The human mutation rate is estimated at 1 per 10,000 genes per generation, and human mutation rates are comparable throughout the world. The only exceptions are populations that have been exposed to factors known as mutagens that cause a change in DNA structure and as a result increase the mutation rate. With approximately 25,000 genes, a typical human contains three mutations. Considering the fact that human genes mutate approximately once every 30,000 to 50,000 times they are duplicated, and in view of the complexity of the gene replication process, it is surprising that so few "mistakes" are made.
Gene size, gene base composition, and the organism's capacity to repair DNA damage are closely linked to how many mutations occur and remain in the genome. Larger genes are more susceptible than smaller ones because there are more opportunities and potential sites for mutation. Organisms better able to repair and restore DNA sequences, such as many kinds of yeast and bacteria, will be less likely to have high mutation rates.
The overwhelming majority of human mutations arise in the father, as opposed to the mother. When compared to egg cells, there are more opportunities for mutation during the many cell divisions needed to produce sperm. Sperm are produced late in the life of males, whereas females produce eggs earlier—during embryonic development and are born with their full complement of eggs. Thus, the frequency of mutations that are passed to the next generation increases with parental age.
The mutation rate is partly under genetic control and is strongly influenced by exposure to environmental mutagens. Some mutagens act directly to alter DNA, and others act indirectly, by triggering chemical reactions in the cell that result in breakage of a gene or group of genes. Radiation or ultraviolet light—from natural sources such as the sun or radioactive material in the earth or from manmade sources such as X-rays—can significantly accelerate the rate of mutations.
The link between radiation and mutation was first identified in the 1920s by Hermann Joseph Muller, who was a student of Thomas Hunt Morgan and worked in the famous "Fly Room." Muller discovered that he could increase the mutation rate of fruit flies more than a hundredfold by exposing reproductive cells to high doses of radiation. His pioneering work prompted medical and dental professionals to exercise caution and minimize patients' exposure to radiation. Because mutagens in the reproductive cells are likely to affect heredity, special precautions such as donning a lead apron to block exposure are used when dental X-rays or other diagnostic imaging studies are performed. Similarly, special care is taken to prevent radiation exposure to the embryo or fetus because a mutation during this period of development when cells are rapidly proliferating might be incorporated in many cells and could result in birth defects.
MODERN SYNTHESIS OF EVOLUTIONARY GENETICS
Present-day theories of evolutionary genetics are indebted to Darwin for his groundbreaking descriptions of organisms, individuals, and speciation. Modern theory differs considerably in that it addresses evolutionary mechanisms at the level of populations, genes, and phenotypes and incorporates understanding of actions, such as genetic drift, that Darwin had not considered.
The modern synthesis of evolutionary theory differs from Darwinism by identifying mechanisms of evolution that act in concert with natural selection, such as random genetic drift. It asserts that characteristics are inherited as discrete entities called genes and that variation within a population results from the presence of multiple alleles of a gene. The most controversial tenet of modern evolutionary theory is its contention that speciation is usually the result of small, gradual, and incremental genetic changes. In other words, macroevolution is simply the cumulative effect of microevolution. Some evolutionary biologists have instead embraced the theory of punctuated equilibrium set forth by the paleontologists Niles Eldredge and Stephen Jay Gould in 1972.
Punctuated equilibrium suggests that long periods of stasis (stability) are followed by rapid speciation. It posits that new species arose rapidly during a period of a few thousand years and then remained essentially unchanged for millions of years before the next period of adaptation. Punctuated equilibrium also proposes that change occurred in a small portion of the population, rather than uniformly throughout the population. Although it is different from Darwin's theory of speciation, it is not inconsistent with natural selection; it simply presumes different mechanisms and timetables for the development of new species.