CONCEPTS IN BIOLOGY
PART IV. EVOLUTION AND ECOLOGY
13. Evolution and Natural Selection
13.5. What Influences Natural Selection?
Now that you have a basic understanding of how natural selection works, we can look in more detail at the factors that influence it. Genetic diversity within a species, the degree of genetic expression, and the ability of most species to reproduce excess offspring all exert an influence on the process of natural selection.
The Mechanisms That Affect Genetic Diversity
For natural selection to occur there must be genetic differences among the individuals of an interbreeding population of organisms. Consider what happens in a population of genetically identical organisms. In this case, it does not matter which individuals reproduce, because the same genes will be passed on to the next generation and natural selection cannot occur. However, when genetic differences exist among individuals in a population and these differences affect fitness, natural selection can take place. Therefore, it is important to identify the processes that generate genetic diversity within a population. Genetic diversity within a population is generated by the mutation and migration of organisms and by sexual reproduction and genetic recombination.
Mutation and Migration
Spontaneous mutations are changes in DNA that cannot be tied to a particular factor. Mutations may alter existing genes, resulting in the introduction of entirely new genetic information into a gene pool. It is suspected that cosmic radiation or naturally occurring mutagenic chemicals might be the cause of many of these mutations. Subjecting organisms to high levels of radiation or to certain chemicals increases the rate at which mutations occur. It is for this reason that people who are exposed to mutagenic chemicals or radiation take special safety precautions.
Naturally occurring mutation rates are low. The odds of a gene mutating are on the order of 1 in 100,000. Most of these mutations are harmful. Rarely does a mutation occur that is actually helpful. However, in populations of millions of individuals, each of whom has thousands of genes, over thousands of generations it is quite possible that a new, beneficial piece of genetic information will come about as a result of mutation. Remember that every allele originated as a modification of a previously existing piece of DNA. For example, the allele for blue eyes may be a mutated brown-eye allele, or blond hair may have originated as a mutated brown-hair allele. In a species such as corn (Zea mays), there are many different alleles for seed color. Each probably originated as a mutation. Thus, mutations have been very important in introducing genetic material into species over time.
For mutations to be important in the evolution of organisms, they must be in cells that give rise to gametes (eggs or sperm). Mutations in other cells, such as those in the skin or liver, will affect only those cells and will not be passed on to the next generation.
Recall that migration is another way in which new genetic material can enter a population. When individuals migrate into a population from some other population, they may bring alleles that were rare or absent. Similarly, when individuals leave a population, they can remove certain alleles from the population.
Sexual Reproduction and Genetic Recombination
Sexual reproduction is important in generating new genetic combinations in individuals. Although sexual reproduction does not generate new genetic information, it does allow for the mixing of genes into combinations that did not occur previously. Each individual entering a population by sexual reproduction carries a unique combination of genes—half donated by the mother and half donated by the father. During meiosis, unique combinations of alleles are generated in the gametes through crossing-over between homologous chromosomes and the independent assortment of nonhomologous chromosomes. This results in millions of possible genetic combinations in the gametes of any individual. When fertilization occurs, one of the millions of possible sperm unites with one of the millions of possible eggs, resulting in a genetically unique individual. This genetic mixing that occurs as a result of meiosis and fertilization is known as genetic recombination.
The new individual has a set of genes that is different from that of any other organism that ever existed. When genetic recombination occurs, a new combination of alleles may give its bearer a selective advantage, leading to greater reproductive success.
Organisms that primarily use asexual reproduction do not benefit from genetic recombination. In most cases, however, when their life history is studied closely, it is apparent that they also can reproduce sexually at certain times. Organisms that reproduce exclusively by asexual methods are not able to generate new genetic combinations but still acquire new genetic information through mutation.
As scientists have learned more about the nature of species, it has become clear that genes can be moved from one organism to another that was considered to be a different species. In some cases, it appears that whole genomes can be added when the cells of two different species combine into one cell. This process of interspecific hybridization is another method by which a species can have new genes enter its population.
The Role of Gene Expression
Even when genes are present, they do not always express themselves in the same way. For genes to be selected for or against, they must be expressed in the phenotype of the individuals possessing them. There are many cases of genetic characteristics being expressed to different degrees in different individuals. Often, the reason for this difference in expression is unknown.
Degrees of Expression
Penetrance is a term used to describe how often an allele expresses itself. Some alleles have 100% penetrance; others express themselves only 80% of the time. For example, there is a dominant allele that causes people to have a stiff little finger. The expression of this trait results in the tendons being attached to the bones of the finger in such a way that the finger does not flex properly. This dominant allele does not express itself in every person who contains it; occasionally, parents who do not show the characteristic in their phenotype have children that show the characteristic. Expressivity is a term used to describe situations in which an allele is not expressed equally in all individuals who have it. An example of expressivity involves a dominant allele for six fingers or toes, a condition known as polydactyly. Some people with this allele have an extra finger on each hand; some have an extra finger on only one hand. Furthermore, some sixth fingers are well-formed with normal bones, whereas others are fleshy structures that lack bones.
Why Some Genes May Avoid Natural Selection
There are many reasons a specific allele may not feel the effects of natural selection. Some genetic characteristics can be expressed only during specific periods in the life of an organism. If an organism dies before the characteristic is expressed, it never has the opportunity to contribute to the overall fitness of the organism. Say, for example, a tree has genes for producing very attractive fruit. The attractive fruit is important because animals select the fruit for food and distribute the seeds as they travel. However, if the tree dies before it can reproduce, the characteristic may never be expressed. By contrast, genes such as those that contribute to heart disease or cancer usually have their effect late in a person’s life. Because they were not expressed during the person’s reproductive years, they were not selected against, because the person reproduced before the effects of the gene were apparent. Therefore, such genes are less likely to be selected against (eliminated from the population) than are those that express themselves early in life.
In addition, many genes require an environmental trigger to be expressed (i.e., they are epigenetic). If the trigger is not encountered, the gene never expresses itself. It is becoming clear that many kinds of human cancers are caused by the presence of genes that require an environmental trigger. Therefore, we try to identify triggers and prevent these negative genes from being turned on and causing disease.
When both dominant and recessive alleles are present for a characteristic, the recessive alleles must be present in a homozygous condition before they have an opportunity to express themselves. For example, the allele for albinism is recessive. There are people who carry this recessive allele but never express it, because it is masked by the dominant allele for normal pigmentation (figure 13.5).
Some genes have their expression hidden because the action of a completely unrelated gene is required before they can express themselves. The albino individual shown in figure 13.5 has alleles for dark skin and hair that will never have a chance to express themselves because of the presence of two alleles for albinism. The alleles for dark skin and hair can express themselves only if the person has the ability to produce pigment, and albinos lack that ability.
FIGURE 13.5. Gene Expression
Genes must be expressed to allow the environment to select for or against them. The recessive allele c for albinism shows itself only in individuals who are homozygous for the recessive characteristic. The man in this photo is an albino who has the genotype cc. The characteristic is absent in those who are homozygous dominant and is hidden in those who are heterozygous. The dark-skinned individuals could be either Cc or CC. However, because the albino individual cannot produce pigment, characteristics for dark skin and dark hair cannot be expressed.
Natural Selection Works on the Total Phenotype
Just because an organism has a “good” gene does not guarantee that it will be passed on. The organism may also have “bad” genes in combination with the good, and the “good” characteristics may be overshadowed by the “bad” characteristics. All individuals produced by sexual reproduction probably have certain genetic characteristics that are extremely valuable for survival and others that are less valuable or harmful. However, natural selection operates on the total phenotype of the organism. Therefore, it is the combination of characteristics that is evaluated—not each characteristic individually. For example, fruit flies may show resistance to insecticides or lack of it, may have well-formed or shriveled wings, and may exhibit normal vision or blindness. An individual with insecticide resistance, shriveled wings, and normal vision has two good characteristics and one negative one, but it would not be as successful as an individual with insecticide resistance, normal wings, and normal vision.
The Importance of Excess Reproduction
A successful organism reproduces at a rate in excess of that necessary to merely replace the parents when they die (figure 13.6). For example, geese have a life span of about 10 years; on average, a single pair can raise a brood of about eight young each year. If these two parent birds and all their offspring were to survive and reproduce at this rate for a 10-year period, there would be a total of 19,531,250 birds in the family.
FIGURE 13.6. Reproductive Potential
The ability of a population to reproduce greatly exceeds the number necessary to replace those who die. Here are some examples of the prodigious reproductive abilities of some species.
However, the size of goose populations and most other populations remains relatively constant over time. Minor changes in number may occur but, if the environment remains constant, a population does not experience dramatic increases in size. A high death rate tends to offset the high reproductive rate and population size remains stable. But this is not a “static population.” Although the total number of organisms in the species may remain constant, the individuals that make up the population change. It is this extravagant reproduction that provides the large surplus of genetically unique individuals that allows natural selection to take place.
If there are many genetically unique individuals within a population, it is highly probable that some individuals will survive to reproduce even if the environment changes somewhat, although the gene frequency of the population may be changed to some degree. For this to occur, members of the population must be eliminated in a non-random manner. Even if they are not eliminated, some may have greater reproductive success than others. The individuals with the greatest reproductive success will have more of their genetic information present in the next generation than will those that die or do not reproduce very successfully. Those that are the most successful at reproducing are those that are, for the most part, better suited to the environment.
13.5. CONCEPT REVIEW
8. What factors can contribute to diversity in the gene pool?
9. Why is over-reproduction necessary for evolution?
10. Why is sexual reproduction important to the process of natural selection?
11. How might a harmful allele remain in a gene pool for generations without being eliminated by natural selection?