NATURAL SELECTION AND EVOLUTION

NATURAL SELECTION AND EVOLUTION

A.    The Role Of Natural Selection

Evolution is the continuous genetic adaptation of a population of organisms to its environment over time. Evolution results when there are changes in genes present in a population. There are three factors that interact to determine how a species changes over time: environmental factors that affect organisms, sexual reproduction among the individuals in the gene pool, and the generation of genetic variety within the gene pool.

The idea that some individuals whose gene combinations favor life in their surroundings will be most likely to survive, reproduce, and pass their genes on to the next generation is known as the theory of natural selection. The theory of natural selection was first proposed by Charles Darwin and Alfred Wallace and was clearly set forth in 1859 by Darwin in his book On the Origin of Species by Means of Natural Selection, or the Preservation of Favored Races in the Struggle for Life

B.     What Influences Natural Selection ?

Spontaneous mutations are changes in DNA that cannot be tied to a particular causative agent. It is suspected that cosmic radiation or naturally occurring mutagenic chemicals might be the cause of many of these mutations. It is knownthat subjecting organisms to high levels of radiation or to certain chemicals increases the rate at which mutationsoccur. In order for mutations to be important in the evolution of organisms, they must be in cells that will become gametes. Mutations to the cells of the skin or liver will only affect those specific cells and will not be passed on to the next generation.

A second very important process involved in generating genetic variety is sexual reproduction. Although sexual reproduction does not generate new genetic information the way mutations do, it allows for the recombination of genes into mixtures that did not occur previously. Each individual entering a population by sexual reproduction carries a unique combination of genes; approximately half donated by the mother and half donated by the father. During meiosis, variety is generated in the gametes through crossing-over between homologous chromosomes and independent assortment of nonhomologous chromosomes. This results in millions of possible combinations of genes 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. The gene mixing that occurs during sexual reproduction is known as genetic recombination. The new individual has a complete set of genes that is different from that of any other organism that ever existed.

The importance of generating new gene combinations is particularly important because the way genes express themselves in an individual can depend on the other genes present. Genes don’t always express themselves in the same way. In order 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 genes expressing themselves to different degrees in different individuals. Often the reason for this difference is unknown. Penetrance is a term used to describe how often an allele expresses itself when present. Expressivity is a term used to describe situations in which the gene expresses itself but not equally in all individuals that have it.

Whenever a successful organism is examined, it can be shown that it reproduces at a rate in excess of that necessary to merely replace the parents when they die. 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 different individuals that allows natural selection to take place. In fact, to maintain itself in an ever-changing environment, each species must change in ways that enhance its ability to adapt to its new environment. For this to occur, members of the population must be eliminated in a non-random manner. Those individuals that survive are those that are, for the most part, better suited to the environment than other individuals. They reproduce more of their kind and transmit more of their genes to the next generation than do individuals with genes that do not allow them to be well adapted to the environment in which they live.

C.    Common Misunderstandings About Natural Selection

There are several common misinterpretations associated with the process of natural selection. The first involves the phrase “survival of the fittest.” Individual survival is certainly important because those that do not survive will not reproduce. But the more important factor is the number of descendants an organism leaves.

Second, the phrase “struggle for life” does not necessarily refer to open conflict and fighting. It is usually much more subtle than that. When a resource such as nesting material, water, sunlight, or food is in short supply, some individuals survive and reproduce more effectively than others.

A third common misunderstanding involves significance of phenotypic characteristics that are not caused by genes. Many organisms survive because they have characteristics that are not genetically determined. The acquired characteristics are gained during the life of the organism; they are not genetically determined and, therefore, cannot be passed on to future generations through sexual reproduction. Therefore, acquired characteristics are not important to the processes of natural selection.

D.    Proccess That Drive Natural Selection

Differential Survival

As stated previously, the phrase “survival of the fittest” is often associated with the theory of natural selection. Although this is recognized as an oversimplification of the concept, survival is an important factor in influencing the flow of genes to subsequent generations. If a population consists of a large number of genetically and phenotypically different individuals it is likely that some of them will possess characteristics that make their survival difficult. Therefore, they are likely to die early in life and not have an opportunity to pass their genes on to the next generation.

Differential Reproductive Rates

Survival alone does not always ensure reproductive success. For a variety of reasons, some organisms may be better able to utilize available resources to produce offspring. If one individual leaves 100 offspring and another leaves only 2, the first organism has passed more copies of its genetic information information on to the next generation than has the second. If we assume that all 102 individual offspring have similar survival rates, the first organism has been selected for and its genes have become more common in the subsequent population.

            Differential Mate Selection

Within animal populations, some individuals may be chosen as mates more n others. This is called “sexual selection.” Obviously, those that are frequently chosen have an opportunity to pass on more copies of their genes than those that are rarely chosen. Characteristics of the more frequently chosen individuals may involve general characteristics, such as body size or aggressiveness, or specific conspicuous characteristics attractive to the opposite sex.

E.     Gene-Frequency Studies and Hardy-Weinberg Equilibrium

In the early 1900s an English mathematician, G. H. Hardy, and a German physician, Wilhelm Weinberg, recognized that it was possible to apply a simple mathematical relationship to the study of gene frequencies. Their basic idea was that if certain conditions existed, gene frequencies would remain constant, and that the distribution of genotypes could be described by the relationship A2 + 2Aa + a2 = 1, where A2 represents the frequency of the homozygous dominant genotype, 2Aa represents the frequency of the heterozygous genotype, and a2 represents the frequency of the homozygous recessive genotype. Constant gene frequencies over several generations would imply that evolution is not taking place. Changing gene frequencies would indicate that evolution is taking place.

The conditions necessary for gene frequencies to remain constant are:

1. Mating must be completely random.

2. Mutations must not occur.

3. Migration of individual organisms into and out of the population must not occur.

4. The population must be very large.

5. All genes must have an equal chance of being passed on to the next generation. (Natural selection is not occurring.)

The concept that gene frequencies will remain constant if these five conditions are met has become known as the Hardy-Weinberg concept. The Hardy-Weinberg concept is important because it allows a simple comparison of allele frequency to indicate if genetic changes are occurring. Two different populations of the same organism can be compared to see if they have the same allele frequencies, or populations can be examined at different times to see if allele frequencies are changing.

F.     A Summary Of The Causes Of Evolutionary Changes

Evolution was described as the change in gene frequency over time. We can now see that several different mechanisms operate to bring about this change. Mutations can either change one allele into another or introduce an entirely new piece of genetic information into the population. Immigration can introduce new genetic information if the entering organisms have unique genes. Emigration and death remove genes from the gene pool. Natural selection systematically filters some genes from the population, allowing other genes to remain and become more common. The primary mechanisms involved in natural selection are differences in deathrates, reproductive rates, and the rate at which individuals are selected as mates. In addition, gene frequencies are more easily changed in small populations because events such as death, immigration, emigration, and mutation can have a greater impact on a small population than on a large population.