12. Diversity Within Species and Population Genetics


12.3. How Genetic Diversity Comes About


Genetic diversity is a term used to describe genetic differences among members of a population. High genetic diversity indicates many different kinds of alleles for each characteristic, and low genetic diversity indicates that nearly all the individuals in the population have the same alleles. A large gene pool with high genetic diversity is more likely to contain some genetic combinations that will allow the organisms to adapt to a new environment; whereas low genetic diversity can have devastating consequences. A number of mechanisms introduce genetic diversity into a population.



Mutations introduce new genetic information into a population by modifying alleles that are already present. Sometimes, a mutation introduces a new allele into the gene pool of a species. At other times, a mutation may introduce an allele that was absent in a local population, although it is present in other populations of the species. All the different alleles for a trait originated as a result of mutations some time in the past and have been maintained within the gene pool of the species as they have been passed from generation to generation during reproduction. Many mutations are harmful, but very rarely one will occur that is valuable to the organism. If a mutation produces a harmful allele, the allele remains uncommon in the population. For example, the Anopheles mosquito is responsible for transmitting malaria in many African countries. At some point in the past, mutations occurred in the DNA of these mosquitoes that made some individuals tolerant to the insecticide Pyrethrin, even before the chemical had been used. These alleles remained very rare in these insect populations until Pyrethrin was used. Then, these alleles became very valuable to the mosquitoes that carried them. Because the mosquitoes that lacked the alleles for tolerance died when they came into contact with Pyrethrin, more of the Pyrethrin-tolerant individuals were left to reproduce the species; therefore, the Pyrethrin- tolerant alleles became much more common in these populations. Scientists have recently found up to 90% Pyrethrin resistance in Anopheles mosquitoes that live in several African countries.




Sexual Reproduction

Although the process of sexual reproduction does not create new alleles, it tends to generate new genetic combinations when the genetic information from two individuals mixes during fertilization, generating a unique individual. This doesn’t directly change the frequency of alleles within the gene pool. However, the new member may have a unique combination of characteristics. This combination may be so superior to those of other members of the population that the new member will be much more successful in producing offspring. In a corn population, for example, there may be alleles for resistance to corn blight (a fungal disease) and to attack by insects. Corn plants that possess both of these characteristics will be more successful than corn plants that have only one of these qualities. They will probably produce more offspring (corn seeds) than the others, because they will survive both fungal and insect attacks. Thus, there will be a change in the allele frequency for these characteristics in future generations.




The migration of individuals from one genetically distinct population to another is also an important way for alleles to be added to or subtracted from a local population. Whenever an organism leaves one population and enters another, it subtracts its genetic information from the population it left and adds it to the population it joins. If it contains rare alleles, it may significantly affect the allele frequency of both populations. The extent of migration need not be great; however, as long as alleles are entering or leaving a population, the gene pool will change.

Many animal populations in zoos are in danger of dying out because of severe inbreeding or line breeding (breeding with near relatives), resulting in reduced genetic diversity (figure 12.5). Often, when genetic diversity is reduced, deleterious recessive alleles in closely related mates are passed to offspring in a homozygous state, resulting in offspring that have reduced chances of survival. Most zoo managers have recognized the importance of increasing genetic diversity in their small populations of animals and have instituted programs of loaning breeding animals to distant zoos in an effort to increase genetic diversity. In effect, they are attempting to simulate the natural migration that frequently introduces new alleles from distant populations. Captive breeding program have been critical in saving many species, including Guam rails, California condors, Przewalski’s horses, scimitar-horned oryx, Partula snails, and Spix’s macaws.




FIGURE 12.5. Captive Breeding of the Black-Footed Ferret

In October 1985, the Wyoming Game and Fish Department, in cooperation with the U.S. Fish & Wildlife Service, started the captive breeding program for North America’s most endangered mammal, the black-footed ferret. Attention was paid to making sure that as much genetic variation was retained as possible—for example, they maintained a sperm bank of particularly valuable males. As a result, the successful return of black-footed ferrets to the plains of the American West began in 1991. The total wild population of black-footed ferrets in 2007 was over 750 individuals in the United States. Biologists hope that a new population census will show a further increase in wild blackfooted ferrets. However, biologists still fear that a lack of genetic diversity may jeopardize the populations.


The Importance of Population Size

The size of the population has much to do with how effective any mechanism is at generating diversity within a gene pool. The smaller the population, the less genetic diversity it can contain. Therefore, migrations, mutations, and accidental death can have great effects on the genetic makeup of a small population. For example, if a town has a population of 20 people and only 2 have brown eyes and the rest have blue eyes, what happens to those 2 brown-eyed people is more critical than if the town has 20,000 people and 2,000 have brown eyes. Although the ratio of brown eyes to blue eyes is the same in both cases, even a small change in a population of 20 could significantly change the frequency of the brown-eye allele. Often, in small populations, random events can significantly alter the gene pool when rare alleles are lost from the population. This process is called genetic drift because the changes are not caused by selection (figure 12.6). This idea will be discussed in greater detail in chapter 13.



FIGURE 12.6. Genetic Drift

The gene pool of a small population may not have the same proportion of alleles as the previous generation. Notice that in the original population, the red frogs were eliminated and failed to breed. Therefore, their genes were not passed on to the next generation. As a result, the frequencies of the genes change in the gene pool.



8. Why can there be greater genetic diversity within a gene pool than in an individual organism?

9. List three mechanisms that contribute to genetic diversity.