CONCEPTS IN BIOLOGY

PART IV. EVOLUTION AND ECOLOGY

12. Diversity Within Species and Population Genetics

12.5. Genetic Diversity in Domesticated Plants and Animals

Humans often work with small, select populations of plants and animals in order to artificially construct specific genetic combinations that are useful or desirable. This is true of plants and animals used for food. If we can produce domesticated animals and plants with genetic characteristics for rapid growth, high reproductive capacity, resistance to disease, and other desirable characteristics, we can supply ourselves with energy in the form of food. Several processes are used to develop such specialized populations of plants and animals. Most have the side effect of reducing genetic diversity.

Cloning

Recall that cloning is the process of reproducing organisms asexually, so that large numbers of genetically identical individuals are produced. These individuals are called clones.

Plants are easy to work with in this manner, because we can often increase the numbers of specific organisms by asexual (without sex) reproduction. Potatoes, apple trees, strawberries, and many other plants can be reproduced by simply cutting the original plant into a number of parts and allowing these parts to sprout roots, stems, and leaves. If a single potato has certain desirable characteristics, it can be reproduced asexually. All of the individual potato plants reproduced asexually would be genetically identical and would show the same desired characteristics. Figure 12.9 shows how a clone can be developed.

FIGURE 12.9. Cloning Plants from Cuttings

All the Peperomia houseplants in the last figure (e) were produced asexually from cuttings and are identical genetically. (a-c) The original plant is cut into pieces. Then, (d) the cut ends are treated with a growth stimulant and placed in moist soil. (e) Eventually, the pieces root and grow into independent plants.

The cloning of most kinds of domesticated animals is much more difficult than the cloning of plants. Even though this is not as yet a practical method of producing animals, in recent years, many kinds of animals have been cloned (table 12.2). The process involves the substitution of a nucleus from a mature animal for the nucleus of an egg. This cell is then stimulated to develop as an embryo. There is a high rate of failure, but the goals of animal cloning are the same as those of plant cloning: the production of genetically identical individuals.

TABLE 12.2. Cloned Animals

Selective Breeding

Humans can bring together specific genetic combinations in either plants or animals by selective breeding. Because sexual reproduction tends to generate new genetic combinations rather than preserve desirable combinations, the mating of individual organisms must be controlled to obtain the desirable combination of characteristics. Selective breeding involves the careful selection of individuals with specific desirable characteristics and their controlled mating, with the goal of producing a population that has a high proportion of individuals with the desired characteristics.

Through selective breeding, some varieties of chickens have been developed that grow rapidly and are good for meat. Others have been developed to produce large numbers of eggs. Often, the development of new varieties of domesticated animals and plants involves the crossing of individuals from different populations. For this technique to be effective, the desirable characteristics in each of the two varieties should have homozygous genotypes. In small, controlled populations, it is relatively easy to produce individuals that are homozygous for a specific trait. To make two characteristics homozygous in the same individual is more difficult. Therefore, such varieties are usually developed by crossing two different populations to collect several desirable characteristics in one organism. Intraspecific hybrids are organisms that are produced by the controlled breeding of separate varieties of the same species.

Occasionally, interspecific hybrids—hybrids between two species—are produced as a way of introducing desirable characteristics into a domesticated organism. Because plants can be reproduced by cloning, it is possible to produce an interspecific hybrid and then reproduce it by cloning. For instance, the tangelo is an interspecific hybrid between a tangerine and a grapefruit. An interspecific hybrid between cattle and the American bison was used to introduce certain desirable characteristics into cattle.

Genetic Engineering

In recent years, scientific advances in understanding DNA have allowed specific pieces of genetic material to be inserted into cells. This has greatly expanded scientists’ ability to modify the characteristics of domesticated plants and animals. The primary goal of genetic engineering is to manipulate particular pieces of DNA and transfer them into specific host organisms, so that they have certain valuable characteristics. These topics were dealt with in greater detail in chapter 11.

The Impact of Monoculture

Although some of the previously mentioned techniques have been used to introduce new genetic information into domesticated organisms, one of the goals of domestication is to produce organisms that have uniform characteristics. In order to achieve such uniformity, it is necessary to reduce genetic diversity. Agricultural plants have been extremely specialized through selective breeding to produce the qualities that growers want. Most agriculture in the world is based on extensive plantings of the same varieties of a species over large expanses of land. This agricultural practice is called monoculture (figure 12.10). It is certainly easier to manage fields in which only one kind of plant is growing, especially when herbicides, insecticides, and fertilizers are tailored to meet the needs of specific crop species. However, with monoculture comes a significant risk. Because these organisms are so similar, if a new disease comes along, most of them will be affected in the same way and the whole population may be killed or severely damaged.

FIGURE 12.10. Monoculture

This wheat field is an example of monoculture, a kind of agriculture in which large areas are exclusively planted with a single crop with a very specific genetic makeup. Monoculture makes it possible to use large farm machinery, but it also creates conditions that can encourage the spread of disease because the plants have reduced genetic diversity.

Our primary food plants and domesticated animal species are derived from wild ancestors that had genetic combinations that allowed them to compete successfully with other organisms in their environment. When humans reduce genetic diversity by developing special populations with certain desirable characteristics, other valuable genetic information is lost from the gene pool. When we select specific, good characteristics, we often get harmful ones along with them. Therefore, these “special” plants and animals require constant attention. Insecticides, herbicides, cultivation, and irrigation are all used to aid the plants and animals we need. In effect, these plants are able to live only under conditions that people carefully maintain (figure 12.11).

Because our domesticated organisms are so genetically similar, there is a great danger that an environmental change or new disease could cause great damage to our ability to produce food. In order to protect against such disasters, gene banks have been established. Gene banks consist of populations of primitive ancestors of modern domesticated plants and animals (figure 12.12). By preserving these organisms, their genetic diversity is available for introduction into our domesticated plants and animals if the need arises.

FIGURE 12.11. Cash Crops Require Constant Attention

The photograph shows a portion of a plantation where native forest has been cleared for the planting of bananas. However, without constant attention, native rainforest plants encroach back into the bananas.

FIGURE 12.12. The Banking of Genes

Plant growers and breeders send genetic material, such as seeds, to the National Seed Storage Laboratory in Ft. Collins, Colorado, where it is stored at extremely cold temperatures to prevent deterioration. Gene banks will play an ever-increasing role in preserving biodiversity as the rate of extinction increases.

12.5. CONCEPT REVIEW

12. How do the genetic combinations in clones and sexually reproducing populations differ?

13. How is a clone developed? What are its benefits and drawbacks?

14. How is an intraspecific hybrid formed? What are its benefits and drawbacks?

15. Why is genetic diversity in domesticated plants and animals reduced?