CONCEPTS IN BIOLOGY

PART IV. EVOLUTION AND ECOLOGY

13. Evolution and Natural Selection

13.3. The Role of Natural Selection in Evolution

Natural selection is a primary process that brings about evolution by selecting which individuals will survive, reproduce, and pass on their genes to the next generation. These processes do not affect genes directly but do so indirectly by selecting individuals for success based on the characteristics an individual displays. Recall that the characteristics displayed by an organism (phenotype) are related to the genes possessed by the organism (genotype). By affecting the reproductive success of individuals, natural selection affects allele frequencies within the population. That change in allele frequency is evolution.

Three factors work together to determine how a species changes over time: environmental factors that affect organisms, sexual reproduction among the individuals in the gene pool, and the amount of genetic diversity within the gene pool. In general, the reproductive success of any individual within a population is determined by how well an individual’s characteristics match the demands of the environment in which it lives. Fitness is the success of an organism in passing on its genes to the next generation, compared with other members of its population. Just because an organism reproduces doesn’t make it “fit.” It can be fit only in comparison with others. Individuals whose characteristics enable them to survive and reproduce better than others in their environment have greater fitness (Outlooks 13.2).

Genetic diversity is important because a large gene pool with great genetic diversity is more likely to contain genetic combinations that allow some individuals to adapt to a changing environment. The characteristics of an organism are not just its structural traits. Behavioral, biochemical, and metabolic traits are also important. Scientists often use behavior, DNA differences, and other chemical differences to assess evolutionary relationships among existing organisms. However, when looking at extinct species, scientists are usually confined to using structural characteristics to guide their thinking.

OUTLOOKS 13.2

Genetic Diversity and Health Care

People turn to their healthcare providers when they experience a medical problem, whether it is the result of an accident, infection, or some abnormality. In many large cities, the emergency rooms (ERs) of large hospitals have become a substitute for a visit to a physician's office or a neighborhood clinic. Medical facilities are thought of as places where everyone always gets better and no one gets sick. However, 2 million people a year get bacterial infections while they are being treated in hospitals as patients. An estimated 90,000 people die from these infections each year.

Many people do not realize that the hospital is a place where patients who have not been able to have their infections resolved by home care bring all of those nasty microbes. Studies have shown that the farther away you are from the hospital, the less dangerous the microbes. What makes this situation worse is the fact that these bacteria are undergoing genetic changes and becoming more unbeatable. Populations of hospital microbes contain mutations that protect them from specific antibiotics— that is, they are antibiotic resistant. If resistant microbes are transmitted, the infected person will find the infection even harder to control. For example, methicillin-resistant Staphylococcus aureus (MRSA) was responsible for over 94,000 potentially fatal infections and nearly 19,000 deaths in the United States in 2005. Eighty-five percent of these deaths were associated with healthcare settings.

13.3. CONCEPT REVIEW

4. Define natural selection.

5. What is fitness, and how is it related to reproduction?