The Case Study of Antibiotic Resistance and Evolutionary Change In February of 2008, a 20-year-old student at Western Washington University died from pneumonia despite being previously healthy. The cause of death was methicillin-resistant Staphylococcus aureus (MRSA), a bacterium that has evolved resistance to many common antibiotics. While Staphylococcus aureus, or "staph," is a common bacterium found on the skin, in the blood, or in the respiratory system, MRSA represents a strain that is increasingly difficult to treat. Historically, MRSA infections occurred mainly in hospitals, but today more than 10% of infections occur in schools, homes, and workplaces. In the United States, MRSA kills approximately 19,000 people annually. Staph is not the only threat; antibiotic resistance has appeared in bacteria causing tuberculosis (killing nearly 2,000,000 people per year), gonorrhea, food poisoning, meningitis, and urinary tract infections. This global onslaught of "supergerms" is a consequence of evolutionary change in bacterial populations driven by natural selection imposed by medical drugs. Understanding the mechanisms of evolution is essential to combatting this crisis by using strategies like reducing antibiotic use. # How Populations, Genes, and Evolution are Related Evolutionary changes do not occur within the lifetime of an individual organism. For example, a pet's fur becoming thicker in winter is not evolution. Instead, evolution occurs from generation to generation, causing descendants to differ from ancestors. It is a property of populations, which are defined as groups including all members of a species living in a given area. While individuals have fates and actions, inheritance provides the link between those lives and the evolution of the population. Traits are determined by the interaction between genes and the environment. A gene is a segment of DNA at a specific chromosomal location. Different versions of a gene are called alleles. An individual whose alleles for a particular gene are the same is homozygous (e.g., BB or bb), while an individual with different alleles is heterozygous (e.g., Bb). The genotype (specific alleles) interacts with the environment to produce the phenotype (physical or behavioral traits). For instance, in hamsters, a dominant black allele (B) codes for an enzyme producing black pigment, while a recessive brown allele (b) codes for an enzyme that lacks this function. Only hamsters with the bb genotype exhibit brown coats. # Population Genetics and the Gene Pool Population genetics is the study of the frequency, distribution, and inheritance of alleles within populations. A central concept is the gene pool, which is the set of all alleles of all genes from all individuals in a population. It is a mental construct, often described as an imaginary bucket containing every allele copy. The number of copies of a specific allele in the gene pool is the sum of individuals with one copy and twice the number of individuals with two copies. Allele frequency refers to an allele's relative proportion in the gene pool. If a population of 25 hamsters has 50 total alleles for coat color and 20 are for black coats, the frequency of the black allele is 20/50=0.40 (or 40%). From this perspective, evolution is defined as the change of allele frequencies within a gene pool over generations. # The Hardy-Weinberg Principle and Evolutionary Equilibrium In 108, Godfrey H. Hardy and Wilhelm Weinberg developed a mathematical model of a non-evolving population, known as the Hardy-Weinberg principle. It states that allele and genotype frequencies remain constant if specific conditions are met. This hypothetical state is called equilibrium population. The five conditions required for this state are: (1) No mutation; (2) No gene flow (no movement of alleles in or out); (3) The population must be very large; (4) All mating must be random; and (5) No natural selection (all genotypes reproduce equally). Natural populations rarely meet all these conditions, but the principle serves as a baseline to understand the forces driving evolution. In Greater Depth, the proportion of individuals with genotypes A1A1, A1A2, and A2A2 can be calculated where p is the frequency of A1 and q is the frequency of A2. The equation is p2+2pq+q2=1. For example, if p=0.7 and q=0.3, then genotype proportions are p2=0.49 (49%), 2pq=0.42 (42%), and q2=0.09 (9%). # Mutations as the Original Source of Genetic Variability Evolution remains at equilibrium only if no DNA changes occur. Mutations usually happen during DNA replication in cell division. While cellular repair systems correct most errors, unrepaired mutations in gamete-producing cells enter the gene pool. Inherited mutations are rare; a typical human gene version appears in approximately 1 out of every 100,000 gametes, or 1 in every 50,000 newborns. Despite this rarity, the cumulative effect is significant because humans have 20,000 to 25,000 genes (40,000 to 50,000 alleles), meaning most newborns carry one or two new mutations. Mutations are not goal-directed; they do not arise because of an organism's needs. They simple provide the potential for change, which environment-dependent natural selection then acts upon. In bacteria, resistance mutations seem common due to massive population sizes (e.g., 150 million bacteria in one drop of saliva) and short generation times (as fast as every 15 minutes). # Gene Flow, Genetic Drift, and Population Size Gene flow is the movement of alleles between populations via interbreeding. Examples include juvenile male baboons leaving the troop and flowering plants spreading pollen or seeds. Gene flow increases the genetic similarity between populations. If blocked, genetic differences may lead to new species. Genetic drift is the process by which chance events change allele frequencies, such as seeds falling into a parking lot or a hailstorm destroying flowers. Genetic drift has a larger impact on small populations. In a small population, random removal of a few individuals can eliminate an entire allele. Computer simulations show that in a population of 20,000, allele frequencies remain stable, but in a population of 8, an allele can reach 100% or 0% frequency within a few generations. # Population Bottlenecks and the Founder Effect A population bottleneck occurs when a catastrophe or overhunting drastically reduces a population to a few individuals. This reduces genetic variability significantly, as seen in Northern elephant seals. Hunted to near extinction by the 1890s, only about 20 seals survived. Although they have increased to 30,000 individuals, they are genetically almost identical, leaving them vulnerable to environment changes. The founder effect occurs when a small group founds an isolated colony. The Amish of Lancaster County, Pennsylvania, descended from approximately 200 immigrants, have a high frequency of Ellis-van Creveld syndrome (short limbs, extra fingers, heart defects) because one founding couple carried the allele. This initial high frequency, combined with genetic drift, led to the prevalence of the syndrome. # Nonrandom Mating and Reproductive Success Organisms seldom mate randomly. Inbreeding, or mating between relatives, increases the proportion of individuals homozygous for many genes, which can expose harmful recessive alleles. Assortative mating occurs when individuals choose mates similar to themselves, such as snow geese preferring partners of the same color phase (white or blue-gray). Neither process changes allele frequencies alone but they alter genotype distribution. Natural selection arises because all genotypes are not equally beneficial. As Alfred Russel Wallace noted, individuals with "some little superiority" are favored. Natural selection is the process where individuals with traits helping them survive and reproduce leave more offspring. Fitness is the measure of an individual's lifetime reproductive success. Selection acts on phenotypes (the displayed traits), which indirectly affects genotypes because the two are tied. # Forces of Selection: Competition, Predation, and Coevolution Selection stems from an environment's nonliving and living components. Competition for scarce resources is most intense among members of the same species because they have near-identical requirements. Selection also arises from predation, where predators consume prey. This often leads to coevolution, a biological arms race where species mutually affect each other's evolution. For example, wolves select for faster deer, and alert deer select for faster wolves. Natural selection is not progressive; it does not make organisms "better," only better adapted to the current environment. This is evident in antibiotic resistance: resistant bacteria are favored only when antibiotics are present and may be at a disadvantage when they are not. # Sexual Selection and Patterns of Evolution Sexual selection acts on traits that help an animal acquire a mate, even if those traits hinder survival, such as the peacock's conspicuous tail or bighorn sheep antlers. This is driven by male-male competition for access to females or female preference for specific male traits. The healthy-male hypothesis suggests elaborate ornaments are outward signs of vigor. Selection influences populations in three ways: (1) Directional selection favors an extreme trait value (e.g., thicker fur during an Ice Age or antibiotic resistance); (2) Stabilizing selection favors the average value and acts against extremes (e.g., intermediate body size in Aristelliger lizards to balance territory defense and owl predation); (3) Disruptive selection favors both extremes and acts against the average (e.g., black-bellied seedcrackers with either large beaks for hard seeds or small beaks for soft seeds). # Balanced Polymorphism and Malaria Resistance Balanced polymorphism occurs when two or more phenotypes are maintained in a population, often because each is favored by different environmental factors. A notable example is sickle-cell anemia in Africa. Being homozygous for the sickle-cell allele causes terminal illness, yet the allele persists because heterozygotes (carrying one normal and one sickle-cell allele) have increased resistance to malaria. In areas where malaria is a high risk, the survival advantage of heterozygotes preserves both alleles in the gene pool. # Questions & Discussion The text includes specific experimental questions. Regarding the streptomycin experiment (Figure 15−3): If mutations occurred in response to antibiotics rather than spontaneously, the surviving colonies would have appeared in random, different positions in each dish rather than the exact same positions. Regarding bighorn sheep (Figure 15−10): If reproduction success was tracked, the difference in the number of offspring would likely be greater for males than for females, because a single dominant male can monopolize many matings while less successful males may have zero. Regarding directional selection limits (Figure 15−12): There is a limit to how extreme a trait can become because eventually, the survival costs of the extreme trait (e.g., excessive size) will outweigh the reproductive benefits, or biological constraints (such as nutrient requirements) will be reached. # Why Antibiotics Cannot Cure a Cold Antibiotics work by inhibiting bacterial metabolic pathways, such as DNA replication, protein synthesis, or cell wall building. Because colds and flus are caused by viruses, which lack metabolism, antibiotics are ineffective against them. Misuse of antibiotics in humans and livestock (over 29 million pounds fed to farm animals in the U.S. annually) has accelerated the evolution of resistant strains, which are now ubiquitous in soil, water, and food supplies.