2.6: Processes and Patterns of Evolution

Natural selection can only take place if there is variation, or differences, among individuals in a population. Importantly, these differences must have some genetic basis; otherwise, the selection will not lead to change in the next generation. This is critical because variation among individuals can be caused by non-genetic reasons such as an individual being taller because of better nutrition rather than different genes.

Genetic diversity in a population comes from two main mechanisms: mutation and sexual reproduction. Mutation, a change in DNA, is the ultimate source of new alleles, or new genetic variation in any population. The genetic changes caused by mutation can have one of three outcomes on the phenotype. A mutation affects the phenotype of the organism in a way that gives it reduced fitness—lower likelihood of survival or fewer offspring. A mutation may produce a phenotype with a beneficial effect on fitness. And, many mutations will also have no effect on the fitness of the phenotype; these are called neutral mutations. Mutations may also have a whole range of effect sizes on the fitness of the organism that expresses them in their phenotype, from a small effect to a great effect. Sexual reproduction also leads to genetic diversity: when two parents reproduce, unique combinations of alleles assemble to produce the unique genotypes and thus phenotypes in each of the offspring.

A heritable trait that helps the survival and reproduction of an organism in its present environment is called an adaptation. Scientists describe groups of organisms becoming adapted to their environment when a change in the range of genetic variation occurs over time that increases or maintains the “fit” of the population to its environment. The webbed feet of platypuses are an adaptation for swimming. The snow leopards’ thick fur is an adaptation for living in the cold. The cheetahs’ fast speed is an adaptation for catching prey.

Whether or not a trait is favorable depends on the environmental conditions at the time. The same traits are not always selected because environmental conditions can change. For example, consider a species of plant that grew in a moist climate and did not need to conserve water. Large leaves were selected because they allowed the plant to obtain more energy from the sun. Large leaves require more water to maintain than small leaves, and the moist environment provided favorable conditions to support large leaves. After thousands of years, the climate changed, and the area no longer had excess water. The direction of natural selection shifted so that plants with small leaves were selected because those populations were able to conserve water to survive the new environmental conditions.

The evolution of species has resulted in enormous variation in form and function. Sometimes, evolution gives rise to groups of organisms that become tremendously different from each other. When two species evolve in diverse directions from a common point, it is called divergent evolution. Such divergent evolution can be seen in the forms of the reproductive organs of flowering plants which share the same basic anatomies; however, they can look very different as a result of selection in different physical environments and adaptation to different kinds of pollinators.

Can divergence occur if no physical barriers are in place to separate individuals who continue to live and reproduce in the same habitat? The answer is yes. The process of speciation within the same space is called sympatric speciation; the prefix “sym” means same, so “sympatric” means “same homeland” in contrast to “allopatric” meaning “other homeland.” A number of mechanisms for sympatric speciation have been proposed and studied.

Real World Example of Natural Selection

Bacteria are living things and therefore evolve just like all other life on Earth. One serious problem facing humans is antibiotic resistance, or when a person either does not take antibiotic medications properly or overuses these medications. If a person does not finish the entire course of medication, only the weakest strains of the bacteria, therefore leaving the stronger strains to survive and multiply. The next generation of bacteria will then not be killed by that medication, making it resistant. These strains are called superbugs and they demonstrate natural selection because the strains best adapted to survive the medication are the ones that
live and reproduce.

By the mid 1940s, penicillin was the treatment of choice for Staphylococcus aureus (S. aureus), a human pathogen that can cause life-threatening infections of skin, blood, bone, heart, and other vital organs; S. aureus resistance to penicillin rapidly evolved in the 1950s. Over the next few decades, resistance to methicillin, which replaced penicillin as the treatment of choice for S. aureus infections, has also emerged. S. aureus strains resistant to the antibiotic are known as methicillin-resistant S. aureus or MRSA.

Antibiotics and other antimicrobial drugs first became widely used in the World War II era, and have saved countless lives and blunted serious complications of many feared diseases and infections. However, some microbes have developed ways to circumvent the effects of antimicrobials. Antimicrobial resistance provides a survival benefit to microbes, making it harder to eliminate infections from the body.

Other diseases including tuberculosis (TB), gonorrhea, malaria, and childhood ear infections are increasingly more difficult to treat due to the emergence of resistance.

Approximately 1.7 million patients in the United States get an infection in the hospital each year, about 99,000 of whom will die as a result. Seventy percent of the bacteria causing such infections are resistant to at least one drug commonly used to treat these infections.

https://report.nih.gov/NIHfactsheets...t.aspx?csid=26

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