Intro to Biology

Compare and contrast directional selection and disruptive selection, and provide an example of each. Both of these show the distribution of phenotypes. In directional selection, the distribution of phenotypes forms a “bell curve.” Selection against one of the extreme phenotypes causes the distribution to move in one direction or the other. An example might be plants whose flower color is determined by incomplete dominance: white, pink, or red. Pink flowers may be the average phenotype, but if we start to remove red flowers from the population, the “mean” phenotype will be shifted toward white flowers. In disruptive selection, the average phenotype is selected against. This produces a “two-humped” bell-type curve, and the greater distribution is split between the two phenotype extremes. If we have the same type of incomplete dominance as mentioned in the previous paragraph, assume that the pink flowers are selected against. This means that the two humps shown in the distribution will be centered on the red and white phenotypes. Many pathogenic bacteria species are becoming resistant to antibiotics.

Explain how such adaptations can develop through the process of natural selection. (Hint: Relate this example to the conditions that are necessary for natural selection to occur.) Antibiotic resistance is the ability of a microorganism to withstand the effects of an antibiotic. It is a specific type of drug resistance. Antibiotic resistance evolves naturally via natural selection through random mutation, but it could also be engineered by applying an evolutionary stress on a population. Antibiotic resistance is a consequence of evolution via natural selection. The antibiotic action is an environmental pressure; those bacteria which have a mutation allowing them to survive will live on to reproduce. They will then pass this trait to their offspring, which will be a fully resistant generation. Several studies have demonstrated that patterns of antibiotic usage greatly affect the number of resistant organisms which develop. What are the major evolutionary trends that developed among major vertebrate groups, specifically those that allowed for the transition from aquatic to terrestrial life?