Question

1. Illustrate and explain the relationship among an organism’s phenotype, environment, and fitness. Outline the specific steps of natural selection. Next, compare and contrast directional, stabilizing, and diversifying selection. Give at least one example of each pattern of natural selection. Finally, list and describe four mechanisms for reducing your risk of becoming infected by antibiotic-resistant bacteria.

2. Define “gene pool.” Next, relate evolution to gene pool and allele frequencies.

Answer 1

1. An organism's phenotype is the collective outcome of its genotype and the environment. Each phenotype has a distinct fitness. Alleles with high fitness are selected and transmitted to the next generations. Deleterious alleles are eliminated from the population.
Natural selection is the driving force of evolution. Alleles that provide a selective advantage to the prevailing environmental conditions are selected and non-adaptive/deleterious alleles are subsequently eliminated from the population. There are three types of natural selection.
1. Stabilizing selection: It is also known as centripetal selection. It operates in a stable environment and eliminates individuals at the two ends of the phenotypic distribution. It preserves individuals with the average phenotypic distribution. It is not a driving force of speciation. However, it eliminates deleterious genotypes from the population. Ex: A greater mortality rate is observed in babies when the birth weight is higher or lesser than the average weight.  
2. Directional selection: It is also known as phyletic evolution or anagenesis. It operates in a gradually changing environment. It eliminates individuals with one end of the phenotypic distribution and preserves individuals with the other/opposite end of the phenotypic distribution. The average value of the phenotype is gradually shifted to either of the ends. It is a good driving force of speciation. Ex: Mosquitoes with high DDT resistance are selected over less resistant strains.
3. Disruptive selection: It is also known as centrifugal selection or adaptive radiation. It operates in heterogeneous environmental conditions. It eliminates individuals with the average phenotypic value and selects individuals with either end of the phenotypic distribution. The phenotypes are moved away from the starting average value. It is a major driving force of speciation. Ex: Evolution of different species of finches on the Galapagos islands.