When natural selection takes place on a population that has single-gene traits, it may affect the allele frequencies and cause some changes in the frequency of phenotypes. For example, the gene for the lizard’s color has two alleles: green or brown color.

When a predator is introduced into the environment, the green lizards are often eaten since they are more visible than the brown lizards when they are perched on tree branches. As a result, the population of green lizards declines, Eventually, the frequency of the allele for green color will decrease and the frequency of allele for brown color would increase. This would result in a higher population of brown lizards on the next generation.

Due to the isolation of the smaller population among the original and larger population, such as the bottleneck or the founder effect, a genetic drift may happen by chance. If the small population carries an allele that may reproduce more offspring than the other group, this kind of allele will become common in a particular population.

Genetic equilibrium is a condition that is characterized by constant allele frequencies in a particular population. Here are the five conditions that would help maintain the genetic equilibrium:
1. individual organisms must mate with others randomly
2. large population
3. no migration to inhibit low gene flow
4. no mutation in the genes
5. low genetic variability to inhibit natural selection

In directional selection, nature favors the individuals at one end of the curve. These individuals have a higher fitness when compared to the others. On the other hand, disruptive selection takes place when two extreme traits or phenotypes are favored by nature, while the average type has a difficulty in adapting to the new condition. This is shown by a curve that has two peaks.

The answer is no. In real life, if any of the 5 conditions that are stated in the Hardy-Weinberg Principle, which include nonrandom mating, small population size, migration, mutations, and natural selection, would take place, the genetic equilibrium is automatically disrupted. Therefore, species are always evolving.

When the antibiotic is introduced to a bacterial community, it kills most of the bacteria due to natural selection. In this process, the stronger species are favored by nature. This is the reason why diversity plays a crucial role in the survival of species. The genetic variability of the bacteria enables them to acquire a higher fitness, which gives them the ability to survive and reproduce in order to continue the existence of their species.

In some cases, bacteria may have genetic mutations which allow them to resist the effects of an antibiotic. This leads to their survival and enables them to transfer the genetic information into the next generation. As a result, the bacteria would evolve and develop defenses that would make them resistant to the drug.