For the Hardy-Weinberg equation to be true, the population in question must be very large. This ensures that coincidental occurrences do not drastically alter allelic frequencies.

Using the Hardy-Weinberg equilibrium equations, you can determine the answer.

The value of  gives us the frequency of the dominant allele, while the value of  gives us the frequency of the recessive allele. The second equation corresponds to genotypes.  is the homozygous dominant frequency,  is the heterozygous frequency, and  is the homozygous recessive frequency.

16% of the population shows the recessive phenotype, and therefore must carry the homozygous recessive genotype. We can use this information to solve for the recessive allele frequency.

We can use the value of  and the first Hardy-Weinberg equation to solve for .

Knowing both  and , you can use the second equation to find the percent of heterozygous organisms in the population.

Genetic drift specifically refers to the change of allele frequencies because of random sampling of gametes. Essentially, this induces genetic bias for particular alleles and can lead to speciation events simply by the chance event of certain gametes producing offspring rather than others.

Changes in mutation frequencies, random mating, and natural selection may lead to changes in allele frequencies, but they are not necessarily the cause of genetic drift. 

Genetic drift occurs when a gene's frequency is changed in a population due to pure chance. Consider a rock rolling down a hill and crushing all flowers that have white petals. The population will now have only red petals because the white ones were destroyed. Were the red petal flowers more suited to their environment? No, it just so happened that all of the white were removed from the gene pool. This shows that evolution can occur due to random chance as well as natural selection, and both forces can have an impact on a population.

It is always difficult to rephrase "survival of the fittest" in some new, clever way. The flowers which BY CHANCE have developed a different color, pattern, or odor that better attracts pollinators are indeed more likely to experience reproductive success and pass on these genes to their offspring. Competing plants might do well for a while, but they are already disfavored, and further environmental changes may put them even more at risk (or have no effect, or again favor them over the presently more attractive plants).

The horse choice is an example of intentional breeding—artificial selection.

The storm option does not imply any condition in any of the plants which conferred an advantage against freezing to death, or even any difference between species of plants; it is more akin to a question about mass extinction than to one about evolution.

The giraffe choice relates to the Lamarckian fallacy of being able to pass on acquired characteristics; species that are more successful just plain "luck out" relative to environmental stresses.  

The bacterial response discusses a mutation without likely survival implications for the bacterium.

Vertebrates have adapted to terrestial living due to their ability to maintain water inside their bodies, despite no longer being immersed in water. The loop of Henle in the nephrons is designed to concentrate urine, reabsorbing water without unnecessarily excreting it. The longer the loops descend into the medulla, the more concentrated the urine becomes. Shorter loops would not concentrate urine as much, and thus would not contribute to a vertebrate's adaptation to land-based life.

Internal lungs, impermeable skin, and internal fertilization would all protect vital processes from interference by the external environment.

In this scenario, the two extreme phenotypes are selected for, while intermediate phenotypes are selected against. This is disruptive natural selection. Over time, disruptive selection results in a decreased frequency of "middle" phenotypes and an increased frequency of the two groups at the extreme ends. This is a process that can eventually lead to speciation.

The opposite is process stabilizing selection, in which the extreme variations are selected against in favor of more "average" phenotypes. Directional selection occurs when only one end of the spectrum is favored, such as sharp teeth but not dull teeth. Artificial selection involves human intervention in selecting desirable traits. Vestigial selection is not a type of natural selection.

Large size is favored in males and small size is favored in females, but intermediate size is always disfavored. The result is an increase in the two extreme phenotypes (either large or small) and a decrease in the average phenotype. This type of trend is known as disruptive selection.

Stabilizing selection occurs when the extreme phenotypes are disfavored, and the average or intermediate phenotype is preferable. Directional selection occurs when only one extreme phenotype is advantageous, for example if only large felines were able to find mates. Genetic drift is the phenomenon by which the allele frequencies of a population change by chance, due to independent assortment or other random events. The bottleneck effect occurs when an outside event, such as disease or natural disaster, diminishes the original population such that the allele frequencies of the new population differ from those of the original.

An organism's evolutionary "fitness" depends on its ability to reproduce and create viable offspring, or contribute its genes to future generations.

Even if an organism is in perfect health, it is considered to have very low fitness if it cannot produce viable offspring. In an evolutionary sense, the perseverence of certain genes in a population defines the favorability of those genes. An increased prevalence of certain genes can be interpreted as evolution. The activities of a single individual (aside from reproductive viability) are relatively ineffective in determining its ability to pass on its genes to future generations.

The higher the taxonomic group, the less similar the members are. This is true for appearance, behavior, and genetics. The order of taxonomic groupings, from most general to most specific is: kingdom, phylum, class, order, family, genus, species.

Of the given answers, phyla are the highest taxonomic rank. Species of different phyla would show the greatest genetic difference. In contrast, genera are the lowest taxonomic rank of the given answers; species of the same genus would show the least genetic difference.