Balancing selection keeps multiple alleles in a population by different mechanisms. In overdominance, the heterozygote genotype has higher fitness than either homozygote, so both alleles are maintained at stable frequencies because offspring with the heterozygous combination are favored. This directly preserves genetic variation since neither allele is lost. In frequency-dependent selection, an allele’s fitness depends on how common it is in the population. When an allele is rare, it has a fitness advantage; as it becomes more frequent, its advantage diminishes. This negative frequency dependence prevents any one allele from sweeping to fixation and can maintain a polymorphism or even cause cycles in allele frequencies over time. So, overdominance maintains both alleles via heterozygote advantage, while frequency-dependent selection maintains alleles because their fitness changes with how common or rare they are.

Disasters are best understood as events rooted in natural Earth processes—geological forces such as earthquakes, landslides, and volcanic eruptions, and meteorological extremes like storms, floods, and droughts. This view captures the broad range of events that cause harm and disruption, which is why describing disasters as geological or meteorological occurrences is the most accurate. Human factors can amplify impacts, and SAMPLEsome disasters involve biological aspects, but the fundamental nature lies in natural processes rather than being caused solely by human error, being entirely unpredictable, or being purely biological.

Selection changes allele frequencies by weighting each genotype by its fitness and then normalizing by the population’s average fitness. Before selection, genotype frequencies are p^2, 2pq, and q^2. After selection, survivors occur in proportions p^2 w_AA, 2pq w_Aa, and q^2 w_aa, and the mean fitness is Wbar = p^2 w_AA + 2pq w_Aa + q^2 w_aa. The frequency of the A allele among survivors is the weighted count of A alleles: p' = [p^2 w_AA + pq w_Aa] / Wbar, since AA contributes two A alleles and Aa contributes one A allele. This gives the correct expression for p' after selection.

The main idea is that in an emergency, you need a clear path to exit. Keeping exits unobstructed lets people evacuate quickly and safely and prevents trips or bottlenecks as they move toward safety. Removing boxes or other objects that block exit routes is the best action because it immediately opens the way for everyone to get out and for responders to reach the area if needed. Leaving things in doorways or in front of exits creates chokepoints and can trap people, dramatically increasing risk. So the correct approach is to remove anything blocking exits to keep those routes open.