Final answer:
Mobile genetic elements can spread through populations even if they decrease the fitness of their carriers due to factors beyond natural selection, such as genetic drift.
Step-by-step explanation:
Regarding mobile genetic elements, it is true that they may spread through populations even if they lower the fitness of the individuals that carry them. Although many think that only beneficial traits are preserved through natural selection, this is not entirely the case. Mobile genetic elements can propagate even if they have deleterious effects, due to factors like genetic drift or linkage to beneficial traits. Beneficial mutations may increase an individual's fitness and spread rapidly through a population. However, deleterious and neutral mutations can still persist due to various evolutionary factors, including mutation rate and genetic drift.