Final answer:
Mobile genetic elements can affect an organism's fitness and may spread through populations even if they are not beneficial due to various evolutionary mechanisms.
Step-by-step explanation:
Mobile genetic elements can have various impacts on the organisms which carry them. They can be beneficial, neutral, or deleterious to an individual's fitness. Natural selection is a key process that can influence the spread or elimination of these elements within a population. If a mutation confers a phenotype that improves an individual's survival or reproductive success, it is likely to become more frequent in the population over time. Conversely, mutations that are harmful may be quickly removed from the gene pool, especially if they are dominant.
However, not all mobile genetic elements will neatly fit into this scenario. Some elements may spread through a population even if they lower the fitness of the individuals. They may do so through mechanisms such as linking themselves with beneficial alleles, or they may persist in the genome at low frequencies due to a balance between mutation and selection. Additionally, mobile genetic elements can affect protein coding regions, potentially having significant effects on fitness. Therefore, the incorrect options are that they will be eliminated unless beneficial and that they do not affect fitness.
So the final answer is that mobile genetic elements may spread through populations even if they lower the fitness of individuals that carry them. This can occur through different means, including the possible linkage with beneficial traits, or simply existing at a balance point where they are not fully eliminated by natural selection.