Final answer:
Unsuccessful genes are generally lost because organisms with these genes tend to die before reproducing, reducing their incidence in the gene pool. This is due to natural selection, which favors genes that increase survival and reproductive success. Over time, this leads to the elimination of harmful genes from the population.
Step-by-step explanation:
Unsuccessful genes are eventually lost primarily because the organisms that carry them are more likely to die before they can reproduce. Natural selection drives this process; it favors individuals with traits that provide a survival advantage in their specific environment. These advantageous genes are then passed down to the next generation. Meanwhile, individuals with disadvantageous, or unsuccessful genes, often have lower survival and reproductive success. Consequently, these harmful genes are less likely to be passed on and can eventually be eliminated from the gene pool.
Natural selection is not about the survival of the 'fittest' in terms of sheer strength or fitness, but rather the fit between an organism's traits and its environment. Sometimes, less advantageous traits might propagate if they are linked with more beneficial genes due to linkage disequilibrium. However, over many generations, traits that lower an organism's overall fitness are less likely to be represented in the population due to reduced reproductive success.
New traits that arise via mutation or evolution can become prevalent if they confer a favorable adaptation. However, because natural selection acts at the level of the individual, a mixture of both beneficial and harmful alleles can exist within an organism, thus influencing the organism's overall fitness and the fate of individual alleles in the gene pool.