The variation which is beneficial is decided by the ecological niche of the organism. It is necessary for the survival of species as only then will the species have the adaptability to survive in rapidly changing external conditions. Any variation which is suitable for the particular niche is selected by nature during its process of evolution. This is spread over several generations. Clearly, what can be seen is that at the species level there is an abundance of time for variation to show its impact. However, at the individual level, this is not so, because of its short lifespan. The individual does not get time to evolve because of changes in the external environment. The only thing that one notices is cellular ageing. This is why is variation beneficial to the species but not necessarily for the individuals.