Levels of organization
Life can be divided into a series of levels of organization, from nucleotide to gene, through cell, organ, and organism, to social group, species, and higher levels. Which, if any, of these levels does natural selection act on and produce adaptations for the benefit of?
An adaptation which benefits one level doesn't always benefit the others:
The levels of organization, from gene through individual to species, are to a large extent bound together in their evolutionary fate, and what benefits one level will usually also benefit the others. However, this is not always so.
Male lions can only join a pride by forcibly evicting the incumbent males. In the fight, lions may get killed or wounded. These fights have losers as well as winners: here the benefit of winning is confined to the individual level and below; the lion species does not benefit. The survival of the species may be little affected by the death of male lions, because the mating system is polygamous and has plenty of males to spare; but the effect is clearly not positive.
Different adaptations, therefore, have different consequences for different units in nature. Does natural selection act to benefit all levels, or does it work to benefit one particular level? If it benefits one level, which is it? In evolutionary biology, this question is expressed as 'What is the unit of selection?'
When a lion hunt is successful, who, or rather what biological entity, benefits?