The units of selection - Adaptation benefits . . . what exactly?
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?
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 elephants fight among themselves for access to females (pictured opposite). In the fight, elephants 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 elephant species does not benefit. The survival of the species may be little affected by the death of male elephants, because the mating system is polygynous and has plenty of males to spare; but the effect is clearly not positive.
The unit of selection
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?'