Individuals maximize their own individual reproductive potential by matching reproductive effort to available resources. Ridley explains conditions under which group selection, carefully defined, could occur. These conditions could in principle be met. However, it seems unlikely that the resulting group selection would be strong enough to override individual selection. The level in the hierarchy of levels of organization that will evolve adaptations is controlled by which level shows heritability.
Selection depends on heritability.
Genes and individuals do have heritable differences. Ordinary groups, such as flocks of birds, do not often seem to have heritable differences. Another sense of 'unit of selection' is the entity whose frequency is adjusted directly by natural selection. The argument here is largely semantic, over how words should be used. One conclusion, favored by many competent biologists, is that with the most careful usage, the only unit of selection is the gene .