Culturing Mammalian Tissues for Cytogenetic Investigations

Biologists often tend to assume that natural selection acts fundamentally on individuals or on species. It is now appreciated that this is an oversimplification. Present understanding of the unit of natural selection, with its pivotal importance in evolution, largely derives from a consideration of the problem of why altruism exists. This paper reviews evidence for and against the theories that evolution acts essentially on genes, on individuals, on kin, or on larger groups.