ABSTRACT
All mammals and most other animals undergo senescence as they get older. The reasons for this are complex and several theories have been put forward to explain the evolutionary significance of senescence. The most widely accepted theory suggests that genes causing senescence do not get exposed to natural selection because environmental hazards kill off individuals before the age at which senescence causes decreased fitness. Thus some genes which are advantageous to an animal’s s survival early in its reproductive life but are deleterious in later life are not eliminated.
