Question

Two theories have been proposed regarding the aging process: the rate-of-living theory and the evolutionary theory of aging. Compare and contrast these two theories. Are there any similarities? What are their differences? Which elements are supported by research, and which elements have been refuted?

Answer 1

Rate of living theory: Postulates that the faster an organisms metabolism, the shorter its lifespan.The theory was originally created by Max Dubner in 1908, after his observation that larger animals outlived smaller ones, and that the larger animals has smaller metabolism. The rate of living theory of aging states living organisms have a finite number of breaths, heartbeats or other measures and that they will die once they have used those up. While the theory is helpful to explain some aspects of aging, it does not really hold up under modern scientific theory.

In ancient times, people believed that just as a machine will begin to deteriorate after a certain number of uses, human body deteriorates in direct proportion to its use. The modern version of this theory recognizes that the number of heartbeats does not predict lifespan. Instead, researchers have focused on the speed at which an organism processes oxygen.In fact, there is little evidence that oxygen metabolism, heartbeat or the number of breaths determine an individual's lifespan. The theory seems to hold up when smaller species with faster metabolisms are compared with larger species with slower metabolisms. The theory can only partially explain the differences in life span between species and it cannot explain the most important factor - what determines lifespan within species.

Evolutionary theory of aging : It explains why mortality rises with age - as individual grow older, less lifetime fertility remains, so continued survival contributes less to reproductive fitness. It also explains the evolution of lower fertility, longer life and increased investments in offspring.

Because aging increases an organism's vulnerability and ultimately leads to its death, it is apparently in contradiction with Darwin's evolutionary theory. Further, how could evolution favour a process that increases mortality and decreases reproductive capacity ? Also how could genes that cause aging evolve ?. The evolutionary theory of aging proposes two models - One derives from Medawar"s ideas in which genetic drift and mutation accumulation lead to loss of late acting beneficial genes or to the appearance of late acting harmful genes. Other models - William's model, aging evolves due to the pleiotropic effect of some genes that are beneficial early in life and then harmful in later ages. At present, both theories are widely accepted and they are not mutually exclusive.