Ageing Mechanism Concepts
If organisms purposely limit their life spans via ageing or semelparous behaviour, the associated evolved mechanisms could be very complex just as mechanisms that provide for mentation, vision, digestion, or other biological function are typically very complex. Such a mechanism could involve hormones, signalling, sensing of external conditions, and other complex functions typical of evolved mechanisms. Such complex mechanisms could explain all of the observations of ageing and semelparous behaviours as described below.
It is typical for a given biological function to be controlled by a single mechanism that is capable of sensing the germane conditions and then executing the necessary function. The mechanism signals all the systems and tissues that need to respond to that function by means of organism-wide signals (hormones). If ageing is indeed a biological function we would expect all or most manifestations of ageing to be similarly controlled by a common mechanism. Various observations (listed below) indeed suggest the existence of a common control mechanism.
It is also typical for biological functions to be modulated by or synchronized to external events or conditions. The circadian rhythm and synchronization of mating behaviour to planetary cues are examples. In the case of ageing seen as a biological function, the caloric restriction effect may well be an example of the ageing function being modulated in order to optimize organism life span in response to external conditions. Temporary extension of life span under famine conditions would aid in group survival because extending life span combined with less frequent reproduction would reduce the resources required to maintain a given population.
Theories to the effect that ageing results by default (mutation accumulation) or is an adverse side effect of some other function are logically much more limited and suffer when compared to empirical evidence of complex mechanisms. The choice of ageing theory therefore is logically essentially determined by one's position regarding evolutionary processes and some theorists reject programmed ageing based entirely on evolutionary process considerations.
Read more about this topic: Evolution Of Ageing
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