Is it possible to program aging (2)
Life span adjustability
Continuation. The beginning of the article is here.
Life expectancy differs not only for different species, in some cases it varies within the species under the influence of external and genetic factors. Therefore, theories of the evolution of aging should explain the regularity of life expectancy. Goldsmith (2012, 2013) argued that the body's ability to precisely regulate its lifespan in response to temporary changes in environmental conditions (for example, a low-calorie diet) is incompatible with theories of unprogrammed aging and, accordingly, testifies in favor of programmed aging. Both publications provide a diagram in which the selection pressure is presented as a function of life expectancy at the current time (Figure 1A). According to Goldsmith, only theories of programmed aging can generate a curve crossing the abscissa (dotted line). Such a model indicates the existence of a value of life expectancy, below which there is a selection pressure aimed at increasing it, and above which there is a decrease in it. However, this statement is erroneous. The theories of disposable soma and antagonistic pleiotropy are based on compromises and therefore give rise to such a model. To illustrate this point of view, Figure 1B demonstrates a clear calculation carried out according to the theory of disposable soma with the substitution of parameter values providing optimal life expectancy corresponding to 8.68. Investing more resources in maintaining the viability of an organism would increase its lifespan, which would simultaneously reduce its evolutionary strength (due to a decrease in fertility) when evaluated using the Malthusian parameter, r. Which, accordingly, would lead to the appearance of selection pressure aimed at reducing the amount of resources invested in maintaining viability, which in turn, it would automatically restore the lifespan to the optimal value.
Figure 1 (A) Selection pressure in the direction of increasing life expectancy as a function of life expectancy at the current time (Goldsmith (2013)). (C) Selection pressure in the direction of increasing life expectancy as a function of average life expectancy at the current time according to the theory of disposable soma (Kirkwood & Rose, 1991). The vertical line corresponds to the optimal value of life expectancy, at which the life-changing selection pressure is zero.
At the same time, the theory of mutation accumulation is not based on compromises and indeed generates a curve corresponding to the dotted line in Figure 1A. Goldsmith states that theories of unprogrammed aging are based on the idea that the pure (without any compromises) evolutionary pressure directed at life and reproduction beyond a certain species-specific age is actually zero. This practically implies that even a slight increase in such pressure would lead to a continuous increase in life expectancy, which is also erroneous. As described above, on an evolutionary scale, there is a constant appearance of destructive genes accumulating in the population to a certain level, regulated by the equilibrium between mutations and selection, indicated as µ/s. Since, according to the theory of accumulation of mutations, the expression of destructive genes is age-specific, its stable level increases with age (since s decreases with aging). Thus, the net effect of the action of two selection pressures – (i) increasing mortality due to destructive genes and (ii) increasing life expectancy because it increases fitness - can provide an increase in age–specific mortality despite maintaining a small selection pressure aimed at increasing life expectancy. This is the very essence of the theory of accumulation of mutations, which can be demonstrated mathematically.
And finally, apparently, Goldsmith believes that only theories of programmed aging allow for the possibility of fine-tuning the lifespan of the body, for example, in the form of a reaction to environmental changes. It is obvious that such a trait can be adaptive and the life-prolonging effect of a low-calorie diet was considered the result of such plasticity that appeared during evolution. The analysis carried out by Shanley and Kirkwood (2001) showed that in the life cycle of a mouse, as in the framework of the theory of disposable catfish, there is a place for adaptive flexibility, providing precise adjustment of investments in life support. In fact, such flexibility is inherent in all characteristics of the life cycle, and the evolution of the optimal aging rate is only one of such characteristics.
Continuation: The ability to develop
Portal "Eternal youth" http://vechnayamolodost.ru 31.10.2016