Aging and Senescence Paper - Focusing on the Distinction and Complex Relationship Between Aging and Senescence, the Evolutionary Theories for Senescence as Well as the Disparities Between Senescence in Different Species

Aging and Senescence Paper - Focusing on the Distinction and Complex Relationship Between Aging and Senescence, the Evolutionary Theories for Senescence as Well as the Disparities Between Senescence in Different Species

People throughout the world are reaching unprecedented ages in history shown by our average life expectancy of 78.2 years. The number of Americans over the age of 65 was 40.3 million on April 1st, 2010, accounting for 13 percent of the total population (Crews 2007). It is projected to be almost double at 88.5 million people by 2050, according to the Census Bureau. The baby boomer generation hitting the age 65 has literally brought about a graying of our society which, of course, brought with it several scientific questions, the ultimate one being “Why do we age?” The question has perplexed generation after generation but now it seems we are on the verge of understanding senescence, the driving mechanism of aging. Scientists have narrowed it down to an evolutionary “trade-off” theory where an organism focuses on early success but faces the consequences later in life (De Loof 2011), but that still does not explain why some species age while others don’t. In this paper, I will focus on the distinction and complex relationship between aging and senescence, the evolutionary theories for senescence as well as the disparities between senescence in different species. Although the research is not complete on this subject, it is through examination of these topics that the reasons behind our aging can be explained.

Before delving in to the research, we must first establish a foundation; what is the difference between aging and senescence? Let’s first start with aging. Aging can be described merely as our passage of time, counted in minutes, days, or years. Because we also say that even rocks and fossils have an “age”, it is now used as a broad term describing a living’s or non-living’s existence through time. The shape of aging (Baudisch 2011), however, is how strongly mortality increases or even decreases as time passes. This is what is now called senescence. It is the biological process of growing older and showing the signs of age. In a more scientific way, it is an event driven process affecting all organisms and bodily systems that results in a loss of reproductive fitness and leads to an increased mortality (Crews 2007). Contrary to every historical belief, senescence is age-independent; we may think that shorter-lived species senescence most quickly and long-lived organisms senescence most slowly, but that would be incorrect (Baudisch 2011). The strength of senescence actually varies between species and ages. Moreover, it is shown that all organisms have extensive repair systems that can heal all kinds of damage and should be able to repair the passage of time (Martins 2011). Why then do some species senescence when others do not? If evolution works that the species that has a distinct disadvantage will be driven to extinction, why do we still age? Research has shown there are a few explanations for this.

Scientists originally thought that there might be a genetic adaptation of aging that provides benefits in early life but damaging in later life, stated in the antagonistic pleiotropy theory (Martins 2011). This theory was proven incorrect as scientists observed no disadvantages to longer living members of a species. In a recent study, great tits, a type of bird, were shown to support strong evidence of the effects of senescence on reproductive success, survival chance, and reproductive value with a relatively low anticipated fitness cost which was dominated mostly by reproductive fitness cost and very little survival fitness cost (Bouwhuis et al. 2012). Then scientists created the disposable soma theory in which species allocate resources in their repair systems to better be able to reproduce early and burn out the functionality of the systems in later life (Geydan et al. 2012). The ecology or environment of a species is crucial because it provides the external causes of death that determine a species’ lifespan, thus creating the need to adapt and evolve internally. As hypothesized by expert scientists, a high age-independent hazard of death should favor the evolution of senescence (Baudisch 2011). This environmentally-caused-mortality causes selection to be weaker on genes that have negative effects later in life than those that have negative effects early in life because this extrinsic mortality will inevitably prevent all members of the species from reaching old age (Carlson et al. 2007). This theory was supported in Carlson’s study of the effect of bear predation on the senescence of salmon. The study also showed evidence for the antagonistic pleiotropy theory by showing that mutations that improve the species fitness early yet harm later in life accumulate within the salmon. However, the disposable soma theory doesn’t apply universally; the American buffalo has almost no age-independent hazards yet ages faster than primates, showing that age-independent hazard is not the only variable in determining the rate of aging. Scientists must consider other variables such as social system, brain size, ability to survive with single vs. multiple disorders, and many more in their results. Scientists proposed two closely tied theories to hopefully fill in the gaps, the kin selection theory, and disease theory. The kin selection theory states that aging, although detrimental to the individual is a benefit to the species as a whole, through a process called kin selection (Martins 2011). The individual ages as to reallocate the resources of the environment to the supposedly more fit descendants. This was shown in Martins study where he used a model to compare a senescent population’s and a non-senescent population’s ability to survive in a changing environment. The aging population survived longer than the non-aging population, showing that when mutation and change are present, the ager population has a better chance to adapt faster and survive better. Aging produces a “pruning” effect, eliminating the individuals that survived by chance. Another form of the kin selection theory, the disease theory states that aging is an adaptation to limit the spread of disease (Mitteldorf and Pepper 2009). The hypothesized benefit is that senescence creates a barrier against infectious epidemics by controlling population density and increasing the diversity of population. Mitteldorf and Pepper argue that if the effects of senescence on individual fitness are disadvantageous, senescence as a form of evolutionary adaptation must be at the group level. Senescence benefits the rate of evolution, increases diversity, and shortens the effective generation time. These evolutionary theories are in the developmental stage but there is substantial evidence to support them. Still, scientists are left with prevalent issues such as the lack of universal accordance to the theories, such as in the case of non-senescent species.

There are some remarkable cases of species disproving all the evolutionary theories of aging, but they occur mostly in modular organisms, or those organisms in which the zygote develops into a discrete unit which then produces more units like itself, rather than developing into a complete organism. These include fungi, sponges, and plants. In one particular study, evidence was found that 300-year old mountain herb, B. pyrenaica, showed no signs of senescence and even showed the opposite, their reproductive fitness increased over time (Garcia et al. 2011). For modular organisms such as plants, it has also been argued that the performance of individuals may improve with age if average size increases with age in a process known as negative senescence. This is also seen in filamentous fungi which are believed to be non-senescent (Geydan et al. 2012). Not only do modular organisms have negligible senescence, some reptiles and fish age very slowly such as turtles, rock fish, and alligators. As I’ve stated before, the ignorance of the disparities between aging and senescence can make scientists believe that long living organisms like turtles don’t experience senescence. Although scientists can tell that different species age differently, they can’t make any claims about their senescence without proper experimentation.

The demographic transition we are facing today is unlike any previous one, with over 50 percent of people reaching their 70th birthday and even more surviving in less-privileged settings (Crews 2007). This unprecedented change in our society has occurred because of our shifted scientific focus to ultimately prolonging life yet we still have not fully explained why we age. Scientists have proposed many answers to this question and, although no one theory is full-proof, together they provide an excellent basis for the evolutionary trade-off theory of aging and senescence. More research in the future will reveal what issues will be resolved and what new issues will arrive.

References
Baudisch, A. 2011. The pace and shape of ageing. Methods in Ecology and Evolution 2:375-382.
Bouwhuis, S., R. Choquet, B. C. Sheldon, and S. Verhulst. 2012. The Forms and Fitness Cost of Senescence: Age-Specific Recapture, Survival, Reproduction, and Reproductive Value in a Wild Bird Population. American Naturalist 179:E15-E27.
Carlson, S. M., R. Hilborn, A. P. Hendry, and T. P. Quinn. 2007. Predation by Bears Drives Senescence in Natural Populations of Salmon. PLoS One 2:e1286.
Crews, D. E. 2007. Senescence, aging, and disease. Journal of Physiological Anthropology 26:365-372.
De Loof, A. 2011. Longevity and aging in insects: Is reproduction costly; cheap; beneficial or irrelevant? A critical evaluation of the "trade-off" concept. Journal of Insect Physiology 57:1-11.
Garcia, M. B., J. P. Dahlgren, and J. Ehrlen. 2011. No evidence of senescence in a 300-year-old mountain herb. Journal of Ecology 99:1424-1430.
Geydan, T. D., A. J. M. Debets, G. J. M. Verkley, and A. D. van Diepeningen. 2012. Correlated evolution of senescence and ephemeral substrate use in the Sordariomycetes. Molecular Ecology 21:2816-2828.
Martins, A. C. R. 2011. Change and Aging Senescence as an Adaptation. PLoS One 6:e24328.
Mitteldorf, J. and J. Pepper. 2009. Senescence as an adaptation to limit the spread of disease. Journal of Theoretical Biology 260:186-195.