Aging Biological Changes - Assignment Example

Aging”, Deboleena Rakshit, Academia Research, April 12, 2008 Aging: Aging is a process of, “irreversible biological changes that occur in all living things with the passage of time, eventually resulting in death” (“Aging”. Microsoft Encarta Encyclopedia Deluxe 2000.) In the evolutionary sense, it means a “progressive decline in fitness (the ability to survive and reproduce) with increasing age.” (Partridge, L. 2001. Evolutionary theories of ageing applied to long-lived organisms.) It is basically caused due to the inability of an organism to combat the usual ‘wear-and-tear’ of tissues with increasing age. Symptoms of Aging in Humans, Animals and Plants: There are certain symptoms of old age that show in all humans, irrespective of gender, racial or geographical affiliation. These include decline in hearing and sight; deteriorating immune system causing weaker resistance to diseases; weakening of muscles; reduction in flexibility of soft tissues and so on. Other visible signs of aging are wrinkled skin, graying of hair or hair loss, loss of vision, hypermetropia, hot flashes (women) and impotence. Symptoms of aging in animals resemble those in humans. Certain symptoms however differ, according to their evolutionary processes and mental ability. For example, an aged lion, which is no longer a contributing member of the pack (i.e. has been ousted from his leadership role by a younger, healthier specimen and is incapable of further reproduction), voluntarily isolates himself and brings on his own death. This is to minimize wastage of resources and to make way for more viable populations. But, humans and elephants (due to their more complex social evolution) take care of their aged members, thereby lengthening the aging process. Thus, they display longer lifespans. This is a direct outcome of evolutionary processes of aging. (This is an evidence for the disposable soma theory, mentioned later) “There are also many species of plants that show clear signs of senescence. Some, such as bamboo, reproduce, age, and die at well defined times indicating a pre-programmed mechanism. Many species of plants also show a slow aging rate. Examples are well known in agriculture such as apple, orange, and other fruit trees.” (http://www.senescence.info/nature.html) Why these symptoms are shown: Taking the example of loss of hair. It is usually observed at a later age, because at old age, hair follicles lose their ability to re-grow. Also, loss of memory, delayed reactions and insomnia generally affect older individuals due to the gradual loss of brain tissues, which are not replaced by newer cells. Another common symptom of old age is joint-aches. This happens because the cartilage becomes cracked and frayed, the cushioning fluid gets thinner, and the tendons and ligaments become less resilient. (http://universe-review.ca/R10-27-ageing.htm#causes). Also, hormonal imbalances at a later age, cause undesirable effects. Menopause in women, for example, is caused due to reduced levels of estrogen in the body. Some other symptoms shown are the decrease in cell count of WBCs and RBCs, increased risk of heart attacks due to arteriosclerosis, reduced efficiency of bladder as the connective tissues weaken and hence a lower capacity of storing and emptying urine. Reasons for Aging: Scientists have not yet determined the exact cause of aging, although there are several theories regarding it. In 1965, Leonard Hayflick, an American microbiologist, observed that under laboratory conditions, human cells can replicate up to 50 times before they stop. He also noticed that when cells stop replicating they begin to senesce, or age. This senescence of cells is responsible for the progressive decline in a person’s vitality i.e., aging. Since this discovery, scientists have been hard at work trying to find out the cause of aging, known as the senescent factor (SF). Early Theories: Alfred Russel Wallace, Darwin’s contemporary and co- discoverer of Natural Selection, proposed the earliest evolutionary theory of aging in 1889. Weismann’s theory suggested that aging was an adaptation meant to limit the population size of the species. Significance & Conclusion: Evidences against this theory include the absence of aging in creatures of the wild. A chaffinch, for example, survives for about a year and a half in the wild. The same bird can live for up to 29 years in captivity! Chimpanzees survive in the wild for 8 years, on an average and for nearly 30 years, when in captivity! This is due to the high rate of early mortality in the wild. Thus, aging is not a prerequisite to control overpopulation. Hence, there would be very little “selection pressure” for the evolution of a particular “aging gene.” The presence of this gene would mean its incorporation within the genome, being an advantageous adaptation. However, since this is not so, scientists now accept three other theories: the accumulation mutation theory, the antagonistic pleiotropy theory and the disposable soma theory. They are not mutually exclusive. The combination actually provides a consistent hypothesis. The Accumulation Mutation Theory: It was first proposed by Peter Medawar in 1952. Mutations occurring spontaneously do not have a chance to express themselves in the wild due to the various selection pressures operating in nature like Natural Selection. Most animals do not have a chance to reach senescence as they fall prey to predators, diseases, natural calamities, starvation, human encroachment of habitats, etc. Thus, the new characters acquired by an organism, which are to be expressed at an old age, are not exhibited during its lifetime. It is passed on from generation to generation without being checked by selection pressures. One gets to observe their effects only when a member of the species, having accumulated all the mutations of previous generations, is observed under captivity. Significance & Conclusion: It is based on the assumption that the forces of Natural Selection weaken with age. In short, it suggests that aging is a by-product of natural selection. Hypothetical Example of application of this theory: Supposing, a mutation has occurred in a line of chimpanzees causing a change in the protein that helps in emulsification of fats and is expressed at an old age. In nature, its significance remains unknown. But in captivity, it is observed that they are unable to assimilate fats from food (like nuts). The Antagonistic Pleiotropy Theory: This theory was formulated by George Williams in 1957, who was then a professor at the Michigan State University. “Antagonistic pleiotropy refers to the expression of a gene resulting in multiple competing effects, some beneficial but others detrimental to the organism.” (“Pleiotropy”. Wikipedia.) It basically suggests the existence of genes which to begin with are beneficial, but become harmful later in life. If these genes grant greater fertility in early years, they are chosen despite the chance that they may cause aging later. Significance and Conclusion: Hence, if you want early reproduction, the tradeoff is that you grow old quicker. However, most of the evidences for the theory have been derived from experiments on Drosophila melanogaster. Whether they apply to other species as well, is yet to be found. Disposable Soma Theory: This theory focuses on the idea that cell maintenance is costly. According to this theory, organisms have evolved in such a way that the amount of energy invested in maintaining the soma (body) is enough to keep it alive till reproduction but not sufficient to keep it alive indefinitely. Significance & Conclusion: This theory also seeks to explain the vast differences in life spans of organisms as the average lifespan varies greatly among different species. In fruit flies (of the order Diptera), for example, the average lifespan is only of 30 to 40 days. Dolphins normally live to age 25, elephants till 50 and tortoises of the Galapagos Islands to 100 years of age. Humans live to an average of 75 years, in the US. These differences arise because of differences in “extrinsic mortality rates.” Examples include birds and bats, which have wings for protection and turtles with shells that reduce the extrinsic mortality rate by protecting them from dangers. Evolutionary Aspects vs. Proximate Explanations: Proximate explanations look for differences between individuals, to explain why one person gets sick while another stays healthy. But Evolutionary or Darwinian Medicine tries to understand why all humans are vulnerable to each disease. (Evolution and Healing: The New Science of Darwinian Medicine, Orion Press, 1996. Nesse and Williams.) For example, In Africa, sickle-cell anemia is very widespread. In this disease, the RBCs become sickle-shaped and in the homozygous recessive condition, become lethal. But in the heterozygous condition, it is beneficial, as it does not allow the malarial parasite Plasmodium to complete its life cycle. This saves the person from Malaria, which is prevalent in tropical Africa. But in regions, where Malaria is not so rampant, people do not suffer from sickle-cell anemia, and this feature is considered to be an evolutionary adaptation for those living in areas where malaria is endemic. This is an example of evolutionary aspect of a disease. In contrast, when an individual contracts Malaria in a population largely unaffected by it, it is an example of a proximate explanation. But, in contexts of determining vulnerability of populations, the evolutionary aspect plays a bigger role. It is therefore, a more holistic approach. Works Cited: 1. http://www.azinet.com/aging/Aging_Book.html#_Toc185660118 2. http://www.pitt.edu/~gmas/1800/READING1_ANS.htm 3. http://www.chester.ac.uk/~sjlewis/EM/Texts/Text1.htm 4. http://www.senescence.info/nature.html 5. http://longevity-science.org/Evolution.htm 6. http://universe-review.ca/R10-27-ageing.htm#causes 7. www.wikipedia.org 8. Microsoft Encarta Encyclopedia Deluxe 2000 (CD) 9. Encyclopedia Britannica: Nature (CD) 10. “Evolutionary Biology of Aging.” Michael R. Rose. 11. “Evolutionary Theories of Aging.” Sharon Phaneuf. University of Florida 12. “Evolution and Healing: The New Science of Darwinian Medicine.” Orion Press, 1996. Nesse and Williams 13. T. B. L. Kirkwood (2008) Understanding ageing from an evolutionary perspective: Journal of Internal Medicine 263 (2) , 117–127 doi:10.1111/j.1365-2796.2007.01901.x 14. Read More