As survival is vital for reproduction, and we have many adaptations to keep us alive, why do we die? Why could natural selection not create mechanisms that allow humans to live forever? Moreover, why do some people commit suicide, which appears to go against everything evolution would advocate? This final portion delves into these perplexing issues.
Selection is most powerful early in life since any events that occur early in life might alter a person's reproductive years for the rest of their life. However, as people age, the ability to select diminishes. In the worst-case scenario, anything that happened to you in your old age immediately before you died would likely not influence your reproductive potential. This indicates that selection will favour adaptations that provide benefits early in life, even if they come at a high cost later. These high expenses increase over time, resulting in the deterioration of all bodily components at about the same rate. Organisms can be considered to be "designed" to perish in this sense.
Senescence theory has helped to unravel some of these riddles. Senescence is not a sickness in and of itself but rather the degeneration of all biological mechanisms as organisms age. Senescence theory begins with an intriguing observation: the force of natural selection diminishes drastically with age. Consider a twenty-year-old lady and a fifty-year-old woman to see why this occurs. Selection is significantly more intense in the younger woman because everything that happens to her might affect most of her future reproductive years.
When triggered at the age of twenty, a gene that impaired a woman's immune system, for example, may harm her entire reproductive capability. If the same harmful gene were triggered in a fifty-year-old woman instead, it would have little effect on her reproductive ability. Selection has little effect on the older woman since most or all of her reproduction has already happened.
Williams (1957) established a pleiotropic hypothesis of senescence based on this fact. Pleiotropy refers to the situation in which a gene can have two or more distinct effects. Assume there is a gene that increases testosterone in males, leading them to be more effective in vying for status with other men in their twenties and thirties.
However, more testosterone has a deleterious effect later in life by raising the chance of prostate cancer. Selection may favour this pleiotropic gene. It becomes more common in successive generations because the early benefit of increased male status overcomes the later cost of reduced longevity due to prostate cancer. We have developed many genes that aid us early in life but have negative consequences later in life when selection is weak or absent.
The pleiotropic hypothesis of senescence explains not just why our organs all wear down at approximately the same time late in life but also why males die seven years earlier than women on average. Because males have more reproductive variation than women, the selection impacts are more pronounced in men. In other words, most fertile women reproduce, yet the most significant number of children they can have is strictly limited to around twelve for all practical purposes.
Men, on the other hand, can have dozens of children or be barred from reproducing at all. Men are more susceptible to selection than women because of their higher diversity in reproduction. Selection will favour genes that allow a guy to compete effectively for mates early in life, allowing him to be one of the few who reproduce a lot or escape being excluded totally.
Men's success in mate competition will be preferred, even if it means that these genes harm survival later in life. Males can and do breed for more extended periods than women. However, senescence theory explains why these later reproductive events have considerably less influence on males than those early in life. Genes that enhance early success in mate competition will be chosen more strongly in men than women at the expense of genes favouring later survival.
This vigorous selection for early advantage results in an increased fraction of pleiotropic genes that cause premature mortality. "It appears likely that males suffer higher mortality than females because in the past they have enjoyed higher potential reproductive success, and this has selected for traits that are positively associated with high reproductive success but at the cost of decreased survival," one researcher noted. Men, in summary, are "designed" to die earlier than women. The notion of senescence helps to explain why.
Denys de Catanzaro, an evolutionary psychologist, devised an evolutionary theory of suicide and tested it on various samples of individuals ranging from the general population to many "high-risk" samples, such as the elderly and those in a mental unit. De Catanzaro's key point is that suicide is most likely to occur when an individual's ability to contribute to his or her inclusive fitness is dramatically limited.
Expectations of poor future health, chronic illness, embarrassment or failure, low chances for successful heterosexual mating, and views of being a burden on one's genetic kin are all indicators of this dramatically diminished potential. Under these conditions, it is at least conceivable that an individual's genes might have a greater chance of replication without him or her present. For example, if a person is a burden to his or her family, the kin's reproduction, and hence the individual's fitness, may suffer due to his or her existence.
To put this evolutionary theory of suicide to the test, de Catanzaro examined suicidal ideation: whether a person had ever considered suicide, had recently considered suicide, intended to kill himself or herself within one year, intended to kill himself or herself at any time, or had previously engaged in suicidal behaviour. The dependent measure was the total of these replies. Of course, suicidal thoughts are not the same as suicide.
Many people experience suicidal thoughts without actually committing themselves. Nonetheless, because suicide is frequently planned, much suicidal thought nearly always precedes an actual suicide. As a result, suicidal ideation is a plausible indicator to investigate as a proxy for actual suicide.
De Catanzaro also asked participants about their perceived burdensomeness to family, perceived significance of contributions to family and society, frequency of sexual activity, success with members of the opposite sex, homosexuality, number of friends, treatment by others, financial welfare, and physical health in another section of the questionnaire.
Participants were asked to rate each item on a seven-point scale ranging from -3 to +3. A broad public sample, a sample of the elderly, a sample from a mental institution, a sample of inmates from a high-security centre housing people who had committed antisocial acts, and two samples of homosexuals were among the participants.
Given the significance of survival in the evolutionary scheme of things, why individuals die (or do not live longer) is intriguing. Senescence theory explains why. Selection is most powerful early in life because early events might affect a person's reproductive years. However, as humans age, the efficacy of selection diminishes; in the extreme, a horrible experience that happened to you just before you died would not influence your reproduction.
This indicates that selection will favour adaptations that benefit early in life, even if they come at a high cost later in life. For example, testosterone production in males appears to bring benefits in early adulthood through mating success. However, it has negative implications later in life in the form of prostate cancer.
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