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Chapter 17: Evolution and the Human Life Cycle
Parent-Offspring ConflictParent-offspring strife occurs as a conflict of interest between mother and offspring; however, this relationship begins earlier in life than might be expected. A child will (obviously) share 100% of its genetic makeup with itself, but potentially only 50% with its siblings (and only up to 25%, if they are half-siblings). Because of this fact, a given child, while it will still wish for its siblings to get attention and nurturing (kinselection), will often be at odds with the mother in terms of the quantity of investment which should be alloted to himself or herself.
As a result of the above situation, natural selection unwittingly created this conflict by favoring those offspring who demanded more investment (and utilized the most effective tactics in manipulating their parents), as well as those mothers who provided an equitable distribution of investment (invariably less than each individual sibling would want for himself or herself). In a sense, this conflict manifests itself even in utero, where the fetus and mother engage in a battle of hormones. The fetus utilizes a barrage of hormones to increase placental bloodflow, while the mother counteracts these hormones with some of her own.
Evolution of SenesenceSenesence, or aging, occurs in all animals. Systems and processes slow down, with physiological deterioration eventually culminating in death. However, while death itself is the irreparable breakdown in organic systems, the inevitability of death is not totally uncontested. All organisms heal wounds; some even regenerate whole limbs (e.g., frogs) or entire bodies (e.g., starfish). Some asexually reproducing organisms do not undergo a period of senesence. Why, then, has natural selection not produced an organism which can live indefinitely? Why do organisms grow old?It is important to keep in mind that the effects of natural selection weaken with age. That is, selection which eliminates young individuals will have much more of an impact on future generations than selection which removes old individuals. This has to do with the reproductive potential of each individual: older individuals have much less reproductive potential left, and their deaths make little mark in a strict Darwinian sense. This being the case, one hypothesis for senesence is called the antagonistic pleiotropy hypothesis. It suggests that the effects of genes which favor youthful fertility also act to decrease longevity; thus, genes producing more of these prolific youths would become more common in the population. Another hypothesis, the mutation accumulation hypothesis, suggests that age comes as a byproduct of accumulations of deleterious mutations. Most mutations will tend to have a deleterious effect, and somatic mutations may play a role in degenerative diseases which plague older individuals, such as osteoporosis, cancer, and heart disease. In fact, both of these hypotheses have received empirical support from lab studies.
Evolution of MenopauseWomen experience another change as a result of senesence, a change which requires a different explanation. Menopause - cessation of reproductive function in females - occurs near the age of 50; however, life expectancy averages nearly 70 years. What evolutionary basis is there to explain menopause?Naturally, menopause may be just an artifact of a greatly lengthened lifespan among humans. Yet, menopause manifests itself as an abrupt decline in reproductive systems. This suggests that its evolutionary history is somewhat different than other systems. Cardiovascular and neurological systems do not experience a similarly abrupt interruption of function in old age. One possible adaptive advantage for menopause is the assistance provided by postmenopausal females to their grandchildren and younger relatives. Perhaps it is more beneficial to care for grandchildren of fecund offspring, rather than engaging in increasingly risky childbirth and chancing higher levels of germ cell mutations. Current models have not provided support for this scenario, leaving the question yet unanswered.
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