Thesis #10 When the forces of natural selection weaken with adult age, declining survival and fertility evolve in adulthood, and thus produce the decline in health which is commonly called ‘aging.’
In animals like mammals and insects, the Force of Natural Selection acting on mortality proportionately scales as follows. Childhood: From birth to the start of reproduction in a population, it acts with 100% of its maximum force. During this phase of life, therefore, natural selection acts strongly to sustain our inborn health, in terms of our capacity to survive, which can be thought of as our basic functional integrity. Even at full strength, natural selection won’t always succeed; accident, contagious disease, and mutation can all degrade health during the childhood years. But it does a damn good job. Aging Phase: From the earliest ages of reproduction until the last age of reproduction, the mortality force steadily declines. During this phase of life, natural selection progressively weakens its evolutionary surveillance of our health, resulting in predictable, general, and sustained declines in our health. Late-life Phase: After the last age of reproduction, and forever after, age-specific natural selection is gone. At this point, our health is sustained only by the age-independent health benefits built by natural selection for earlier ages. However, in many animal populations, this late-life phase features relatively reasonable health, and the preservation of some useful function, under protected conditions. But it does not among humans in industrial countries, an issue that we will discuss in detail later in the 55.
In animals like mammals and insects, the Force of Natural Selection acting on fecundity scales as follows. Childhood: From birth to the start of reproduction in a population, the fecundity force usually progressively falls. However, since we do not reproduce during this period, this has no observable effect. Aging Phase: During the earliest ages of reproduction, there is usually a transitional period during which fecundity increases rapidly with adult age. This can be thought of as the developmental result of all the quantitative effort that natural selection is putting into enabling the adult organism to develop the anatomical structures, physiological functions, and reproductive behaviors required for successful reproduction. Thus, in teen-aged humans, the first years of basic reproductive capacity are marked by relatively poor fertility, inappropriate or misdirected sexual behavior, and poor parenting behavior. But quickly, the adult human develops its full reproductive capacity, at around the age of 20 years in men, somewhat later in women. After that, fertility falls with age, the other side of declining health during adult life. Note that in mammalian species, which have a great deal of parental care of offspring, an infertile adult caring for their immediate offspring is still effectively reproducing. That is to say, in evolutionary terms female reproductive function extends past menopause. Late-life Phase: After the last age of survival in the evolutionary history of a population, age-specific natural selection for the maintenance of fecundity is gone. During this epoch of life, our reproductive functions are sustained only by age-independent benefits built by natural selection for earlier ages. However, in many animal populations, this late-life phase features measureable reproductive function. But it does not among humans in industrial countries, an issue that we will discuss in detail later in the 55.