Thesis #35 – Severe antagonistic pleiotropy can cause the evolution of zero late-adult survival probability even under ideal conditions, when genetic trade-offs between early reproduction and subsequent adult survival are sufficiently strong.
Not all organisms have three phases to their lives, with development, aging, and late life following each other in order, when they are protected by humans from the deadly slings and arrows of life in the wild. Some have life cycles with just two phases, regardless of how well we take care of them. In these cases, development is followed by a brief and tumultuous adulthood, characterized by an unremitting focus on reproduction, leading to an early death. That is, nature has its share of tragic young “shooting stars,” species that exhibit neither protracted aging nor a post-aging phase of relative stabilization, when kept under good conditions.
This is a common pattern among annual plants, which die shortly after their flowering. Among invertebrates, it is famously the pattern of mayflies, which may have an adult phase that lasts just hours. Less famous are the many moth species that emerge from pupal cases without mouthparts, spending their brief adult lives seeking mates and dying shortly after the females lay their eggs. The Pacific salmon are well-known for their strenuous and brief adulthood, spent swimming upstream to their natal streams. On reaching their destination they furiously spawn, and quickly die, over the course of days.
Yet some of these species can be given protracted lives by the simple expedient of castration, as already mentioned. Pacific salmon can thereby lives for years longer, in laboratory aquaria, after careful excision of their gonads.
What is occurring here is that antagonistic pleiotropy between early reproduction and subsequent survival is so severe that later survival is entirely precluded if the physiological mobilization for reproduction is allowed to occur. There is nothing about the workings of evolution by natural selection that is solicitous for the survival of the adults of any species, if that survival impinges on the net rate of transmission of genes into the next generation. This “gene-level” view of what natural selection provides in turn explains many paradoxes in biology: the sterility of worker honeybees, the male praying mantis allowing his mate to devour him, and the attenuated adult lives of the “big-bang” reproducers of the plant and animal worlds. Thus, from an evolutionary standpoint, the Dionysian rock-star who has many sexual conquests and a short lifespan is not necessarily the tragic failure that those of us with more pedestrian lifestyles might think, at least evolutionarily.
A protracted aging phase followed by a distinct late life phase is not a necessary product of lifecycle evolution. It is merely one of several possible life cycles, along with big-bang suicidal reproduction and a fissile life cycle with no aging whatsoever. Evolution by natural selection rules the shapes of our lives, with an unremitting force that defines what the limits of our lives will be, absent radical medical intervention. Castration, awkwardly enough, shows the extent to which such drastic interventions can surmount evolution limitations. While I am not in the business of recommending castration, my point would be that it does show the promise of what we might eventually attain using different means.