Thesis #11 – If the forces of natural selection are strengthened during later adulthood, improved later health will evolve if natural selection is not impeded by very small population sizes, environmental change, or an absence of heritable variation.

There are a variety of ways to show that the Forces of Natural Selection provide the key factor in determining the evolution of the pattern of functional decline called aging.  Some of them will be discussed in subsequent theses of the 55.  But there is a strikingly simple pattern of comparative variation among different species of animal which makes the point fairly well, and you don’t need much biological background to understand it.

Consider turtles.  Turtles come in very different shapes and sizes, some of which live out almost all their lives in oceans, like green sea turtles.  Other types of turtle live in marshy wetlands.  And we can consider tortoises, like the giant Galapagos Tortoises, as part of the overall turtle group, too.  They almost never go swimming.

Some turtles and tortoises have very thick shells.  Furthermore, many of them can retract their head and limbs inside these shells when they are attacked.  Some of them, furthermore, have powerful biting mouths, which you would be ill-advised to place your fingers inside.  When turtles and tortoises with these thick shells are maintained in zoos, they can live for very long periods of time, during which some of them maintain active sex lives.  I have seen Galapagos tortoises at the San Diego zoo which are much older than I am, some probably at least 90 years of age.  They still live with only modest medical care provided by their veterinarians.

On the other hand, soft-shelled turtles have much shorter lifespans, often less than twenty years, even when they are compared with hard-shelled turtles and tortoises of similar size.  Why should this be so?

It is a basic feature of the Force of Natural Selection acting on mortality that a population which has suffered higher rates of mortality in its evolutionary history will tend to have its force decline faster with adult age, when everything else is equal.  Soft-shelled turtles are not as good at defending themselves against predators as turtles with harder shells.  Therefore, evolutionary theory predicts that they will evolve shorter lifespans, because of a faster decline in their forces of natural selection acting on mortality.

This concept readily generalizes to the following contrasts:  similar species with and without venom, the venom giving better defense against potential predators, and the evolution of longer lifespans; similar species with and without flight, flight allowing ready escape from predators, and the evolution of longer lifespans; in general, larger animals are harder for predators to subdue and eat, so those species too tend to live longer.  Sometimes multiple factors differ between species, such as larger snakes with no venom (like boid snakes) which may or may not live longer than smaller species with venom (like cobras).  But when other factors are similar, an attribute that strongly reduces mortality levels will usually lead to the evolution of longer lifespans, when similar species are compared under good laboratory or zoo conditions.