Thesis #16 – Among the most important physiological constraints on the action of natural selection are trade-offs between the biological functions underlying age-specific rates of survival and reproduction.
One of the most important features of physiology that affects the evolution of aging is trade-offs between functions at early versus later ages. George C. Williams felt that this was the central requirement for the evolution of aging, as he argued in his famous 1957 paper on the topic. His conclusion was that, given his then-crude understanding of the declining Forces of Natural Selection, evolution would give up later deterioration for early vigor whenever it was given the chance.
There is a great deal of theoretical, comparative biological, and experimental evidence in favor of this hypothesis. Some of that evidence will be discussed in subsequent theses. But in this thesis the chief focus will be to develop the basic idea, particularly why so many biologists think that it is important.
The idea of evolutionary and functional trade-offs is one of the ubiquitous themes of biology. And it makes perfect sense in terms of physics and chemistry. Even in terms of engineering.
Think about automobiles. It would be very hard to build a car that can carry many passengers, get great gas mileage, and have outstanding acceleration or handling, all at the same time. There are material trade-offs that impinge on the automotive engineering of such a vehicle, where many of these constraints arise from basic features of physics, such as momentum and other corollaries of the laws of motion, as well as chemistry, such as the efficiency with which chemical combustion of gasoline can generate force.
In the same way, there are material constraints that affect how quickly a large animal can develop: how big it is at birth, how quickly it can be fed, how efficiently its food can be converted into growing tissue, and so on. Biology is full of constraints, some of which arise from basic features of physics and chemistry, some of which are more specifically biological, such as rates of cell division and the time required for cells to undergo differentiation.
Thus the basic concept of trade-offs can be naturally extended to relationships among such biological processes as survival and reproduction. An insect that provisions its eggs with fats from its abdominal fat body, and then lays those eggs and flies away, won’t have those fats with which to survive a subsequent period of starvation. It has used them up. It can get more nutrients, if it has mouthparts – which mayflies and some moths do not. But there is a material trade-off between resources sequestered for reproduction at early ages and resources conserved for survival to later ages. From this trade-off, as well as many other kinds of trade-off, the idea of a material antagonism between early reproduction and later survival is entirely natural.