August Weismann Programmed Death Theory

Weismann's Programmed Death Theory

August Weismann (1834 - 1914) was a noted German evolutionary biologist considered by some to be the most important 19th century evolutionary thinker after Darwin. In 1882 he published a theory of mammal aging to the effect that programmed death accomplished an evolutionary purpose and that therefore animals were designed to have a limited life span.

Darwin had earlier proposed that the evolution process was extremely incremental and proceeded in "tiny steps." If this was so, younger animals in any evolving population would tend to be minutely more evolved (adapted) than older animals. Weismann's idea was that programmed death assisted the evolution process by removing older animals from the population and thus freeing resources (food supply, habitat) for younger, more evolved, animals.

Other theorists had two major objections:

First, Weismann was emphasizing death, per se, as the "beneficial" quality that caused evolution of the death mechanism. Other theorists pointed out that few animals in the wild live long enough to die of old age and that therefore a programmed death mechanism would very seldom actually operate. If it did not operate there would be no evolutionary motivation to develop the mechanism.

Second, programmed death was, of course, contrary to basic Darwinian evolutionary mechanics (survival of the fittest). How would animals that committed biological suicide be more likely to pass their designs to descendents than those that did not?

Some efforts were made (limited by 19th century technology) to find a "death gland" or other suicide mechanism in mammals but were unsuccessful. Weismann did not have effective arguments against the objections and eventually recanted. Weismann's theory was subsequently largely ignored or taught as a scientific curiosity.

However, in the late 20th century there was a renewed interest in programmed aging (design-limited life span) because of a number of developments that spoke to the major objections to Weismann's concept:

First, There is now substantial agreement that aging (as opposed to death, per se) does have evolutionary consequences. Aging causes weakness, increased susceptibility to disease and environmental conditions, reduced mobility, and reduced sensory acuity, all of which plausibly reduce survival and reproduction potential. These effects begin to occur at rather young ages.

Second, our certainty in the absolute truth of traditional evolutionary mechanics theory has dramatically declined because of conflicts between the theory and a wide variety of observations. This led to development of alternative evolutionary mechanics theories that support Weismann's concept.

Third, experimental evidence (such as aging genes) increasingly favors programmed aging.

These developments are discussed in detail elsewhere on this web site.

Weismann's theory would now be considered an evolvability theory and is one of a number of evolvability explanations for programmed aging.