Evolvability Theory of Aging – Overview
T. C. Goldsmith September 2003
This is a brief overview of the evolvability theory of aging. A much more comprehensive treatment may be found in the short book The Evolution of Aging (PDF version) (HTML version)(paperback version ISBN: 0595280692). See http://www.azinet.com/aging/.
All living organisms on Earth share common characteristics or "fundamental properties of life" inherited from the original primordial organism that appeared about 4 billion years ago. Characteristics that differ from species to species, such as the characteristics distinguishing an amoeba from an elephant, are presumed to be the result of the process of evolution, that is, evolved characteristics or adaptations.
Life spans of different animals vary dramatically between species and seem to fit with other evolved characteristics such as reproductive behavior. Other peculiarities in life span behavior and recent discoveries such as aging genes and hormone mediated aging further suggest that life span is an evolved characteristic, a part of an organism’s design.
However, the traditional theory of natural selection, introduced by Darwin in 1859, is incompatible with an evolved characteristic that limits life span. Darwin’s theory holds that an evolved characteristic must increase an organism’s ability to survive or reproduce. Aging, suicidal behavior, or other characteristic which limits life span or otherwise reduces the probability that an organism will reproduce can not, therefore, be an evolved characteristic under traditional theory. Although it is accepted that tradeoffs can exist between the survival and reproductive functions no reproductive benefit has been demonstrated that could compensate for the survival disadvantage of biological aging.
A number of other animal characteristics also appear to be incompatible with natural selection. For example, sexual “selection” and mating rituals reduce the probability that an animal will reproduce and are therefore adverse characteristics and not selectable under natural selection theory.
Because of the incompatibility between aging and natural selection, various theories in which aging was not an evolved characteristic or was only partly the result of natural selection were developed, some of which are currently widely respected. These theories have difficulties explaining the very diverse life span behavior of different species, are incompatible with subsequent discoveries, or have internal logical inconsistencies.
Research in genetics has revealed an internal inconsistency in the natural selection theory. Darwin’s theory is based on the idea that the basic ability of organisms to evolve is a constant. All organisms have, as a fundamental property, the capacity for evolution and that capacity is a constant that cannot be affected by any evolved characteristic. If this were not so, an organism could evolve a characteristic which gave it the ability to adapt more rapidly than a competing organism, an obvious evolutionary advantage which is not handled by the natural selection theory.
A very central part of Darwin’s theory was that “natural variation” in characteristics between members of a species was an essential component of this constant evolutionary capacity or evolvability. According to Darwin, evolution took place by means of variation.
However, it was subsequently determined that “natural variation” in genetically transmittable characteristics was actually largely the result of a family of very complex and obviously evolved characteristics such as sexual reproduction, meiosis, and gene crossover, and that the degree of “natural variation” itself therefore varies extensively between different species. The basic, fundamental, primordial, genetic mechanism is essentially digital in nature and is equipped to make exact copies of genetic information but little equipped for structured variation in genetically transmitted characteristics. At least in the case of variation, characteristics could evolve which affected the capacity for evolution itself. Variation as an evolved characteristic is itself incompatible with traditional natural selection theory because organisms possessing a large degree of variation are, on average, less fit than other equally adapted organisms possessing less variation. According to Darwin's theory, variation would "select out" when in fact, it has "selected in".
The evolvability theory therefore proposes that there is an additional factor in determining whether a characteristic is evolved and retained in an organism’s genome, namely evolvability. A characteristic can be evolved if it increases an organism’s ability to survive, reproduce, or evolve. Tradeoffs can exist between all three factors and a characteristic such as aging or sexual selection (or a characteristic producing variation) can evolve even though adverse to survival and reproduction if it has a sufficient compensating benefit to evolvability. Complex organisms can be expected to have complex and diverse evolvability characteristics just as they have complex and diverse survival and reproductive characteristics. In addition to characteristics producing variation, many characteristics of organisms, especially more complex organisms, plausibly improve evolvability.
It is apparent in such a scheme that life span would have an evolvability impact. According to Darwin, organisms evolve incrementally in tiny steps. Each generation of an evolving species is only minutely more adapted than the previous generation. A species with a shorter life span would accumulate such incremental improvements in adaptation more rapidly and therefore would have an advantage over a species with a longer life span. A species needs a life span sufficient to fully develop, breed at least once, and nurture and protect progeny (if applicable). A longer life span would have a progressively smaller survival and reproduction advantage and a progressively larger evolvability disadvantage. This fits with observed animal characteristics. Aging has at least five other evolvability benefits that are described in the book.
The evolvability theory is one of several new or resurgent theories that hold that aging is an evolved characteristic, an adaptation, and a design feature of organisms, rather than a defect, disorder, or fundamental property of life. The distinction between evolved characteristic or not has enormous implications for the prospects and direction of anti-aging research as described in the book and summarized in Aging Theory Research Implications.
Copyright 2003 T. Goldsmith