Capacity for Further Evolution - Evolvability
Darwin defined certain qualities that organisms needed to possess in order to enable the evolution process: Inheritable mutational changes to organism designs had to occur. Natural variation in designs among members of a species had to consequently exist. Organisms had to be in competition for resources. Natural selection therefore operated on all populations of all species.
Darwin had no reason to believe that all these qualities were not fundamental qualities that applied, equally, to all living organisms. All species were subject to mutations, competitive forces, and natural selection. All organisms therefore possessed the same capacity for evolution. Evolutionary capacity or evolvability was a constant that did not need to be specifically handled by an evolutionary mechanics theory. Traditional evolutionary mechanics still takes this position.
However, genetics science eventually produced a long list of discoveries that cast doubt on the idea of constant evolvability in a number of different ways. Recall that biological inheritance is critical to evolutionary mechanics. Any mutational change has to be inherited myriad times in order to propagate to a population, much less an entire species. If inheritance mechanisms handled all mutational changes in exactly the same way, inheritance could be disregarded as a variable factor in the evolutionary process. Genetics discoveries have definitively determined that this is not in fact the case. For example, a mutational change to an X or Y chromosome or to mitochondrial DNA propagates in an entirely different way from a change to the other chromosomes. Some organisms (e.g. bacteria) only possess one haploid set of genetic data. More complex diploid organisms possess two sets of genetic data that jointly specify organism design. Mutational changes propagate in very different ways in these two cases. A mutation to genetic data in a diploid organism might have essentially no phenotypic effect unless both sets of genetic data possessed the mutation. This tends to retard propagation of beneficial changes and enhance propagation of adverse changes relative to the haploid case. Another example: some genes are essentially duplicated such that a mutational change to one would have proportionally less effect than a similar change to some other gene that was less redundant, thus changing the way such a mutation would propagate and introducing robustness or resistance to mutational change. The above are only a few of the many instances of genetics discoveries that cast doubt on the idea that evolvability is a constant or otherwise present issues for traditional evolutionary mechanics theory. See How Genetics Discoveries Affect Evolution Theory for more information.
Theorists suggest that not only does evolvability vary between organisms, but that it is a selectable property. Organisms can acquire through the evolution process increases in their evolvability just as they can acquire increases in fitness. Most of the evolvability possessed by complex organisms may result from evolved characteristics such as sexual reproduction and design-limited life span. Organism characteristics that increase evolvability tend to decrease individual fitness. Therefore, fitness is a trade-off with evolvability. There are evolvability explanations for all of the observed conflicts with traditional evolutionary mechanics.
Some followers of traditional evolutionary mechanics theory have criticized evolvability theories as simply another form of group selection. However, the mechanics of evolvability propagation are logically rather different and valid criticism would need to deal with the differences.
G. P. Wagner (1996) is generally credited with formal introduction of the evolvability concept.
Evolvability Theories of Aging
A number of evolvability advantages have been proposed for a design-limited life span including the following:
Increases genetic diversity by decreasing potential for a relatively few older individuals to dominate the gene pool.
Increases the adult death rate. Adult death rate is the equivalent to adult lives lived per unit time. Increases the number of organisms participating in the evolution process.
Allows evolution of intelligence, immunity, and other organism design characteristics whose utility depends on acquisition of something that non-genetically adds to fitness.
Some theorists suggest that gradual aging seen in most mammals and larger animals has additional evolvability benefit over biological suicide: Gradual deterioration provides a challenge that can be overcome by fitness thereby increasing the apparent differential between more fit and less fit organisms and thus increasing evolution rate.
The earliest formal evolvability theory was the programmed death theory developed by August Weismann in 1882. Programmed aging theories based on evolvability have been developed by Vladimir Skulachev and Theodore Goldsmith. See Resources for journal articles.
Many organism characteristics appear to have evolvability implications including age of sexual maturity and other restrictions on reproduction such as some mating rituals and behaviors. An evolvability-based programmed-aging version of the Maintenance Theory of Aging suggests a special relationship between aging and sexual maturity age.
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