Evolvability

Evolvability is a concept within the Darwinian understanding of biological evolution. Darwin's theory of evolution by natural selection requires that plants, animals, and other organisms be able to produce offspring that are sometimes better adapted to the circumstances of life than the parents are. It is these offspring that survive and reproduce, and the adaptive traits thus increase in number if they are passed down to the offspring. If the only changes to be found in offspring were deleterious, adaptive evolution could not occur. So, the ability to produce enough advantageous variation to allow adaptive evolution to occur is what is called "evolvability".

Wagner (2005) describes two definitions of evolvability which have two main meanings. The first one is: A biological system is evolvable and
 * if its properties show heritable genetic variation,
 * if natural selection can thus change these properties.

The second one is: A biological system is evolvable
 * if it can acquire novel functions through genetic change, functions that help the organism survive and reproduce.

These definitions can be applied on all levels of biological organisation, from macromolecules to mammals. The two meanings are not synonymous. Not all systems that are evolvable in the first sense are evolvable in the second sense. An example is given by Wagner (2005).

Example
Consider an enzyme-coding gene that undergoes different mutations in different individuals of a population. Because of the mutations, the activity of the enzyme fluctuates among different individuals. If this mutation is heritable and influences fitness than natural selection can act on the enzyme’s activity. The enzyme’s activity is thus evolvable in the first sense. However, after millions of years, no mutation might give this enzyme a trait which might permit survival in a new environment. Thus, although the enzyme’s activity is evolvable in the first sense, that does not mean it is evolvable in the second sense. The opposite, does not work. Every innovative, evolvable system can evolve by natural selection.

Organisms are incredibly complex, yet also highly robust to genetic change on all levels of organization. This robustness is one of a few aspects that can affect evolvability in the first and the second sense.

Robustness and evolvability
Robustness will not increase evolvability in the first sense. In organisms with a high level of robustness, mutations will have smaller phenotypic effects than in organisms with a low level of robustness. Thus, robustness reduces the amount of heritable genetic variation on which selection can act. One can see this conclusion in two ways: The first way is that robustness causes mutations to be neutral and therefore no innovation will occur. The second way gives neutral mutations an important function in innovation. Although many neutral mutations do not change primary functions, they can change other system features for future evolution. So, robustness can facilitate exaptation. From this point of view, robustness implies that many mutations are neutral and such neutrality promotes innovation.