Editing Evolvability (section)

== Alternative definitions ==

[[Andreas Wagner]]<ref name="Wagner 2005">{{cite book |author=Wagner A |title=Robustness and evolvability in living systems |publisher=Princeton University Press |series=Princeton Studies in Complexity |year=2005 | isbn=978-0-691-12240-3}}</ref> describes two definitions of evolvability. According to the first definition, a biological system is evolvable:
* if its properties show heritable genetic variation, and
* if natural selection can thus change these properties.

According to the second definition, a biological system is evolvable:
* if it can acquire novel functions through genetic change, functions that help the organism survive and reproduce.

For example, consider an [[enzyme]] with multiple [[alleles]] in the population. Each allele catalyzes the same reaction, but with a different level of activity. However, even after millions of years of evolution, exploring many sequences with similar function, no mutation might exist that gives this enzyme the ability to catalyze a different reaction. Thus, although the enzyme's activity is evolvable in the first sense, that does not mean that the enzyme's function is evolvable in the second sense. However, every system evolvable in the second sense must also be evolvable in the first.

[[Massimo Pigliucci]]<ref name="Pigliucci 2008">{{cite journal | vauthors = Pigliucci M | title = Is evolvability evolvable? | journal = Nature Reviews Genetics | volume = 9 | issue = 1 | pages = 75–82 | date = January 2008 | pmid = 18059367 | doi = 10.1038/nrg2278 | s2cid = 3164124 | author-link = Massimo Pigliucci | url = https://philpapers.org/archive/PIGIEE.pdf }}</ref> recognizes three classes of definition, depending on timescale. The first corresponds to Wagner's first, and represents the very short timescales that are described by [[quantitative genetics]].<ref>{{cite journal | vauthors = Houle D | title = Comparing evolvability and variability of quantitative traits | journal = Genetics | volume = 130 | issue = 1 | pages = 195–204 | date = January 1992 | doi = 10.1093/genetics/130.1.195 | pmid = 1732160 | pmc = 1204793 }}</ref><ref>{{cite journal | vauthors = Hansen TF, Pélabon C, Houle D | title = Heritability is not evolvability. | journal = Evolutionary Biology | date = September 2011 | volume = 38 | issue = 3 | pages = 258–277 | doi = 10.1007/s11692-011-9127-6 | bibcode = 2011EvBio..38..258H | s2cid = 11359207 }}</ref> He divides Wagner's second definition into two categories, one representing the intermediate timescales that can be studied using [[population genetics]], and one representing exceedingly rare long-term innovations of form.

Pigliucci's second definition of evolvability includes Altenberg's<ref name="Altenberg 1994" /> quantitative concept of evolvability, being not a single number, but the entire upper tail of the fitness distribution of the offspring produced by the population. This quantity was considered a "local" property of the instantaneous state of a population, and its integration over the population's evolutionary trajectory, and over many possible populations, would be necessary to give a more global measure of evolvability.