Editing Evolvability (section)

== Exploration ahead of time ==
When [[mutational robustness]] exists, many mutants will persist in a cryptic state. Mutations tend to fall into two categories, having either a very bad effect or very little effect: few mutations fall somewhere in between.<ref>{{cite journal | vauthors = Eyre-Walker A, Keightley PD | title = The distribution of fitness effects of new mutations | journal = Nature Reviews Genetics | volume = 8 | issue = 8 | pages = 610–8 | date = August 2007 | pmid = 17637733 | doi = 10.1038/nrg2146 | s2cid = 10868777 }}</ref><ref>{{cite journal | vauthors = Fudala A, Korona R | title = Low frequency of mutations with strongly deleterious but nonlethal fitness effects | journal = Evolution; International Journal of Organic Evolution | volume = 63 | issue = 8 | pages = 2164–71 | date = August 2009 | pmid = 19473394 | doi = 10.1111/j.1558-5646.2009.00713.x | s2cid = 12103318 | doi-access =  }}</ref> Sometimes, these mutations will not be completely invisible, but still have rare effects, with very low [[penetrance]]. When this happens, natural selection weeds out the very bad mutations, while leaving the others relatively unaffected.<ref>{{cite journal | vauthors = Masel J | title = Cryptic genetic variation is enriched for potential adaptations | journal = Genetics | volume = 172 | issue = 3 | pages = 1985–91 | date = March 2006 | pmid = 16387877 | pmc = 1456269 | doi = 10.1534/genetics.105.051649 | author-link = Joanna Masel }}</ref><ref>{{cite journal | vauthors = Rajon E, Masel J | title = Evolution of molecular error rates and the consequences for evolvability | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 108 | issue = 3 | pages = 1082–7 | date = January 2011 | pmid = 21199946 | pmc = 3024668 | doi = 10.1073/pnas.1012918108 | bibcode = 2011PNAS..108.1082R | author2-link = Joanna Masel | doi-access = free }}</ref> While evolution has no "foresight" to know which environment will be encountered in the future, some mutations cause major disruption to a basic biological process, and will never be adaptive in any environment. Screening these out in advance leads to [[preadaptation |preadapted]] stocks of cryptic genetic variation.

Another way that phenotypes can be explored, prior to strong genetic commitment, is through learning. An organism that learns gets to "sample" several different phenotypes during its early development, and later sticks to whatever worked best. Later in evolution, the optimal phenotype can be [[genetic assimilation |genetically assimilated]] so it becomes the default behavior rather than a rare behavior. This is known as the [[Baldwin effect]], and it can increase evolvability.<ref>{{cite journal |vauthors=Hinton GE, Nowlan SJ | title=How learning can guide evolution | journal=Complex Systems | volume=1 |pages=495–502 | year=1987}}</ref><ref>{{cite journal | vauthors = Borenstein E, Meilijson I, Ruppin E | title = The effect of phenotypic plasticity on evolution in multipeaked fitness landscapes | journal = Journal of Evolutionary Biology | volume = 19 | issue = 5 | pages = 1555–70 | date = September 2006 | pmid = 16910985 | doi = 10.1111/j.1420-9101.2006.01125.x | s2cid = 6964065 | doi-access = free }}</ref>

Learning biases phenotypes in a beneficial direction. But an exploratory flattening of the [[fitness landscape]] can also increase evolvability even when it has no direction, for example when the flattening is a result of random errors in molecular and/or developmental processes. This increase in evolvability can happen when evolution is faced with crossing a "valley" in an [[fitness landscape |adaptive landscape]]. This means that two mutations exist that are deleterious by themselves, but beneficial in combination. These combinations can evolve more easily when the landscape is first flattened, and the discovered phenotype is then fixed by [[genetic assimilation]].<ref>{{cite journal | vauthors = Kim Y | title = Rate of adaptive peak shifts with partial genetic robustness | journal = Evolution; International Journal of Organic Evolution | volume = 61 | issue = 8 | pages = 1847–56 | date = August 2007 | pmid = 17683428 | doi = 10.1111/j.1558-5646.2007.00166.x | s2cid = 13150906 | doi-access =  }}</ref><ref>{{cite journal | vauthors = Whitehead DJ, Wilke CO, Vernazobres D, Bornberg-Bauer E | title = The look-ahead effect of phenotypic mutations | journal = Biology Direct | volume = 3 | issue = 1 | pages = 18 | date = May 2008 | pmid = 18479505 | pmc = 2423361 | doi = 10.1186/1745-6150-3-18 | doi-access = free }}</ref><ref>{{cite journal | vauthors = Griswold CK, Masel J | title = Complex adaptations can drive the evolution of the capacitor [PSI], even with realistic rates of yeast sex | journal = PLOS Genetics | volume = 5 | issue = 6 | pages = e1000517 | date = June 2009 | pmid = 19521499 | pmc = 2686163 | doi = 10.1371/journal.pgen.1000517 | doi-access = free }}</ref>