Editing Phenotype (section)

==Phenotypic variation==
{{See also|Ecophenotypic variation}}
Phenotypic variation (due to underlying heritable [[genetic diversity|genetic variation]]) is a fundamental prerequisite for [[evolution]] by [[natural selection]]. It is the living organism as a whole that contributes (or not) to the next generation, so natural selection affects the genetic structure of a population indirectly via the contribution of phenotypes. Without phenotypic variation, there would be no evolution by natural selection.<ref name="Lewontin70">{{cite journal | vauthors = Lewontin RC |author-link=Richard Lewontin |date=November 1970 |title=The Units of Selection |url=http://joelvelasco.net/teaching/167/lewontin%2070%20-%20the%20units%20of%20selection.pdf |journal=[[Annual Review of Ecology, Evolution, and Systematics|Annual Review of Ecology and Systematics]] |volume=1 |pages=1–18 |doi=10.1146/annurev.es.01.110170.000245 |jstor=2096764|s2cid=84684420 }}</ref>

The interaction between genotype and phenotype has often been conceptualized by the following relationship:

:genotype (G) + environment (E) → phenotype (P)

A more nuanced version of the relationship is:

:genotype (G) + environment (E) + genotype & environment interactions (GE) → phenotype (P)

Genotypes often have much flexibility in the modification and expression of phenotypes; in many organisms these phenotypes are very different under varying environmental conditions. The plant ''[[Hieracium umbellatum]]'' is found growing in two different [[habitat]]s in [[Sweden]]. One habitat is rocky, sea-side [[cliff]]s, where the plants are bushy with broad leaves and expanded [[inflorescence]]s; the other is among [[dune|sand dunes]] where the plants grow prostrate with narrow leaves and compact inflorescences. The habitats alternate along the coast of Sweden and the habitat that the seeds of ''Hieracium umbellatum'' land in, determine the phenotype that grows.<ref name=Botanyonline>{{cite web | vauthors = von Sengbusch P |url=http://www.biologie.uni-hamburg.de/b-online/e37/37b.htm |title= Phenotypic and Genetic Variation; Ecotypes | work = Botany online: Evolution: The Modern Synthesis - Phenotypic and Genetic Variation; Ecotypes |access-date=2009-12-29 |url-status=dead |archive-url=https://web.archive.org/web/20090618051236/http://www.biologie.uni-hamburg.de/b-online/e37/37b.htm |archive-date=2009-06-18 }}</ref>

An example of random variation in ''[[Drosophila]]'' flies is the number of [[ommatidia]], which may vary (randomly) between left and right eyes in a single individual as much as they do between different genotypes overall, or between [[cloning|clones]] raised in different environments.{{citation needed|date=March 2018}}

The concept of phenotype can be extended to variations below the level of the [[gene]] which affect an organism's fitness. For example, [[silent mutations]] that do not change the corresponding amino acid sequence of a gene may change the frequency of [[guanine]]-[[cytosine]] base pairs ([[GC content]]). The base pairs have a higher thermal stability (''melting point'') than [[adenine]]-[[thymine]], a property that might convey, among organisms living in high-temperature environments, a selective advantage on variants enriched in GC content.{{citation needed|date=March 2018}}

===The extended phenotype===
{{main |The Extended Phenotype}}

[[Richard Dawkins]] described a phenotype that included all effects that a gene has on its surroundings, including other organisms, as an extended phenotype, arguing that "An animal's behavior tends to maximize the survival of the genes 'for' that behavior, whether or not those genes happen to be in the body of the particular animal performing it."<ref name=r1>{{cite journal | vauthors = Dawkins R | title = Replicator selection and the extended phenotype | journal = Zeitschrift für Tierpsychologie | volume = 47 | issue = 1 | pages = 61–76 | date = May 1978 | pmid = 696023 | doi = 10.1111/j.1439-0310.1978.tb01823.x | author-link = Richard Dawkins }}</ref> For instance, an organism such as a [[beaver]] modifies its environment by building a [[beaver dam]]; this can be considered an [[Gene expression|expression of its genes]], just as its [[incisor]] teeth are—which it uses to modify its environment. Similarly, when a bird feeds a [[brood parasite]] such as a [[cuckoo]], it is unwittingly extending its phenotype; and when genes in an [[orchid]] affect [[orchid bee]] behavior to increase pollination, or when genes in a [[peacock]] affect the copulatory decisions of peahens, again, the phenotype is being extended. Genes are, in Dawkins's view, selected by their phenotypic effects.<ref>{{Cite book | vauthors = Dawkins R |author-link=Richard Dawkins |title=The Extended Phenotype |publisher=Oxford University |year=1982 |page=[https://archive.org/details/extendedphenotyp0000dawk/page/4 4] |isbn=978-0-19-288051-2 |url=https://archive.org/details/extendedphenotyp0000dawk/page/4 }}</ref>

Other biologists broadly agree that the extended phenotype concept is relevant, but consider that its role is largely explanatory, rather than assisting in the design of experimental tests.<ref name="Hunter2009">{{cite journal | vauthors = Hunter P | title = Extended phenotype redux. How far can the reach of genes extend in manipulating the environment of an organism? | journal = EMBO Reports | volume = 10 | issue = 3 | pages = 212–215 | date = March 2009 | pmid = 19255576 | pmc = 2658563 | doi = 10.1038/embor.2009.18 }}</ref>