The phenotype (from Greek o- (faino-)’showing’ and (tpos) ‘type’) is a set of observable features or qualities of an organism in genetics. The phrase refers to an organism’s morphology, or physical shape and structure, as well as its developmental processes, biochemical and physiological features, behaviour, and behavioural outcomes. The expression of an organism’s genetic code, or genotype, and the effect of environmental variables are the two primary components that determine its phenotype. Both factors may interact, altering phenotype even more. When two or more clearly different phenotypes exist in the same population of a species, the species is called polymorphic.
Phenotypic variation (due to underlying heritable 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.
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 (see ecophenotypic variation). The plant Hieracium umbellatum is found growing in two different habitats in Sweden. One habitat is rocky, sea-side cliffs, where the plants are bushy with broad leaves and expanded inflorescences; the other is among sand dunes where the plants grow prostrate with narrow leaves and compact inflorescences. These habitats alternate along the coast of Sweden and the habitat that the seeds of Hieracium umbellatum land in, determine the phenotype that grows.
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 clones raised in different environments.
The concept of phenotype can be extended to variations below the level of the gene that 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). These 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.
The extended phenotype
Main article: 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.” For instance, an organism such as a beaver modifies its environment by building a beaver dam; this can be considered an 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.
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.