Phenotypic integration
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Phenotypic integration is a metric for measuring the correlation of multiple functionally-related traits to each other.[1] Complex phenotypes often require multiple traits working together in order to function properly. Phenotypic integration is significant because it provides an explanation as to how phenotypes are sustained by relationships between traits. Every organism's phenotype is integrated, organized, and a functional whole. Integration is also associated with functional modules. Modules are complex character units that are tightly associated, such as a flower.[2] It is hypothesized that organisms with high correlations between traits in a module have the most efficient functions.[3] The fitness of a particular value for one phenotypic trait frequently depends on the value of the other phenotypic traits, making it important for those traits evolve together. One trait can have a direct effect on fitness, and it has been shown that the correlations among traits can also change fitness, causing these correlations to be adaptive, rather than solely genetic.[4] Integration can be involved in multiple aspects of life, not just at the genetic level, but during development, or simply at a functional level.
Integration can be caused by genetic, developmental, environmental, or physiological relationships among characters.[5] Environmental conditions can alter or cause integration, i.e. they may be plastic.[6] Correlational selection, a form of natural selection can also produce integration. At the genetic level, integration can be caused by pleiotropy, close linkage, or linkage disequilibrium among unlinked genes.[7] At the developmental level it can be due to cell-cell signaling such as in the development of the ectopic eyes in Drosophila. It is believed that the patterns of genetic covariance helped distinguish certain species.[8] It can create variation among certain phenotypes, and can facilitate efficiency. This is significant because integration may play a huge role in phenotypic evolution. Phenotypic integration and its evolution can not only create large amounts of variety among phenotypes which can cause variation among species. For example, the color patterns on Garter snakes range widely and are caused by the covariance among multiple phenotypes.