Phenotypic Integration is the term used to describe when multiple functionally-related traits are correlated with each other.Pigliucci, Massimo. "Phenotypic integration: studying the ecology and evolution of complex phenotypes." Ecology Letters 6.3 (2003): 265-272. 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.Wagner, Günter P. "Homologues, natural kinds and the evolution of modularity." American Zoologist 36.1 (1996): 36-43. It is hypothesized that organisms with high correlations between traits in a module have the most efficient functions.Brock, Marcus T., and Cynthia Weinig. "PLASTICITY AND ENVIRONMENT‐SPECIFIC COVARIANCES: AN INVESTIGATION OF FLORAL–VEGETATIVE AND WITHIN FLOWER CORRELATIONS." Evolution 61.12 (2007): 2913-2924. 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.Schlichting, Carl D., and Massimo Pigliucci. Phenotypic evolution: a reaction norm perspective. Sinauer Associates Incorporated, 1998. 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.Wagner, Gunter P., and Lee Altenberg. "Perspective: Complex adaptations and the evolution of evolvability." Evolution (1996): 967-976. Environmental conditions can alter or cause integration, i.e. they may be plastic.Schlichting, Carl D. "The evolution of phenotypic plasticity in plants." Annual Review of Ecology and Systematics 17 (1986): 667-693. 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.Murren, C. J., and P. X. Kover. "QTL mapping: a first step toward an understanding of molecular genetic mechanisms behind phenotypic complexity/integration." Phenotypic integration: studying the ecology and evolution of complex phenotypes (M. Pigliucci and K. Preston, eds.). Oxford University Press, New York (2004): 195-212. 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.Murren, Courtney J., Nicole Pendleton, and Massimo Pigliucci. "Evolution of phenotypic integration in Brassica (Brassicaceae)." American Journal of Botany 89.4 (2002): 655-663. 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. thumbnail|Phenotypic Integration is important for many traits. One specifically is the development of fewer bones in the neurocranium. Shortly after the structure of DNA was uncovered, Olson and Miller (1958) wrote the first book regarding the topic of phenotypic integration.Olson E.C. & Miller R.L. (1958) Morphological Integration. University of Chicago Press, Chicago. The term integration was first used  by Olson and Miller to describe correlations among characters that are influenced by selection.Olson, Everett C., and Robert L. Miller. Morphological integration. University of Chicago Press, 1999. Following Olson and Miller, botanical studies on coherence between characters were done spanning over many years.Clausen, J. & Hiesey, W.M. (1960). The balance between coherence and variation in evolution. Proc. Natl Acad Sci U S A, 46, 494–506. Its first expansion was in the construction of a morphological integration genetic model constructed by Lande (1980). However, the term "Phenotypic Integration" was first coined by Pigliucci and Preston, in their book, Phenotypic Integration, which helped elucidate the observed laws of correlation and some theoretical issues regarding the topic.Pigliucci, Massimo, and Katherine (Katherine A.) Preston. Phenotypic integration. Oxford: Oxford University Press, 2004.
Phenotypic Integration occurs when multiple functionally-related traits are correlated with each other.Pigliucci, Massimo. "Phenotypic integration: studying the ecology and evolution of complex phenotypes." Ecology Letters 6.3 (2003): 265-272. 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.Wagner, Günter P. "Homologues, natural kinds and the evolution of modularity." American Zoologist 36.1 (1996): 36-43. It is hypothesized that organisms with high correlations between traits in a module have the most efficient functions.Brock, Marcus T., and Cynthia Weinig. "PLASTICITY AND ENVIRONMENT‐SPECIFIC COVARIANCES: AN INVESTIGATION OF FLORAL–VEGETATIVE AND WITHIN FLOWER CORRELATIONS." Evolution 61.12 (2007): 2913-2924. 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.Schlichting, Carl D., and Massimo Pigliucci. Phenotypic evolution: a reaction norm perspective. Sinauer Associates Incorporated, 1998. 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.Wagner, Gunter P., and Lee Altenberg. "Perspective: Complex adaptations and the evolution of evolvability." Evolution (1996): 967-976. Environmental conditions can alter or cause integration, i.e. they may be plastic.Schlichting, Carl D. "The evolution of phenotypic plasticity in plants." Annual Review of Ecology and Systematics 17 (1986): 667-693. 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.Murren, C. J., and P. X. Kover. "QTL mapping: a first step toward an understanding of molecular genetic mechanisms behind phenotypic complexity/integration." Phenotypic integration: studying the ecology and evolution of complex phenotypes (M. Pigliucci and K. Preston, eds.). Oxford University Press, New York (2004): 195-212. 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.Murren, Courtney J., Nicole Pendleton, and Massimo Pigliucci. "Evolution of phenotypic integration in Brassica (Brassicaceae)." American Journal of Botany 89.4 (2002): 655-663. 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. thumbnail|Phenotypic Integration is important for many traits. One specifically is the development of fewer bones in the neurocranium. Shortly after the structure of DNA was uncovered, Olson and Miller (1958) wrote the first book regarding the topic of phenotypic integration.Olson E.C. & Miller R.L. (1958) Morphological Integration. University of Chicago Press, Chicago. The term integration was first used in reference to genetics by Olson and Miller , referring to correlations among characters that are influenced by selection.Olson, Everett C., and Robert L. Miller. Morphological integration. University of Chicago Press, 1999. Following Olson and Miller, botanical studies on coherence between characters were done spanning over many years.Clausen, J. & Hiesey, W.M. (1960). The balance between coherence and variation in evolution. Proc. Natl Acad Sci U S A, 46, 494–506. Its first expansion was in the construction of a morphological integration genetic model constructed by Lande (1980). However, the term "Phenotypic Integration" was first coined by Pigliucci and Preston, in their book, Phenotypic Integration, which helped elucidate the observed laws of correlation and some theoretical issues regarding the topic.Pigliucci, Massimo, and Katherine (Katherine A.) Preston. Phenotypic integration. Oxford: Oxford University Press, 2004.
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