What_is_PIC

Phylogenetic Independent Contrasts (PIC) is a subset of phylogenetic comparative methods, which use information on the evolutionary relationships of organisms (phylogenetic trees) to test for correlated evolutionary changes in two or more traits. PIC is a statistically-based approach that uses the phylogenetic tree and evolutionary branch lengths as a guide to determine whether two or more quantitative characters are evolutionarily correlated. By using a phylogeny, it avoids being misled by correlations that are due to inheriting similar characters, rather than characters changing together. For example, primates have proportionally larger brains than other mammals and are more intelligent, but these two traits could have just happened to evolve in the ancestor of all primates once and so do not truly reflect correlation (it's effectively one data point duplicated many times by speciation). It would be more convincing if we found large brains and great intelligence evolved together in several different groups, like whales, primates, parrots, and crows. PIC can correctly distinguish correlations due to the same states being inherited from correlations due to traits genuinely evolving together using just the trait values and a tree with branch lengths.

For example, Kelly and Southwood (1999) used PIC to determine that tree species' abundance is positively correlated with insect-species richness, where more common trees have greater diversity of associated insect fauna. A PIC study of seals and sea-lions (Mottishaw et al., 1999) examined diving adaptation characters in relation to increasing dive times. Although they found that characters comprising the 'dive reflex', such as apnea (suppression of breathing) and bradycardia (reduced heart rate) showed no correlation with dive times, other characters, such as body mass, spleen size and blood volume were significantly correlated with longer dive times.

For someone who is doing data analysis, PIC can be considered as a new set of "characters" with evolution history subtracted. Thus the correlation between two or more PIC characters becomes meaningful.

References

Comparative Methods: http://en.wikipedia.org/wiki/Phylogenetic_comparative_methods

Independent constrasts: http://www.biology.ualberta.ca/courses.hp/biol606/OldLecs/Lecture2K.03.Penney.html

Kelly, C.K. and T.R.E. Southwood. 1999. Species richness and resource availability: A phylogenetic analysis of insects associated with trees. Proc. Natl. Acad. Sci. USA 96: 8013-8016.

Mottishaw, P.D., S.J. Thornton, and P.W. Hochachka. 1999. The diving response mechanism and its surprising evolutionary path in seals and sea lions. Amer. Zool. 39: 434-450.