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I am testing the correlation between two physiological parameters in plants using the pic function in R. I am a bit stuck on the interpretation of the phylogenetic independent contrast (PIC). Without considering the PIC, I got a significant correlation, but there is no correlation between the PIC. What does the absence of correlation between the PIC mean:

  • the correlation without PIC is the effect of phylogeny

OR,

  • There is no phylogenetic effect on the correlation.

Thank you

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  • $\begingroup$ Welcome to Biology.SE! Can you add information about what tools you are using to do your analysis? ——— Please also take the tour and then go through the help pages starting with How to Ask questions effectively on this site. Thanks! 😊 $\endgroup$
    – tyersome
    Commented Jan 22, 2020 at 5:53
  • $\begingroup$ Hello, I am using the pic function in R $\endgroup$
    – Ariana7521
    Commented Jan 23, 2020 at 12:14
  • $\begingroup$ Comments are ephemeral, so in the future please edit your question to include that kind of information — that should increase your chances of getting an answer. In addition, the Bioinformatics site might be a more appropriate place for this type of question, especially since it isn't getting any response here. $\endgroup$
    – tyersome
    Commented Jan 25, 2020 at 21:58

1 Answer 1

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Your first option is the correct interpretation. The correlation you observe in the raw data is the effect of phylogeny.

Without using PICs, you might expect to see phylogenetic autocorrelation, i.e. your raw data correlate as a function of the amount of shared evolutionary history between different species. Using PICs is one way of testing whether the correlation still exists when the effect of phylogenetic autocorrelation is removed. This is exactly what you observe—you see the correlation in raw data (which are not phylogenetically independent) but it disappears when you use PICs. Without knowing any details of your study beyond what's in the question, it sounds like a textbook case of phylogenetic autocorrelation.

The PIC method originates with Felsenstein (1985). I'd encourage checking that paper out for further details on the method and interpretation of the results.

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