Karla,
The units of sigsq are in your trait units^2 per time unit of your
phylogeny. So if your trait is in cm and your phylogeny in millions of
years, then the units of sigsq are in cm^2/my.
Comparing BM to OU units is more complicated. Gene Hunt discussed this
in the following paper:
http://paleobiol.geoscienceworld.org/content/38/3/351.short
Long story short, they are in the same units, but it may not be
particularly meaningful to compare them between OU and BM models. For
example, you can have an extremely high sigsq in the OU model, but if
alpha is very high as well, you may end up with very slow rates of
evolution when observed over the lifetime of a phylogeny (specifically,
the stationary variance of the OU process is sigsq^2/(2*alpha), thus if
alpha is high enough the stationary variance will tend towards 0, and
all the traits across taxa will be virtually identical).
Hope this helps.
Josef Uyeda
On 12/07/2015 03:35 PM, Karla Shikev wrote:
Dear all,
I am fitting a BM model using fitContinuous, a vector of trait values and a
time-calibrated tree and then I use the sigsq parameter as my measure of
the rate of evolution of the trait in question. My questions are:
(1) what is the unit of the sigsq parameter?
(2) to what extent is the sigsq in a BM model comparable to sigsq in an OU
model, for instance? are they on the same units?
Thanks!
Karla
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