I am running several phylogenetic least squares analyses in R, where I'm taking an existing data set for several species, and adding two new species for which I have data. I want to do is test whether the new all-species regression is significantly different than the old regression. But I am unsure of the appropriate test to use.
The existing data set establishes a regression that should be applicable to all species. I can add data on two species, and what I find is that one of the two species falls more or less on regression predicted by the original data set. The other species fall way below the regression predicted by the original data set. So I guess what I'm trying to determine is if there are significant differences in both slope and intercept, as well as significant differences in the strength of the regression.