Models of evolution applied to primate tetherin sequences.

*The likelihood ratio test (LRT) was used to compare two nested models of sequence evolution, in which one model is a more complex version of the other. The LRT is used to justify the additional assumptions that are incorporated in the more complex model (or to reject the more complex model in favor of the simpler model) by asking whether the additional assumptions result in a statistically significant improvement in fit to the data. For the three comparisons indicated, the second model only differs from the first in permitting some form of positive selection to act on individual sites. The LRT is conducted by calculating twice the difference in maximum likelihood scores for the two models under comparison and then finding the chi-square critical value. For a recent discussion of these models and their evaluation using the LRT, see reference [24].
d.f. - degrees of freedom.