nucleotide substitution model

Question

[sayyarsehrish]

in nucleotide substitution models following terms are used; invariable sites (+I), rate of variation across sites (+G), gamma distribution, rate heterogeneity any one knw definition of these terms ? plz ur help will be highly appreciated.

[microfuge]

I could be wrong but rate heterogeneity is typically for variation in mutation rate across genome. The mutation rate itself is a distribution as it varies (usually modelled as a gamma distribution).