What's the difference between Homogeneous and non-homogeneous gamma models?

•homogeneous gamma model: -gives rise to two descendent populations (D1 and D2) each of which has the same site- specific rates as the ancestral population. • non- homogeneous gamma model: -gives rise to two descendent populations (D3 and D4) in which the site-specific rates can be different from those in the ancestral population. -each descendent population D3 and D4 contains the same number of slow, moderate and fast sites. that means the same for each of these individual descendent populations .