My cousin's mtDNA was a match to both the Mende and the Temne of Sierra Leone. This says to me that at some point, these were the same people, or some of them were the same people.

Over time, they have split off into two groups, but genetically there is overlap between the groups. They are not all the same, because I have seen other results posted where the person matched just Mende or just Temne. My cousin's results match people belonging to both groups.

I have seen info online where Dr. Kittles has said you can almost look at the geography and match it to the "alikeness" or differences between neighboring groups. For example, if a canyon separates them, you will find larger differences than if just a river separates them.... I may not be quoting him exactly, but that's the gist of it. It is easier to cross a river (and therefore mix genes) than it is to cross a canyon.