Nearly every Somali speak the "northern Somali dialect". There are barely any dialects of af-soomali to begin with. The only major one that exists is Maay and only Rahanweyn speak it.
Genetically, culturally and linguistically, Somalis from north to south are homogenous. Its impossible to tell difference between a Hawiye and an Isaaq or a Somali from Djibouti or Garissa. But of course qabiliyaad prompt many Somalis to believe there is a difference. If you tell a German from Berlin that he is a different ethnicity from a German from Hamburg, he would laugh at your face because he isn't poisoned by tribal thoughts.
This cursed civil war has turned brother against brother. In the absolute worst case scenario, we may see Isaaqs become like Macedonians like
@Som had once suggested.