I am looking at the family trees of Dwarves and ran into this question: how did non-Durin folk end up in the company of Thorin? Ori, Dori, Nori, Bifur Bofur and Bombur are all not of the Durin clan, but all other Dwarves of the company are from the line of Durin.
Is this ever explained by Tolkien? Is there a difference between the dwarves of separate clans? Did the Durin separate themselves from the others at any point after or during the events of The Hobbit?
These chaps were of Durin's line, but presumably remote enough that they didn't figure into the (simplified) family tree. This is revealed in a footnote to the Dwarvish family tree printed in Appendix A:
Of the other companions of Thorin Oakenshield in the journey to Erebor Ori, Nori, and Dori were also of the House of Durin
Return of the King Appendix A "Annals of the Kings and Rulers" III "Durin's Folk"
This is unknown, but there was a kinship between them and Thorin, mentioned in the same footnote I referenced above:
[M]ore remote kinsmen of Thorin: Bifur, Bofur, and Bombur were descended from the Dwarves of Moria but were not of Durin's line.
Return of the King Appendix A "Annals of the Kings and Rulers" III "Durin's Folk"
They most likely accompanied Thorin because of that Kinship. However, it's worth noting that this isn't the first time the other clans have assembled for the benefit of the Line of Durin; in Appendix A, it's said that they came to help recover Khazad-dûm from the Orcs:
Durin's Folk fathered all their host, and they were joined by great forces sent from the Houses of other Fathers; for this dishonour to the heir of the Eldest of their race filled them with wrath.
Return of the King Appendix A "Annals of the Kings and Rulers" III "Durin's Folk"
Not a significant one it's principally historical. All Dwarves are descended from one of the Seven Fathers, which is where the clan distinction came from.
As one of the Seven Fathers of the seven races of the dwarves, all of the Longbeards should be descended in the paternal line from Durin. Thus we might suppose that Bifur, Bofur, and Bombur were also patrilinaly descended from Durin but that descent was no longer recorded since their families split off from the line of the kings thousands of years ago.
Of course many dwarves of the other six races and kingdoms might have migrated to Moria and become subjects of the King of Durin's Folk over the thousands of years since Durin's time. In fact the appendixes to The Return of the King say that many dwarves from Belegost and Nogrod settled in Moria at the end of the First Age more than six thousand years before The Hobbit, so Bifur, Bofur, and Bombur could easily have been descended from father to son from some others of the Seven Fathers of the Dwarves.
