The Dunedain of the North and of the South

[Olmer]

Hello back to you, Eärniel Your intuition did not fail you. Here I am on my favorite subject - the Contriving Elves.

Whereas in Gondor the Nazghûl themselves could come to Eärnur and gaud him into a duel on his own doorstep.

It was personal. Remember who defeated the Witch-King and destroyed Angmar? Other than that Nazghul had never bothered with whoever inheriting the crown of Gondor.

Mordor would have known fairly accurately who was ruler in Gondor....but would Sauron have possessed that same knowledge of the Northern line?

Sauron was not giving a flying pig about Arvedui's line. I suspect, he knew quite well what kind of "help" they have got from Gondor and was fairly sure that Gondor won't make any efforts to let them rise again.
However he was not aware of the Oathbreakers and the powerful curse. Otherwise he would be much more interested in Isildur's heirs.
The elves knew and kept it secret, taking care of Dunedain's line.

... but their safety lay in secrecy and hiding. They had no court, no obvious abode, no official treaties with other people, they were far harder to pin down.

Oh, they had abodes besides Elves's court. All right. And I assume it were quite large, so for 16 generation they were managing to keep the blood pure, which means it was more than enough of available purebred maidens, not too close relatives and with king's blood.
Gondor, seems, with all their potentials and intentions did not have such luxury of choices.

And coupled with the protection (even if only for the formative years) of the heirs in Rivendell, as well as some good education they could get there, the Northern line had an edge.

Agree on education. Teaching an assembly of very well trained, experienced in combat and grateful Numenorian’s descendants to an absolute loyalty to the elves .
Protection...not much. Only to keep the existence of heirs secret up to the point, and then they will be on their own. Fighting, dying...I think Elrond entirely secured his secret weapon by making sure that in absence of a direct heir it will be someone next in line, who will evoke the summons of the Dead.

[Valandil]

Sorry Olmer... I just can't imagine Elrond running a nursery/day care for all the little Dunedain descendants of Arvedui.

[Lefty Scaevola]

Originally Posted by Olmer

And I assume it were quite large, so for 16 generation they were managing to keep the blood pure, which means it was more than enough of available purebred maidens, not too close relatives and with king's blood. .

yeah, apparently they had a large enough population to avoid ill effects from inbreeding. Either that or they were lucky emnough to have a gene pool with few or no bad recessive genes. Or at least, Tolkien never bothered to write about "a Habsburg jaw" hereditary deformity.

[Valandil]

On your original question, Grey Wolf, it may be partly that the northerners were more careful about who they married - but also, I think their realistic marriage partners were almost exclusively other Dunedain.

In the first thousand years of the Third Age, Arnor did not make attempts to expand the kingdom. And they were hemmed in - arctic areas (with some bands of Lossoth) to the north, ocean to the south, Misty Mountains to the east (with some hill people) and Elves to the west. About all they could have done was expand into Dunland, or else try to expand over the Misty Mountains. They kept to themselves, whereas Gondor was always expanding in this time. In Gondor, while they were rare, political royal marriages to other peoples would have always been a possibility. In Arnor, there was no apparent need for this (until Arvedui and Firiel married in 1940).

In the next thousand years, there was first prolonged civil war among Arnor's daughter kingdoms, then the long campaign against the upsurging land of Angmar. Again - no others to marry with besides their own people.

And in the last thousand years of the Third Age, I think Eriador was largely depopulated altogether - with most Dunedain living in small bands, probably striving to stay connected with one another.

In the north, the idea of intermarriage would have been a tough thing to happen anyway. In Gondor, it may have been almost inevitable.