Talos is considered to be a Nord.

However, all of his descendants are all Imperials. Uriel and Martin Septim - the only Septims to make in-game appearances, are confirmed by their games' Construction Sets to be Imperials.

How does that work? How can a family just change their race like that? You can change your race with console commands, but those things are part of Elder Scrolls lore.

Evolution isn't an answer, either, because evolution takes place over millions of years. The third era only lasted for 433 years. From Talos to Martin Septim, we have only 22 generations; I can trace my OWN ancestry back longer than that!

So, unless we adopt the South Park explanation of evolution (where we're all "the [censored] offspring of monkeys having? buttsix with a fish squirrel"), then I don't buy it, and if that IS the Elder Scroll's version of evolution, then why can't we observe evolution in real-time? Or at the very least, from game-to-game, e.g. new races appearing in Skyrim that weren't in Oblivion? Ok, we have Falmer, but they're supposed to have always been underground, and are only just now resurfacing, not created from hyper-evolution.

So what gives?

Your looking at it from the wrong perspective and missing many pieces. First off, we don't know that he was a nord, but lets say he was. Nords and Imperials share a common origion as one of the various Nedic peoples. As a matter of fact, Imperial and Nord are more cultural representations rather than racial because it wasn't until Morrowind there was an 'Imperial' race. Fundamentally, theres not a difference between Imperials and Nords, because their the same thing. Human.

Where as the Nedes who became 'Imperials' remained on Tamriel, the Nedes who became Nords wandered north and settled in Atmora.

Uriel and Martin aren't direct descendants of Talos.

And even if they were, race is determined by the female, not the male.

The Septim line as we know it are not direct descendants. Tibers line died with his grandson, the rest of the Septims are of his brothers stock.

Most Cyrodillic people already have at least some Nord blood, especially in the Colovian West where many lineages of Imperial rule originated.

Also, Racial Phylogeny seems to indicate that children are primarily the race of their mother, not their father. While Tiber was a Nord it's logical he took a Heartlander wife to appease his subjects, as did all the Septim heirs after him with a few notable exceptions.

Another way to look at it is that if you're a man, and the Emperor, you're an Imperial even if you had a Breton father and Nord mother or some other combination. Imperial is about more than just blood - it's what it says on the tin. You'd literally be an Imperial, being the head of the Imperial state. Human, raised in Cyrodill, educated in Imperial schools, etc. Nedic and Nordic blood...close enough for government work.

Also, as others have said, Tiber's direct blood line ended with his grandson, and it's been many many generations since his brother's bloodline took over. I'd say well over 90% of the Septim blood at the time of Uriel and his sons was pure Heartlander blood, with trace amounts of Breton, Nordic and just a little Dunmer thrown in for good measure. Imperial enough, so to speak.

One Word - War of the Red Diamond.

Wait no One Term.

What made Tiber Septim Dragonborn was the soul gifted to him by Akatosh - nothing else

I don't think being dragonborn involved in the question, but how could Tiber be a nord and his descendents Imperial was.