[mytailorisrich]

A few instances of "new blood" in a population can be enough because it will then spread over generations.

In many countries the proportion of foreign-born is >10%, so the mixing is quite intense in terms of evolutionary time scales.

If that continues for a few hundreds of years, diversity will have significantly reduced. Indeed, some suggest that e.g. skin colour may largely homogenise within a thousand years.

A thousand years might sound like a very long time wrt. a single individual's lifespan, but for these issues it is nothing at all. In addition, some traits (e.g. pale skin colour) are quite recent from what we know.

[bryanrasmussen]

So Science Fiction Story time, immortal finds it difficult to blend into future less diverse population because their personal characteristics are no longer widely enough found that they can just wander off and go live somewhere else.

The last X is an easy target.

on edit: for anyone unfamiliar with this trope, see https://en.wikipedia.org/wiki/Requiem_for_Methuselah or https://en.wikipedia.org/wiki/The_Man_from_Earth (both written by Jerome Bixby, but many other examples by Neil Gaiman, etc. available) for easy clarification. One of the common ideas of the trope is that an immortal sooner or later becomes problematic to have in one place, they must wander of people will attempt to experiment on them to figure out the secret of immortality. Of course the second bit of the trope is the wandering immortal leave traces, and more intense searchers after immortality will follow them and attempt capture. If diversity decreases in the future based on present trends the story then becomes that it becomes increasingly difficult for the wandering immortal to wander without everyone being aware of them.

[nobody9999]

There are a number of other stories which address this as well.

Sometimes without the trope you mention (L. Sprague de Kamp's The Gnarly Man[0])

And sometimes with a variation on it (Heinlein's Methuselah's Children[1]), which he follows up with some of the same characters, but not invoking the trope (Time Enough for Love[2]).

There are many others of course, those are just the ones that came off the top of my head.

[0] https://en.wikipedia.org/wiki/The_Gnarly_Man

[1] https://en.wikipedia.org/wiki/Methuselah%27s_Children

[2] https://en.wikipedia.org/wiki/Time_Enough_for_Love