The modern world with intercontinental travel is a relatively new thing. Even with that, many people don’t travel far: https://www.forbes.com/sites/lealane/2019/05/02/percentage-of-americans-who-never-traveled-beyond-the-state-where-they-were-born-a-surprise/
There’s plenty of reproductive isolation; especially when we’re talking about timelines on the orders of thousands of years here. It’s a myth that humans are some sort of ‘melting pot’, as we really haven’t been until this latest generation; due to ease of access to travel, better living conditions, access to birth control, and evolving social acceptance.
Doesn’t matter if not everyone is traveling far to reproduce, it only takes a few people to introduce a blob of diversity into an otherwise isolated population and suddenly all their ancestors become contributors to that areas gene pool. Without repeated introductions it won’t form a large part but it will form part. For example most people have direct neanderthal and denisovian ancestors and it’s not estimated that pairing between modern humans and those populations were all that regular an event and yet their genes are everywhere.
It only takes a few people to introduce a blob of diversity into an otherwise isolated population and suddenly all their ancestors become contributors to that areas gene pool.
I researched into this, and was incredibly surprised. The only compatible offspring of neanderthals and modern day humans, were with a male homo neanderthalensis, and a female homo sapien. Estimates are you’d only need 1 successful interbreeding every 70 generations or so.
If it’s that low, that makes me ponder: How is it we had both enough genetic isolation for speciation to occur, but enough proximity for pairing with modern humans? Denisovans also interspersed among our population, much the same way.
And if it takes that little, why do we see so many distinct genetic populations today? We see distinct differences in bone structure, birth canal shape, height, metabolism, and the article says even specific notable changes to immune system.
It’s because there aren’t distinct populations like you perhaps imagine them being, it’s more like a smeared colour pallet where one area might be a bit more red or a bit more blue but it’s hard to say a specific area is pure blue. The distinct features or populations exist as statistical probabilities based on likely ancestry for a given area. Any given individual in a population probably doesn’t express all the “unique” features, but over the total population those features are most prevalent.
Regarding Neanderthals and denisovian populations, they were probably more like what we’d call subspecies in other animals than truly distinct species from modern humans, isolated long enough to build up some unique genetic markers but not quite long enough to be fully separate.
