I learned that if animals are separated by a geographic barrier and if over time they inbreed, it introduces a new species. Why does this not occur with humans which are separated by continents and have been around for such a long time.
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Because it has not been a long time. We left Africa about 65000 years ago, and in the grand scheme that is nothing.
Besides, there have been adaptations to the areas.
It did, to a degree. There are quite obvious genetic differences between humans that live in different parts of the globe, precisely because the evolutionary pressures in those different areas rewarded different traits. Humans that remained in Africa have different traits than humans that migrated north vs. migrated east.
The reason we are still the same species is that not enough time has really passed for the genetic differences to get _so_ pronounced that we would say we are a different species. Humans only really started migrating around the globe 60-90k years ago; combined with our very long lifespans, there just haven’t been all that many generations of people. **If** we kept those geographic boundaries in place then likely we would have separated into multiple species at some point, but modern technology has made travel fairly routine.
In a large population like humans have in most places, it takes a long time for changes to propagate. A small population can evolve much faster, because you have fewer carriers of each allele or characteristic. If they die, the trait is gone.
Also, evolutionary time is measured in generations. We have a fairly long generation compared to most animals and especially compared to bacteria and viruses. Every generation is one opportunity for random variations and the environment to affect your reproductive prospects. So everything takes longer for us.
Also, we’re humans. We don’t let the environment affect our reproductive prospects very much, because we have a lot of control over our environment. We make sure everybody is fed and cared for. It takes a lot for an environmental factor to kill off a subgroup of us that has a heritable trait.
Also, regional human populations have never been completely reproductively isolated. There’s always been some flow of people or genes across the border areas, even if it was slow at times.
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