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For 1. No, it is a lot more complicated than that. For instance, Dogs, Wolves, and Coyotes can all interbreed with each other. But even if you treat all dogs as one species, they can't actually all interbreed with eachother without human intervention due to size differences. Then there are ring species, which can breed with two separate species that cannot breed with each other. That isn't even getting into species that can asexually reproduce.

There was a nice article in The Economist a few months back describing the issues with coming up with a good definition of "species": https://www.economist.com/schools-brief/on-the-origin-of-spe...

That concept (populations which can share genes) is for species which have sexual reproduction. For populations which require special conditions, you can talk about subspecies.

But not for the rest of lifeforms. Bacteria reproduce by cloning themselves and they can share genes laterally (between adults). And viruses are really more similar to computer viruses with intended replication errors than a lifeform.

Well some species have both sexual and asexual reproduction, many trees can self fertilize or fertilize with another trees pollen for instance.

Look into the wonderful world of Gene McCarthy, PhD.

http://www.macroevolution.net/

"Biology, hybrids, human origins and more"

> I think the effort to minimise the differences between early humans is admirable, and should be continued. We have a tiny, farcically small sample size of evidence to base any conjectures about early humans on. Being inclusive about Homo Sapiens because it stops nazis being dicks about race is a good thing.

Science should not be corrupted by the goal of countering human irrationality. It's no use: racists will use "science" to rationalize their positions whether or not mainstream science agrees. And it's harmful: the more science deviates from the pursuit of truth, the less we all can trust it.

The ability to mate is just one among numerous phenotypical characteristics. Whether to elevate it to anything special of course depends on the fuzzy boundary of whether or how much it is actioned on.

Canis latrans can mate with canis lupus and they are not considered the same species, just fyi.

> if two populations can mate, then they're the same species

Wouldn't that mean homo sapiens and neanderthals are the same species, given that many people have neanderthal genes and interbreeding is even a hypothesis for the extinction of the neanderthals?

1. Common counter-example: Horses and donkeys are considered separate species. They can produce offspring. There have been rare cases of that offspring being fertile. Not so clear.

Traditionally, two members of a genus (but of different species) make infertile offspring.

Two members of a species make fertile offspring.

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This isn't a "perfect" definition, but the delineation is clear in most cases. In your case, a horse and a donkey make infertile offspring (aka: a mule), and are therefore different species but the same genus.

What I'm saying is there are documented cases of mules being fertile and having healthy offspring.

"because it stops nazis being dicks about race"

Nothing is effective enough for those extremes. But for the rest of society, specially those during school years, this is enlightening and hopefully enough to push agaisnt our inherent racism.