Wouldn’t the limited genetics cause weaker offspring? AND is it truly the same species if it originated from the same two parents?
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Geneticist here.
So biology is always a game of chances. Children of siblings or close relatives do indeed have higher chances of having birth defects, but not always and not necessarily.
Narrowing down the genetic pool (called bottleneck effect) more often leads to extinction than not, but not necessarily.
It depends on a lot of things. If the initial parent/siblings had, per chance, rather good genetic variability (like, heterozygotes on most genetic position), or they didn’t have too many bad alleles, or the environment wasn’t too challenging until they could restore variability by de novo mutations, then there’s a practical chance to get back from bottleneck. It’s just rare.
A bit more Eli5. If you are lucky enough and didn’t inherit the bad genes from your parents, nor did your sibling, then you cannot pass it down even if you have children with the sibling. In reality you have to be really lucky for that, because most people do have some inherited ticking bomb in their genes (we just don’t know about it), and there’s a *bigger* chance that our siblings have the same than a stranger has it.
Because we have all genes it two copies and usually one is enough to be healthy, the real problem starts if we inherit two bad versions. And simply, inheriting two of the same bad copies within a genetic pool of close relatives is more probable than from a stranger.
As you see, there’s chance to come back from a really narrow bottleneck, but that species must be very lucky. And yes, it still counts the same species.
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