I think what "we observe in nature" is debatable. Kropotkin was taking part in that debate, although it was over 100 years ago.

But apparently in 1998, respected evolutionary biologist Stephen Jay Gould found Kropotkin to be "no crackpot" too. (Article found from wikipedia reference).

https://www.marxists.org/subject/science/essays/kropotkin.ht...

There's plenty of cooperation between population members not directly related observed in nature, that's literally what Kropotkin's whole book is about.