As a child I remember having a book with a collection of novellas that revolved around a common theme that went like this:
Survivors of an ill-fated expedition arrive on a distant planet and create, via genetic engineering, miniaturized versions of humans to live in the shallow waters of a local ocean. The mini humans farmed amoebas and fought a fish (or a lizard, can't recall).
Another batch of humans was adopted to live in tree tops above a canopy in a heavy vegetation. There was also a third story, can't recall what about.
I apologize for not being able to recall more. I hope this is enough. Thanks all!