Sea caves are formed by erosion
Lava tubes are formed by pressure/flow
Limestone caves are formed by solution
"chloroplast and animals" are not a match, since chloroplast only exists in plants and smalls cells for the purpose of utilizing photosynthesis. Animals do not require this process.
The allele for sickle cell anemia would be an example of natural selection. In most situations, having crescent shaped red blood cells would hold less oxygen & would not be ideal. But in places that are affected by diseases like malaria, which attack red blood cells, having crescent blood cells minimizes the threat malaria can cause.
Hope this helps!
Because if the human population grow the Arctic grows faster too. It had to be the same to balance it,