The branch connected with nomenclature,identification,and classification is:

Why does the problem of classification become more complex at higher taxonomic categories?

Consider the following diagram and fill in the blank spaces/boxes.

$A :$ Species is a genetically closed system.
$R :$ Because the reproductive isolation constitutes the most important boundary between different species.

If an organism belongs to different orders but the same phylum,it may also belong to the same ......... .

Give a scientific reason: Classification is essential for understanding the inter-relation among living organisms.