Biological classification is a dynamic and ever-evolving phenomenon that keeps changing with our understanding of life forms. Justify, the statement by taking any two examples.

Classification has been a centre piece of biology ever since Linnaeus, providing a frame work on which existing knowledge can be organised and predictions about unknown traits can be made.
But the basis of biological classification has gone through a series of upheavals over the last 3 centuries, from being considered a plan in mind of the creator, to the neutral assessment of over all similarity, to a reflection of evolutionary niches and finally to a phylogenetic mapping of the tree of life. e.g., in two kingdom classification their are two groups, i.e., Plantae and Animalia.
These groups are made on the basis of structural and cellular difference like plant possess cell wall, central vacuole, whereas animals lack cell wall and central vacuole but the drawback in this system classification is that eukaryotic and prokaryotic organisms like Euglena, bacteria and fungi are not included in this kingdom classification. 
That are later added in three kingdom classification that in (kingdom–Protista) finally 1969, Whittaker purposed five kingdom classification. 

These classifications are done on the basis of increase in understanding regarding organism by the advancements made in scientific techniques and by observing minute detailing that has led this classification more detailed and scientifically correct.