Why does the cladistic approach to classification suggest that the class reptilian (reptiles) is not a phylogenetic classification? Explain.

The class reptilian is not a phylogentic classification because it is incomplete as a subgroup. Phylogenetic classification (in the cladistic approach of classification) refers to complete groupings of organisms that are identified as groups or subgroups and that derive from a common ancestor, shown by a shared genetic stem.  If a subgroup is incomplete, meaning it has lost, through divergence of genetic traits and characteristics, part of its subgroup, then that incomplete subgroupis classified...

The class reptilian is not a phylogentic classification because it is incomplete as a subgroup. Phylogenetic classification (in the cladistic approach of classification) refers to complete groupings of organisms that are identified as groups or subgroups and that derive from a common ancestor, shown by a shared genetic stem.  If a subgroup is incomplete, meaning it has lost, through divergence of genetic traits and characteristics, part of its subgroup, then that incomplete subgroup is classified in the cladistic approach as paraphylogentic (incomplete), as reptiles are, and not monophylogenetic as birds and mammals are.  It is because the subgroup of reptiles is incomplete, having lost birds as they diverged genetically away from reptiles, that reptiles are not a phylogenetic classification.

The cladistic approach is a classification approach to assign organisms to a group that share similar inherited characteristics from a common ancestor.  These groups of organisms are called clades.  The group mammals would be a clade.  The members of this group are warm-blooded, generally have fur, and give live birth to young.  The group aves (birds) is also a clade.  The members of this group are also warm-blooded, have feathers, and give birth to young by laying eggs.  Both these groups share inherited physical characteristic from one common ancestor. Reptiles are in the same cladistic grouping under the same phylogentic classification of Amniotica even though Reptiles are cold-blooded, have neither fur nor feathers, and give birth to their young by laying eggs.  They share enough inherited characteristics from a common ancestor to be included in the same phylogenetic ranking.  But the Reptile subgroup loses the extension subgroup of birds and so is excluded from being a complete phylogentic group as mammals and birds are in phylogentic classification according to the cladistic approach.