In the construction of a phylogenetic tree, a considers the overall similarities among a group of species without trying to understand their evolutionary history, whereas a considers the possible pathways of evolution to find the best possible tree.

The given blanks can be filled with phenetic approach and cladistic approach.

Explanation:

The study of the pathways of evolution is termed as cladistics. A person who deals with cladistics looks for queries like the number of branches present among the group of organisms, the branching sequence, and the associations between the branches.  

On the other hand, the study of the associations between the group of organisms based on the extent of similarity among them is termed as phenetics, in such case similarity can be based on phenotype, molecular or anatomical level.  

Hence, to construct a phylogenetic tree, the overall similarities amongst the group of species can be done by using a phenetic approach, and the possible pathways of evolution can be predicted by using a cladistic approach.