Final answer:
Homologous characteristics are the only phenotypic feature used in constructing phylogenies to identify ancestor-descendant relationships. Analogous traits are not reliable for this purpose because they may result from convergent evolution. Phylogenetic trees are based on derived homologous characteristics that indicate shared ancestry.
Step-by-step explanation:
The only phenotypic feature that can be used in constructing phylogenies to identify close ancestor-descendant relationships is the presence of homologous characteristics. These are traits that species have inherited from a common ancestor. In comparison, analogous characteristics might appear similar but do not indicate a close evolutionary relationship because they are the result of convergent evolution, arising independently in different lineages. Constructing a phylogenetic tree involves sorting homologous and analogous traits and organizing the homologous traits using cladistics to form clades or monophyletic groups.
Modern phylogenetic analysis also considers genetic and molecular analyses in addition to morphological and fossil data. Thus, the construction of a phylogenetic tree that accurately represents evolutionary relationships is largely based on derived homologous characteristics, which have emerged after the split from a common ancestor and are unique to specific lineages or groups of organisms.