A phylogenetic tree represents an hypothesis about the evolutionary relationships between different organisms. It is built using characteristics shared between or unique to the organisms in question, whether they are behavioural, morphological or molecular. The layout of the tree groups the organisms based on these characteristics or their absence. Therefore the first step to constructing a catgirl phylogenetic tree is to choose catgirls and score their features (Table 1).

Luna



Sailor Moon

Limbs: Quadruped

Nose: Feline

Paws/hands: Feline

Tail: Present

Cat ears: Dorsal

Haru



The Cat Returns

Limbs: Biped

Nose: Feline

Paws/hands: Feline

Tail: Present

Cat ears: Dorsal

Natsuki



Hyperpolice

Limbs: Biped

Nose: Human

Paws/hands: Human

Tail: Present

Cat ears: Dorsal

Ichigo



Tokyo Mew Mew

Limbs: Biped

Nose: Human

Paws/hands: Human

Tail: Present

Cat ears: Dorsal

Meruru



The Vision of Escaflowne

Limbs: Biped

Nose: Human

Paws/hands: Human

Tail: Present

Cat ears: Lateral

Alicia Rue



Sword Art Online

Limbs: Biped

Nose: Human

Paws/hands: Human

Tail: Present

Cat ears: Lateral

Catherine



Gintama

Limbs: Biped

Nose: Human

Paws/hands: Human

Tail: Absent

Cat ears: Dorsal

Neko-Musume



Gegege no Kitaro

Limbs: Biped

Nose: Human

Paws/hands: Human

Tail: Absent

Cat ears: Absent

Yoruichi



Bleach

Limbs: Biped

Nose: Human

Paws/hands: Human

Tail: Absent

Cat ears: Absent

Lina Inverse



Slayers

Limbs: Biped

Nose: Human

Paws/hands: Human

Tail: Absent

Cat ears: Absent

Using Luna as the representative of the outgroup, it is clear that the first derived trait is bipedality, and the second trait is the presence of more human facial features and human hands. Haru represents a rare example of a catgirl with human shape but paws and a cat nose, although this may be because she is a human partly transformed into a cat.

Natsuki and Ichigo display the typical catgirl form. There are plenty of other examples, such as Blair and Izutsumi. Dejiko was excluded because her ears and paws are a hat and gloves, making it impossible to tell whether she has human hands and ears underneath or not, thus she couldn't be easily placed on the tree.

There are two separate groups which differ from the typical form. One has their ears on the side of their heads, exemplified by Meruru and Alicia Rue. The other branch is separated by the loss of the tail, while their ears remain on top of their head, as seen in Catherine. A further development can be seen in Yoruichi and Neko-Musume, both of whom lack cat ears as well as the tail (Figure 1).