The above diagram
shows the evolutionary and morphological relationships among
four taxa A, B, C, & D:
A & B
are most similar phenetically: the morphological difference
measured between them on the horizontal axis is smallest.
B & C are most similar patristically:
the amount of change that
separate them as measured along the branches of the tree is
smallest.
C & D are most closely
related cladistically: of the three branch points in the
tree, they have a more recent common ancestor (MRCA)
than any other pair in the tree.
By analogy with an actual tree, A & B are twig tips that are closest together on the outside of a tree ("as the crow flies"), despite being on different branches. B & C are closest together "as the ant runs". C & D are on the same stem "as the branches grow," despite the tips being very far apart.
Substituting real
biological examples for the hypothetical taxa: lizards and crocodiles have a superficial phenetic similarity due to their
elongate, scaly bodies and four-footed stance. However, it has
long been recognized that crocodiles are more closely related to
dinosaurs and birds as Archosauria, which accounts for a
number of shared characteristics,e.g., crocs, birds, and
(presumably) dinosaurs have four-chambered hearts and
make vocal sounds. Within this group, crocodiles and dinosaurs have undergone
relatively little morphological change since the Mesozoic, which
results in their patristic
similarity (e.g., crocodilians and dinosaurs are scaly,
terrestrial, diapsid tetrapods). Among Archosauria,
one of the two orders of Dinosauria
and the many orders of Aves (modern
birds) have the closest cladistic
relationship (with an MRCA sometime in the Cretaceous
period), but because of adaptations associated with the
evolution of flight, birds are morphologically distinctive.
Concepts of
morphological similarity and degree of change
are rather subjective. Morphological similarity depends on
features chosen: crocodile and lizards limbs are quite
different, more so that the differences between the latter and
the remnants of snake limbs. Limb structures are also quite
different between the two orders of dinosaurs, the four-legged Saurischia
("lizard-hipped") and two-legged Ornithischia ("bird-hipped")
(the hip joint of the latter is only superficially similar to
that of modern birds (Aves), which evolved from the Saurischia).
The subjectivity of perceptions of similarity is
well-illustrated by films in the "Jurassic Park"
franchise. Dinosaurs are depicted so as to resemble living
birds, rather than as distinctive terrestrial megafauna in their
own right. The recent discovery that many dinosaur species had
feathers reinforces the impression that they are bird-like,
rather than that birds are dinosaur-like, because of our long
association of feathers exclusively with birds.