The several groups and subgroups are particularly situated as branches of the tree to indicate ordered lineal relationships between all the organisms shown.
Further, any group may (or may not) be considered a taxon by modern systematics, depending upon the selection of its members in relation to their common ancestor(s); see second and third diagrams.
The term monophyly, or monophyletic, derives from the two Ancient Greek words μόνος (mónos), meaning "alone, only, unique", and φῦλον (phûlon), meaning "genus, species",[4][5] and refers to the fact that a monophyletic group includes organisms (e.g., genera, species) consisting of all the descendants of a unique common ancestor.
By comparison, the term paraphyly, or paraphyletic, uses the ancient Greek prefix παρά (pará), meaning "beside, near",[4][5] and refers to the situation in which one or several monophyletic subgroups are left apart from all other descendants of a unique common ancestor.
The concepts of monophyly, paraphyly, and polyphyly have been used in deducing key genes for barcoding of diverse group of species.
A
phylogenetic tree
: both blue and red groups are
monophyletic
. The green group is
paraphyletic
: it is missing a monophyletic subgroup – the blue group – that shares a common ancestor with itself. In this form,
monophyletic
means "no sideways stems leaving the group".
A cladogram of the
primates
, showing a
monophyletic
taxon: the
simians
(in yellow); a
paraphyletic
taxon: the
prosimians
(in cyan, including the red patch); and a
polyphyletic
group:
the night-active primates, i.e., the
lorises
and the
tarsiers
(in red).
A cladogram of the vertebrates showing phylogenetic groups. A
monophyletic
taxon
(in yellow):
the group of "reptiles and birds"
, contains its most recent common ancestor and all descendants of that ancestor. A
paraphyletic
taxon (in cyan):
the group of reptiles
, contains its most recent common ancestor, but does not contain all the descendants (namely Aves) of that ancestor. A
polyphyletic
"group" (in red):
the group of
all warm-blooded amniotes
(Aves and Mammalia), does not contain the most recent common ancestor of all its members; this group is not seen as a taxonomic unit and is not considered a taxon by modern
systematists
.
