Reconstructing Evolutionary Relationships
Phylogeny of Terrestrial Vertebrates
In this example we will construct a phylogenetic hypothesis of
relationships between seven taxa of terrestrial vertebrates: frogs, turtles,
mammals, birds, crocodiles, lizards, and snakes. We will use another
taxon (ray-finned fish) – which has some similarities with your terrestrial
vertebrates but which is not part of that group – as our out-group to
polarize the characters chosen for the analysis.
The first step in every analysis is identification of the characters. This
is often the most important and most difficult part of the work (not to
mention time and labour consuming). Below is the list of the characters,
each of which is accompanied by the corresponding number used in Table
1. Table 1
contains a list of the character states found in each taxon.
(You are not expected to learn these characters and their states for the
quiz or test.)
: Does the egg have the amnion, an
extra-embryonic membrane that evolved to over-
Possible states: amnion present, absent.
Appendages (2): Early vertebrates evolved fins to
swim more efficiently in the water. Limbs allowed
movement on land. Although snakes lack external
limbs, several groups of snakes have rudimentary
appendages, proof of the fact that the ancestors of
snakes did have limbs.
Possible states: fins, limbs.
Body covering (3):
Possible states: scales, scutes,
Metabolism (4): Maintenance of a steady internal
state (homeostasis) is critical. In vertebrates, body
temperature is controlled either internally by
metabolic processes (endothermy) or is dependent on
the external environment (ectothermy).
Possible states: endothermy, ectothermy.
Internal nostrils (5): The nostrils of most fishes do
not connect with the mouth. The nostril of lungfishes
and of all terrestrial vertebrates do.
Possible states: internal nostrils present, absent.
Temporal fenestrae (6): Number of temporal holes
in the skull, which allow for the expansion of jaw
muscles (enhancing biting strength).
Possible states: none, one, or two.
Male genitalia (7): Presence or absence of a
hemipenes, or split penis. Snakes and lizards are
unusual in that they have a double penis, whereas
other vertebrate groups have a single penis.
Possible states: hemipenes present, absent.
Digestive system (8):
Presence or absence of
a specialized gizzard, a very muscular portion of
the stomach. The muscle action, together with
a tough lining of cuticle, and especially grit that
is ingested, aid in the grinding of fibrous foods
such as seeds.
Possible states: gizzard present, absent.
Nitrogenous waste (9):
Type of nitrogenous
waste. Ammonia is converted to either urea or
uric acid before excretion.