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CLADISTICS

 ANATOMY AND PHYSIOLOGY

   Carolus Linnaeus devised a system of classification for living things.   In this system, similar species are grouped in the same genus; similar genera are grouped in the same family, similar families are grouped together in the same order, similar orders are grouped together in the same class; similar classes are grouped into phyla; similar phyla are grouped into one of six kingdoms.

 

 

 

TYRANNOSAUR

 

HUMAN

 

KINGDOM

 

Animalia

 

Animalia

 

PHYLUM

 

Chordata

 

Chordata

 

CLASS

 

Reptilia

 

Mammalia

 

ORDER

 

Theropoda

 

Primates

 

FAMILY

 

Tyrannosauridae

 

Pongidae

 

GENUS

 

Tyrannosaurus

 

Homo

 

SPECIES

 

rex

 

sapiens

 

 

1.  Related species are classified in the same genus.

GENUS

2.      Related genera are classified in the same family.
FAMILY

Two additional comments to this family: It could be split into two subfamilies: Paninae (with Gorilla, Pan, and Homo) and  Ponginae (with Pongo).  The second comment is that while the genus and species classifications are constant (humans will always be Homo sapiens), different systems of higher classification can be used.  A more conservative classification would place humans in their own family Hominidae.  Such a classification is not consistent with cladistic and molecular evidence.

3.      Related families are classified in the same order.

ORDER

We can further subdivide an order into suborders, infraorders and superfamilies.  Humans belong to the Superfamily Hominoidea with all the apes;  apes and Old World monkeys belong to the Infraorder Catarrhini; apes, Old World monkeys, and New World monkeys belong to the suborder Anthropoidea.

 4.      Related orders are classified in the same class.

CLASS

We can further divide a class into subclasses, infraclasses, and superorders.  Humans belong to the infraclass Eutheria (with all placental mammals) and the subclass Theria (placental and marsupial mammals).

5.      Related classes are classified in the same Phylum.

We could divide Phylum chordata into Subphyla, superorders, and other groups as well.  Mammals are classified as amniotes along with reptiles and birds, all amniotes are classified as tetrapods along with amphibians, all tetrapods are classified as gnathostomes along with Osteichthyes and Chondrichthyes, all gnathostomes and some agnathans are classified as Vertebrates (subphylum), and all vertebrates and hagfish are classified as craniates.

At time of Darwin, most naturalists classified humans in their own order Bimana.  Huxley classified humans as primates but put them in their own suborder.  Darwin in 1871 boldly stated that the humans should be a family or subfamily of catarrhines.  Simpson in 1945 classified humans in the separate family Hominidae

Now Hominidae/Pongidae contains humans and apes; Homininae/Ponginae all apes except gibbons, tribe Hominini African apes, and subtribe Hominina humans and chimps (Goodman, 1994)

 

     In our Evolutionary model, all species in a genus are more closely related to each other than any of them are to any other species because all members of a genus evolved from a common ancestor fairly recently.  All the genera of a given family are more closely related to each other than any of them are to any genus outside this family because they all shared a common ancestor more recently than any share an ancestor with any other group.  In our second model, biological groups are not necessarily real. Even though we group humans, chimpanzees, monkeys, and lemurs in the same order (Primates), this doesn’t reflect ancestry.  The origin of humans had nothing to do with the origin of chimpanzees and was a completely separate event.  The origin of chimpanzees had nothing to do with the origin of monkeys and was a completely separate event.  The origin of monkeys had nothing to do with the origins of humans, chimps, lemurs, or any other group of primates because it was a completely separate event.  Even though we group primates, rodents, bats, and whales together in a class called Mammalia; these animals are not related because the origin of each group was a completely separate event.  Even though we group mammals, reptiles, birds, amphibians, and fish in a Phylum called Chordata, these animals are not related.

      Unfortunately, it is difficult to make testable predictions from the second hypothesis.  If the origin of humans, chimps, snakes, and fish were separate events, how many middle ear bones would each have?  They could all have the same number, they could vary in a way that reflects their classification (that is, the mammals could be similar to each other), or they could vary in a way that doesn’t reflect their classification.  ANYTHING COULD BE OBSERVED.  What bones make up the skull?   If all of their origins were completely separate, anything could be observed.  Is there a renal portal vein? If all of their origins were completely separate, anything could be observed. 

CLADOGRAM
CLADOGRAM

      If evolution occurred, we can draw a relationship based on cladistic phylogeny which depicts relationships between organisms in a testable cladogram.  Only similarities are represented without reference to time.  Monophyletic groups arise from a common ancestor and there should be evolutionary novelties that all members of the group share.  Such a group is a clade or a natural group.  In the second cladogram above, dinosaurs form a clade or monophyletic group. Polyphyletic groups are groups which do not have a common ancestor included in the group.  In the first cladogram, the two major groups of dinosaurs evolved separately from different groups of thecodonts (which are not dinosaurs) and dinosaurs would be a polyphyletic grouping.

      Paraphyletic groups are lineages that do not include all descendants of a common ancestor.  In both of the dinosaur cladograms, “thecodonts” are a paraphyletic group because not all of the descendants of the first thecodonts are considered thecodonts (some are called dinosaurs).  Are humans apes? Are we amphibians? Are we fish?  If your answer is “no”, then apes, amphibans, and fish are paraphyletic groups since the group “fish” does not include modified descendants of fish (us) and the group “apes” does not include modified descendants of apes (us) (Fastovsky, 1996; Lucas, 2004).

 

     Cladograms (developed by William Hennig in 1950) have changed the way we look at phylogenetic relationships.  Since they are not built on ancestral and descendent species only, they are not necessarily affected by gaps in the fossil record and it is not necessary to show all the members of a monophyletic group.  They are based on a hierarchy of characteristics.  Refer to the second dinosaur cladogram above.  If dinosaurs and thecodonts form a clade, there would be general characteristics that all thecodonts and dinosaurs share (that would have evolved in the first thecodonts and would not have appeared in the more primitive reptiles that gave rise to them). If all dinosaurs form a clade, there would be derived characteristics that all dinosaurs would share.  If the Saurischian dinosaurs are a clade, there would be derived characteristics that all Saurischian dinosaurs would share.  Organisms are not grouped based on overall similarity but on whether they possess new derived features (Fastovsky, 1996).