If you're seeing this message, it means we're having trouble loading external resources on our website. Show If you're behind a web filter, please make sure that the domains *.kastatic.org and *.kasandbox.org are unblocked. BACK TO DINOSAURS 2001 HOME PAGE CLADISTICSHOW, IN DETAIL, DO WE WORK OUT RELATIONSHIPS?There are several methods that have been used to work out the evolutionary relationships of organisms. The most successful of these is the so-called "cladistic" or "phylogenetic" method. This method is based on an older concept, that of "homologous characters".To begin, we supply some definitions: Character:Homologous characters are the fundamental basis of cladistics. In cladistics we look for characters that we can hypothesis are shared because they were inherited from a common ancestor. The premise is that a species develops a new character and passes that character down to its daughter species. Each of the daughter species then can add new characters, but each adds a different one, since they are now separate species and on their own evolutionary trajectory. This continues to happen through time as more and more species split off.A feature or thing we can examine or label. It is important that the feature be heritable.Homologous Character:Character which is shared by taxa by descent.Analogous CharacterShared resemblance between characters by other means than descent, such as adaptation. This is often called a homoplastic character. We see in retrospect characters shared between two species. How do we decide if a character is homologous or analogous? First we hypothesize them to be so. Then we look at the preponderance of other characters to test our hypothesis. Cladistics gives us a framework in which to do this. Cladistics was basically invented by Willi Hennig who was a specialist in flies. Here is a link to a site that describes in detail and in a different way how we do cladistics: http://www.ucmp.berkeley.edu/clad/clad1.html In cladistics we assume that we wish to focus on genealogical relationships and that our classifications of taxa should depend on our analysis of these genealogical relationships. Of prime importance is the historical sequence in which the taxa descended from a common ancestor. Hence, our cladistic hypotheses are based on our estimate of the historical sequence of the acquisition of novel characters. Next, some more definitions: Primitive Character = PlesiomorphyThe method in cladistics is to build and test relationships based on the distribution of the states of characters and to build groups by the recognition of synapomorphies.A character which is in the state shared by the common ancestor of the group.Derived Character = ApomorphyA character in a new state, not the primitive one.Shared Character:A character shared by all the members of the group.Shared Derived Character = SynapomorphyA character which is in a new state and shared by all the member of the group.Unique Derived Character = AutapomorphyA character found only in that taxon. Is a synapomorphy when discussed at the level of the members of a taxon. Let´s look at a specific example of a cladogram VERTEBRATESCICHLID -NOTE THAT FOR EACH SHARED DERIVED CHARACTER THERE IS A PRIMITIVE CHARACTER.has backbone, paired appendages (fins), dorsal nerve cord and aorta which are shared derived characters uniting the cichlid with frogs, turtles, kangaroos, mice, and humans.FROG -has all those plus legs.TURTLE -has all those plus a hard shelled egg called an amniotic egg.KANGAROO -all those plus hair, warm blood, and egg develops inside.MOUSE -all those plus placental developmentHUMAN -all those plus very large brain and loss of hair.For example, kangaroos, mice and humans all share having hair. The corresponding primitive character is "not having hair". Fish, frogs and turtles lack hair, but these shared primitive characters are not evidence of relationship, because many other organisms lack hair that are outside the group we are considering - clams, for example.NESTED SETS OF SHARED DERIVED CHARACTERS ARE DEPICTED ON THE CLADOGRAM OF THESE SYNAPOMORPHIES ALONG WITH THEIR CORRESPONDING SHARED PRIMITIVE CHARACTERS.Cladogram:IF WE PUT THESE CHARACTERS ON THE CLADOGRAM, YOU CAN SEE HOW THEY DO NOT MAKE A CLEAR PATTERN.A branching diagram depicting the hierarchical arrangement of taxa defined by the distribution of shared derived characters.But there are also characters which are unique to each group (autapomorphies), not shared with the othersCICHLID -WE CAN PUT THESE CHARACTERS ON A CLADOGRAM AS WELL.symmetrical tail fin, spiny fins.FROG -long sticky tongue, tadpole.TURTLE -shell covering body, no teeth.KANGAROO -big grinding teeth, thick tail.MOUSE -ever-growing incisors.HUMAN -opposable thumb, reduced toes, huge brain. What characters are used for constructing cladograms?The characteristics used to create a cladogram can be roughly categorized as either morphological (synapsid skull, warm blooded, notochord, unicellular, etc.) or molecular (DNA, RNA, or other genetic information). Prior to the advent of DNA sequencing, cladistic analysis primarily used morphological data.
What is the main characteristic used to construct cladograms?Constructed cladograms all typically share certain key features: Root – The initial ancestor common to all organisms within the cladogram (incoming line shows it originates from a larger clade) Nodes – Each node corresponds to a hypothetical common ancestor that speciated to give rise to two (or more) daughter taxa.
What 4 things are used to make a cladogram?How to Make a Cladogram. Introduction: How to Make a Cladogram. ... . Step 1: Pick Organisms for Your Cladogram. ... . Step 2: Pick One Ancestral and One Derived Characteristic to Designate the Outgroup. ... . Step 3: Pick Derived Characteristics for the Ingroup (Part 1) ... . Step 4: Pick Derived Characteristics for the Ingroup (Part 2). What is a character on a cladogram?“A character is a feature of an organism that can be evaluated as a variable with two or more mutually exclusive and ordered states” (Pimentel & Riggins 1987) “A character is a theory that two attributes which appear different in some way are nevertheless the same.” (Platnick 1979)
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