Post on 19-Dec-2015
28 May 2015 Cladistics2108.ppt 3
Cladistics
• Grouping by common descent.
• 3-taxon statement: A and B are more
closely related to each other than either is to C.
28 May 2015 Cladistics2108.ppt 4
MONOPHYLETIC group(s):
• Grouping of species including all descendants of a common ancestor and the common ancestor.
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POLYPHYLETIC group
• [Linnaeus] states as follows: “On account of their warm bilocular heart, their lungs, their moveable eyelids, their hollow ears, penem intrantem feminam mammis lactantem, …”
• “I take the good old-fashioned ground that the whale is a fish… a whale is a spouting fish with a horizontal tail.” o Melville, H. 1851. Moby Dick, Chapter XXXII.
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POLYPHYLETIC group(s)
• Include one or more species not descended from the same ancestor as the other members.
• TO BE AVOIDED!!!!!!!
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• BUT, lineages, true common ancestors, are unknown. ALL we really know are modern species.
• Fossil specimens could be ancestors, or extinct side branches, of the family tree.
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HOW TO RECOGNIZE A MONOPHYLETIC GROUP:
• Classification by shared derived characters only. • primitive &/or convergent similarities will
mislead.
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Shared derived character:
= Homology
= “Distinguishing character.” Trait, characteristic, or feature of a group found in every member of the group and not in any other group.
28 May 2015 Cladistics2108.ppt 10
Cladistics
• Avoids bias of researcher, authority.
• Grouping (classification) = scientific hypothesis.Testable by congruence of charactersRepeatable.
Cladistics
• Reading a cladogram• X-axis
• Y-axis
• Line, line segment
• Node, branching point
• Line end
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