BIO 3 Cladogenesis - BIO153: Lecture 3 Cladistics January...

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1 BIO153: Lecture 3 Cladistics January 12, 2009 You may find it helpful to refer to the Glossary of terms used in Phylogenetic analysis (in the folder entitled “Supporting Material for Lectures”). Topics: comparing phenetics, Linnaean taxonomy and cladistics steps in conducting a cladistic analysis the principle of parsimony some practical applications of phylogenetic analysis Recall from lecture 2 that characters can be shared through convergence (two unrelated organisms resemble each other because they have been shaped in the same way by natural selection); characters can be shared by descent (two organisms resemble each other because they are closely related). (Example: horses and zebras share many characters because they are closely related (the ancestor of zebras and horses was around not long ago, and there has been relatively little time for horses and zebras to diverge from each other). On the other hand, zebras and tigers share the character of having striped fur, but this does NOT indicate a close evolutionary relationship – the ancestor that gave rise to both zebras and tigers was not striped, and stripes were acquired independently in both lineages.) Only characters shared by descent help us reconstruct evolutionary relationships. Characters shared through descent may be symplesiomorphies or synapomorphies Symplesiomorphies = ancestral homologies shared characters inherited from a distant common ancestor
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2 Character is usually not found in all descendents (Why? If the common ancestor is distant, a lot of time has passed and there has been a lot of opportunity for the character to be modified in at least some of the descendents.) Synapomorphies = derived homologies Shared character is inherited from a recent common ancestor Character is usually found in all descendents (Why? If the common ancestor is recent, little time has passed and thus the character has probably not been modified in the descendents.) Because synapomorphies are usually found in all descendents, synapomorphies are reliable indicators of shared ancestry. Because symplesiomorphies are often not found in all descendents, symplesiomorphies are not reliable indicators of shared ancestry.
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This note was uploaded on 04/25/2010 for the course BIOLOGY BIO153 taught by Professor Cordon during the Fall '10 term at University of Toronto- Toronto.

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BIO 3 Cladogenesis - BIO153: Lecture 3 Cladistics January...

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