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Lecture 16

BIOL 243 Lecture Notes - Lecture 16: Paraphyly, Molecular Clock, Ingroups And Outgroups

Course Code
BIOL 243
Gordon Chua

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[THEME 5A Phylogeny]
Phylogeny is the history of decent - as species proliferate and their evolutionary
relationships branch out, evolutionary and genetic relationships between organisms.
The last common ancestor is identified as a node. If two or more species share a
common ancestor not shared by any other species, they are considered closest
relatives., or sister groups.
"Groups" are also called taxon. They usually emphasize monophyletic groups in which
all individuals share a common ancestor. For example, tetrapods share a common
ancestor. In contrast, a group such as "reptiles", which excludes birds (whom do share
the same ancestor), is simply a paraphyletic group. Finally, polyphyletic groups do
not include the last common ancestor of all members (ex/ birds and bats).
For birds and bats, they are grouped together because of their wings (which is an
example of a character state). However, birds wings and bat wings developed
independently of eachother and are therefore analogous. If a character trait were to be
similar because of descent from a common ancestor, it is homologous.
A synapomorphy or synapomorphic character state is a trait that is shared by two
or more taxa and inferred to have been present in their most recent common ancestor,
whose own ancestor in turn is inferred to not possess the trait. Phylogenetic
reconstructions based on synamorphic character states is called cladistics.
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