Biology 1001A Lecture Notes - Lecture 17: Ingroups And Outgroups, Hybrid Swarm, Convergent Evolution

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30 Apr 2012
Biology Lecture 17: Phylogeny
Speciation is Reversible:
o Speciation can be reversed, if we can go back to the formation of a hybrid swarm. This
can occur if the isolating mechanisms breakdown.
o Phylogeny: an attempt to reconstruct the evolutionary history of all life on earth
o We know that all life on earth came from one common ancestor, however different
species had more recent common ancestors than others
Homology vs. Homoplasy:
o Homology: similarity due to a recent common ancestor
o Homoplasy: not all similarity reflects common ancestory
misleading similarity (convergence): two species do not have a common recent
ancestor, but they appear similar because they share the same habitat
misleading dissimilarity (divergence): two species are closely related, but since
they inhabit different environments or have different selection pressures they
do not appear to be similar
Homology vs. Homoplasy:
o Homologous traits: lead you to uncover the true phylogeny
o Homoplasious traits: mislead you to the wrong phylogeny
o To determine whether traits are homologous or homoplasious , you can use cladistic
o In cladisitic analysis, only shared and derived traits (synapomorphies) are informative
o Traits common or shared among two or more groups
o Presumably, these traits are similar due to homology because they both inherited a trait
from a common ancestor
o Must also be derived (of recent evolutionary origin)
Types of traits:
o Synapomorphies:
Shared and derived
o Symplesiomorphies:
Shared, but ancestral (present in common ancestor) not informative
o Autapomrphies:
Unique to one species (not shared), derived within the group not informative
Outgroup Analysis:
o Ancestral trait: trait is present in the outgroup and in all members of the ingroup
o Ancestral trait: present in the outgroup and some of the ingroup
o Derived trait: absent in the outgroup, and present in some of in group
o If the trait is present in the outgroup, and completely absent in the ingroup we have no
idea, thus it is not informative
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