BIOL 2040 Lecture Notes - Lecture 13: Paraphyly, Distant Relatives, Branch Point
27 Apr 2018
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Lecture 13 - Phylogenies
April 10, 2018
1:33 PM
Constructing phylogenies
• How to figure out what phylogenies fit in
• Phylogenetic trees depict the core of what makes a phylogeny
• Patterns of common ancestry
o Mothers' mitochondrial DNA is shared with us and our siblings
o To figure out when our cousins' mitochondrial DNA is shared with us, we have to go back
further in the tree to a distant ancestor
• To find shared mitochondrial DNA with someone who is less related to us, we have to
go back even further
o Pattern depicts a phylogeny - it's a pattern of having a common ancestor
Phylogeny and Common Ancestry
• Close relatives share a more recent common ancestor
• Distant relatives share a more distant (older, further in the past) common ancestor
Nesting Common ancestors
• A small family is nested into a larger phylogeny branch
• Ex. a family of humans is nested into a branch of homosapiens, nested into a branch of primates,
and nested into larger branches all the way back to the beginning of life
• Branch point: most recent ancestor diverges from these points
Dichotomous branching
• After something branches off, changes from the most recent branching points are independent of
each other
• Ancestral branch reflects many characteristics existing in the branched off species
Sister taxa
• Taxon (plural taxa)
o A named group of organisms at any level of phylogeny
o Ex. eukaryotes, pines, mice, amniotes
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• Sister taxa
o Lineages that diverge from the same ancestral node on a phylogenetic tree
o Even if you 'rotate' the nodes, if you changed what descendent is on what end of the sister
taxa at the same node, it still means the same thing - that those two species diverged from
the same node
• What species is read from left to right doesn't tell you what came first - just that they
descended from the same common ancestor
• What matters is the pattern of common ancestry
Some Terminology
• Monophyletic:
o All of the descendants of a common ancestor
o Question: What is the sister taxon to group 3?
• Sister taxon means the other branches from the most recent common ancestor node
• Sister taxon to group 3 would be groups 1 and 2
• Paraphyletic:
o A group that includes a common ancestor and all but one of its' descendent lineages
o It is the most common occurrence
o Ex. prokaryotes (archaea and bacteria) are paraphyletic to eukaryotes
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Homology of Bone structures
• It is the basic structure that arose in a common ancestor and has been modified in different
lineages
• Homology defined: similarity that is caused by inheritance from a common ancestor
o Things similar in structure even if function is different
• Things not a homology?
o Between a shark and an orca: similarity is functionally necessary, but not inherited from a
common ancestor
• The similarity is due to repeat or convergent evolution
Shared genetic defect
• Implies defect arose in common ancestor before splitting off into groups
• Ex. the same repeated gene was found in humans and chimpanzees in the same chromosome
Shared derived characters
• Pseudogenes: a section of a chromosome that is an imperfect copy of a functional gene
o If these were developed before the lineage split, the groups split from the node shares the
same character
o They make up the derived characters from the common ancestor
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find more resources at oneclass.com
