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Lecture 11

BIOLOGY 1M03 Lecture Notes - Lecture 11: Hox Gene, Even-Toed Ungulate, Homeobox

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Ben Evans

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Biology 1M03 Lecture 11
The cladistic (maximum parsimony) approach to inferring a phylogeny focuses on
synapomorphies, which are shared derived characters
o A synapomorphy is a novel trait that a clade of organisms have and that others outside this
clade lack
Homology and Homoplasy
Similarities are inherited from a common ancestor
Homeobox (Hox) genes regulate differentiation within the adult and embryo
o How do we know that Hox genes of humans and fruit flies are homologous?
Similar DNA sequences (including homeobox domain)
Similar genomic organization
Similar expression patterns during development
Similar function
Other species closely related to fruit flies and humans also have Hox gene with these
Homology is something that is similar amongst species because they inherit this trait from a
common ancestor
o It occurs when traits are similar due to shared ancestry
o Occurs when traits are similar for reasons other than common ancestry
o Similar traits can evolve independently in two distant species
o Eg. Birds and bats
Bird and bat wings are not homologous even though the bones are homologous
Convergent evolution
o Occurs when natural selection favours similar solutions to the problems posed by a similar
way of life
o Is a common cause of homoplasy
o Eg. Dolphins and ichthyosaurs both have streamline bodies, long jaws, fins and flippers, but
their ancestors did not
Maximum parsimony assumes that the best explanation or pattern is the one that implies the
least amount of change
Convergent evolutions and other causes of homoplasy should be rare compared with similarity
due to shared ancestry
o The tree that implies the fewest overall evolutionary changes should be the one that most
accurately reflects what happened during evolution
Whale Evolution: Case Study
Traditionally, cladograms are based on morphological data place whales outside of the
artiodactyls, mammals that have hooves, an even number of toes, and an unusual pulley-shaped
ankle bone (astragalus)
The astragalus is a proposed synapomorphy that previously identified artiodactyls as a
monophyletic group
Support for the "whippo hypothesis"
o Recent data on gene sequences called short interspersed nuclear elements (SINEs) shows
that whales and hippos share SINE genes that are absent in other artiodactyl groups
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