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Class notes introductory biology (BIOL1010) Lesson 12
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Phylogeny, Systematics and Microbial DiversityPhylogeny
● A hypothesis for the evolutionary history of a species/ group of species
Systematics
● A discipline focused on classifying organisms and determining their evolutionary
relationships
Linnaean Classification Systems
Binomial nomenclature
- Two-part format of a scientific name
- The first part is the genus, the second part is the specific epithet
Hierarchical classification
- Groups species into a hierarchy of increasingly inclusive categories
Classification Issues
- A phylogenetic tree represents a hypothesis about evolutionary relationships
- If a species has lost a key feature shared by its close relatives it will be
reclassified to reflect evolutionary history
Reconstruction of phylogeny
● Focus on features that result from a common ancestry (homologies)
● Do not be tricked by analogies because these are convergent evolutions and do
not represent a common ancestor
Homology vs. Analogy
Homology: Similarity in characteristics from shared ancestry
Analogy: Similarity between two species due to convergent evolution
Homologous forelimbs but Analogous as wings (Birds & Bats)
● Had common tetrapod ancestor
● Skeletal systems of bats and birds are homologous
● Bats have stretch membrane for wings, birds have feathers
● Wings arouse independently
● Wings are analogous to another
, Molecular systematics: the discipline that uses data from DNA and other molecules to
determine evolutionary relationships.
Problems with nucleic acid comparisons
- Species closely related differ only at a few sites. This can be from an insertion or
deletion causing the bases to shift a notch.
- However, the computer will not take this into account
- Difficult to distinguish between homology and analogy
Cladistics: common ancestry is the primary criterion used to classify organisms.
Species placed in groups called clades, which includes an ancestral species and all of
its descendants
Monophyletic clade
● Consists of ancestral species and all of its descendants
Paraphyletic clade
● Consists of some ancestral species and some of the descendants
Polyphyletic clade
● Includes taxa with different ancestors
Shared ancestral characters
● A character that originated in an ancestor of the taxon
Shared derived characters
● An evolutionary novelty unique to a clade
Phylogeny and shared derived characters
● Characters unique to particular clades
● Possible to determine the clade where each shared derived characters first
appeared
● Infers evolutionary relationships
Ingroup
● The species being studied
Outgroup
● Species/group of species form evolutionary lineage that is known to have
diverged before the lineage of the studied species
○ By comparing we can determine which characters are derived at various
branch points of vertebrate evolution
○ Identifies early branch point
How systematist use principles in reconstructing phylogenies
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