Cladistics
An approach for systematics to classify organisms using ancestral relationship as the primary criteria
Clades
Group of organisms which are classified together
Monophyletic group
A clade where all members are descendants of their recent common ancestor
Concept is relative to reference point
Paraphyletic group
A clade where all members are descendants of their most recent common ancestor but there are more descendants of that ancestor which were not included
Is an incomplete representation of organisms with close evolutionary relatedness
Polyphyletic group
A clade which includes members belonging to a different evolutionary lineage
Concept is also relative to reference point
Algae
Eukaryotic photoautotrophs labelled algae
Is a polyphyletic group
Guidelines to keep in mind during classification:
- Keep track of shared ancestral and shared derived
characters as they get passed on from the ancestor to descendants - Very complicated traits, shared across organisms,
usually do not occur due to convergent evolution
Ingroup vs outgroup
- Ingroup: organisms you want to classify (frog, cow, dog)
- Outgroup: a species closely related to the ingroup, but does not share any common trait (fish)
Build a phylogenetic tree assuming these traits
arouse only
once during evolution
Maximum parsimony
- When there are multiple ways to describe a phenomena, the simplest explanation is most likely the correct one
- Organisms are classified with maximum parsimony as the guiding principle
Classification using genes
- To resolve the discrepancy, evolution events for all sites are mapped onto all possible
scenarios - The scenario which requires the least number of changes overall is the most parsimonious explanation
Why are computers essential for molecular genetics
- Amount of calculation required to resolve the tree increase drastically with more species
Maximum likelihood
resolve molecular phylogenetics while taking into account the different chances of mutations
Why is taxonomy not straight forward
- Horizontal gene transfer (HGT): Genes (which represent morphological and biochemical traits) can jump from one lineage to another unrelated lineage
What is the fundamental assumption during classification of traits
a trait gets transferred from ancestor to descendant via reproduction
* Horizontal gene transfer breaks this assumption and
makes it much harder to discern relationships between
organisms
