- Uses
o Binomial nomenclature
o Hierarchical arrangement
- Incompatible with some aspects of evolution
o But still widely used
o Forms basis of vertebrate biology
Binomial Nomenclature
- Father of modern taxonomy = Carolus Linnaeus
- Standardised naming system
- Identify to every known species
- Latinized
o Universal language for early scientists
o Also to avoid confusion between different languages
- Old system approach= inappropriate
o Species were treated as inanimate objects
o Does not give a general idea of what species may and may not be related
to other species
- Therefore: Evolutionary information incorporated into biological classification
Hierarchical arrangement
- Hierarchical groups= grouping of species into higher categories
- Based on anatomy (+ physiology and behaviour)
- King Phillip Came Over For Good Soup
o Kingdom
o Phylum
o Class
o Order
o Family
o Genus
o Species
▪ Contentious because the biological species concept (reproductive
isolation) cannot be applied to extinct species, thus, classification
today relies heavily on genetic data as well as morphology,
behaviour, and anatomy
Phylogenetic Systematics
- Traditional biological classification replaced by Phylogenetic systematics
- Systematics
o The theory and practice of classification (incorporates identification,
nomenclature, and the practice of classification)
- This branch of science aims to discover the structure of the branching patterns
(tree) of relationships that is created by speciation
1
, - Hypothesis about evolutionary relationships expressed as a tree = Phylogeny
- Tree terminology
o A tree consists of nodes, branches, and
‘leaves’/terminal nodes (figure to the
right)
o The 2 black dots on the left-hand side of
this tree are terminal sister taxa because
they are connected to each other by the
common ancestor represented by the
white dot below them.
o The same tree can be drawn in different
ways
▪ C and D are sister taxa- they are the most recently derived taxa in
the tree
▪ C and D are also closely related to B
▪ A is basal to the clade containing B, C and D
Cladistics
- Pioneer = Willi Hennig
- Approach: Evolutionary lineages based on derived characteristics
- Monophyletic lineages = Clades
o A clade implies that there is common ancestry for everything that is
connected by monophyletic lineage
o The way we classify using cladistics is by differentiating what is
ancestral and what is derived
o Apomorphic character
▪ Derived/advanced.
▪ Shared between two or more species/taxa.
OR
▪ Unique to a single species/taxon
o Synapomorphic
▪ Shared, derived characteristics.
▪ Both shared and derived
▪ Evidence of common ancestry (accounts for all similarities)
o Autoapomorphic
2
, ▪ Unique
▪ Shared but not derived.
o Pleisiomorphic character
▪ Primitive (Ancestral)
▪ Usually shared (except where extinctions have occurred)
▪ Shared
Typical question for Cladistics
1. Given the following morphological characteristics for three ingroup taxa (A-C),
and one outgroup taxon (D), and assuming that only one stripe can be gained
at a time, infer the evolutionary relationships of these taxa, in the form of a
tree, using a cladistic approach:
Taxon number Tail Number of toes Number of dorsal
stripes
A Absent 5 3
B Absent 4 4
C Present 4 4
D Present 5 2
Indicate all mutational steps on the tree, with a description of each change
Answer:
1) For a four-taxon case, there are 3 possible trees/ evolutionary hypotheses
→ Draw 3 trees
2) Insert outgroup
3) Add ingroup taxa ensuring that all possible terminal sister taxa relationships
are represented
3
, 4) Map characteristics
→ Tail: Taxon A and B do not have a tail
→ Toes: Taxon B and C have 4 toes (the outgroup D= 5 toes)
→ Stripes: outgroup has 2 stripes, A has 3 stripes, C has 4 stripes and B has 4
stripes
5) Identify best evolutionary hypothesis (principle of parsimony- simplest is best)
→ Tree with least amount of steps will be best evolutionary hypothesis
→ The last tree (with 5 steps) is most parsimonious
6) What else can this tree tell us?
→ Common ancestors to terminal sister taxa (from most parsimonious
explanation)
→ Characteristics of common ancestor to the terminal taxa
o 4 stripes
o 4 toes
o Tail
Synapomorphy: Shared, derived character
Autoapomorphy: Unique, derived character
Symplesiomorphy: Shared, ancestral character
Cladistic trees guide Taxonomy:
- Partition primitive and derived characteristics
- How?
o Use close relatives (outgroup) to the group under study (ingroup) → this
gives direction or polarity.
o Once there is direction then ancestral and derived characteristics can be
identified
o Competing hypotheses/trees can be considered and the best tree is
selected using the principle of parsimony.
- Monophyletic groups (clades)
- Why?
4
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