Topic 5.4: Cladistics Flashcards
Cladistics definition
Classification of organisms into groups of species (clades)
Clade definition
Group of organisms that have evolved from a common ancestor (Clade = Ancestral organism + All evolutionary descendants)
Cladograms definition
Tree diagrams that show the most probable sequence of divergence in clades
Elements of cladograms (4)
a) Root
b) Node
c) Outgroup
d) Clade
Root
Initial ancestor common to all organisms within a cladogram
Node
Hypothetical common ancestor that speciated to more taxa
Outgroup
The most distantly related species in the cladogram
Clade
A common ancestor and all of its decendants
Construction of cladograms using structural evidence (2)
1) Organize selected organisms according to defined characteristics
2) Sequentially order organisms according to shared characteristics to construct a cladogram
a) Each characteristic is represented by a node, with more common characteristics representing earlier nodes
How have cladograms been constructed historically?
Based on structural characteristics
Why is it not reliable to establish evolutionary connections based on structural characteristics? (2)
a) Closely related organisms can exhibit very different structural features (homologous) due to adaptive radiation
b) Distantly related organisms can display very similar structural features (analogous) due to convergent evolution
Homologous traits (5)
a) Similar internal structure
b) Different external structure due to selection pressures
c) Species have a common ancestry
d) Evidence for divergence evolution
e) Pentadactyl limb
Analogous traits (5)
a) Different internal structure
b) Similar external structure due to common selection pressures
c) Species do not share a common ancestry
d) Evidence for convergent evolution
e) Wings in insects, birds, and bats
How are cladograms being constructed nowadays?
Biochemical evidence (DNA / Amino acids)
What demonstrates the degree of evolutionary divergence between two species?
The number of differences between compatible base sequences
A greater number of differences between comparable base sequences suggest…
more time has passed since two species diverged
The more similar the base sequence of two species are, …
the more closely related the two species are expected to be
Using DNA / RNA sequences to construct cladograms (2)
a) To compare closely related organisms
b) Mutations occur more readily in non-coding sequences than in gene sequences
Using amino acid sequences to construct cladograms (2)
a) To compare distantly related species
b) Slowest rate of change due to degeneracy
What happens if a sequence accumulates mutations at a constant rate?
There is a positive correlation between the number of differences between two species and the time since they diverged from a common ancestor.
Clade reclassification of figworts
a) It was the 8th largest family of angiosperms
b) After examining a chloroplast gene, they decided to split the figworts into 5 different clades and become into the 36th largest group
