5.4 Cladistics Flashcards
What is cladistics?
Cladistics is a method of classifying organisms into groups of species called clades (from Greek ‘klados’ = branch)
What does each clade consist of?
Each clade consists of an ancestral organism and all of its evolutionary descendants
What will members of a clade share?
Members of a clade will possess common characteristics as a result of their shared evolutionary lineage
What is the purpose of organising clades?
Clades can be organised according to branching diagrams (cladograms) in order to show evolutionary relationships
What are cladograms and what do they represent?
Cladograms are tree diagrams where each branch point represents the splitting of two new groups from a common ancestor
What does each node represent?
Each branch point (node) represents a speciation event by which distinct species are formed via divergent evolution
What do cladograms show in terms of divergence?
Cladograms show the probable sequence of divergence and hence demonstrate the likely evolutionary history (phylogeny) of a clade
What does a fewer number of nodes show?
The fewer the number of nodes between two groups the more closely related they are expected to be
What do cladograms show in terms of evolution?
Cladograms can show evolutionary relationships and demonstrate how recently two groups shared a common ancestry
What evolutionary history do humans share with primates i.e. what common clade?
Humans, chimpanzees, gorillas, orangutans and gibbons all belong to a common clade – the Hominoids
What larger clade are hominoids a part of?
The Hominoid clade forms part of a larger clade – the Anthropoids – which includes Old World and New World monkeys
What 4 features do cladograms share?
root
nodes
outgroup
clades
What is a root on a cladogram?
Root – The initial ancestor common to all organisms within the cladogram (incoming line shows it originates from a larger clade)
What is a node on a cladogram?
Nodes – Each node corresponds to a hypothetical common ancestor that speciated to give rise to two (or more) daughter taxa
What is an outgroup on a cladogram?
Outgroup – The most distantly related species in the cladogram which functions as a point of comparison and reference group
What are clades on a cladogram?
Clades – A common ancestor and all of its descendants (i.e. a node and all of its connected branches)
Based on what two factors can cladograms be constructed?
Cladograms can be constructed based on either a comparison of morphological (structural) features or molecular evidence
Historically, structural features were used to construct cladograms, but molecular evidence is now more commonly used
How to construct a cladogram
https://ib.bioninja.com.au/standard-level/topic-5-evolution-and-biodi/54-cladistics/cladograms.html
What chemical similarity do all organisms have?
All organisms use DNA and RNA as genetic material and the genetic code by which proteins are synthesised is (almost) universal
Due to DNA almost being universal, what can be compared to ascertain evolutionary relationships?
This shared molecular heritage means that base and amino acid sequences can be compared to ascertain levels of relatedness
What will accumulate in DNA over the course of time?
Over the course of millions of years, mutations will accumulate within any given segment of DNA
What does the number of comparable base sequences demonstrate?
The number of differences between comparable base sequences demonstrates the degree of evolutionary divergence
What does a greater difference between comparable base sequences suggest?
A greater number of differences between comparable base sequences suggests more time has past since two species diverged
Therefore from base sequences, how can we determine whether organisms are evolutionarily related?
Hence, the more similar the base sequences of two species are, the more closely related the two species are expected to be
What molecular sequences can scientists compare?
When comparing molecular sequences, scientists may use non-coding DNA, gene sequences or amino acid sequences
Why is non-coding DNA compared?
Non-coding DNA provides the best means of comparison as mutations will occur more readily in these sequences
Why do gene sequences mutate at a slower rate?
Gene sequences mutate at a slower rate, as changes to base sequence may potentially affect protein structure and function
What sequence has the slowest rate of change?
Amino acid sequences may also be used for comparison, but will have the slowest rate of change due to codon degeneracy
Therefore what are amino acid sequences used to compare?
Amino acid sequences are typically used to compare distantly related species (i.e. different taxa)
What are DNA/RNA base sequences used to compare?
while DNA or RNA base sequences are often used to compare closely related organisms (e.g. different haplogroups – such as various human ethnic groups)
Can mutations accumulate at a constant rate?
YES
Some genes or protein sequences may accumulate mutations at a relatively constant rate (e.g. 1 change per million years)
What can constant mutation be used for?
If this rate of change is reliable, scientists can calculate the time of divergence according to the number of differences
E.g. If a gene which mutates at a rate of 1 bp per 100,000 years has 6 bp different, divergence occurred 600,000 years ago
What is using mutations at a constant rate to date divergence called?
This concept is called the molecular clock
What are 3 limitations of a molecular clock?
Different genes or proteins may change at different rates (e.g. haemoglobin mutates more rapidly than cytochrome c)
The rate of change for a particular gene may differ between different groups of organisms
Over long periods, earlier changes may be reversed by later changes, potentially confounding the accuracy of predictions
Historically, what was classification based off of?
Historically, classification was based primarily on morphological differences (i.e. structural characteristics)
Closely related species were expected to show similar structural features, indicating common ancestry
What are two key limitations of using morphological differences as a basis for classification?
Closely related organisms can exhibit very different structural features due to adaptive radiation (e.g. pentadactyl limb)
Distantly related organisms can display very similar structural features due to convergent evolution
What is convergent evolution?
Convergent evolution is the independent evolution of similar features in species with distinct lineages
When may convergent evolution occur?
It may occur when different species occupy the same habitat and are thus subjected to the same selection pressures
What is an example of convergent evolution?
The shared conditions cause common adaptations to be selected in different species, resulting in structural similarity
An example of convergent evolution is the development of wings in birds, bats and insects
Why are structural features not commonly used to determine clades?
Structural traits are not commonly used to determine clades as such features may not necessarily indicate shared heritage
What are homologous structures?
Traits that are similar because they are derived from common ancestry are termed homologous structures
What are analogous structures?
Traits that are superficially similar but were derived through separate evolutionary pathways are termed analogous structures
By what process do homologous structures arise?
arise via divergent evolution
by what process do analogous structures arise?
arise via convergent evolution
What evidence has debunked many evolutionary similarities (based on morphology)
Using molecular evidence, scientists have discovered that many species thought to be closely related based on shared structural characteristics actually demonstrate distinct evolutionary origins
Give two examples of molecular evidence debunking previous evolutionary relationships
Crocodiles have been shown to be more closely related to birds than lizards, despite closely resembling lizards in structure
Many species of plants previously classified as figworts have been reclassified based on molecular evidence
What are figworts?
Until recently, figworts were the 8th largest family of flowering plants (angiosperms), containing 275 different genera
Why was the figwort group not perfect?
This was problematic as many of the figwort plants were too dissimilar in structure to function as a meaningful grouping
How was the figwort split into different clades?
Taxonomists examined the chloroplast gene in figworts and decided to split the figwort species into five different clades
How has reclassification affected the figwort family?
Now less than half of the species remain in the figwort family – which is now the 36th largest among angiosperms
