5.4 Cladistics Flashcards
What is cladistics?
Cladistics is a method of classifying organisms into groups of species called clades
What does each clade consist of?
An ancestral organism and all of its evolutionary descendants
What will members of a clade possess?
Common characteristics as a result of their shared evolutionary lineage
What are cladograms?
They are tree diagrams where each branch point represents the splitting of two new groups from a common ancestor
What is a branch point also called?
Node
What does each node represent?
A speciation event by which distinct species are formed via divergent evolution
What do cladograms show?
Cladograms show the probable sequence of divergence, the likely evolutionary relationships and how recently they showed a common ancestor
The fewer the number of nodes between two groups…
The more closely related they are expected to be
What clade do humans, chimpanzees, gorillas, orangutans and gibbons all belong to?
The hominoids
What does the hominoid clade belong to?
A larger clade called the anthropoids
What are the four key features of a cladogram?
Root
Nodes
Outgroup
Clades
What is a root?
The initial ancestor common to all organisms within the cladogram
What is a node?
Each node corresponds to a hypothetical ancestor that speciated to give rise to two or more daughter taxa
What is an outgroup?
The most distantly related species in the cladogram which functions as a point of comparison and reference group
What are clades?
A common ancestor and all of its descendants
What can cladograms be constructed based on?
Structural evidence or molecular evidence
What is the preferred evidence used when constructing cladograms?
Molecular evidence
What are the steps to constructing a cladogram based on structural evidence?
- Organise selected organisms according to defined characteristics
- Sequentially order organisms according to shared characteristics to construct a cladogram
What are the steps to constructing a cladogram based on molecular evidence?
- Select a gene or protein common to a range of selected organisms
- Copy the molecular sequence for each of the selected organisms
- Run a multiple alignment to compare molecular sequences
- Generate a cladogram from multiple alignment data
What does a shared molecular heritage mean?
Base and amino acid sequences can be compared to ascertain levels of relatedness
What do all organisms use DNA and RNA for?
Genetic material
What demonstrates the degree of evolutionary divergence?
The number of differences between comparable base sequences
What does 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
What do scientists use when comparing molecular sequences?
Non coding DNA
Gene sequences
Amino acid sequences
Tell me about non coding DNA when comparing molecular structures?
Non coding DNA is the best for comparison as mutations will occur more often in the sequences
Tell me about gene sequences when comparing molecular structures?
Gene sequences mutate at a slower rate as changes in base sequences may affect protein structure and function
Tell me about amino acid sequences when comparing molecular structures?
Whilst amino acid sequences are used for comparison they have the slowest mutation rate due to codon degeneracy
What are amino acid sequences typically used to compare?
Distantly related species
What are DNA and RNA base sequences typically used to compare?
Closely related organisms
What is there a positive correlation between?
The number of differences between two species and the time since they diverged from a common ancestor
If the rate of change is reliable what can scientists calculate?
Scientists can calculate the time of divergence according to the number of differences
What is the molecular clock?
How scientists can calculate the time of divergence according to the number of differences using the reliable rate of change of mutations
What are factors that limit the molecular clock?
- different genes and proteins change at different rates
- The rate of change for a particular gene may differ between different groups of organisms
- Earlier changes may be reversed by later changes potentially impacting the accuracy of predictions
What was classification historically based on?
Structural characteristics
What were closely related species expected to show?
Similar structural features
What are the two limitations of using structural differences for classification?
Closely related organisms can show different structural features due to adaptive radiation
Distantly related organisms can show similar 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?
When different species have the same habitat and have the same selection pressures
What does shared conditions cause?
Common adaptations in different species causing similar structural features
What are homologous structures?
Traits that are similar because of common ancestry
What are analogous structures?
Traits that are superficially similar due to common selection pressures but happen through separate evolutionary pathways
What has molecular evidence helped scientists show?
That species once thought to be closely related due to similar structural features actually showed very different evolutionary origins
Why were figworts problematic?
Many of the figwort plants had too different structures to function as a meaningful grouping
What did the taxonomists examine to split the figwort species into smaller clades?
The chloroplast gene
What size were figworts before and after reclassification?
8th largest to 36th largest
