5.4 Cladistics

Clades

Consists of an ancestral organism and all of its evolutionary descendants

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Members will possess common characteristics as a result of their shared evolutionary lineage

Node

A branch point in a cladogram which represents a speciation event by which distinct species are formed via divergent evolution

Hominoids

Common clade consisting of humans, chimpanzees, gorillas, orangutans and gibbons

Anthropoids

Consists of hominoid clade and includes old world monkeys and new world monkeys.

Key features of cladograms

Root, nodes, outgroup, clades

Root (feature of cladogram)

The initial ancestor common to all organisms within the cladogram

Nodes (feature of cladogram)

Each node corresponds to a hypothetical common ancestor that speciated to give rise to two (or more) daughter taxa

Outgroup (feature of cladogram)

The most distantly related species in the cladogram which contains functions as a point of comparison and reference group

Clades (feature of cladogram)

A common ancestor and all of its descendants

Two ways to construct a cladogram

1. Structural evidence
2. Using molecular evidence

Structural evidence (creating a cladogram)

1. Organise selected organisms according to defined characteristics
2. Sequentially order organisms according to shared characteristics to construct a cladogram

Molecular evidence (creating a cladogram)

1. Select a gene or protein common to a range of selected organisms
2. Copy the molecular sequence (DNA or amino acid) for each of the selected organisms
3. Run a multiple alignment to compare molecular sequences (DNA or amino acid)
4. Generate a phylogeny tree (cladogram) from multiple alignment data

How is DNA used to see the degree of evolutionary divergence?

* The number of differences between comparable base sequences, demonstrates the degree of evolutionary divergence.
* More differences = more divergence
* More similar = more closely related species

Why is non-coding DNA used as a means of comparison?

Because mutations will occur more readily in these sequences

Mutations in gene sequences

These mutate at a slower rate, as changes to base sequence may potentially affect proteins structure and function

Mutations amino acid sequences

have the slowest rate of change due to codon degeneracy

When are amino acids used vs. DNA or RNA to analyze species

Amino acids = compare distantly related species

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DNA/RNA base sequences = used to compare closely related organisms

Molecular clock

When genes or protein sequences may accumulate mutations at a relatively constant rate

Limitations molecular clock

* Different genes or proteins may change at different rates
* 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

Limitations in using morphological differences as a basis for classification (structural evidence)

* Closely related organisms can exhibit very different structural features due to adaptive radiation
* Distantly related organisms can display very similar structural features due to convergent evolution

Convergent evolution

The independent evolution of similar features in species with distinct lineages

Homologous structures

Traits that are similar because they are derived from common ancestry

Analogous structures

Traits that are similar but were derived through separate evolutionary pathways. (same function, different structure)

Reclassification of figworts

* Figworts were the 8th largest family of flowering plants containing 275 different genera
* This was problematic as many of the figwort plants were too dissimilar in structure to function as a meaningful grouping
* Taxonomists examined the chloroplast gene in figworts and decided to split the figwort species into five different clades
* Now less than half of the species remain in the figwort family – which is now the 36th largest among angiosperms