Topic 5.4 Cladistics

Question 1

Cladistics

Answer 1

A method of classifying organisms into groups of species called clades.

Question 2

Clade

Answer 2

Consists of one ancestral organism and all of its evolutionary descendants.

• Members of one clade will possess common characteristics (because of shared evolutionary lineage)
• Organized according to branching diagrams (cladograms) to show evolutionary relationships

Question 3

Cladograms

Answer 3

Tree diagrams where each branch point represents the splitting of two new groups from a common ancestor.

• One node (branch point) represents one speciation event (formation of distinct species formed via divergent evolution)
• Show probable sequence of divergence, demonstrating likely evolutionary histories (phylogenies) of clades

Question 4

Cladograms with humans and other primates

Answer 4

Evolutionary relationships between humans and other primates: they share a common ancestry.

• Humans, chimpanzees, gorillas, orangutans, and gibbons belong to a common clade (hominoids)
• Hominoid clae forms a part of a larger Anthropoids clade (including Old and New World monkeys)

Question 5

Features of cladograms

Answer 5

• Roots: an initial ancestor common to all organisms within the cladogram
• Nodes: each corresponds to a hypothetical common ancestor that speciated
• Outgroup: the most distantly-related species in the cladogram; functions as a point of comparison and reference group
• Clades: common ancestor and all of its descendants

Question 6

Construction of cladograms

Answer 6

Done so utilizing structural evidence (ex. physical characteristics) or molecular evidence.

Question 7

Molecular evidence in evolution

Answer 7

All organisms use DNA and RNA, genetic material and the genetic code by which proteins are synthesized is (almost) universal.
- Base and amino acid sequences can thus be compared to ascertain levels of relatedness

Question 8

Comparing molecular evidence

Answer 8

• Number of differences between comparable base sequences demonstrates the degree of evolutionary divergence (greater differences suggest more time since divergence, lesser differences suggest closer relation)
• Non-coding DNA occur more readily, providing best means of comparison (typically utilized to compare closely-related organisms)
• Gene sequences mutate slower, as changes in base sequence can affect protein structure/function
• Amino acid sequences have slowest rate of change due to codon degeneracy (typically utilized to compare distinctly-related species)

Question 9

Molecular clock

Answer 9

Genes or protein sequences may accumulate mutations at a relatively constant rate. If it is reliable, scientists can calculate the time of divergence according to the number of differences.

Question 10

Limiting factors of a molecular clock

Answer 10

• 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