IB Environmental systems and societies 2.1L Classification Methods Notes

Classification: Cladistics Groups Life by Shared Ancestry, Not Just Appearance

Modern classification asks who is truly related by descent, not just who looks alike.

Clades and Monophyletic Groups

1. A clade is a monophyletic group: one ancestral species plus every one of its descendants.
2. Members of a clade share synapomorphies, inherited traits derived from that common ancestor that set the group apart from others.

Cladistics vs Traditional Taxonomy

1. Traditional Linnaean taxonomy groups organisms mainly by morphological similarity, which can mislead when unrelated species look alike.
2. Cladistics instead groups by shared derived traits (synapomorphies) that reflect true evolutionary relationships.
3. It expresses these relationships as cladograms, branching diagrams that are hypotheses about evolutionary ancestry.

Cladograms and Phylogeny

1. Each node on a cladogram marks a point of divergence, where one lineage split into two.
2. The branching pattern is a phylogeny, a reconstructed hypothesis of evolutionary history.
3. Placing extinct and living species on the same tree helps rebuild the tree of life.

Advantages of Cladistic Classification

1. Reflects true evolutionary history.
   1. Trees trace how organisms diverged from common ancestors, mapping actual phylogeny rather than surface resemblance.
2. Reduces (though does not remove) subjectivity.
   1. Cladistics can draw on quantifiable molecular data such as DNA and amino acid sequences that can be statistically analysed and independently re-checked.
   2. It still uses morphological characters, and choosing which characters to score and which tree best fits the data both involve judgement, so it reduces rather than eliminates subjective bias.

Advantages of Cladistic Classification (continued)

1. Has predictive power.
   1. If one species in a clade has a trait such as a metabolic pathway or bone structure, relatives in the same clade probably share it.
   2. This guides work in medicine, paleontology, and conservation biology, predicting features of poorly known or extinct species.

Advantages: Patterns, Data, and Comparison

1. Reveals patterns of divergence.
   1. Nodes show when lineages diverged and how new species arose, clarifying links between extinct and living species.
2. Integrates multiple lines of evidence.
   1. Fossils, genetic data, and shared structures can all feed one hypothesis, and new evidence refines clades without breaking the system.
3. Enables comparative and evolutionary studies.
   1. Researchers can trace where complex traits such as flight, vision, or endothermy first emerged across clades.

The Linnaean Hierarchy: A Ranked System Built on Morphology

1. The Linnaean system ranks organisms in a fixed hierarchy: Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species.
   1. The Bengal tiger runs Eukarya, Animalia, Chordata, Mammalia, Carnivora, Felidae, Panthera, Panthera tigris.
2. Historically the ranks were assigned from morphology, visible traits rather than evolutionary lineage.
3. This makes the system inflexible, and it does not always match the divergence patterns that molecular data reveal.

Limitations of Traditional Classification

1. Reliance on morphology.
   1. Grouping by observable features misleads when convergent evolution gives unrelated species similar traits under similar pressures.

Limitations: Evolution, Genes, and Species Boundaries

1. Convergence and divergence confuse morphology.
   1. Convergent evolution makes unrelated groups look alike, while divergent evolution makes close relatives look different.
   2. Either can cause misclassification when only appearance is used.

Limitations: Gene Flow Across Boundaries

1. Horizontal gene transfer breaks linear descent.
   1. Microbes can swap genes across species boundaries, for example via plasmids, so evolution can look more like a web than a single tree.

Limitations: Fuzzy Species and Rigid Ranks

1. Hybridisation and introgression blur boundaries.
   1. Interbreeding produces hybrids, and genes crossing between species (introgression) is common in plants, birds, and early human ancestors.
2. The species concept itself is contested.
   1. The Biological Species Concept (interbreeding to produce fertile offspring) fails for asexual organisms, fossil species, and ring species.
3. Ranks are inflexible.
   1. Fixed ranks impose artificial divisions, and moving one group to a new rank can disrupt the whole hierarchy.

Limitations: Constant Molecular Revision

1. Molecular advances force frequent reclassification.
   1. DNA sequencing keeps revealing unknown relationships, so the traditional system rarely stays stable.

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