IB Environmental systems and societies (ESS) - Old 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.