A3.2 Classification and cladistics

What is taxonomy?

The science of classifying organisms into groups—taxa, based on observable physical traits.

Why do we need classification in biology?

The immense diversity of species, classification enables a broad range of further study.

Define ‘taxon’

A group of ant rank, e.g. domain, kingdom, phylum etc.

Recall the hierarchy of taxa (8)

• Domain

• Kingdom

• Phylum

• Class

• Order

• Family

• Genus

• Species

Whats a paradigm shift—how is it relevant to cladistics?

A fundamental change in common understanding.

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The change from traditional classification systems based on arbitrary taxa to classification with clades based on common ancestors is a paradigm shift.

Define ‘evolution’

The change in heritable characteristics of a population of a species over time.

Whats cladistics, cladograms?

A method of classifying organisms into clades according to shared traits, all organisims in clades have evolved from a common ancestor.

Cladograms are tree diagrams showing the most probable divergence between species.

What are the (2) conditions for cladistic classification?

1. All organisims in a clade evolved from the same common ancestor.

2. All orgamisims sharing a common ancestor have to be in the same taxonomic group.

What is the most objective evidence for placing organisms in the same clade?

The base sequences of genes aka amino acid sequences of proteins.

Scientists estimate relationships by comparing DNA or RNA or protein sequences.

What is a molecular clock?

As organisims evolve their DNA sequences accumulate mutations.

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Since muations accumulate at a constant rate they can be used to estimate when species diverged from a common ancestor.

State and explain (3) limitations of Molecular clocks

1. Generation time

2. Population size

3. Selective pressure

What’s a ‘node’ on a cladogram

The point at which two or more species diverge from a common ancestor.

What is ‘parisimony analysis’

A method used to select the most probable cladogram

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Operates on the assumption that a simple hypothesis with a small number of evolutionary change is more beleivable than a complex hypothesis.

Deduce, with reason, if Neanderthals can be classified into the same clade as modern humans.

There is a greater number of mutations between neanderthals and modern humans than between any two modern humans, suggesting the common ancestor for neanderthals and common humans is further back than that of all modern humans.

Thus they arent in the same clade.

Explain why reptiles do not exist according to cladistics.

The traditional classification of reptiles includes lizards, snakes, turtles and crocodiles.

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However, lizards and snakes have a different recent common ancestor than turtles and crocodiles. The clade that includes lizards, snakes, turtles and crocodiles also includes the birds. If birds are not considered reptiles, then all of the other groups cannot be considered reptiles either.