A3.2: Classification and Cladistics

What is taxonomy

The science of naming, describing, and classifying organisms.

What are the traditional levels of classification

Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species.

Why do biologists classify organisms

To organize biodiversity, identify organisms, standardize communication, and reflect relationships among organisms.

What are the benefits of a classification system

It makes identification easier, avoids confusion caused by local names, predicts shared characteristics, and helps show evolutionary relationships.

What are limitations of the traditional classification system

Similar appearances may result from convergent evolution, and some groups do not accurately reflect evolutionary ancestry.

Why is it better for classification to correspond to evolutionary relationships

It groups organisms by common ancestry, making classification more biologically meaningful and predictive.

What is a clade

A group consisting of a common ancestor and all of its descendants.

How is a clade shown on a cladogram

As any branch that includes a node and all branches arising from it.

What evidence is used to place organisms in a clade

DNA sequences, amino acid sequences, morphology, embryology, and fossil evidence.

How do biological sequences change over time

Mutations gradually accumulate, increasing sequence differences between lineages.

What is the source of differences in DNA or protein sequences

Mutations in nucleotide sequences, which may alter amino acid sequences.

What is a molecular clock

A method that estimates divergence time by comparing the number of sequence differences between organisms.

What data can be used to construct cladograms

DNA base sequences or amino acid sequences of proteins.

What is parsimony in biology

The principle that the simplest explanation with the fewest evolutionary changes is preferred.

How does parsimony help build cladograms

The most likely tree is the one requiring the fewest mutations or trait changes.

How are bioinformatic tools used to create cladograms

Sequences are aligned, differences are counted, and software generates the most parsimonious evolutionary tree.

What is a cladogram

A branching diagram that shows hypothesized evolutionary relationships based on shared derived characteristics.

What is the root of a cladogram

The common ancestor from which all organisms in the diagram descend.

What is a node on a cladogram

A branching point representing a common ancestor and a divergence event.

What is a terminal branch

The end of a branch representing a current species or taxonomic group.

How can a cladogram be analyzed

By identifying which organisms share the most recent common ancestors and which groups form clades.

Why did cladistics lead to reclassification of some organisms

Molecular evidence sometimes revealed that traditional classifications did not match true evolutionary relationships.

Give an example of reclassification using molecular data.

Archaea were separated from bacteria after rRNA comparisons showed they are fundamentally distinct.

What are the three domains of life

Bacteria, Archaea, and Eukarya.

What evidence led to the three-domain system

Comparisons of ribosomal RNA (rRNA) base sequences.

Who proposed the three-domain system

Carl Woese.

How did rRNA evidence change classification

It showed that prokaryotes consist of two deeply distinct groups: Bacteria and Archaea.

Which domains are more closely related

Archaea and Eukarya are more closely related to each other than either is to Bacteria.

Why is rRNA useful for studying evolutionary relationships

It is present in all organisms, performs the same function, and evolves slowly enough to compare across all life forms.