A3.2 Classification and cladistics (Organisms, HL): Practice Questions

In the binomial nomenclature system, what do the two parts of the scientific name represent, in order?

A cladogram shows three species: X, Y, and Z. Species X and Y share a common ancestor that is not shared with Z. Species Y and Z share a homologous feature, but molecular analysis indicates this feature evolved independently in both lineages. What does this suggest about the feature, and how would it be treated in a strictly cladistic analysis using molecular data?

Consider the use of ribosomal RNA (rRNA) sequences in constructing phylogenetic trees. Why are rRNA genes particularly useful for determining evolutionary relationships among distantly related organisms, but less effective for resolving relationships among very closely related species?

Evidence from the reclassification of the Figwort family (Scrophulariaceae) using DNA sequences showed that many plants previously grouped together based on flower morphology did not share a recent common ancestor. What does this suggest about the use of morphology in classification?

Explain the significance of the principle of parsimony when biologists are faced with multiple possible cladograms for the same set of species based on morphological data.

Cladistics is a system of classifying organisms based on shared derived characteristics (synapomorphies).

(a) Explain the difference between homologous and analogous structures, and describe why only homologous structures are used to construct a cladogram.
(b) Consider a cladogram constructed for a group of organisms. Explain the principle of parsimony in the context of cladogram construction, and discuss how molecular data, such as DNA sequences, can be used to refine or challenge hypotheses derived from morphological cladistics.

The shift from traditional taxonomy to cladistics has significantly altered our understanding of biological relationships.

(a) Explain the principle of the 'molecular clock' and how it is used to estimate the time of divergence between two different clades.
(b) Using the reclassification of the Figwort family (Scrophulariaceae) as an example, describe how molecular evidence revealed that the group was not a monophyletic clade.
(c) Compare the utility of homologous structures and analogous structures in cladistics, explaining why structural similarities do not always reflect close evolutionary relationships.