Unity and Diversity
This domain explores how life is simultaneously unified and diverse: shared molecules, membranes and genetic mechanisms recur across organisms, while variation and evolution generate biodiversity. The exam tests molecular evidence for common ancestry, cell structure, classification and cladistics, evolutionary processes, and biodiversity conservation.
Core rules and concepts
- Water is polar and forms hydrogen bonds, giving cohesion, adhesion, a high specific heat and solvent properties that support transport and metabolism.
- Nucleic acids store and transmit information through complementary base pairing; proteins are polypeptides whose folded structure determines function.
- Cells are the basic unit of life. Prokaryotic and eukaryotic cells share membranes, ribosomes and DNA, but eukaryotes have a nucleus and compartmentalisation.
- Classification uses taxa and binomial nomenclature: genus capitalised, species lowercase, both italicised. Cladistics groups organisms by shared derived characters; analogous structures arise by convergent evolution.
- Evolution by natural selection requires heritable variation, selection pressure, differential survival and reproduction, and change in allele frequencies. Biodiversity includes genetic, species and ecosystem diversity.
Worked example
An outgroup lacks a backbone. Species P has a backbone; Q has a backbone and limbs; R has a backbone, limbs and an amniotic egg; S has all these plus hair. Backbone is a shared derived character grouping P, Q, R and S. Limbs group Q, R and S; the amniotic egg groups R and S; hair is unique to S. Therefore S is the most nested ingroup, R is its closest relative in this character set, and the outgroup roots the tree. The reasoning depends on shared derived characters, not on how “advanced” an organism appears.
Traps that cost marks
Do not call organisms “more evolved.” Do not confuse homologous structures, inherited from a common ancestor, with analogous structures. Remember that viruses are not cells and do not independently metabolise. Biodiversity is not just species richness; it also includes genetic and ecosystem variation.