Unity and Diversity
This section covers the IB Biology SL domain Unity and Diversity. The exam tests how a shared molecular and cellular basis unifies life, and how classification, cladistics, evolution and conservation explain and measure diversity.
Core rules
- Water supports life through cohesion, adhesion, a high specific heat capacity, a high latent heat of vaporisation and solvent properties; hydrogen bonding explains these properties.
- Nucleic acids store information in a sugar-phosphate backbone with complementary bases: A pairs with T in DNA or U in RNA, and C pairs with G; strands are antiparallel.
- All cells share a plasma membrane, cytoplasm, ribosomes and genetic material; viruses are acellular, so their status as living is debated.
- Classification uses the hierarchy domain, kingdom, phylum, class, order, family, genus, species; binomial names italicise genus and species.
- Cladistics groups organisms by shared derived characters and common ancestry; molecular sequence similarity provides quantitative evidence.
- Evolution by natural selection requires heritable variation, selection pressure and differential survival and reproduction; speciation follows reproductive isolation.
Worked example
A 1,000-base DNA sequence differs by 18 bases between species B and C, by 30 bases between A and B, and by 45 bases between A and C. Percentage similarity = (1,000 - differences) / 1,000 x 100. B-C = 98.2%, A-B = 97.0%, A-C = 95.5%. B and C are most closely related in this sample because they have the fewest differences, so a cladogram should place them in the same clade.
Traps
- Homologous structures indicate common ancestry; analogous structures indicate convergent evolution, not close relatedness.
- A cladogram shows branching pattern, not a complete fossil timeline or levels of advancement.
- Do not call a virus a prokaryote; it has no cell structure or independent metabolism.
- Write binomials correctly: Homo sapiens, genus capitalised, species lower case.