IB DP Biology- D4.1 Natural selection- IB Style Questions For HL Paper 2 -FA 2025

(a)

Homologous structures are anatomical features in different organisms that share a similar basic layout, even if they serve different purposes. The classic example is the pentadactyl limb seen in mammals, birds, and reptiles. Although adapted for tasks like flying, grasping, or swimming, the bone arrangement is fundamentally the same. This structural similarity suggests that these organisms inherited the design from a shared ancestor. Over time, the structure was modified through divergent evolution to suit various environmental demands, supporting the idea of evolutionary change.

(b)

Conditions:
The Hardy–Weinberg principle states that allele frequencies remain constant only when the following criteria are met:
• Very large population size (no genetic drift)
• No migration in or out (no gene flow)
• No mutations
• Random mating
• No natural selection acting on the population

Use of the equation:
The Hardy–Weinberg equation is:
\( p^2 + 2pq + q^2 = 1 \) where • \(p\) = frequency of the dominant allele • \(q\) = frequency of the recessive allele The terms represent genotype frequencies: • \(p^2\) = homozygous dominant • \(2pq\) = heterozygous • \(q^2\) = homozygous recessive Knowing any one value (often \(q^2\)) allows calculation of the full set of allele and genotype frequencies.

(c)

Modern classification increasingly relies on evolutionary relationships rather than simple physical similarities. Cladograms visually display these relationships by showing how species branch from a common ancestor.

• A clade includes an ancestor and all of its descendants.
• A node represents a divergence point where a new lineage forms.
• Species sharing a more recent node are considered more closely related.

By analysing DNA, amino acid sequences, or shared derived traits, scientists build cladograms that form the basis of natural (phylogenetic) classification, grouping organisms according to evolutionary history.