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How-to-approach-D4.2: Stability and Change

Keywords: IB Biology Topic D4.2, Stability and Change, Selection Pressures, Stabilizing Selection, Directional Selection, Disruptive Selection, Speciation, Reproductive Isolation, Punctuated Equilibrium, Gradualism.

Welcome to the dynamics of the gene pool: Topic D4.2 Stability and Change. In the new IB Biology syllabus, this unit explores the 'Bio-Logic' of how populations stay the same or diverge into new species. While D4.1 covered the mechanism of selection, D4.2 looks at the patterns of that selection and the barriers that lead to the birth of new species.

This topic is a favorite for Paper 2 data-analysis questions involving bell curves and distribution shifts. You must be able to distinguish between stabilizing, directional, and disruptive selection. Furthermore, you need to understand 'Speciation'—the process by which one species splits into two—and the different types of reproductive isolation (temporal, behavioral, and geographic) that make it possible.

Before we look at the graphs, remember the evolutionary trade-off: Stability is great for an organism in a static environment, but change is the only way to survive a shifting world. Evolution is a constant balancing act between maintaining what works and innovating to meet new challenges.

1. The Three Patterns of Natural Selection

Selection pressures can affect a population's distribution of traits in three distinct ways. You should be able to sketch and identify these curves.

• Stabilizing Selection: Favors the intermediate phenotype and acts against extreme variations (e.g., human birth weight).
• Directional Selection: Favors one extreme phenotype over the other, causing the mean to shift (e.g., antibiotic resistance).
• Disruptive Selection: Favors both extremes and acts against the intermediate (e.g., bird beaks specialized for either small or large seeds, but not medium).

2. Reproductive Isolation: The Barriers to Breeding

Speciation requires that two populations become 'isolated' so they no longer exchange genes.

• Geographic Isolation: Physical barriers like mountains or water separate populations (Allopatric).
• Temporal Isolation: Populations breed at different times of the day or year.
• Behavioral Isolation: Differences in mating calls or rituals prevent interbreeding.

[Image showing examples of geographic, temporal, and behavioral isolation]

3. The Pace of Evolution: Gradualism vs. Punctuated Equilibrium

How fast does change happen? There are two main models in the IB syllabus:

• Gradualism: Evolution occurs at a slow, constant rate with small, incremental changes over long periods.
• Punctuated Equilibrium: Long periods of stability (stasis) are interrupted by 'bursts' of rapid significant change, often triggered by environmental shifts.

[Image comparing the gradualism model and punctuated equilibrium model]

4. Polyploidy: Instant Speciation (HL Focus)

In some cases, particularly in plants (like the genus Allium), speciation can happen in a single generation.

• Occurs due to total non-disjunction during meiosis, resulting in an extra set of chromosomes.
• A tetraploid (4n) individual cannot successfully breed with a diploid (2n) individual, creating immediate reproductive isolation.

5. Exam Strategy: Graph Analysis

If you are presented with a graph showing a change in a population over time:

• 1. Identify the Initial State (the 'before' curve).
• 2. Identify the Final State (the 'after' curve).
• 3. Describe the change: Did the mean move (Directional)? Did the peak get taller and narrower (Stabilizing)? Or did the peak split (Disruptive)?
• 4. Link it to the Selection Pressure mentioned in the text (e.g., a change in climate or predator).