Mini-Lesson

Continuity & Change: Ecosystems (HL)

Populations evolve and ecosystems respond to change. This HL lesson covers D4.1 Natural selection, D4.2 Stability & change and D4.3 Climate change, including the Hardy-Weinberg equation.

You will study types of selection, use Hardy-Weinberg to track allele frequencies, and link CO2 to climate change and biodiversity.

Work through each screen, answer the questions as you go (some are wordy, some are calculations) and collect ⭐ stars. Press Start when you are ready.

D4.1 Natural selection

Selection and the gene pool (HL)

The gene pool is all the alleles of all genes in a population. Natural selection changes allele frequencies over generations.

Directional selection favours one extreme (e.g. darker peppered moths in pollution).
Stabilising selection favours the intermediate.
Disruptive selection favours both extremes.

D4.1 Hardy-Weinberg

The Hardy-Weinberg model (HL)

For a large, randomly mating population with no selection, mutation, migration or drift, allele and genotype frequencies stay constant:

p + q = 1
p² + 2pq + q² = 1p² = homozygous dominant, 2pq = heterozygous, q² = homozygous recessive

Deviations from the predicted frequencies are evidence that evolution is occurring.

Calculate

Your turn — calculate

1In a population at Hardy-Weinberg equilibrium the recessive homozygote frequency q² = 0.16. Calculate the frequency of heterozygotes (2pq).

2pq

Hint: q = √0.16 = 0.4, so p = 0.6; 2pq = 2 × 0.6 × 0.4.

Calculate

Your turn — calculate

2In a population the dominant allele frequency p = 0.7. Calculate the expected frequency of homozygous dominant individuals (p²).

p-squared

Hint: p² = 0.7 × 0.7.

Calculate

Your turn — calculate

3A community has three species with 12, 18 and 20 individuals (N = 50). Using D = N(N−1) ÷ Σn(n−1), calculate Simpson's diversity index D.

D

Hint: Σn(n−1) = 12×11 + 18×17 + 20×19 = 818; N(N−1) = 50×49 = 2450.

Sort it

Sort each item into its group

Sort each item into the correct category.

Type of natural selection

Greenhouse gas

Hardy-Weinberg term

D4.2-D4.3 Change

Stability, change and climate

Ecosystems can be stable yet vulnerable: keystone species and tipping points can trigger large shifts, and higher biodiversity often improves resilience.

Climate change: rising CO2 and methane trap infrared radiation. Effects include shifting species ranges and timings, and ocean acidification. Positive feedback (melting ice lowers albedo) amplifies warming.

Quick check

Question

?One assumption of the Hardy-Weinberg model is that there is...

Quick check

Question

?Peppered moths becoming darker in polluted areas is an example of...

Quick check

Question

?A positive-feedback process that amplifies global warming is...

Match it

Match each item to its partner

Match each term to its correct description.

Item

Partner

Quick check

Question

?The gene pool of a population is...

Recap

The big ideas to take away

D4.1 (HL): gene pool; directional, stabilising and disruptive selection

D4.1 (HL): Hardy-Weinberg: p + q = 1 and p² + 2pq + q² = 1

D4.2-D4.3: tipping points, keystone species; CO2/methane, ocean acidification, albedo feedback

Skills: Hardy-Weinberg allele and genotype frequencies; Simpson's index

Press Finish to see your score.

🏆

Mini-lesson complete!

⭐⭐⭐

You have worked through Theme D: Continuity & Change — Ecosystems for IB Diploma Biology. 🎉

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