Senior Biology | Evolution and Natural Selection Prep Guide (for tutors) | Four conditions, allele frequencies and speciation in three steps

2026-11-05

Tutor lesson-prep guides · Secondary Biology

Giraffes stretched their necks to reach high leaves appears every year. This lesson separates use and disuse from natural selection with four conditions, then puts numbers on allele frequencies so the mechanism is visible.

Objectives — Students can: (1) describe natural selection with variation, inheritance, overproduction and differential survival; (2) use the Hardy-Weinberg equation to turn allele frequencies into genotype numbers; (3) explain directional selection in resistant bacteria and state three lines of evidence for evolution.

01 Prerequisites (3-minute check)

If any of these fails, fix it before the lesson starts.

Skill	Quick check	Pass mark
Alleles	What do we call two versions of the same locus	Alleles
Genotype	What are AA, Aa and aa called	Homozygous, heterozygous, homozygous
Probability	What is 0.8 times 0.8	0.64

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-8 min	Opening myth	Ask what caused the giraffe neck to lengthen and collect answers	Hands up, compare later
8-22 min	Four conditions	Variation, inheritance, overproduction, differential survival and reproduction	Re-explain the giraffe with all four
22-36 min	Hardy-Weinberg	p = 0.8, q = 0.2; AA 0.64, Aa 0.32, aa 0.04	Square it themselves
36-46 min	Population conversion	In 1000 animals: AA 640, Aa 320, aa 40	Multiply the proportions
46-56 min	Resistance and speciation	1e+09 bacteria at mutation rate 1e-06 give 1000 resistant cells; 1 per cent survive, so 10 remain	Describe directional selection
56-60 min	Exit check	Collect and mark three items	Work alone

03 Teaching script (read it out)

Scene 1: All four conditions are needed

Natural selection is not change on demand. Individuals vary in an inherited way, they produce more offspring than the environment can support, and variants differ in survival and reproduction, so the next generation carries more of the favourable variant.
Against the giraffe story: the neck did not stretch from reaching. Short and long necks already varied; long necks survived food shortage better and passed those alleles on.
Method: make students retell the giraffe story with all four conditions and stop them the moment they say so the neck grew.

Students say: retell the giraffe with four conditions, without saying the neck grew.

Scene 2: Hardy-Weinberg: one squaring does it

Let the dominant allele frequency be p = 0.8 and the recessive q = 0.2; they sum to 1. Under random mating genotype frequencies are (p + q) squared: AA = 0.64, Aa = 0.32, aa = 0.04, and the three add to 1.
In a population of 1000 that is 640, 320 and 40 individuals.
If selection pushes q from 0.2 to 0.1, the recessive homozygote falls from 0.04 to 0.010, a bigger drop than q itself because it is squared.

Students do: set q to 0.3, recompute the three genotypes, and say which falls fastest.

Scene 3: Resistance is evolution on fast forward

A population of 1e+09 bacteria at a mutation rate of 1e-06 already contains about 1000 resistant cells before any drug is used.
The drug kills 99 per cent, leaving 10 resistant cells that keep dividing, so the next generation is almost entirely resistant.
That is directional selection: the environment does not create the variation, it filters what is already there. Speciation needs isolation and time; resistance is only its first step.

Students say: why is the claim that bacteria learned resistance after the drug wrong? They must note the mutation pre-existed.

04 Question types and worked steps

Type	Example	Steps	Answer
Four conditions	Explain the giraffe with natural selection	Map each of the four in turn	All four stated
Frequency	p = 0.8, q = 0.2, three genotypes	Expand the square, then scale	640, 320, 40
Resistance	1e+09 cells, mutation 1e-06	Multiply for resistant cells, then by survival	1000 then 10
Evidence	Give three lines of evidence	Fossils, homologous structures, molecular sequences	One sentence each

Sanity check (teach this): Sanity check: 0.8 plus 0.2 is 1; 0.64 plus 0.32 plus 0.04 is 1; 640 plus 320 plus 40 is 1000; 1e+09 times 1e-06 is 1000, the right order of magnitude; when q falls from 0.2 to 0.1, q squared falls from 0.04 to 0.010, a larger relative drop as expected from squaring.

05 Independent practice (3 questions)

Question	Answer	What to watch
Four conditions, two items	Explain the peppered moth	All four present
Frequency, three items	Different p and q values	Any missing square term
Population conversion, two items	Proportions times total	Do the three counts sum to the total
Resistance, one item	Explain rising resistance after drug use	Is pre-existing variation mentioned

06 Common mistakes and what to say

Mistake	Student writes	Say this
Use and disuse	Giraffes reached high leaves so their necks lengthened	The neck did not lengthen; short and long necks already existed, and short-necked animals left fewer offspring.
Treating q as a genotype frequency	Saying aa is 0.2	q is an allele frequency; genotype aa is q squared.
Omitting 2pq	Writing only 0.64 and 0.04	Heterozygotes are 2pq, the largest term; without it the total cannot reach 1.
Mutation caused by the drug	The drug made the bacteria mutate	Mutation is random and pre-exists; the drug only selects.

07 Homework (paper, section and question numbers)

Secondary biology chapter index — All chapters at a glance
Prep guide master index — Compare similar chapters
Redo this chapter — One four-condition item, two frequency items and one resistance item

08 Exit check (last 5 minutes)

Question	Answer	Decision
Four conditions of natural selection	Variation, inheritance, overproduction, differential survival	Missing one sends you back to act one
p = 0.8, what is the aa proportion	0.04	Answering 0.2 sends you back to act two
When do resistant bacteria appear	Before the drug is used	Answering after sends you back to act three

09 Tutor FAQ

Must the Hardy-Weinberg assumptions be memorised?

Know the five: no mutation, no migration, random mating, large population and no selection. Questions usually ask why reality departs, so naming a violated assumption is enough.

Does speciation always need geographic isolation?

No, sympatric speciation occurs, but DSE mostly examines allopatric cases; the key idea is interrupted gene flow.

How much evidence is needed?

Three categories suffice: fossils as a time series, comparative anatomy such as homologous structures, and molecular similarity of DNA and protein sequences.

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A TutorZone lesson-prep guide. Written by the TutorZone team; questions and lesson flow are original and are not taken from any school or publisher material.