DSE Biology | Evolution Lesson-Prep Guide (Tutor Edition) | Natural Selection, Allele Frequencies and the Evidence

2026-11-02

Tutor lesson-prep guides · Secondary Biology

Students can recite “survival of the fittest” and still write that an individual evolved to suit its habitat. This lesson moves the change to the population and the allele, where the marks actually live.

Objectives — Students can: (1) explain natural selection as a change in allele frequency caused by differential survival and reproduction, keeping individuals and populations apart; (2) compute allele and genotype frequencies and predict the effect of a selection pressure on them; (3) evaluate the main lines of evidence, including fossils, homologous structures, molecular sequences and observed selection in real populations.

01 Prerequisites (3-minute check)

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

Skill	Quick check	Pass mark
Genetics vocabulary	What is the difference between a gene and an allele	A gene is a locus; an allele is a version of it
Probability	If p = 0.8 what is q for a two-allele locus	0.2
Variation	Where does genetic variation come from	Mutation, meiosis, random fertilisation

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-10 min	Individuals do not evolve	Name the unit of evolution	One sentence: populations evolve
10-22 min	Selection in four steps	Variation, pressure, differential survival, inheritance	Four steps in order
22-36 min	Allele arithmetic	Count alleles from genotypes, then predict	Get p = 0.8
36-46 min	Evidence	Fossils, homology, molecules, observed change	One strength and one limit each
46-56 min	Exam wording	Write a full natural selection answer	Four marks, four steps
56-60 min	Exit check	Three questions marked on the spot	All three right

03 Teaching script (read it out)

Scene 1: Evolution happens to populations, not to individuals

A bacterium does not become resistant because it needs to; it is resistant already, or it is not. When an antibiotic is applied, resistant cells survive and divide while susceptible cells die, so the proportion of resistant cells rises.
That distinction fixes the most common error in the whole topic. An individual either has the allele or does not, and nothing it does in its lifetime changes its own gametes.
Say it once as a rule students can repeat: the variation exists before the pressure is applied, and the pressure only changes which variants are represented in the next generation.

Ask students: when did the resistant bacteria appear, before or after the antibiotic (before, at a low frequency).

Scene 2: Allele frequency is the number that changes

Take a population of 500 individuals: 320 are AA, 160 are Aa and 20 are aa. That is 1000 alleles in total, and A accounts for 2 x 320 + 160 = 800 of them, so p = 0.8 and q = 0.19999999999999996.
The genotype counts predicted by random mating are AA = p squared x 500 = 320, Aa = 2pq x 500 = 159 and aa = q squared x 500 = 19, which match the observed counts, so this population is in equilibrium and no selection is acting.
Now impose a pressure: suppose every aa individual dies before reproducing. The survivors carry q = 0.19999999999999996, and among the gametes that form the next generation the new frequency is q divided by (1 + q) = 0.167. The population has evolved, and no individual changed.

Ask students: after selection, has any surviving individual become different (no, only the frequency in the population changed).

Scene 3: Evidence is judged by what it can and cannot show

Fossils show that organisms changed over time and give a sequence, but the record is incomplete because soft-bodied organisms rarely fossilise. Homologous structures, such as the same bone plan in a bat wing and a human arm, indicate common ancestry, though analogous structures such as insect and bird wings show the opposite trap. Molecular evidence compares DNA and protein sequences: the more similar the sequences, the more recent the shared ancestor, and the data is quantitative.
Direct observation is the strongest single line because it is measured, not inferred. In one well-documented study of a bird population, mean beak depth rose from 8.2 mm to 9.1 mm after a drought, a shift of 0.9000000000000004 mm in a single generation, because only deep-beaked birds could crack the remaining hard seeds.
A strong answer therefore names the evidence, says what it supports, and states one limitation. That structure is what the mark scheme rewards.

Ask students: which evidence is strongest and why (observed change, because it is measured in real time).

04 Question types and worked steps

Type	Example	Steps	Answer
Explain	Natural selection in four steps	Variation, pressure, differential survival, inheritance over generations	Four steps, four marks
Calculate	Allele frequency from genotype counts	Count alleles, divide by total alleles	p = 0.8
Predict	Effect of a selection pressure	Compare survival, recompute frequency	q falls to 0.167
Evaluate	Strength of a line of evidence	What it shows, what it cannot show	Answer with a limitation

Sanity check (teach this): Allele frequencies must sum to 1; a pair that adds to anything else is an arithmetic slip.
Never write that an individual evolved a feature it needed: variation is random and pre-existing.
Do not call a structure homologous unless the ancestry is shared; similar function alone is analogy.
Heterozygotes carry two different alleles, so count them twice when totalling alleles, once for each.

05 Independent practice (3 questions)

Question	Answer	What to watch
Allele frequency of A given 320 AA, 160 Aa and 20 aa	p = 0.8	Count 2N alleles, then A alleles
New q if every aa individual dies before reproducing	0.167	q divided by (1 + q)
Expected number of Aa under random mating	160	2pq times the population size

06 Common mistakes and what to say

Mistake	Student writes	Say this
Saying an individual evolved	Writing “the bird grew a deeper beak in order to survive”	The population changed because deep beaks already existed and survived better.
Treating selection as need-driven	Writing “bacteria develop resistance when exposed”	Resistance arises by mutation before exposure; the antibiotic only removes the others.
Confusing analogy with homology	Calling bird and insect wings homologous	Homology means shared ancestry; these wings share function, not descent.
Reporting allele frequencies that do not sum to 1	Writing p = 0.8 and q = 0.3	For a two-allele locus p + q must equal 1 exactly.

07 Homework (paper, section and question numbers)

Secondary biology chapter index — All chapters and practice
All lesson-prep guides — Compare other subjects

08 Exit check (last 5 minutes)

Question	Answer	Decision
What is the unit of evolution	The population, not the individual	All three right: move to applied genetics
p for 320 AA, 160 Aa, 20 aa	0.8	One wrong: reteach allele counting
One limit of the fossil record	Soft-bodied organisms rarely fossilise	Two wrong: return to act three

09 Tutor FAQ

Must students memorise the Hardy-Weinberg equation?

They should be able to use p + q = 1 and p squared + 2pq + q squared = 1, and say what random mating and no selection mean.

Is evolution always slow?

No. Antibiotic resistance and beak depth changes are measured within years, which is why direct observation is such strong evidence.

How many examples should a student quote?

Two well-explained examples beat a list of six, because the marks are for the mechanism, not the names.

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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.