Steps of natural selection

Variation exists, more offspring are produced than survive, competition happens, the better adapted survive and reproduce, and they pass on their alleles, so allele frequencies change.

Common mistake: do individuals evolve?

No. Individuals do not change their genes to adapt. Populations evolve as allele frequencies change across generations.

Abiotic vs biotic selection pressures

Abiotic: non-living factors (temperature, drought, salinity). Biotic: living factors (predators, pathogens, competitors, mates).

Selection of traits that increase mating success, e.g. bright plumage or courtship displays, even if they reduce survival.

(HL) Hardy–Weinberg equations

p + q = 1 (allele frequencies) and p² + 2pq + q² = 1 (genotype frequencies). Start from q², the recessive phenotype frequency.

All the alleles of all the genes in an interbreeding population at a given time.

Biology

D4.1 Natural selection

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Natural Selection: Fitness in 8 Min (6 Marks)

Natural Selection Explained (6-Mark Question)

Natural selection acts on heritable variation when more offspring are produced than can survive, so better-adapted individuals reproduce more and allele frequencies change over generations. Study guide, 5 practice questions with markschemes and 6 flashcards. Free.

variation, overproduction and selection pressure · abiotic and biotic selection factors · sexual selection · modelling selection · (HL) gene pools and allele frequency · (HL) directional, stabilising and disruptive selection · (HL) Hardy-Weinberg equation and artificial selection

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Study guide

The logic of natural selection

Natural selection follows from a few observations that can be checked in real populations:

flowchart TD
  A["Heritable variation exists in a population"] --> B["More offspring are produced than the environment can support"]
  B --> C["Competition for limited resources"]
  C --> D["Individuals with favourable traits survive and reproduce more"]
  D --> E["They pass on their alleles"]
  E --> F["Favourable alleles become more common over generations"]

Sources of variation

Variation must be heritable to matter. Its sources are mutation (the only source of new alleles), meiosis (independent assortment and crossing over make new allele combinations) and sexual reproduction (random fertilisation combines alleles from two parents). Variation that comes only from the environment, such as size from diet, is not passed on.

Selection pressures

A selection pressure is any environmental factor that affects which individuals survive and reproduce.

Abiotic factors: temperature, water availability, salinity, soil pH.
Biotic factors: predators, pathogens, competitors, food availability.

Competition is greatest when populations overproduce, which is common: many fish and plants produce huge numbers of offspring, and most die young. Fitness in biology means reproductive success, not strength.

Sexual selection

Some traits spread because they improve mating success rather than survival. Females may choose mates with bright colours or elaborate displays, as in birds of paradise or guppies, and males may compete directly using antlers or size. These traits can even reduce survival, but they persist because they increase the number of offspring.

Modelling selection

Simple models help show selection: computer simulations, or class activities where "predators" pick coloured beads against a background. Classic field studies include guppies in Trinidad, where populations with fewer predators evolved brighter males over a few generations after being moved to new streams. Models must be interpreted with care: they simplify reality and must be tested against data.

Gene pools and allele frequency (HL)

A gene pool is all the alleles of all genes in an interbreeding population. Evolution can be measured as a change in allele frequency, the proportion of all copies of a gene that are a particular allele.

Patterns of selection (HL)

Type	Which phenotypes are favoured	Effect on distribution
Directional	One extreme	Mean shifts in one direction
Stabilising	Intermediate	Mean unchanged, range narrows
Disruptive	Both extremes	Distribution can split into two peaks

Examples: antibiotic resistance spreading in bacteria (directional), human birth mass clustering around a middle value (stabilising), and beak sizes favoured at two extremes when two seed sizes are common (disruptive).

Hardy-Weinberg and artificial selection (HL)

For a gene with two alleles at frequencies p and q:

\[p + q = 1 \qquad p^2 + 2pq + q^2 = 1\]

where \(p^2\) and \(q^2\) are the homozygous genotype frequencies and \(2pq\) the heterozygote frequency. The equation predicts genotype frequencies in a population that is not evolving. Its assumptions are a large population, random mating, no mutation, no migration and no selection. If observed frequencies differ from predictions, one of these assumptions is being broken. Start calculations from the recessive phenotype, since its frequency equals \(q^2\).

In artificial selection, humans choose which individuals breed, so the selection pressure is deliberate. It has produced many crop varieties and domestic breeds and works on the same principle of heritable variation.

Common mistakes

Saying organisms "adapt because they need to" or mutate in response to the environment.
Equating fitness with strength or speed rather than reproductive success.
In Hardy-Weinberg questions, taking the square root of the dominant phenotype frequency. Use the recessive phenotype.
Forgetting the heterozygote term is 2pq, not pq.

Exam tips

Explain natural selection as a chain: variation, overproduction, competition, differential survival and reproduction, inheritance, change in allele frequency.
Calculate with Hardy-Weinberg: find q from the recessive phenotype, then p = 1 − q, then each genotype frequency; show all steps.
Distinguish the three patterns of selection with a sketch-style description of each graph.
Use the word "allele" correctly: selection changes allele frequencies in the population, not genes in an individual.

Cheat sheet

Steps: heritable variation → overproduction → competition → differential survival/reproduction → inheritance → allele frequencies change
Variation sources: mutation (new alleles), meiosis, random fertilisation
Selection pressures: abiotic (temperature, water) and biotic (predators, pathogens, competitors)
Fitness = reproductive success
Sexual selection: mate choice or competition; can lower survival
(HL) Directional = one extreme; Stabilising = middle; Disruptive = both extremes
(HL) p + q = 1; p² + 2pq + q² = 1
(HL) q² = frequency of recessive phenotype → q = √(q²)
(HL) Hardy-Weinberg assumptions: large population, random mating, no mutation, no migration, no selection
(HL) Artificial selection: humans choose the breeders

Key definitions

Natural selection: The process by which individuals with heritable traits better suited to their environment survive and reproduce more, so those traits become more common.
Selection pressure: An environmental factor that affects the survival or reproduction of individuals with different traits.
Fitness: An individual's relative success in surviving and producing fertile offspring.
Sexual selection: Selection for traits that increase success in obtaining mates, through mate choice or competition.
Adaptation: A heritable trait that increases an organism's chance of surviving and reproducing in its environment.
Gene pool (HL): All the alleles of all the genes present in an interbreeding population.
Allele frequency (HL): The proportion of all copies of a gene in a population that are a particular allele.
Stabilising selection (HL): Selection that favours intermediate phenotypes and reduces variation around the mean.
Hardy-Weinberg principle (HL): A model predicting stable allele and genotype frequencies in a large, randomly mating population with no mutation, migration or selection.

Practice questions

SLPaper 1AFoundation

Question 1

(a) [1]

Which is a source of new alleles in a population?

A. Natural selection
B. Mutation
C. Overproduction of offspring
D. Competition

Markscheme

1 B — only mutation creates new alleles; selection acts on variation that already exists.

SLPaper 1BStandard

Question 2

Pale and dark forms of a ground beetle were marked, released onto two habitats and recaptured one week later.

Habitat	Form	Number released	Number recaptured
Pale sand	Pale	120	54
Pale sand	Dark	120	21
Dark volcanic soil	Pale	120	17
Dark volcanic soil	Dark	120	49

(a) [1]

Calculate the percentage of dark beetles recaptured on pale sand.

Markscheme

1 \(21 \div 120 \times 100 = 17.5\) %

(b) [4]

Explain the pattern in the recapture data in terms of natural selection.

Markscheme

1 Beetles matching their background were recaptured more (survived better)
1 Camouflaged beetles are less likely to be seen and eaten by predators (biotic selection pressure)
1 Survivors reproduce and pass on the allele(s) for their colour
1 Over generations the frequency of the matching colour increases in each habitat

(c) [1]

Suggest one limitation of using recapture rate as a measure of survival.

Markscheme

1 Beetles may have migrated away/been harder to find rather than died / marks may affect predation

HLPaper 2Challenging

Question 3

In a population of 2500 snails, shell banding is controlled by one gene with two alleles. Unbanded shells are recessive; 400 snails have unbanded shells. Assume Hardy–Weinberg equilibrium.

(a) [2]

Calculate the frequency of the recessive allele (q).

Markscheme

1 \(q^2 = 400 \div 2500 = 0.16\)
1 \(q = 0.4\)

(b) [2]

Calculate the expected number of heterozygous snails.

Markscheme

1 \(p = 1 - 0.4 = 0.6\); \(2pq = 2 \times 0.6 \times 0.4 = 0.48\)
1 \(0.48 \times 2500 = 1200\) snails

(c) [2]

State two conditions required for Hardy–Weinberg equilibrium.

Markscheme

1 Large population / no genetic drift
1 Random mating / no selection / no mutation / no migration (any second condition)

Key facts

Steps of natural selection: Variation exists, more offspring are produced than survive, competition happens, the better adapted survive and reproduce, and they pass on their alleles, so allele frequencies change.
Common mistake: do individuals evolve?: No. Individuals do not change their genes to adapt. Populations evolve as allele frequencies change across generations.
Abiotic vs biotic selection pressures: Abiotic: non-living factors (temperature, drought, salinity). Biotic: living factors (predators, pathogens, competitors, mates).
Sexual selection: Selection of traits that increase mating success, e.g. bright plumage or courtship displays, even if they reduce survival.
(HL) Hardy–Weinberg equations: p + q = 1 (allele frequencies) and p² + 2pq + q² = 1 (genotype frequencies). Start from q², the recessive phenotype frequency.
(HL) Gene pool: All the alleles of all the genes in an interbreeding population at a given time.