4.1.1 Evolution by Natural Selection · 4. Natural Selection and Genetic Modification | Edexcel GCSE Combined Science 1SC0: Biology

Section 1 of 9The steps of natural selection

Stabilizing selection on human birth weight

Vertebrate Evolution Cladogram — Semantic Redraw

Disruptive selection on beak size in birds

1Variation

Variation individuals in a population differ in their features, because of differences in their alleles. New alleles arise by mutation.

2Overproduction

Overproduction more offspring are produced than can survive, so there is competition for food, mates and space.

3Selection

Selection individuals with characteristics that suit the environment are more likely to survive; this is sometimes called survival of the fittest.

4Reproduction

Reproduction survivors breed and pass on the beneficial alleles to their offspring.

5Change over time

Change over time in later generations a greater proportion of the population has the useful allele.

Note that selection acts on the phenotype, but what changes in the population is the frequency of alleles. An individual does not change; a population does.

Section 2 of 9Example: peppered moths

The peppered moth exists as a pale form and a dark form.

1

Before the Industrial Revolution, most moths were pale, camouflaged against lichen-covered tree bark, and the dark form was rare because birds saw it easily.

2

As soot from factories killed lichen and blackened the trees, pale moths were more visible and were eaten more often, while dark moths were camouflaged.

3

The dark allele became more common, and by the mid-1800s dark moths dominated in industrial areas.

4

After clean air laws reduced pollution in the twentieth century, the pale form increased again.

5

The allele for dark colour arose by mutation, and the environment determined whether it was an advantage.

Section 3 of 9Example: antibiotic resistance

Peppered moth natural selection on light and polluted dark bark

How Antibiotic Resistance Can Spread in the Gut — Animated

How antibiotic resistance can spread in the gut

How Antibiotic Resistance Develops in a Bacterial Population

Antibiotic resistance: resistant bacteria survive and multiply

Bacteria reproduce very quickly, so evolution can be seen in a short time.

In a population, a random mutation can give one bacterium resistance to an antibiotic.

When the antibiotic is used, most bacteria die, but the resistant one survives and divides, and passes on the resistance gene to its descendants.

MRSA

After a few generations most of the population is resistant. MRSA is a strain of Staphylococcus that is resistant to methicillin and other antibiotics.

To slow this process, doctors should not prescribe antibiotics unnecessarily, and patients should complete their courses so that all the bacteria are killed.

	Peppered moth	Antibiotic resistance
Source of variation	Mutation to dark colour	Mutation giving resistance
Selection pressure	Predation by birds	Antibiotic
Result	Dark allele common in polluted areas	Resistant strain spreads

Section 4 of 9Adaptation and speciation

Five-stage speciation by behavioural change from food preference in insects

Features that increase an organism's chance of survival and reproduction are adaptations: structural (the thick fur and fat of a polar bear), behavioural (migration), or physiological (producing a toxin). Over many generations, selection leads to a population that is well adapted to its habitat.

If two populations of the same species become separated, for example on different islands, they experience different conditions and different selection pressures.

Over time their genes diverge so much that they can no longer interbreed to produce fertile offspring: a new species has formed.

Speciation

This is called speciation.

Extinct

Species that fail to adapt to a change in conditions, such as new predators, disease or loss of habitat, may become extinct.

Section 5 of 9Common misunderstandings

Natural selection does not occur because organisms need or want to change.

1

Mutations are random; they are not caused by the need.

2

The environment selects among existing variants.

3

Evolution is also not the same as an individual getting stronger from exercise.

4

Finally, 'fittest' means best suited to the environment and able to leave the most offspring, not simply the strongest.

5

Describe selection using the sequence of variation, selection pressure, survival, reproduction and inheritance, and avoid language implying purpose.

Section 6 of 9Selection pressures and how they act

Anything that affects the survival or breeding success of individuals is a selection pressure.

Selection pressures and how they act

Common ones are predation, disease, competition for food or mates, climate and the availability of water.

The same population can be affected by several pressures at once, and a feature that helps with one may be a disadvantage for another: bright feathers help attract a mate but also attract predators.

The balance between them determines how an allele changes in frequency.

When the environment is stable, selection tends to keep a population close to the best-suited form. When the environment changes, selection favours different variants and the population changes. A sensible approach to an exam question is to identify the selection pressure, state which variant has the advantage, say why, and then describe the change in allele frequency over generations.

Section 7 of 9Exam-style questions

Natural selection in peppered moths (pollution and camouflage)

Genetic Variation in a Species — Interactive 2D Simulation

Explain how natural selection led to dark-coloured peppered moths becoming more common in industrial areas. [4]4 marks
Explain why antibiotic-resistant strains of bacteria have become more common. [3]3 marks
Suggest how two populations of the same species on different islands might become different species. [3]3 marks

Section 8 of 9Key terms

Term	Meaning
Natural selection	The process by which individuals with advantageous features survive and reproduce more.
Selection pressure	An environmental factor that affects survival, such as predation or disease.
Adaptation	A feature that helps an organism survive in its habitat.
Speciation	The formation of a new species.
Extinction	The permanent loss of all members of a species.
Antibiotic resistance	The ability of bacteria to survive an antibiotic.
Camouflage	Colouring that makes an organism hard to see.
MRSA	A strain of bacteria resistant to many antibiotics.

Section 9 of 9Exam technique

1

Saying an organism changes because it wants or needs to.

2

Forgetting that mutation produces the variation before selection.

3

Writing that individuals evolve, not populations.

4

Writing 'survival of the fittest' as strongest, instead of best adapted.

5

Missing out the step in which the survivors reproduce and pass on alleles.