Natural Selection in Action

In this lesson we look at how natural selection works step by step, examine real-world examples, and explore how it can lead to the formation of entirely new species.

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The Mechanism of Natural Selection — Step by Step

It is essential that you can describe the process of natural selection clearly and in the correct order. Here is the full mechanism:

Step 1: Genetic Variation Exists

Within any population of a species, individuals differ from one another. This genetic variation arises from:

• Mutations — random changes in DNA that create new alleles
• Sexual reproduction — meiosis and the random fusion of gametes produce new combinations of alleles

Step 2: A Selection Pressure Acts

A selection pressure is any factor in the environment that affects an organism's ability to survive. Examples include:

• Predation
• Disease
• Competition for food or mates
• Climate or habitat change
• Human activity (e.g., use of antibiotics or pesticides)

Step 3: Differential Survival

Some individuals have characteristics that make them better adapted to the selection pressure. These individuals are more likely to survive — this is what "survival of the fittest" means.

Step 4: Differential Reproduction

Individuals that survive are more likely to reproduce and pass on their alleles to the next generation.

Step 5: Change in Allele Frequency

Over many generations, the frequency of the advantageous allele increases in the population, while disadvantageous alleles become less common. The population gradually changes.

Exam tip: When describing natural selection, always make these five points in order: variation, selection pressure, survival, reproduction, change in allele frequency. This is the structure examiners are looking for in extended-response questions.

graph TD
    A["Step 1: Genetic variation exists in population"] --> B["Step 2: Selection pressure acts on population"]
    B --> C["Step 3: Individuals with advantageous traits survive"]
    C --> D["Step 4: Survivors reproduce and pass on alleles"]
    D --> E["Step 5: Frequency of advantageous allele increases"]
    E -->|"Over many generations"| F["Population adapts to environment"]

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Example 1: The Peppered Moth (Biston betularia)

The peppered moth is a classic example of natural selection observed in Britain during and after the Industrial Revolution.

Before Industrialisation (Pre-1800s)

• Most peppered moths were light-coloured (pale with dark speckles).
• A rare dark (melanic) form also existed due to a natural mutation.
• Light moths were well camouflaged on the pale, lichen-covered bark of trees.
• Dark moths were easily spotted by bird predators and were quickly eaten.
• The light form was therefore much more common.

During Industrialisation (1800s)

• Soot from factories blackened tree bark and killed lichens.
• Now, dark moths were better camouflaged on the dark bark.
• Light moths became conspicuous and were eaten more often.
• The dark form survived and reproduced more, and its frequency increased dramatically.
• In industrial areas like Manchester, up to 98% of peppered moths were dark by the late 1800s.

After the Clean Air Acts (1956 onwards)

• Pollution declined, lichens regrew, and tree bark became lighter again.
• The light form regained its advantage and its frequency increased once more.

Why This Is Important

Feature of Natural Selection	How It Applies to Peppered Moths
Variation	Light and dark forms exist in the population
Selection pressure	Bird predation — birds eat the more visible form
Differential survival	Better-camouflaged moths survive
Reproduction	Survivors pass on their alleles
Change in allele frequency	Dark allele increases during pollution; decreases after

Exam tip: The peppered moth example is excellent for showing that natural selection can change direction when the environment changes. Make sure you state that the variation (dark form) existed BEFORE the selection pressure changed.

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Example 2: Antibiotic Resistance in Bacteria

This is one of the most important examples of natural selection for your GCSE and has real implications for human health.

How Resistance Develops

1. A population of bacteria shows genetic variation. A random mutation gives one bacterium a gene for resistance to an antibiotic (e.g., penicillin).
2. When the antibiotic is used, it acts as a selection pressure — it kills the non-resistant bacteria.
3. The resistant bacterium survives because the antibiotic has no effect on it.
4. With no competition from dead bacteria, the resistant bacterium reproduces rapidly by binary fission. It can divide as often as every 20 minutes.
5. The entire new population carries the resistance gene. The antibiotic is now ineffective against this population.
6. Bacteria can also transfer resistance genes to other bacteria through horizontal gene transfer (e.g., via plasmids), spreading resistance further.

MRSA

MRSA (methicillin-resistant Staphylococcus aureus) is a well-known example:

• It is resistant to multiple antibiotics, including methicillin and flucloxacillin.
• It causes serious infections, particularly in hospitals where patients are vulnerable.
• Treating MRSA requires alternative, often more expensive and toxic, antibiotics.

What Can Be Done?

Slowing the development of antibiotic resistance is a major public health priority:

Action	Why It Helps
Complete the full course of antibiotics	Ensures all bacteria are killed; partially treated infections leave resistant survivors
Do not use antibiotics for viral infections	Antibiotics only work on bacteria; unnecessary use increases the chance of resistance developing
Restrict agricultural use	Overuse in farming creates resistance that can transfer to human pathogens
Develop new antibiotics	Provides alternatives when existing drugs become ineffective
Improved hygiene in hospitals	Reduces spread of resistant bacteria between patients

Exam tip: When explaining antibiotic resistance, never say the bacteria "learn" to resist or "become resistant because of" the antibiotic. The mutation happens randomly BEFORE the antibiotic is applied. The antibiotic selects for the resistant bacteria — it does not cause the resistance.

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Speciation

Speciation is the formation of a new species. It occurs when populations of the same species become so different that they can no longer interbreed to produce fertile offspring.

How Speciation Occurs (Allopatric Speciation)

The most common type is geographical (allopatric) speciation:

Step 1: Geographical Isolation A population is separated into two (or more) groups by a physical barrier such as a mountain range, river, ocean or desert. This prevents the groups from interbreeding.

Step 2: Different Selection Pressures The two isolated populations experience different environmental conditions (different climates, food sources, predators, diseases).

Step 3: Natural Selection Acts Differently In each population, different characteristics are advantageous. Natural selection favours different alleles in each population.