1. Selective Breeding (Artificial Selection)

Selective breeding, also called artificial selection, is the process where humans deliberately choose organisms with desirable characteristics and breed them together so that those characteristics become more common in future generations.

This process works because:

• Individuals in a species show variation

• Many traits are genetic and can be passed on

• Choosing specific parents increases the chance that offspring will inherit those traits

Humans have used selective breeding for thousands of years, long before genetics was understood.

Commonly Selected Traits

• Faster growth

• Larger size

• Higher yield (milk, meat, wool, grain)

• Better taste or quality

• Disease resistance

• Calm or manageable behavior

Problems with Selective Breeding

Selective breeding can reduce genetic diversity, which may:

• Increase risk of inherited diseases

• Make populations less adaptable to environmental changes

Despite this, selective breeding is widely used in agriculture, farming, and horticulture.

2. Selective Breeding in Sheep (Detailed)

Sheep are one of the best examples of selective breeding because humans have heavily modified them.

Traits Farmers Select For

Farmers may breed sheep that:

• Produce large amounts of wool

• Have fine, soft wool fibers

• Grow quickly for meat production

• Are resistant to parasites and diseases

• Can survive in harsh environments

How It Works

1. Sheep with the best traits are identified

2. These sheep are chosen as parents

3. Less useful sheep are not allowed to breed

4. Over many generations, the desired traits become common

Result

Modern sheep:

• Produce far more wool than wild sheep

• Often cannot survive without human care (shearing)

• Are genetically different from their ancestors

This shows how powerful selective breeding can be when applied over many generations.

3. Natural Selection (Very Important)

Natural selection is the process by which organisms better adapted to their environment are more likely to survive and reproduce.

It was proposed by Charles Darwin.

Key Conditions for Natural Selection

Natural selection occurs when all of these are present:

1. Variation – individuals are different

2. Overproduction – more offspring are produced than can survive

3. Competition – limited resources cause struggle

4. Inheritance – traits are passed genetically

5. Survival of the fittest – best-adapted individuals survive and reproduce

(“Fittest” means best adapted, not strongest)

Result

• Helpful traits become more common

• Harmful traits become rarer

• Over long periods, this can lead to evolution and new species

Natural selection is not planned and has no goal — it simply happens due to environmental pressures.

4. Comparison: Selective Breeding vs Natural Selection

Similarities

• Both rely on variation

• Both involve inheritance of traits

• Both cause populations to change over generations

5. Bacteria and Antibiotics (Natural Selection in Action)

Bacteria reproduce very quickly, sometimes every 20 minutes. This allows evolution to happen fast.

What Happens When Antibiotics Are Used

1. A bacterial population has variation

2. Most bacteria are killed by the antibiotic

3. A few have natural resistance due to mutations

4. Resistant bacteria survive

5. These bacteria reproduce

6. The population becomes resistant

Important Points

• Antibiotics do not create resistance

• Resistance already exists due to random mutations

• Overuse of antibiotics increases the problem

This is a real-world example of natural selection, happening right now.

6. Peppered Moths (Classic Example)

Peppered moths show how environmental change affects natural selection.

Before the Industrial Revolution

• Trees were light colored

• Light moths were camouflaged

• Dark moths were eaten more

• Light moths were common

During Industrial Revolution

• Pollution darkened trees with soot

• Dark moths were camouflaged

• Light moths were eaten more

• Dark moths became common

After Pollution Controls

• Trees became lighter again

• Light moths increased once more

Why This Matters

• No moth chose to change color

• Birds caused the selection

• The environment determined survival

This perfectly demonstrates natural selection.