18. Variation and selection

Factors causing genetic variation

• Mutation

• Meiosis

• Random mating

• Random fertilisation

What is gene mutation

Random change in base sequence of DNA

CAN OCCUR BC OF:

• Chemicals

• Ionizing radiation

Adaptive features of hydrophytes

• Wide flat leaves to increase surface area

• Small roots because they can absorb water directly

• Stomata found on upper side allowing gas exchange

Adaptive features of xerophytes

• Thick waxy cuticle to prevent water loss

• Sunken stomata to reduce water loss

• Small leaves to reduce surface area for evaporation

• Extensive roots to access water from deep

• Thick stem or leaves to store water

Natural selection

1. Genetic variation within population

2. Production of many offspring

3. Struggle for survival, competition for resources

4. Greater chance of reproduction by individuals who r better adapted to environment

5. These individuals pass on their alleles to next generation

Selective breeding

• Selection by humans of individuals with desirable features

• Crossing these individuals to produce next generation

• Selection of offspring showing desirable features

Why do humans selectively breed plants

• Disease resistant crops

• Increased crop yield

• Adaptation to challenging weather conditions

• Improvement in taste and flavor of fruits

• Cultivation of plants with large or unique flowers

Why do humans selectively breed animals

• Cows goats and sheep that produce lots of milk and meat

• Chickens that lay large eggs

• Domestic dogs that r gentle

• Sheep with good quality wool

• Horses with fine features and speed

Difference between natural selection and artificial selection

• Natural occurs naturally whereas artificial occurs with humans

• Natural results in adaptation for better survival whereas artificial results in desires for humans

• Natural leads to gradual change over time while artificial results in rapid changes in short time