Natural Selection in Biology

Natural selection is a fundamental mechanism of evolution that explains how species change over time. It focuses on how certain traits become more common because they help organisms survive and reproduce.

1. Overview of Natural Selection

Key Points

• It is a non-random process.

• Acts on existing variations within a population.

• Leads to adaptation over many generations.

• Proposed by Charles Darwin.

2. Conditions Required for Natural Selection

For natural selection to occur, four main conditions must be met:

a. Variation in Traits

• Individuals in a population differ from one another.

• Variations can be:

  • Physical (size, color, speed)

  • Behavioral (hunting style, mating rituals)

  • Physiological (resistance to disease)

b. Inheritance

• Traits must be heritable.

• Offspring resemble parents more than unrelated individuals.

c. Overproduction of Offspring

• Organisms produce more offspring than the environment can support.

• Creates competition for:

  • Food

  • Water

  • Shelter

  • Mates

d. Differential Survival and Reproduction

• Individuals with beneficial traits are more likely to survive.

• These individuals produce more offspring.

• Advantageous traits become more common over time.

3. How Natural Selection Operates

Key Processes

• Selection Pressure: Environmental factors that influence survival (predators, climate, disease).

• Adaptation: Traits that improve survival.

• Fitness: The ability to survive and produce fertile offspring.

Steps in the Process

1. Variation exists.

2. Environment presents challenges.

3. Individuals with beneficial traits survive better.

4. They reproduce more.

5. Population gradually changes.

4. Types of Natural Selection

a. Directional Selection

• Favors one extreme trait.

• Example: Peppered moths becoming darker during industrial pollution.

b. Stabilizing Selection

• Favors the average traits.

• Example: Human birth weight (extremes are riskier).

c. Disruptive Selection

• Favors both extreme traits over the average.

• Can lead to two distinct groups.

5. Examples of Natural Selection

Animals

• Giraffes: Longer necks allowed better access to food.

• Finches: Different beak shapes evolved on different islands.

Plants

• Drought-resistant plants: Survive better in dry climates.

Microorganisms

• Antibiotic-resistant bacteria: Survive drug treatment and multiply.

6. Why Natural Selection Is Important

Key Points

• Explains the diversity of life.

• Helps understand:

  • Adaptations

  • Evolutionary relationships

  • Survival strategies

• Essential for fields like medicine, ecology, and genetics.