Variation and Selection

Variation in Living Things

  • Variation refers to the differences between individuals of the same species.
  • Causes of variation:
    • Genetic Variation: Derived from mutations (random changes in genes/chromosome structure) and sexual reproduction, leading to different phenotypes.
    • Environmental Variation: Influenced by environmental factors such as diet or sunlight.

Types of Variation

  • Continuous Variation:

    • Characteristics change gradually with no distinct categories.
    • Examples: height, length.
    • Graphical representation: Histogram (bars touch).

  • Discontinuous Variation:

    • Individuals can be categorized into distinct groups.
    • Examples: tongue rolling, eye color.
    • Graphical representation: Bar Chart (individual bars separated).

Natural Selection and Evolution

  • Natural Selection:

    • A process where the better adapted individuals survive and reproduce, passing on their advantageous traits to offspring.
    • Key elements include:
    1. Variations in phenotypes (e.g., antibiotic resistance).
    2. Differential survival (e.g., antibiotic-resistant bacteria survive).
    3. Differential reproductive success (survivors pass on traits).

  • Theory of Evolution:

    • Proposed by Charles Darwin; describes gradual changes in organisms over time, potentially leading to new species.
    • Natural selection is the mechanism driving evolution.
    • Extinction occurs when species fail to adapt to changes in their environment.

Antibiotic Resistance

  • A process where bacteria evolve to resist the effects of antibiotics (e.g., penicillin).
  • Most bacteria are killed, but some survive due to advantageous mutations.
  • Resistant bacteria proliferate and pass on their mutation, leading to increased resistance (e.g., MRSA).

Selective Breeding

  • Process where humans select individuals for desirable traits (appearance, food quality, disease resistance).
  • Leads to offspring consistently displaying these traits across generations.
  • Not a natural process; rather a form of artificial selection.
  • Examples:
    • In crops: Shorter stalks for easier harvest and higher yield in wheat.
    • In animals: Selectively bred dogs may face inbreeding issues, leading to genetic diseases.

Historical Context of Evolution

  • Darwin's theory faced initial skepticism due to contradictions with religious beliefs and the vast timescales of evolution.
  • Fossil records provide evidence of many extinct species (e.g., dinosaurs, mammoths) and highlight the importance of adaptation to survive environmental changes.