Natural Selection Flashcards

Natural Selection and Its Types

• Natural Selection Overview: Natural selection is the process by which certain traits become more or less common in a population due to the effects of those traits on the individuals' ability to survive and reproduce in their environment.

Types of Natural Selection

• Directional Selection:

  • Definition: Favors one extreme phenotype over the other.

  • Example: If a population of beetles has light green, medium green, and dark green beetles, and they move into a dark environment, the dark green beetles are favored because they blend in better with their surroundings.

• Stabilizing Selection:

  • Definition: Favors the average/mean phenotype and selects against extremes.

  • Example: A population of beetles in a medium green environment will see that medium green beetles survive better than light or dark green ones, as they are less visible to predators.

• Disruptive Selection:

  • Definition: Favors both extreme phenotypes and selects against the average/mean.

  • Example: In an environment where light green and dark green beetles can hide effectively, both extremes would thrive, while medium green beetles would be at a disadvantage due to higher visibility.

Speciation

• Definition: The process by which new species arise.

• Criteria for Defining a Species: Two organisms are considered different species if they cannot interbreed and produce fertile offspring.

• Major Mechanisms of Speciation:

  • Geographic Isolation: Physical barriers divide populations, leading to divergence over time.

  • Reproductive Isolation: Differing mating behaviors or timings prevent interbreeding.

Models of Speciation

• Gradualism: Speciation occurs slowly over time with small changes leading to new species.

• Punctuated Equilibrium: Rapid changes occur in a species followed by long periods of stability; often sparked by environmental changes or disasters.

Patterns of Evolution

Divergent Evolution

• Definition: Two or more species develop different traits from a common ancestor as they adapt to different environments.

• Example: Darwin's finches originating from a common ancestor and evolving distinct beak shapes suited to their specific feeding needs.

Convergent Evolution

• Definition: Different species evolve similar traits independently, often because they occupy similar environments or ecological niches.

• Example: The evolution of wings in bats, birds, and insects despite their separate evolutionary paths.

Coevolution

• Definition: The process where two or more species influence each other’s evolution, often in a mutualistic relationship.

• Example: Bats evolving longer noses to access nectar from flowers, while flowers evolve features that attract bats for pollination.

Hardy-Weinberg Principle

• Purpose: This principle helps understand conditions under which a population does not evolve. It defines the genetic equilibrium in a population in absence of evolutionary forces.

• Factors: Populations must meet certain criteria such as large size, no mutations, random mating, no natural selection, and no gene flow to maintain genetic stability.