Natural Selection

Rucete ✏ Biology In a Nutshell

1. Darwin’s Theory of Natural Selection

  1. Populations possess enormous reproductive potential.
  2. Population size remains relatively stable over time.
  3. Resources are limited, leading to competition.
  4. Individuals compete for survival and reproduction.
  5. There is variation among individuals in a population.
  6. Much of this variation is heritable (passed on to offspring).
  7. Only the most fit individuals survive and reproduce.
  8. Favorable traits accumulate, leading to evolution over generations.

2. Types of Selection

a) Stabilizing Selection

  • Selects against extreme traits, favoring average phenotypes.
  • Reduces variation in the population.
  • Example: human birth weight (very low or high weight has higher mortality).

b) Directional Selection

  • Favors one extreme of a trait.
  • Leads to a shift in allele frequency over time.
  • Example: evolution of insecticide resistance, or dark-colored peppered moths during the Industrial Revolution.

c) Disruptive (Diversifying) Selection

  • Favors both extremes of a trait over the average.
  • Can lead to speciation if the population splits.
  • Example: birds with either very small or very large beaks favored over medium beaks.

d) Sexual Selection

  • Occurs when traits increase mating success, even if they don’t increase survival.
  • Often results in sexual dimorphism (males and females look different).
  • Example: bright plumage in male peacocks.

e) Artificial Selection

  • Human-directed breeding for desired traits.
  • Example: selective breeding of dogs, crops, or livestock.

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