16.3 Natural Selection and Adaptation

Natural selection shapes populations over time, favoring traits that enhance survival and reproduction. This process leads to adaptation, where organisms become better suited to their environment. From antibiotic-resistant bacteria to camouflaged prey, natural selection drives evolutionary change.

Different types of selection—directional, stabilizing, and disruptive—influence trait distribution in populations. Genetic variation fuels this process, providing the raw material for adaptation. Examples like mimicry in insects and antifreeze proteins in Arctic fish showcase nature's incredible adaptive abilities.

Natural Selection and Adaptation

Process of natural selection

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• Natural selection is a mechanism of evolution that results in the adaptation of populations to their environment over time
  • Occurs when individuals with certain heritable traits survive and reproduce more successfully than others in a population
  • These advantageous traits are passed on to offspring, increasing their frequency in the population over generations
• Key components of natural selection
  • Variation: Individuals within a population differ in their traits (color, size, behavior)
  • Inheritance: Some of these traits are heritable and can be passed from parents to offspring (eye color, height)
  • Differential fitness: Individuals with advantageous traits have higher survival and reproductive success (fitness) compared to those with less advantageous traits
  • Selection pressure: Environmental factors that influence the survival and reproduction of individuals (predation, climate, resource availability)
• Over time, the accumulation of advantageous traits through natural selection leads to adaptive evolution, resulting in populations becoming better suited to their environment (Darwin's finches, antibiotic resistance in bacteria)

Types of natural selection

• Directional selection
  • Occurs when one extreme of a trait is favored over the other
  • Shifts the population's mean phenotype in one direction
  • Example: Antibiotic resistance in bacteria, where resistant individuals survive and reproduce more successfully
• Stabilizing selection
  • Favors intermediate phenotypes over extreme ones
  • Reduces phenotypic variation in the population
  • Example: Human birth weight, where both very low and very high birth weights have lower fitness
• Disruptive selection
  • Favors extreme phenotypes over intermediate ones
  • Increases phenotypic variation in the population and may lead to speciation
  • Example: Beak size in African finches, where both large and small beaks are advantageous for different food sources (seeds, insects)

Genetic variation in adaptation

• Genetic variation is the foundation for adaptive evolution through natural selection
  • Mutations and genetic recombination during sexual reproduction generate new alleles and genotypes (point mutations, chromosomal rearrangements)
  • This genetic diversity provides the raw material for natural selection to act upon
• Populations with higher genetic variation have a greater potential to adapt to changing environments
  • Different alleles may confer advantages under different environmental conditions (heat tolerance, drought resistance)
  • As the environment changes, individuals with advantageous alleles will have higher fitness and contribute more to future generations
• Genetic drift and gene flow also influence genetic variation within and among populations
  • Genetic drift can lead to the loss of rare alleles, reducing genetic variation (founder effect, bottleneck effect)
  • Gene flow can introduce new alleles into a population, increasing genetic variation (migration, hybridization)

Examples of adaptive traits

• Camouflage in prey animals
  • Matching coloration or patterns that blend with the environment, making it difficult for predators to detect them (snowshoe hare, leaf insects)
  • Increases survival by reducing the risk of predation
• Mimicry in insects
  • Some harmless species evolve to resemble toxic or dangerous species, deterring predators
    1. Batesian mimicry: Harmless species mimic toxic or dangerous species (viceroy butterfly mimicking monarch butterfly)
    2. Müllerian mimicry: Two or more toxic or dangerous species evolve to resemble each other, reinforcing the warning signal to predators (heliconius butterflies)
• Antifreeze proteins in Arctic fish
  • Proteins that prevent ice crystals from forming in the fish's body fluids, allowing them to survive in sub-zero temperatures (cod, sculpin)
  • Enables fish to inhabit cold environments and access food sources unavailable to other species
• Drought tolerance in plants
  • Adaptations such as deep roots, waxy leaves, and efficient water storage mechanisms (cacti, succulents)
  • Allows plants to survive and reproduce in arid environments with limited water availability