Unit 7: Natural Selections Notes

Concepts of Evolution

• Evolution: The process of organisms changing over time as a result of environmental changes.

• Natural Selection: Driving mechanism of evolution where the most fit organisms survive and reproduce, passing on advantageous traits.

• Artificial Selection: Opposite of natural selection; traits selected by humans rather than nature.

• Key Principle: Individuals do not evolve; evolution occurs through variation in a population, primarily due to mutations.

Types of Selection

1. Directional Selection:

   • One extreme phenotype is advantageous.

   • Population shifts towards this beneficial trait over time.

2. Stabilizing Selection:

   • Average phenotype is favored; extremes are selected against.

   • Population becomes more similar over time.

3. Disruptive Selection:

   • Extremes are favored over the average.

   • Leads to divergence within a population.

Forces of Evolution

• Microevolution: Changes in gene frequencies in populations due to various forces:

  • Mutations: Random changes in DNA.

  • Natural Selection: Favorable traits increase fitness.

  • Migration/Gene Flow: Movement of organisms between populations.

  • Nonrandom Mating: Mate selection based on certain traits.

  • Genetic Drift: Random changes impacting small populations; includes effects such as:

  • Bottleneck Effect: Catastrophic events reduce a population's size drastically.

  • Founder Effect: Small groups colonize new areas, leading to limited genetic variation.

Hardy-Weinberg Equilibrium

• A theoretical state indicating no evolution occurring.

• Requires stability of allele frequencies (p and q) and understanding of genotype frequencies ($p^2$, $q^2$, and $2pq$).

Speciation

• Definition: Process leading to evolution of new species through reproductive isolation.

  • Allopatric Speciation: Occurs when populations are geographically isolated.

  • Sympatric Speciation: Occurs in overlapping locations without physical barriers.

Isolating Mechanisms

• Prezygotic Barriers: Prevent fertilization.

  • Temporal Isolation: Differences in breeding times.

  • Behavioral Isolation: Differences in mating behaviors.

  • Habitat Isolation: Different habitats limit interaction.

  • Mechanical/Chemical Isolation: Incompatibility of reproductive structures.

• Postzygotic Barriers: Affect hybrid offspring viability:

  • Reduced Hybrid Fertility: Hybrids may be sterile.

  • Reduced Hybrid Viability: Hybrids may be weak and unhealthy.

Patterns of Speciation

• Speciation can occur quickly (punctuated equilibrium) or gradually (gradualism), often resulting in populations that cannot interbreed successfully.

Evidence Supporting the Relationship of All Living Things

• Types of data supporting common ancestry include:

  • Geographic Data

  • Geologic Data

  • Morphological Data (homologous, analogous, vestigial structures)

  • Molecular Data

• Understanding and constructing phylogenetic trees is crucial for illustrating biological relationships.

Miller-Urey Experiment

• Provides evidence for abiogenesis, showing organic molecules can form from inorganic substances under specific conditions:

  • Simulated early Earth atmosphere with inorganic gases and water.

  • Heat and electricity led to the formation of organic molecules in a week.

Practice & Review

• MC Practice: 12 questions in 18 minutes.

• Unit 7 Review scheduled for 4/19.

• Password for review: convergent.