Charles Darwin's Theory of Evolution Through Natural Selection
- Darwin's theory explains how species have descended from common ancestors through natural selection.
- Key Concepts of Natural Selection:
- Genetic Variation: All species possess inherited genetic variations.
- Overproduction of Offspring: Organisms produce more offspring than can survive.
- Competition: Competition exists for limited environmental resources.
- Survival of the Fittest: Individuals best suited to their environment survive.
- Descent with Modification: Species descend from others with changes over time.
Before Darwin
- Jean-Baptiste Lamarck proposed that living things evolve to become more complex over time through acquired traits.
- Traits evolved by "use or disuse".
- Acquired traits are not passed on; only traits determined by DNA are inherited.
Species
- A species is a group of organisms that can mate and produce fertile offspring.
Darwin's Observations in the Galapagos Islands
- Darwin's theory arose from observations of wildlife in the Galapagos Islands.
- Finches on the islands adapted to different environments, leading to speciation.
- Speciation or Reproductive Isolation: Birds became so different they could no longer interbreed and produce fertile offspring.
Evolution
- Evolution is the change in a species over time
- Driven by natural selection.
- Evolution explains the diversity and relationships of living things.
- Natural Selection: Organisms best adapted to their environment survive and pass on their genes.
Mechanics of Evolution
- Artificial Selection: Selective breeding for desired traits.
- Mutation: Changes in genetic material that may provide an advantage.
- Gene Flow (Migration): Transfer of genes between populations.
- Increases genetic variation.
- Genetic Drift: Random changes in trait frequency, especially in small populations.
- Decreases genetic variation.
- Bottleneck effect: Population size is reduced for at least one generation, reduces variation and adaption ability.
- Founder's effect: New colony started by few population members, reduces genetic variation.
Hardy-Weinberg Principle
- States that allele frequencies remain constant if these conditions are met:
- No mutations
- Large breeding population
- Random mating
- No immigration or emigration
- No natural selection
Population Evolution
- Changes in population size are affected by immigration, births, emigration, and deaths.
- Exponential growth: Rapid increase due to abundant resources.
- Logistic growth: Population faces limited resources.
- Carrying capacity: Maximum population size an environment can support.
- Limiting factors:
- Density-dependent: Affected by population density (e.g., predation).
- Density-independent: Not affected by density (e.g., natural disasters).
Natural Selection
- Fitness: Ability to survive and reproduce.
- Microevolution: Observable change in allele frequencies within a population.
- Single-gene traits: Natural selection leads to changes in phenotype frequencies.
- Polygenic traits:
- Directional selection: Favors one extreme.
- Stabilizing selection: Favors the average.
- Disruptive selection: Favors both extremes.
Evidence of Evolution
- Biogeography: Study of species distribution and adaptation. Divergent evolution and Parallel Evolution.
- Fossil Record: Documents the evolution of species from extinct ancestors.
- Comparative Anatomy:
- Homologous Structures: Share common ancestry with adapted structures.
- Vestigial Structures: Reduced body parts with no function.
- Embryology: Similar patterns of development, suggests shared ancestry.
- Biochemical (DNA): Similar genetic code reveals common descent.
- Observable Events: Direct observation of evolutionary changes.
Models of Evolution
- Gradualism: Gradual change over long periods leading to new species.
- Punctuated Equilibrium: Rapid change separated by periods of stasis.