Bio 101 - Module 8 : Natural Selection and Evolution

1. Darwin, Wallace, and Natural Selection

• Charles Darwin: Traveled on the H.M.S. Beagle (1831–1836); observed Galápagos finches with varied beak shapes specialized for distinct food sources, which inspired his idea of natural selection.

• Alfred Russel Wallace: Naturalist who independently developed the theory of natural selection; presented joint papers with Darwin in 1858.

• Natural Selection ("Survival of the Fittest"): The process where individuals with favorable inherited traits survive environmental pressures and reproduce more successfully than others.

• Three Principles of Natural Selection:

  1. Inheritance: Traits are passed down from parent to offspring.

  2. Overproduction & Competition: More offspring are produced than can survive, creating competition for limited resources (inspired by Thomas Malthus).

  3. Variation: Offspring vary in their inherited traits; those with competitive advantages reproduce more.

2. Genetics, Adaptation, and Evolutionary Patterns

• Variation Requirement: Natural selection can only act on traits with a genetic basis.

  • Mutation: Ultimate source of all new alleles and novel genetic variation (changes in DNA).

  • Sexual Reproduction: Blends parental alleles to generate unique combinations of genotypes and phenotypes.

• Adaptation: A heritable trait that aids survival and reproduction in a specific environment. If the environment changes, the trait may no longer be advantageous.

• Patterns of Evolution:

  • Divergent Evolution: Two related species evolve in different directions from a common ancestor.

  • Convergent Evolution: Distantly related or unrelated species independently evolve similar traits due to similar environmental pressures.

3. Evidence for Evolution

• Physical Evidence:

  • Fossil Record: Shows chronological progression of life and structural changes over geological time.

  • Anatomy: Homologous structures share common structural architecture from a common ancestor; vestigial structures are nonfunctional remnants of ancestral traits.

  • Embryology: Developing embryos display ancestral structures (e.g., pharyngeal pouches) that disappear before maturity.

• Biological Evidence:

  • Biogeography: The geographic distribution of species reflects evolutionary history combined with continental drift (plate tectonics).

  • Molecular Biology: Shared DNA, universal genetic code, and conserved transcription/translation mechanisms point to a single common ancestor for all life.

4. Common Misconceptions to Avoid on Quizzes

• "Evolution is just a theory": In science, a theory is a thoroughly tested and verified explanatory framework, not an untested guess or hypothesis.

• "Individuals evolve": Individuals do not evolve; populations evolve over generations as allele frequencies change.

• "Evolution has a goal or organisms evolve on purpose": Organisms do not consciously adapt or become "better"; natural selection simply tracks changing environments.

• "Evolution explains how life began": Evolutionary theory explains how populations diversify and change over time, not the origin of the first living cells.

5. Population Genetics and Fitness

• Microevolution vs. Macroevolution:

  • Microevolution: Change in allele frequencies within a population over generations.

  • Macroevolution: Large-scale evolutionary changes over long periods, such as the emergence of new species.

• Key Definitions:

  • Allele: A variant form of a gene.

  • Allele Frequency: The proportion of a specific allele in a gene pool.

• Fitness:

  • Darwinian Fitness: An individual's genetic contribution to the next generation's gene pool.

  • Relative Fitness: An individual's survival and reproductive success compared directly to other members of the population.

6. Modes of Selection and Population Mechanisms

• Modes of Natural Selection:

  • Stabilizing Selection: Favors average/intermediate phenotypes; selects against extremes.

  • Directional Selection: Favors one extreme phenotype when environmental conditions change.

  • Diversifying (Disruptive) Selection: Favors two or more extreme phenotypes; selects against intermediate types.

  • Frequency-Dependent Selection: Phenotype fitness depends on its frequency (positive = common is favored; negative = rare is favored).

  • Sexual Selection: Selective pressure to obtain mates, often producing secondary sexual characteristics and sexual dimorphism.

• Mechanisms That Alter Allele Frequencies:

  • Natural Selection: Adaptive changes favoring beneficial alleles.

  • Genetic Drift: Chance events that randomly alter allele frequencies (impacts small populations most):

  • Bottleneck Effect: A catastrophic drop in population size leaves a small, non-representative gene pool.

  • Founder Effect: A small group leaves to colonize a new area, carrying an unrepresentative sample of alleles.

  • Gene Flow: Movement of alleles between populations via migration and interbreeding.

7. Phylogenetics and Taxonomy

• Phylogenetic Tree: A diagram that hypothesizes evolutionary relationships among organisms.

  • Rooted Tree: Features a single lineage at the base representing a shared common ancestor.

  • Unrooted Tree: Shows relationships between species without showing the common ancestor.

  • Branch Point (Node): Point where a single lineage splits into distinct new lineages.

  • Sister Taxa: Two lineages that originate from the exact same branch point.

  • Basal Taxon: An early-diverging lineage that remains unbranched near the root.

  • Polytomy: A branch point with more than two descendant lineages, indicating unresolved relationships.

• Key Tree Limitations:

  • Closely related organisms do not always look alike, and similar-looking organisms may not be closely related (convergent evolution).

  • Branch lengths do not necessarily represent chronological time.

• Taxonomy: Hierarchical classification system grouping organisms into progressively broader categories; updated continuously using modern molecular genetics.