Biology: Darwinian Evolution Notes

Sexual Selection

• Definition: A form of natural selection influencing mating preferences.
• Mechanisms:
  • Female choice: Females select mates based on appealing traits.
  • Choosy females yield more offspring.
  • Example species:
    • Male peacock with ornate tail feathers indicates fitness.
    • Male mandrill and anole lizard exhibit bright coloration as indicators of quality.
  • Male competition: Males compete for access to females.
  • Larger size or weapons (like antlers) aid in success in contests.
  • Examples include:
    • Antlers on male deer.
    • Male stag beetle with horns used in fighting.
    • Male fiddler crab with oversized claws for display.

Sexual Dimorphism

• Definition: Differences in appearance between males and females resulting from sexual selection.
• Examples:
  • Stag beetles: Distinct size and weaponry differences.
  • Deer: Large antlers in males.
  • Elephant seals: Size variance;
  • Mandarin ducks: Colorful ornamentation in males.

Modes of Natural Selection

• Types:
  1. Stabilizing Selection: Favors intermediate phenotypes.
  • Example: Infants born at moderate birth weights are more likely to survive.
  1. Directional Selection: Favors one extreme phenotype.
  • Example: Turkey farmers breed for larger breast muscles.
  1. Disruptive Selection: Favors both extremes, leading to two or more contrasting phenotypes.
  • Example: Salmon exhibit both small and large sizes, with intermediate sizes being less common.

Microevolution vs. Macroevolution

• Microevolution: Small evolutionary changes within a species over a short time scale. Example: Evolution of beak sizes in finches.
• Macroevolution: Large-scale evolutionary changes, including the emergence of new species (speciation).
• Speciation: Formation of new species from an ancestral population due to divergence. Example: Modern wheat forming from hybrids that self-fertilize.

Defining a Species

• Common Definition: A group capable of interbreeding to produce healthy, fertile offspring.
• Exceptions: Hybrid species such as ligers (lion-tiger cross) and grolars (grizzly-polar cross).

Reproductive Barriers

• Types:
  1. Behavioral Isolation: Species recognize each other through specific rituals.
  2. Habitat Isolation: Species live in different habitats and never meet.
  3. Mechanical Incompatibility: Anatomical differences prevent mating.
  4. Gametic Incompatibility: Sperm and egg from different species typically cannot fertilize each other.
  5. Hybrid Weakness: Offspring of two species may be sterile or less fit.

Speciation Processes

• Allopatric Speciation: Occurs when a population is split by a physical barrier leading to divergence.
  • Example: The Grand Canyon separating a population into two species.
• Sympatric Speciation: New species can arise within the same area due to other isolating mechanisms (like behavioral changes).

Biodiversity and Taxonomy

• Taxonomy: Classification, naming, and identification of species.
• Hierarchical classification:
  1. Domain: Eukarya
  2. Kingdom: Animalia
  3. Phylum: Chordata
  4. Class: Mammalia
  5. Order: Carnivora
  6. Family: Felidae
  7. Genus: Panthera
  8. Species: Panthera tigris

Phylogenetic Trees

• Represent evolutionary relationships among organisms as branching diagrams.
• Usage: Hypothesis about the evolutionary history of related species; the closer two species are on the tree, the more closely related they are.

Study Tips

• Understand definitions and examples for each type of selection and speciation.
• Familiarize yourself with reproductive barriers and why they contribute to speciation.
• Remember taxonomic ranks and apply them to specific examples, like the tiger's classification.