Speciation and Extinction Notes

Evolution and Diversity

What is a Species?

  • Evolution and Species: Evolution involves the formation of new species and the extinction of others.
  • Defining Species: Species are considered distinct groups of organisms, but the definition has evolved.
  • Microevolution: Small evolutionary changes accumulating in a population, occurring rapidly (few generations).
  • Macroevolution: Slower, large-scale changes resulting in the formation of new species.

Linnaeus and Species Classification

  • Carolus Linnaeus (1700s): Developed a naming scheme for species.
  • Binomial Nomenclature: Combines Genus and species (e.g., Homo sapiens for humans).
  • Format: Genus species (Genus capitalized, species lowercase, both italicized).
  • Example: Canis lupus (wolf).

Biological Species Concept

  • Modern Definition: Species are defined by their ability to interbreed and produce fertile offspring.
  • Speciation: New species arise when individuals can no longer interbreed with the original group.

Limitations of the Biological Species Concept

  • Asexual Species: Cannot be defined by interbreeding.
  • Fossil Species: Reproduction cannot be observed.
  • Potential vs. Reality: Some species may not interbreed in nature despite the ability to do so.
  • Incomplete Isolation: Reproductive isolation is not always absolute, especially in plants.

DNA Analysis in Defining Species

  • Genetic Comparison: Comparing nucleotide sequences of shared genes.
  • Similarity Threshold: Organisms with >97% DNA sequence identity are considered the same species.

Reproductive Barriers and Speciation

  • Divergence: Subgroups unable to interbreed undergo independent evolutionary paths.
  • Speciation: The formation of new species.

Types of Reproductive Barriers

  • Pre-zygotic Barriers: Occur before zygote formation, preventing fertilization.
  • Post-zygotic Barriers: Occur after zygote formation, reducing offspring fitness.

Spatial Patterns and Speciation

  • Geographical Setting: Reproductive barriers arise from physical or nonphysical separation.
Allopatric Speciation
  • Geographical Barrier: Physical separation prevents interbreeding (e.g., rivers, mountains).
  • Allele Frequencies: Diverge due to lack of gene transfer.
  • Examples:
    • Galapagos Tortoises: Populations on different islands.
    • Devil's Hole Pupfish: Isolated in a hot spring, leading to genetic divergence.
Sympatric Speciation
  • Shared Habitat: Divergence occurs within the same habitat.
  • Microenvironments: Different phenotypes favored by different microenvironments.
  • Example: Fish in a deep lake with distinct ecological zones.
Polyploidy

  • Polyploid Organism: Gametes from two species unite, forming a new species.

  • Common in Plants: Offspring have more chromosomes than parents.

  • Hybrid Inviability: Polyploid organisms are reproductively isolated.

  • Diploid: Receiving a copy of every chromosome from each parent, two copies of every chromosome.

  • Gray Treefrog Example

    • Hyla chrysoscelis: Diploid (2N)
    • Hyla versicolor: Tetraploid (4N)

  • Evolutionary Scenario: Tetraploid gray treefrog (Hyla versicolor) evolved from genetic contributions of H. chrysoscelis and two extinct diploid species.

Determining Type of Speciation

  • Continuum: Nature ranges from complete reproductive isolation to intermingling.
  • Barrier Detection: Barriers may be difficult to detect.
  • Species Specificity: Barriers are not the same for all species.

Pace of Speciation

  • Fossil Record: Supports gradualism and punctuated equilibrium models.
Gradualism
  • Slow and Steady: Evolution occurs through small, incremental changes.
  • Example: Microscopic ocean protists.
Punctuated Equilibrium
  • Short Bursts: Rapid evolution followed by stability.
  • Example: Fossil Trilobites.
  • Allopatric Speciation Connection: Geographical changes trigger bursts of evolution.

Speciation After Mass Extinction

  • Mass Extinction: Major environmental changes cause widespread species death.

  • Extinction: When all members of a species have died.

  • New Resources: Surviving organisms exploit new resources and diversify.

  • Background Extinction Rate: Species go extinct due to gradually changing environments.

  • Plate Tectonics: Shifting land masses leading to climate changes and coastline rearrangement.

Biological Classification Systems

  • Common Descent: Classification based on evolutionary history.

Taxonomy

  • Definition: Science of describing, naming, and classifying species.
  • Taxonomic Levels (inclusive to exclusive): Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species.
  • Example: Aloe vera
    • Domain: Eukarya
    • Kingdom: Plantae
    • Phylum: Anthophyta
    • Class: Liliopsida
    • Order: Liliales
    • Family: Asphodelaceae
    • Genus: Aloe
    • Species: Aloe vera
  • Modern Taxonomy: Based on the work of Linnaeus.
  • Hierarchical Grouping: Species grouped based on shared characteristics.
  • Taxon: Each group at any level.

Phylogenetics

  • Definition: Study of evolutionary relationships among species.

Systematics

  • Definition: Determination of evolutionary relationships using various characteristics.

Cladistics

  • Definition: Systematics based on assumptions of how to analyze characteristics
  • Shared Derived Traits: Focuses on traits that are derived (not inherited from an ancestor).
    • Derived Characteristics: Traits resulting from beneficial genetic mutations passed on.
  • Cladistic Analysis:
    • Determines the history of evolutionary change & species divergence
    • Is based on patterns of modification
  • Willi Hennig: Developed Cladistics
  • Plesiomorphic: Original/ancestral condition of a character
  • Apomorphic: Mutated/derived condition of a character
  • Phylogenetic Bracketing: Inferring characteristics of extinct species by examining characteristics of extant ancestors and descendant species.
    • Example: crocodiles (ancestors of dinosaurs) and birds (descendants of dinosaurs).

Cladograms

  • Definition: Diagrams showing evolutionary relationships.
  • Clade: Group of organisms with a common ancestor and all descendants (monophyletic group).
  • Monophyletic Group: Birds, dinosaurs, and crocodiles share a common ancestor.
  • Paraphyletic Group: Excludes some descendants of an ancestor (e.g., Linnaean class Reptilia excluding birds).
  • Polyphyletic Group: Excludes the most recent common ancestor of its members (e.g., endotherms).

Evolutionary Relationships

  • Common Ancestors: Indicate relatedness.

Key Concepts

  • Species Definition: Distinct groups; cladistics or taxonomy.
  • Speciation: Result of genetic differences preventing successful reproduction.
  • Reproductive Isolation: Pre-zygotic or post-zygotic barriers.
  • Genetic Differences: Arise from mutations.
  • Habitat Changes: Influence allele frequencies; allopatric (physical separation) or sympatric (within population) speciation.