20.1 Organizing Life on Earth

Classifying life helps scientists organize and understand the vast diversity of organisms on Earth. From broad domains to specific species, this system reveals evolutionary relationships and patterns among living things.

Phylogenetic trees visually represent these relationships, showing how species are connected through time. By interpreting these diagrams, we can infer common ancestors, evolutionary events, and the complex web of life's history.

Classification and Taxonomy

Importance of biological classification

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• Provides standardized way to categorize and name organisms worldwide
  • Enables effective communication among scientists (common language)
  • Facilitates study and understanding of vast diversity of life on Earth (millions of species)
• Identifies patterns and relationships among organisms
  • Helps understand evolutionary history and adaptations of species (shared characteristics)
  • Allows predictions about characteristics and behavior of newly discovered or poorly studied organisms (inference based on related species)

Levels of taxonomic classification

• Domain: Broadest level of classification (Archaea, Bacteria, Eukarya)
• Kingdom: Major divisions within domains (Animalia, Plantae, Fungi, Protista, Archaea, Bacteria)
• Phylum: Groups organisms based on general body plan (Chordata, Arthropoda)
• Class: More specific than phylum, based on shared physical characteristics (Mammalia, Aves)
• Order: Groups organisms with similar features within a class (Primates, Carnivora)
• Family: More closely related group within an order (Hominidae, Felidae)
• Genus: Group of closely related species (Homo, Felis)
• Species: Most specific level, organisms capable of interbreeding and producing fertile offspring (Homo sapiens, Felis catus)

Systematics and evolutionary relationships

• Systematics studies diversification of living forms and their relationships through time
  • Classifies organisms into groups based on shared characteristics and evolutionary history (cladistics)
• Taxonomy names, describes, and classifies organisms into groups
  • Uses morphological, behavioral, and genetic data to identify and classify species (phenotype and genotype)
• Constructs phylogenetic trees to represent evolutionary relationships
  • Depict common ancestry and degree of relatedness between species (branching patterns)
  • Infer evolutionary events and history (speciation, extinction)

Phylogenetic Trees and Evolutionary History

Interpretation of phylogenetic trees

• Branching diagrams represent evolutionary relationships
  • Root represents common ancestor of all organisms on tree (earliest point)
  • Branches represent divergence of lineages over time (splitting events)
  • Tips represent present-day organisms (extant species)
• Closely related organisms grouped together on same branch
  • More recent common ancestor indicates closer relationship (shorter branches)
• Nodes represent divergence of two or more lineages from common ancestor
  • Branch length can indicate amount of genetic change or time since divergence (molecular clock)
• Infer evolutionary events from tree structure
  • Shared derived characters (synapomorphies) indicate common ancestor (homology)
  • Convergent evolution identified when distantly related organisms share similar traits due to similar environmental pressures (analogy)
• Examples:
  • Phylogenetic tree of vertebrates shows mammals and birds share more recent common ancestor than with reptiles or amphibians
  • Molecular phylogenies reveal whales are more closely related to hippopotamuses than to other marine mammals (convergent evolution of aquatic adaptations)

Evolution and Biodiversity

Mechanisms of evolution

• Natural selection drives adaptation to environmental pressures
  • Organisms with advantageous traits are more likely to survive and reproduce
  • Genetic variation within populations provides raw material for selection
• Genetic drift can lead to changes in allele frequencies, especially in small populations
• Gene flow between populations can introduce new genetic variations

Biodiversity and its importance

• Refers to the variety of life forms within a given ecosystem, biome, or the entire Earth
• Contributes to ecosystem stability and resilience
• Provides a wide range of ecosystem services and resources for human use

Historical perspective

• Charles Darwin's theory of evolution by natural selection revolutionized biology
  • Proposed common descent of all living organisms from a single ancestor
  • Explained the mechanism for evolutionary change through natural selection
• Modern synthesis integrates Darwin's ideas with genetics and population biology