Chapter 15: Tracing Evolutionary History

Phylogeny and the Tree of Life

Phylogeny and Evolutionary History

• Phylogeny is the evolutionary history of a species or group of species.
• Inferred from:
  • Fossil record
  • Morphological homologies
  • Molecular homologies
• Homologies are similarities due to shared ancestry, evolving from the same structure in a common ancestor.
  • Organisms with similar morphologies are usually closely related.
  • Convergent evolution: Similar adaptations due to a common environment, not shared ancestry.
  • Analogy: Similarity due to convergent evolution.

Systematics and Classification

• Systematics is the discipline focusing on classifying organisms and determining evolutionary relationships.
• Taxonomy, introduced by Carolus Linnaeus, is the system of naming/classifying species.
  • Each species gets a two-part scientific name (binomial):
    • Genus
    • Specific epithet
• Taxonomic hierarchy: Genera → Families → Orders → Classes → Phyla → Kingdoms
  • Each unit is called a taxon.
• Phylogenetic trees depict hypotheses about the evolutionary history of species.
  • Branching diagrams show hierarchical classification.
  • Indicate evolutionary relationships and descent patterns from common ancestors.

Constructing Phylogenetic Trees

• Cladistics: Method grouping organisms into clades.
  • Clades are monophyletic groups, including ancestral species and all descendants.
• Based on Darwinian concepts:
  • Shared ancestral characteristics: Originated in an ancestor.
  • Shared derived characteristics: Unique evolutionary novelties.
• Comparison of ingroup (taxa being investigated) and outgroup (taxa diverged before ingroup lineage) to identify derived characters.
• Phylogenetic tree constructed from branch points or nodes with new derived characters.
• Scientists use parsimony for the simplest explanation.
• Trees are revised with new data.
  • Example: Crocodilians are closest relatives to birds, sharing features like four-chambered hearts, singing, nesting.

Molecular Systematics

• Uses DNA and other molecules to infer relatedness.
• Over 110 billion DNA bases sequenced from thousands of species.
• Clarifies evolutionary relationships:
  • Recently branched species have more similar DNA.
  • Longer separate evolutionary paths show more DNA divergence.
• Different genes evolve at different rates:
  • rRNA changes slowly, useful for long-term divergences.
  • mtDNA evolves rapidly, useful for recent events.
• Molecular biology shows a commonality, supporting Darwin's theory of descent with modification.

These notes cover the evolutionary history, systematics, and the role of molecular data in phylogeny. They provide a summary of how species are classified and related through evolutionary history, using various forms of evidence and methodologies such as cladistics and molecular systematics.