Classification and Cladistics

Classification and Cladistics

• Classification systems group organisms by traits or evolutionary origins.
• Organisms are classified using a hierarchical system.
• Domains are the broadest groups, with species representing the most basic level.

The Taxon Concept

• A taxon is any classificatory group.
• Taxonomy is the process of assigning organisms to groups.
• Biologists classify organisms into a hierarchy of taxa, ranging from species to kingdom.
• Species in a genus that diverge enough become separate genera.
• The point when separation happens cannot be determined, which creates a boundary paradox.

Advantages of Classification

• Classification should reflect the evolutionary origins of species.
• Criteria include:
  • Organisms sharing a common ancestor belong to the same taxonomic group.
  • Species within a taxonomic group evolve from the same common ancestor.
• Members of a taxonomic group share inherited traits called synapomorphies.
• Knowing that bats are classified as mammals allows for predictions about newly discovered bat species.

Clades as Groups of Organisms with Common Ancestry

• Species evolve and split over time, forming new species that can be identified by shared characteristics.

• A clade is a group of organisms that evolved from a common ancestor.

• Clades include all living species and their ancestral species and vary in size.

• It's not always clear which species belong to a clade.

• The best evidence for clade membership comes from base sequences of genes or amino acid sequences of proteins.

• Morphological traits are considered when sequence data is unavailable.

• Every species is part of multiple clades.

Gradual Accumulations of Sequence

• Mutations cause differences in DNA base sequences, leading to variations in amino acid sequences of proteins.
• These differences accumulate gradually over time.
• A molecular clock estimates the time since two species diverged from a common ancestor, based on the number of differences, assuming a roughly constant mutation rate.
• The mutation rate varies based on generation time, population size, and selective pressure.
• NCBI website provides tools for comparing protein and DNA sequences.

Base Sequences of Genes or Amino Acid Sequences

• Species within a clade can be more or less related based on how recently they diverged.
• Species that diverged more recently have fewer differences in base or amino acid sequences.
• Sequence analysis helps in constructing cladograms.
• Cladograms are branching diagrams showing ancestor-descendant relationships.

Analyzing Cladograms

• A cladogram uses branches to represent relationships.
• Terminal branches represent individual clades.
• Branching points are called nodes, and signify the split of a hypothetical ancestral species into two or more clades.
• Clades linked by a node are closely related, while those connected via multiple nodes are less related.
• The root is the base of the cladogram.
• Cladograms are evidence, but not proof, of evolutionary history.
• They are built on the assumption that the fewest mutations account for base or amino acid sequence differences.

Classification of All Organisms in Three Domains

• Determining the base sequences of ribosomal RNA revealed two distinct groups of prokaryotes.
• The three domains of life are Eubacteria, Archea, and Eukaryota.