ch 19

Systematics: Seeking Order Amid Diversity

19.1 How Are Organisms Named and Classified?

Systematics is the branch of biology concerned with naming and classifying organisms. It provides a framework for understanding the diversity of life and its evolutionary relationships.

• Taxonomy: The science of naming, describing, and classifying organisms. A taxon is a named group of organisms.

• Binomial Nomenclature: Developed by Carl von Linne (Carolus Linnaeus), this system assigns each species a two-part Latin name (genus and species).

• Genus: A group of closely related species.

• Species: Populations of organisms that can potentially interbreed under natural conditions.

• Scientific Name Format: The genus name is capitalized, the species name is lowercase, and both are italicized or underlined (e.g., Gavia immer).

• Evolutionary Descent: Modern classification emphasizes evolutionary relationships, as first proposed by Charles Darwin.

• Phylogeny: The evolutionary history of a group of organisms. Classification is now based on phylogeny rather than just morphology.

• Systematics: The science of reconstructing phylogeny and classifying organisms based on evolutionary relationships.

Example: Bluebirds

• The genus Sialia includes three species: Sialia sialis (eastern bluebird), Sialia mexicana (western bluebird), and Sialia currucoides (mountain bluebird).

19.1.1 Methods in Systematics

• Morphological Similarities: Systematists compare anatomical features among living organisms to infer evolutionary relationships.

• Convergent Evolution: Similar features may evolve independently in unrelated groups (e.g., legless lizards and snakes), so not all similarities are useful for classification.

• Anatomical Traits: Historically, anatomical features have been most important for classification, including both external and internal structures.

• Microscopic Structures: Features such as pollen grain structure can be used to classify plants.

Figure: Microscopic Structures

• Morning glory pollen and bitter melon pollen are examples of microscopic features used in classification.

19.1.2 Genetic Methods in Systematics

• Molecular Systematics: Relies on genetic similarities, especially DNA sequences, to reconstruct phylogeny.

• Genetic Similarities: Fewer differences in DNA sequences indicate closer evolutionary relationships.

• Mutations: Systematists compare nucleotide sequences and count mutations to infer relatedness.

• Example: Human and chimpanzee chromosomes are extremely similar, reflecting a recent common ancestor.

19.1.3 Clades and Hierarchies

• Clade: A group of organisms that includes an ancestor and all its descendants.

• Hierarchy: Clades can be nested within larger clades, forming a hierarchical structure.

• Taxonomic Ranks: Traditional ranks include domain, kingdom, phylum, class, order, family, genus, and species.

• Nested Hierarchy: Each level includes all levels below it.

19.1.4 Modern Trends in Classification

• Decline of Taxonomic Ranks: Modern systematics has de-emphasized traditional ranks in favor of clades and domains.

• Domains: The broadest taxonomic group, including Bacteria, Archaea, and Eukarya.

19.2 What Are the Domains of Life?

Classification systems have evolved to reflect new scientific discoveries, especially in molecular biology.

• Early systems divided life into prokaryotes and eukaryotes, with prokaryotes placed in a single kingdom.

• Carl Woese established that DNA and ribosomal RNA (rRNA) sequences could be used to distinguish major groups.

• Bacteria and Archaea: Differences in rRNA genes revealed two distinct prokaryotic domains, each with its own evolutionary history.

• Three Domains of Life: Bacteria, Archaea, and Eukarya.

• Domain Bacteria: Includes most prokaryotes.

• Domain Archaea: Includes prokaryotes with unique cell wall and RNA polymerase features.

• Domain Eukarya: Includes all organisms with membrane-bound nuclei and organelles (plants, animals, fungi, protists).

Figure: The Tree of Life

• The tree of life shows three major branches (domains): Bacteria, Archaea, and Eukarya.

• Each domain contains kingdoms, indicated by major branches.

19.3 Why Do Classifications Change?

Classification systems are subject to revision as new data become available, especially at the species level.

• Revisions: Changes at the top levels (kingdom, domain) are rare, but species designations are frequently revised.

• Example: African elephants are now divided into two species based on genetic evidence.

• Biological Species Concept: Defines species as groups of interbreeding natural populations. Difficult to apply to asexual organisms and some protists.

• Phylogenetic Species Concept: Defines species as the smallest group descended from a common ancestor, sharing distinguishing characteristics.

19.4 How Many Species Exist?

Estimating the number of species on Earth is challenging due to the vast diversity and many undiscovered organisms.

• About 15,000 new species are identified annually, mostly in the tropics.

• The total number of named species is around 1.6 million.

• Estimates suggest that 8.7 million species may exist.

• Most undiscovered species are Bacteria, Archaea, and Protozoans.

• Biodiversity: The number and variety of Earth's species.

• Of all species identified, about 70% are prokaryotes and protists, 20% are plants and fungi, and the rest are animals.

Table: Major Taxonomic Ranks and Domains

Rank	Description
Domain	Largest, most inclusive group (Bacteria, Archaea, Eukarya)
Kingdom	Major groups within domains (e.g., Animalia, Plantae, Fungi, Protista)
Phylum	Groups of related classes
Class	Groups of related orders
Order	Groups of related families
Family	Groups of related genera
Genus	Groups of closely related species
Species	Basic unit of classification; populations that can interbreed

Additional info: Modern systematics increasingly relies on molecular data, and the concept of domains has replaced kingdoms as the highest taxonomic rank. The biological species concept is not universally applicable, especially for asexual organisms.