Classification and Taxonomy Lecture Notes

Introduction to Taxonomy and Biological Classification

  • Definition of Taxonomy: Taxonomy is the formal scientific study involving the naming, defining, and classifying of groups of biological organisms. These classifications are based on shared characteristics.

  • Importance of Classification: Scientists have identified approximately 2×1062 \times 10^6 (2 million2 \text{ million}) species, though estimates suggest there may be between 5 million5 \text{ million} and 20 million20 \text{ million} species in existence. The primary reasons for classification include:

    • Establishing a Universal Language: Provides a consistent naming system used globally by scientists.

    • Predicting Characteristics: Helps in medical fields (e.g., identifying specific bacteria for correct antibiotic use) and pharmacology (e.g., finding medicinal qualities in species related to known beneficial plants).

    • Learning Evolutionary History: Traces relationships between different life forms over time.

    • Aiding in Conservation: Identifying species and tracking populations is essential for determining endangerment and developing protection strategies.

The Historical Evolution of Classification Systems

  • Aristotle (Over 2,000 years ago): The Greek philosopher was the first to classify organisms. He established two main kingdoms:

    • Kingdom Plantae: Organisms that make their own food. He further subdivided them by size into herbs, shrubs, and trees.

    • Kingdom Animalia: Organisms that do not make their own food and are capable of movement. He grouped them by habitat: land, sea, and air.

    • Limitations of Aristotle’s System: Certain organisms did not fit easily. For example, sponges do not move, and coral does not produce its own food.

  • The Impact of the Microscope (17th Century17^{th} \text{ Century}): The invention of the microscope revealed organisms that challenged simple categories. Examples include Euglena, which uses locomotion (animal-like) but also performs photosynthesis (plant-like). This necessitated more complex kingdoms.

  • Ernst Haeckel (18661866): This German biologist added a third kingdom for microorganisms.

    • Kingdom Protista: Includes algae, protozoa, slime moulds, and water moulds.

  • Further Expansion of Kingdoms:

    • Kingdom Fungi: Originally placed under Plantae, but moved because mushrooms and moulds do not use photosynthesis.

    • Kingdom Bacteria (Monera/Eubacteria): Single-celled organisms that lack a nucleus. They grow and obtain energy from diverse environments.

    • Kingdom Archaea: Identified separately from bacteria in the 1990’s1990\text{'s}. These are microorganisms that inhabit extreme environments.

Modern Classification: The 6 Kingdoms and 3 Domains

Organisms are currently organized into a system featuring three Domains containing six Kingdoms:

  • Domain Bacteria: Contains Kingdom Bacteria.

  • Domain Archaea: Contains Kingdom Archaea.

  • Domain Eukaryota: Contains the four remaining kingdoms:

    • Kingdom Protista

    • Kingdom Fungi

    • Kingdom Plantae

    • Kingdom Animalia

Binomial Nomenclature: The Scientific Naming System

  • Carolus Linnaeus: Developed binomial nomenclature, a two-word naming system usually derived from Latin or Greek roots.

  • Standard Formatting Rules:

    • The first word is the genus name. It must be capitalized and italicized.

    • The second word is the species name. It must be all lowercase and italicized.

    • Example: Lagenorhynchus cruciger (Hourglass Dolphin).

  • Advantages of the System:

    • Consistency: The name remains the same regardless of geographic location or local language.

    • Relationship tracking: Shared genus names indicate that organisms are closely related. For example, Panthera leo (lion), Panthera tigris (tiger), and Panthera pardus (leopard) all belong to the same genus.

The Taxonomic Hierarchy

Organisms are classified using a phylogenetic hierarchy, where they are placed into groups based on increasing similarities. The system consists of 8 ranks (or taxa):

  1. Domain (broadest)

  2. Kingdom

  3. Phylum

  4. Class

  5. Order

  6. Family

  7. Genus

  8. Species (most specific)

  • Organizational Trend: As you move from Domain to Species, there is an increasing similarity between organisms but a decreasing number of species within the group.

  • Mnemonic Devices:

    • "King Philip Came Over For Good Soup"

    • "Kids Prefer Chips Over Fancy Green Salad"

  • Example Classification: Human (Homo sapiens):

    • Kingdom: Animalia (Organisms able to move on their own)

    • Phylum: Chordata (Animals with a backbone)

    • Class: Mammalia (Chordates with fur/hair and milk glands)

    • Order: Primates (Mammals with collar bones and grasping fingers)

    • Family: Hominidae (Primates with flat faces and 3D vision)

    • Genus: Homo (Hominids with upright posture and large brains)

    • Species: Homo sapiens (Genus Homo members with a high forehead and thin skull bones)

  • Example Classification: Monarch Butterfly:

    • Kingdom: Animalia, Phylum: Arthropoda, Class: Insecta, Order: Lepidoptera, Family: Danaidae, Genus: Danaus, Species: plexippus.

  • Example Classification: Koala:

    • Kingdom: Animalia, Phylum: Chordata, Class: Mammalia, Sub-class: Marsupialia, Order: Diprontodontia, Sub-order: Phascolarctomorphia, Family: Phascolarctidae, Genus: Phascolarctos, Species: cinereus.

Comparative Systems and Species Concepts

Cladistics and Cladograms

  • Definition: Also known as a Phylogenetic Tree, this system is based on similarities and differences between DNA and RNA sequences.

  • Purpose: It shows when different groups diverged from a common ancestral line and represents evolutionary relationships.

  • Example Characteristics used in Cladograms: Presence of vertebrae, lungs, amniotic eggs, exoskeleton, molting, or the ability to see UV light.

The Three Species Concepts

  1. Morphological Species Concept: Distinguishes species based on physical appearance and structure.

    • Advantages: Simplest method, non-disruptive to living species, easy to use for fossils and field guides.

    • Disadvantages: Difficulty in "drawing the line," issues with mimicry, and sexual dimorphism (males and females looking different).

  2. Biological Species Concept: Defines species as members of a population that actually or potentially interbreed in nature.

    • Advantages: Widely used and easy to determine with living species.

    • Disadvantages: Does not apply to asexual reproduction or fossils; complicated by physical separation of populations.

  3. Phylogenetic Species Concept: Defines species as the smallest set of organisms sharing an ancestor; focuses on distinct evolutionary history and DNA.

    • Advantages: Most accurate; applicable to extinct species if DNA is available.

    • Disadvantages: Evolutionary history is not known for all species; DNA access is limited and can be expensive.

Dichotomous Keys

  • Function: A tool used to identify a specific species through a series of choices.

  • Construction Rules:

    • Use two questions (couplets) at each stage.

    • Objective traits only: Questions must not be subjective or behavioral (e.g., do not ask if an animal "likes grapes" or "is friendly").

    • Avoid behavioral questions because they are useless if the animal is dead or not currently active.

    • Start with broad characteristics and move toward specific details.

  • Sample Dichotomous Key Logic:

    1. Does it have a bony shell? (Yes: Blanding's or Snapping Turtle; No: Go to 3)

    2. Does it have hair? (Yes: Wolf, Squirrel, Skunk, or Chipmunk; No: Frog)

    3. Physical traits like height (< 75\,cm) or specific stripe patterns (longitudinal vs. eye-crossing) then separate the remaining species.

  • Primate Identification Exercise: Included species for key creation are the Saki monkey, Golden lion-tamarin, Two-toed sloth, Lowland gorilla, Ring-tailed lemur, Orangutan, White-handed gibbon, and Human. Keys must use traits like orange/red hair rather than behavioral traits like climbing trees.