Tree Thinking and Dichotomous Branching The Three Domains of Life

Overview

  • Three recognized domains:

    • Bacteria

    • Archaea

    • Eukaryota

  • Key concepts include the Last Universal Common Ancestor (LUCA) and the Last Eukaryotic Common Ancestor (LECA).

The Tree of Life

Cultural Significance

  • The Tree of Life:

    • An archetype in various world mythologies and religions.

    • Symbolizes the origin and continuity of life.

Biological Importance

  • Model and Research Tool:

    • Essential for exploring the evolution of life.

    • Highlights relationships among organisms.

  • Assumption in Biology:

    • All organisms are related, tracing back to LUCA.

Phylogenetic Tree of Life Structure

Overview of Tree Composition

  • Branches of Life:

    • Bacteria:

    • Sub-divisions include:

      • Spirochetes

      • Proteobacteria

      • Cyanobacteria

      • Planctomyces

      • Bacteroides

      • Cytophaga

      • Thermotoga

      • Aquifex

    • Archaea:

    • Examples include:

      • Methanosarcina

      • Methanobacterium

      • Halophiles

      • Methanococcus

      • Thermoproteus

      • Pyrodicticum

    • Eukaryota:

    • Includes:

      • Animals

      • Fungi

      • Plants

      • Protists (Ciliates, Flagellates, Trichomonads, Microsporidia, Diplomonads)

  • LUCA (Last Universal Common Ancestor):

    • Common ancestor for all current and past organisms.

Eukaryotic Specifics

  • Eukaryotes:

    • Have organelles and a nuclear membrane.

  • LECA (Last Eukaryotic Common Ancestor):

    • Represents the common ancestor of all eukaryotic organisms.

Differences Between Prokaryotes and Eukaryotes

  • Prokaryotes (Bacteria and Archaea):

    • Lack cell organelles.

    • Many biochemical and rRNA markers separate Archaea from Bacteria.

Dichotomous Trees and Keys

Definition and Origin

  • Dichotomous Trees:

    • The term derives from Greek:

    • Di- = two

    • -tomia = cutting

    • Represents the division of organisms into groups through a stepwise approach.

Biological Implications

  • Illustrates an evolutionary assumption: Organisms split into groups one at a time.

  • Emphasizes the sequence of evolutionary events, with proximity to the base/root signifying earlier origins.

Stepwise Approach to Sorting

  • Utilized in dichotomous keys, valuable for identification or diagnosis.

  • Reflects evolutionary relationships among organisms.

Examples of Dichotomous Keys (Plant Identification)

Basic Structure of a Dichotomous Key

  • Example Inquiry:

    • Is the plant woody?

    • Yes → Proceed to step 1.

    • No → Proceed to step 1’. (herbaceous plants)

Further Example Steps for Classification

  1. If woody:

    • Does the woody plant have flat and broad leaves?

      • Yes → Go to step 2.

      • No → Go to step 2’.

  2. If flat and broad leaves:

    • Is the plant taller than 3 m?

      • Yes → (identifies as trees).

      • No → (identifies as shrubs).

  3. If not flat and broad:

    • Are the leaves waxy and needle-like?

      • Yes → (identifies as conifers).

      • No → End classification based on other attributes.

Full Classification Example in Steps

  • Plant Types Identified:

    • Herbaceous plants

    • Broadleaf trees

    • Broadleaf shrubs

    • Conifers

Key Takeaways

  • Three Domains of Life:

    • Bacteria, Archaea, Eukaryota.

  • LUCA:

    • A shared ancestor for all current and extinct organisms.

  • LECA:

    • Represents the common ancestor to all eukaryotes.

  • Importance of Tree Diagrams & Dichotomous Keys:

    • Illustrate sequential changes in evolutionary history.

    • Crucial tools for identifying and classifying organisms.