Biological Diversity: Phylogenies and the History of Life (Lectures 8 & 9)

Biological Diversity: Phylogenies and the History of Life

Introduction

This section introduces the study of biological diversity through the lens of phylogenetics, focusing on how scientists reconstruct evolutionary relationships and interpret the history of life on Earth. Key topics include the construction and interpretation of phylogenetic trees, adaptive radiation, and mass extinction events.

Phylogenetic Trees

Definition and Components

Phylogenetic trees are diagrammatic representations of evolutionary relationships among organisms. They help scientists visualize how species are related through common ancestry.

• Root: The most ancestral branch in the tree.

• Branches: Represent evolutionary lineages.

• Nodes: Points where branches split, indicating a common ancestor.

• Tips: Represent current species or taxa.

• Outgroup: A taxon outside the group of interest, used for comparison.

Example: A phylogenetic tree showing relationships among lungfish, lizard, dog, and human, with lungfish as the outgroup.

Constructing Phylogenetic Trees

Phylogenetic trees are built using data matrices that compare characteristics (morphological or genetic) among taxa.

• Data matrix example: Presence (1) or absence (0) of traits such as skull, limbs, hair, and lactation.

• Outgroup selection is crucial for rooting the tree and determining trait polarity.

Example: Table showing trait distribution among lungfish, lizard, dog, and human.

Taxon	Skull	Limbs	Hair	Lactation
Lungfish (outgroup)	1	0	0	0
Lizard	1	1	0	0
Dog	1	1	1	1
Human	1	1	1	1

Binomial Nomenclature and Classification

Carolus Linnaeus developed a universal system for naming organisms, known as binomial nomenclature (Genus + species), and established a hierarchical classification system:

• Domain (added by Carl Woese)

• Kingdom

• Phylum

• Class

• Order

• Family

• Genus

• Species

Example: Homo sapiens (human species)

Domains of Life

Carl Woese introduced the concept of Domain to distinguish major groups of life:

• Bacteria: Prokaryotes without a membrane-bound nucleus.

• Archaea: Prokaryotes with unique modes of protein synthesis.

• Eukarya: Eukaryotes with membrane-bound nuclei.

Domain	Membrane-bound Nucleus	Protein Synthesis
Bacteria	No	Standard
Archaea	No	Unique
Eukarya	Yes	Standard

Patterns in Life’s History

Adaptive Radiation

Adaptive radiation is the rapid evolution of diversely adapted species from a common ancestor, often following the colonization of new environments or the emergence of new traits.

• Example: Hawaiian silverswords evolved from a single tarweed ancestor, resulting in over 30 species in ~2 million years.

• Example: African cichlid fishes diversified in isolated lakes after geological events created new habitats.

• Drivers include competition, trait variability, and ecological opportunities.

Mass Extinction Events

Mass extinction refers to the rapid loss of a large number of species due to catastrophic environmental changes (e.g., volcanic eruptions, asteroid impacts).

• Short-term changes (e.g., volcanic eruptions) can block sunlight, disrupt food chains, and cause widespread die-offs.

• Long-term effects include altered climate and ecosystem structure.

• Extinction events can be natural (e.g., Ice Ages) or anthropogenic (e.g., deforestation).

Geological Time Scale and Major Events

Major Eons and Eras

The history of life is divided into major eons and eras, each marked by significant evolutionary and extinction events.

• Hadean Eon: Formation of Earth (~4.6 billion years ago)

• Archaean Eon: Origin of life (~3.5 billion years ago)

• Proterozoic Eon: Rise of complex life forms

• Phanerozoic Eon: Diversification of animal and plant life

Example: The Permian extinction event marked the end of the Paleozoic era, with a significant percentage of species lost.

Applications and Implications

Importance of Phylogenetics

Understanding phylogenetic relationships helps scientists:

• Classify organisms accurately

• Trace evolutionary history

• Predict traits and behaviors

• Conserve biodiversity

Adaptive Traits and Speciation

Trait variability and ecological opportunities can lead to new species through mechanisms such as natural selection and reproductive isolation.

• Examples include changes in feeding strategies, habitat use, predation risk, and mate selection.

• Plant reproduction can diversify through new pollination modes, aiding colonization of new habitats.

Summary Table: Key Concepts

Concept	Definition	Example
Phylogenetic Tree	Diagram of evolutionary relationships	Tree showing lungfish, lizard, dog, human
Binomial Nomenclature	Genus + species naming system	Homo sapiens
Adaptive Radiation	Rapid diversification from a common ancestor	Hawaiian silverswords, African cichlids
Mass Extinction	Rapid loss of species due to environmental change	Permian extinction

Additional info: Some context and examples were inferred to clarify fragmented notes and ensure completeness.