Lecture 2 - The Tree of Life Overview

Lecture 2 - The Tree of Life - Part 1: Importance of the Tree of Life

• The Great Chain of Being (500-1500 AD) viewed life as a stairway, ascending from lower forms to higher forms.
• Higher beings in the hierarchy possessed all attributes of lower beings.
• The medieval view, La scala naturae, placed minerals at the bottom, then fire, plants, animals, humans, and God at the top.
• Linnaeus' Kingdom of Animals (1735) is a classification system.

Modern Linnaean View

• The Linnaean system classifies organisms into taxa: Kingdom, Phylum, Class, Order, Family, Genus, Species.
• Modern classification still shows influences of Medieval perspective, focusing on higher being, despite understanding of evolution.

Darwin and Diverging Lineages

• Darwin recognized extinction as a necessary aspect of evolution, needing many living species of the same genus to exist.
• He visualized species as branching lineages, although he lacked the mechanism of genetics.
• Darwin's sketch showed branching lineages, demonstrating relationships between organisms, and extinction of some forms.

Not Just Darwin: Edward Hitchcock

• Edward Hitchcock (1840) similarly envisioned branching lineages of extinct and living organisms, highlighting the concept of extinction.
• Hitchcock's work predated Darwin's publication, reflecting a similar understanding of evolution.

The Modern Synthesis of Genetics and Evolution

• Theodosius Dobzhansky (1937) redefined evolution as a change in allele frequency in a gene pool.
• Evolutionary biology has incorporated genetics and knowledge of mutations in creating a modern understanding.
• Evolutionary processes are explained in terms of genetic changes at the population level and understanding that genetics drives evolution.

Lineage Changes and Divergence Over Time

• Evidence from biology and fossil records show that lineages have changed and diverged over time.
• Various disciplines like behavioral ecology, morphology, and molecular biology help examine lineage changes over extended periods.
• Using morphological evidence from fossils to track lineages and observe evolutionary changes.
• The study of Whales used morphological data from fossils and extant whales to trace evolutionary changes.

Cetaceans: Whales, Dolphins, and Porpoises

• Cetaceans (whales, dolphins, porpoises) have two suborders: Mysticetes (baleen whales) and Odontocetes (toothed whales) and both emerged approximately 34 million years ago.
• Baleen whales filter feed using keratin-based baleen plates.
• Toothed whales are predators that use echolocation and have teeth.

Which Features Are Good for Classification?

• Homologous traits (similar traits due to shared ancestry) are useful for classification.
• Derived traits, or apomorphies, are features that are unique to a clade, offering valuable insights into evolutionary relationships.

How Can A Fin And A Hand Be Homologous?

• Similarities in developmental processes and underlying skeletal structures (e.g. bone patterns in the forelimbs of vertebrates) can highlight homologous traits of organisms in divergent branches.

Derived Traits Can Be Reduced or Lost

• The presence or absence of traits can give clues to the evolutionarily pathways and relationships of species.
• Certain traits present in ancestral organisms may be reduced or lost in later descendant species, for example, hind limbs in whales.
• Hidden traits can be re-discovered or used in combination with other data to determine evolutionary relationships.

Dorudon and Pakicetus

• Dorudon (40 million years old) and Pakicetus (50 million years old) are fossil whales demonstrating evolutionary progression from terrestrial ancestors.

Synapomorphies: Shared Derived Traits

• Synapomorphies are derived traits shared by two or more lineages.
• Using the traits of Pakicetus, whales and their mammalian relatives to highlight evolutionary relationships.
• A synapomorphy can be a trait acquired at a particular point in the evolution of a group.

Other Synapomorphies and Classification

• Shared traits are useful in classifying groups, establishing relationships and the order in which traits evolved.
• Groups identified from the ancestor and its descendants (cladistics), providing better understanding of the evolutionary pathways of organisms.

Autoapomorphies

• Autapomorphies are traits unique to a specific taxon and do not support broader groupings. Ex: platypus laying eggs (a trait that evolved after branching from other mammals).
• Using these traits for broader, general grouping of organisms isn't beneficial.

This quiz explores the significance of the Tree of Life, tracing historical perspectives from the Great Chain of Being to modern Linnaean classification systems. It highlights Darwin's contributions to evolutionary biology and the concept of branching lineages. Test your knowledge on the essential classifications and ideas that shape our understanding of biodiversity.