In-Depth Notes on Evolution and Cladistics

Introduction to Evolution and Cladistics

• Evolution and cladistics are vital in understanding biodiversity and the relationships between species.
• The Tree of Life illustrates ancestry and relationships among species.

Learning Goals

• Understand the Linnean system for naming and grouping species.
• Outline how natural selection changes the diversity and forms of life over time.
• Interpret phylogenetic trees to understand evolutionary relationships.
• Distinguish between homologous and analogous traits.
• Apply parsimony in hypothesis selection.

Classification

• Definition: Categorizing organisms based on similarities and differences.
• Purpose:
  • To describe natural categories (taxonomy).
  • To understand evolutionary relationships (systematics).
  • To facilitate effective communication about species.

Taxonomy and Systematics

• Taxonomy: Naming species (e.g., Linnaean system by Carolus Linnaeus).
• Systematics: Organizing categories based on evolutionary relationships, not just appearance.
• Linnaeus' approach involves:
  • Binomial Naming: Genus + trivial name (e.g., Homo sapiens for humans, Canis familiaris for dogs).

Hierarchical Classification

1. Kingdom: Animalia
2. Phylum: Chordata
3. Class: Mammalia
4. Order: Primates
5. Family: Hominidae
6. Genus: Homo
7. Species: Homo sapiens

Modern Concepts of Species

• Biological Species Concept: Defined by interbreeding capability under natural conditions.
• Morphospecies Concept: Based on physical features for extinct species.

Evidence of Evolution

• Fossils show links between extinct and modern species (Cuvier).
• Gregor Mendel's work on heredity laid the groundwork for understanding trait inheritance.

Darwin's Natural Selection

• Principles outlined in On the Origin of Species (1859):
  1. More offspring produced than can survive.
  2. Variations among offspring exist.
  3. Competition for resources.
  4. Favorable traits increase chances of survival and reproduction.
  5. Beneficial traits are inherited.
• Fossil evidence shows species and adaptations over time.

Homology vs. Analogy

• Homologous Traits: Similar structures arising from a common ancestor (e.g., whale fin vs. human hand).
• Analogous Traits: Structures that perform similar functions but do not share an evolutionary history (e.g., bee wings vs. bird wings).
• Examples of Homologous Structures: Vertebrate forelimbs similar due to shared ancestry.

Cladistics

• A method of classification that emphasizes evolutionary relationships based on shared derived characteristics.
• Cladograms visually represent these relationships, indicating how species are grouped based on evolutionary changes.

Assigning Characters in Cladistics

• Not all characters are useful for distinguishing evolutionary relationships.
• Criteria must be applicable to separate groups effectively; for instance, feathers indicate birds, but cannot separate bird species.

Parsimony in Analysis

• The simplest explanation (with the fewest steps) is favored as the best hypothesis for evolutionary relationships.

Definition of Clades

• Clades (or monophyletic groups) include species that share a common recent ancestor, emphasizing evolutionary relatedness (e.g., dinosaurs and lizards share common ancestry distinct from turtles).

Type Specimens

• The type specimen is the designated example of a species, kept for comparison in museums, defining species identity regardless of how it compares with others.

Summary

• Evolution and classification are foundational in understanding biodiversity. The Linnaean system, cladistics, and natural selection provide frameworks for studying the evolutionary history and relationships of organisms.

Upcoming Topics

• Next class will cover dating dinosaurs and Mesozoic life with practical applications in cladistics.