Lecture 15 - Cladistics

Introduction to Botany and Plant Evolution

  • Definition of Botany: The study of plants, covering their structure, properties, and biochemical processes.

  • What is a Plant?: Characteristics defining plants include the presence of chlorophyll and a cell wall made of cellulose. A central theme is distinguishing between plant and non-plant.

Linnaeus and Classification

  • Binomial Naming System: Developed by Carl Linnaeus; uses two names for species:

    • Genus: The first part of the name, capitalized.

    • Species: The second part, lowercase. Examples include Homo sapiens.

  • Classification Hierarchy: Organisms are classified in a hierarchical structure from broad (Domain) to specific (Species), akin to a pyramid.

Cladistics Overview

  • Definition: The study of evolutionary relationships among a group of organisms.

  • Taxon: A grouping of organisms at any classification level.

  • Important Concepts:

    • Sister Taxa: Groups that share a most recent common ancestor.

    • Clade: A group consisting of an ancestor and all its descendants.

Reading Phylogenetic Trees

  • Branches and Nodes:

    • Branching Points: Represent common ancestors.

    • Sister Taxa Information: Helps to understand relationships (e.g., wolves and dogs are more closely related than to jackals).

  • Polytomies: These indicate uncertainty in evolution where multiple species diverge at once.

Homology vs. Analogy

  • Homology: A shared trait inherited from a common ancestor; reflects true evolutionary relationships.

  • Analogy: Similar traits that arise independently due to similar environmental pressures (convergent evolution).

  • Principle of Parsimony: Favor explanations that require the fewest changes or assumptions; important for establishing correct relationships between traits.

Evolution of Photosynthetic Life

  • Evolutionary progression from bacteria to land plants shows critical steps in the development of photosynthetic organisms.

  • Key Groups:

    • Eukaryota: Includes animals, fungi, and plants.

    • Archaeplastida: Includes land plants and red/green algae.

Origin of Land Plants

  • Timeline:

    • Origin of plants: Approximately 470 million years ago.

    • Origin of vascular plants: About 425 million years ago.

    • Seed plants: Evolved roughly 360 million years ago, leading to gymnosperms and angiosperms.

Summary of Plant Evolutionary Milestones

  • Nonvascular Plants: Liverworts, mosses, hornworts.

  • Vascular Plants: Lycophytes, Monilophytes, Gymnosperms, Angiosperms (flowering plants).

  • Key evolutionary adaptations: Development of vascular tissues allowed plants to grow taller and colonize various environments.

Important Notes

  1. Identification of Outgroups: Used in phylogenetic studies to determine ancestral traits.

  2. Shared Ancestral Features: Traits present in the most recent common ancestor of a clade, essential for identifying relationships.

  3. Unresolved Evolutionary Relationships: Indicated by polytomies, highlighting areas where further research is needed.