In-Depth Notes on Chordata and Evolutionary Biology

Phyla Overview
- Chordata
- Cephalochordata (e.g., amphioxus)
- Urochordata (e.g., tunicates)
- Vertebrata (e.g., fish, mammals)
- Hemichordata (e.g., acorn worms)
- Echinodermata (e.g., starfish, sea urchins)
- Arthropoda
- Annelida
- Mollusca
Embryological Linkages
- Metazoa – Multicellular organisms
- Key characteristics include:
- Cell layers and tissues: ectoderm, mesoderm, endoderm
- Nervous system present
- Heterotrophic nature

Types of Germ Layers
- Ectoderm
- Form outer layers of skin and nervous system
- Endoderm
- Forms majority of the digestive system lining
- Mesoderm (appears last)
- Forms notochord, most muscles, circulatory system, and urogenital systems
Gastrulation is critical:
- Formation of germ layers occurs during embryonic development

Key Features of Bilateria
- Three cell layers
- Organs with bilateral symmetry
- Presence of coelom within mesoderm, supporting organ development
- Gut tube open at both ends: mouth and anus
Symmetry
- Radial vs. bilateral symmetry

Cell Differentiation
- Autonomous specification (invertebrates)
- Fixed fate for cells during development
- Conditional specification (vertebrates)
- Cell fate is influenced by interactions with adjacent cells
Hox Genes
- Regulatory genes that determine body plan and segment identity
- Duplication and diversification of these genes contributed to complexity in vertebrate evolution

Essential Features of Chordates:
1. Notochord - Provides support and develops into vertebral column
2. Dorsal hollow nerve tube - Develops into central nervous system
3. Pharyngeal slits - Used in filter feeding or gas exchange
4. Endostyle - Glandular tissue involved in feeding
5. Post-anal tail - Extends beyond the anus and serves various functions

General Features:
- Small, fish-like shape
- Active larval stages, sedentary adults
Anatomy:
- Myomeres: segmented muscle blocks contributing to swimming
- Notochord extends to head aiding in support for burrowing
- Pharyngeal slits for feeding and gas exchange through diffusion

Structural Differences:
- Cephalochordates are simpler, lacking a cranium or complex sensory organs compared to vertebrates
- Vertebrates have a more complex digestive system and circulatory system with neural control
- Locomotion differences: Cephalochordates use myomeres for movement while vertebrates utilize more efficient muscular systems with fins and tails

Genetic Duplication and Hox Genes:
- Significant evolutionary innovations arise from gene duplications
- Jawed vertebrates possess four clusters of Hox genes, facilitating complex body plan developments
Developmental Asymmetry:
- Mechanisms of asymmetry established through paracrine and autocrine signaling pathways in early development

Stages:
- Zygote: initial fertilized cell
- Cleavage: rapid cell division
- Blastula: hollow ball of cells
- Gastrulation: formation of germ layers
- Gastrula: archenteron and differentiated layers emerge
Deuterostome Classification:
- Mouth forms secondarily; anus is first

Evolution from simple structures (like non-vertebrate chordates) to complex organisms (vertebrates) marked by increased size, functionality, and specialization
Increased predation and environmental awareness correlated with anatomical changes in vertebrates, including more efficient respiratory and circulatory systems.