Overview of Animal Diversity: Key Concepts and Phylogeny

Unikonta and Animal Phylogeny

Major Clades of Unikonta

• Unikonta is a supergroup of eukaryotes that includes two main clades:

  • Amoebozoans: Includes amoebas and slime molds.

  • Opisthokonts: Includes fungi, animals, and their closest protistan relatives.

Interpreting Phylogenetic Trees

• Phylogenetic trees illustrate evolutionary relationships.

• The most recent common ancestor is found at the node where two lineages diverge.

• The closest relative is the group sharing the most recent common ancestor with the group of interest.

Major Characteristics of Animals

Key Features of Animals

• Heterotrophy: Animals obtain energy by consuming other organisms.

• Presence of Tissues: Most animals have specialized tissues (e.g., muscle, nervous tissue).

• Digestive System: Animals typically have internal digestive systems for processing food.

• Nerve and Muscle Cells: Enable movement and response to stimuli.

  • Skeletal muscle enables somatic (body) movement.

  • Note: There are exceptions and unique adaptations in biology.

Structural Proteins

• Collagen is a structural protein found in animal cells, providing strength and support.

Animal Development

Stages from Zygote to Gastrula

• Zygote: Fertilized egg cell (diploid).

• Cleavage: Rapid cell divisions without growth.

• Blastula: Hollow ball of cells formed after cleavage.

• Gastrulation: Formation of embryonic tissue layers (gastrula).

Sexual Reproduction in Animals

• Male and female haploid gametes (sperm and egg) fuse to form a diploid zygote.

Animal Origins and Molecular Evidence

Choanoflagellates and Sponges

• Phylogenetic trees show choanoflagellates as the closest living relatives to animals (especially sponges).

Unique and Shared Genes

• Certain genes (e.g., CCD gene) are unique to humans.

• Cadherin proteins are found in both choanoflagellates and animals; they function in cell adhesion, helping cells stick together and communicate.

Symmetry and Tissue Organization

Types of Symmetry

• Radial symmetry: Body parts arranged around a central axis; common in sessile or planktonic animals (e.g., jellyfish).

• Bilateral symmetry: Body has right and left halves; typical of actively moving animals (e.g., humans, insects).

Germ Layers

• Diploblastic organisms: Have two germ layers (ectoderm and endoderm).

• Triploblastic organisms: Have three germ layers (ectoderm, mesoderm, endoderm).

Body Cavities

• Most triploblastic animals possess a body cavity (coelom).

• A true coelom is a fluid-filled cavity completely lined by mesoderm.

• Functions of a body cavity:

  • Cushions and protects internal organs.

  • Allows organs to grow and move independently of the body wall.

Types of Body Cavities

Type	Description	Location
Acoelomate	No body cavity between digestive tract and outer body wall	Solid mesoderm fills space
Hemocoel	Body cavity partially lined with mesoderm and filled with hemolymph	Between tissues/organs
Coelom	True body cavity completely lined by mesoderm	Surrounds internal organs

Developmental Modes: Protostomes vs. Deuterostomes

• Protostomes:

  • Spiral, determinate cleavage

  • Mouth develops from the blastopore

• Deuterostomes:

  • Radial, indeterminate cleavage

  • Anus develops from the blastopore

Modern Phylogenetic Analysis

• Phylogenetic relationships are now inferred using molecular data, including:

  • rRNA sequences

  • Hox genes

  • Whole genome analysis

  • Mitochondrial genes

  • Other molecular markers

Major Clades of Bilaterians

Clade	Major Members	Key Characteristics
Deuterostomia	Chordates, Echinoderms	Deuterostome development, radial cleavage
Ecdysozoa	Arthropods, Nematodes	Molting (ecdysis) of exoskeleton
Lophotrochozoa	Mollusks, Annelids, Flatworms	Lophophore feeding structure or trochophore larva

Summary Table: Key Animal Features

Feature	Description
Symmetry	Radial or bilateral
Germ Layers	Diploblastic or triploblastic
Body Cavity	Acoelomate, hemocoel, or coelomate
Development	Protostome or deuterostome
Molecular Data	rRNA, Hox genes, genome analysis

Example: Human Development

• Humans are bilateral, triploblastic, coelomate deuterostomes in the clade Deuterostomia.

Additional info: The stages of animal development (zygote, cleavage, blastula, gastrula) are fundamental to understanding embryology and are frequently tested on exams such as the MCAT. The distinction between protostomes and deuterostomes is a classic topic in comparative zoology. Modern phylogenetics relies heavily on molecular data, which has reshaped our understanding of animal relationships.