Animal Diversity: Phylogenetic Relationships

Animal Diversity: Phylogenetic Relationships

• Animals are multicellular, heterotrophic eukaryotes lacking cell walls.
• Most animals possess tissues. A tissue is a group of cells arranged in a specific pattern or layer that perform similar functions. These tissues originate from embryonic germ layers.
• Animals have two unique tissue types: nervous tissue and muscle tissue.
  • Nervous tissue is comprised of neurons and supporting cells which conduct signals.
  • Muscle tissue is responsible for movement, via contraction, which results from the interaction of actin and myosin proteins. Different types of muscle tissue exist (smooth, cardiac, skeletal)

Phylogeny

• Phylogeny is a classification scheme based on evolutionary relationships.
• A phylogeny is a hypothesis explaining the relationships between organisms, based on available evidence. Researchers analyze many data sources to generate the evolutionary relationships represented by the phylogenetic trees.

Body Plans

• Animals' body plans are categorized by their symmetry.
  • Poriferans (sponges) are asymmetrical.
  • Radial symmetry, found in some animals, means there is more than 2 planes that can divide the animal in half, creating similar halves. The body has a single main axis around which body parts are arranged.
  • Bilateral symmetry means the animal can be divided into mirror image halves along a mid-sagittal (longitudinal) plane. Bilateral animals often exhibit cephalization, which is an evolutionary trend in which nervous and sensory structures are concentrated in the anterior (front) end of the organism.

Overview of Animal Development

• Fertilization of an egg results in a diploid zygote.
• Cleavage: Zygote undergoes many cell divisions to form a blastula.
• Blastula: Embryo in the stage where it is a hollow ball of cells.
• Gastrulation: Formation of a layered embryo (gastrula) which can happen by invagination.
• Gastrula stage embryo is characterized by blastocoel, archenteron and blastopore.
• Germ layers: Ectoderm forms the outer covering and nervous system; endoderm forms the gut, liver, and lungs; and mesoderm forms the muscle.

Animal Classification Based on Development

• Animals can be categorized as diploblastic (2 germ layers) or triploblastic (3 germ layers).
• Protostomes have a mouth that develops from the blastopore; Deuterostomes have an anus that develops from the blastopore. Protostomes have spiral cleavage and deuterostomes have radial cleavage.

Animal Classification Based on Body Cavities

• Animals can be classified based on presence or absence of a body cavity, and the organization/structure of it. Acoelomates lack a body cavity entirely; pseudocoelomates have a body cavity, but it is not completely lined with tissue; and coelomates have a body cavity completely lined by tissue.

• Ecdysozoans are a group of protostomes that shed their exoskeletons to grow. Arthropods and nematodes (roundworms) are examples of ecdysozoans.

• Lophotrochozoans are a group of protostomes that have a lophophore (feeding structure) and/or a trochophore larva. Ectoprocts, molluscs, and annelids are examples of lophotrochozoans.

• The study of animal development, and diversification, often revolves around the organization and types of body cavities present in different types of animals. The organization and structure is different in some animals, and the underlying mechanisms may explain the evolutionary relationships of different animal groups.

Explore animal diversity, focusing on multicellular, heterotrophic eukaryotes. Learn unique tissue types like nervous and muscle tissues. Understand phylogeny as a classification scheme based on evolutionary relations and data analysis.