Invertebrate and Vertebrate Evolution

Objectives

• Outline invertebrate evolution
• Overview of vertebrate evolution
• Explain how animal behavior evolves

Invertebrates evolved several traits before vertebrates appeared. They are found in most animals.

Traits

• Multicellularity
• Tissues
• Radial symmetry (sponges lack this)
• Cephalization - concentration of nerve tissue

More advanced traits

• Bilateral symmetry
• Mesoderm - third layer of cells
• Complete digestive system
• Pseudocoelom and coelom
• Segmented body

Notochord

• Stiff support rod in a chordate
• Some invertebrate chordates evolved into vertebrates

The earliest vertebrates (500-600 million years ago) were jawless fish similar to hagfish. They had a cranium but no vertebral column.

After hagfish appeared, fish similar to lampreys evolved a partial vertebral column.

Until about 450 million years ago, fish had endoskeletons of cartilage.

About 400 million years ago the first bony fish appeared. This could support a larger body.

The first amphibians evolved from lobe-finned fish about 365 million years ago.

Reptiles evolved from amphibians. They laid amniotic eggs and had internal fertilization.

Mammals, then birds evolved from reptiles.

Until mammals and birds evolved, all vertebrates were ectothermic, meaning body temperature had to be regulated by the environment.

Mammals and birds evolved endothermy, the regulation of body temperature from the inside from metabolic changes.

Thus, metabolic rate and activity can remain high regardless of temperature, but maintaining the temperature requires more energy.

Animal Behavior

Animal behavior includes all the ways an animal interacts with other animals and the environment.

The study of animal behavior is called ethology.

Ethology Questions

• What is the stimulus for the behavior?
• How does the behavior develop?
• Why did the behavior evolve?
• How did the behavior evolve?

If behaviors increase fitness, they are likely to become more common over time.

Nature v. Nurture