General Biology/Animal Evolution

The Evolutionary Tree in Animals

There are many competing hypotheses for the form of the evolutionary tree of animals. A traditional hypothesis is that the tree resembles a tuning fork: it has a short base and two main branches. However, there is recent molecular evidence that challenges part of this traditional hypothesis.

Under the tuning fork model, the "base" of the tree includes structurally simple animals like sponges, corals, and their relatives.

One main branch includes arthropods, molluscs, annelids, and nematodes. This branch, or a large part of it, usually is called the protostomes.

The second main branch includes vertebrates (phylum Chordata), and starfish, sea urchins, and their relatives (phylum Echinodermata). This branch usually is called the deuterostomes.

Flatworms (phylum Platyhelminthes), which include free living planarians as well as parasitic flukes and tapeworms may be placed very low on the protostome branch, or high on the trunk just below the protostome - deuterostome branching.

Support for the "Tuning Fork" Model

The features of animals that have been interpreted as suggesting a tuning fork model are extremely basic characteristics of body organization and early embryonic development.

Body Symmetry

Asymmetry

Lack of any symmetry. Many sponges are asymmetric.

Bilateral Symmetry

There is only one plane of symmetry, and it is anterior-to-posterior, dorsal-to-ventral, through the midline.

Characteristic of most protostomes and the higher deuterostomes.

Presence of True Tissues

Tissues are defined as an integrated group of cells that share a common structure and a common function (for example, nervous tissue or muscle tissue).

Sponges are described as lacking true tissues.

True tissues are present in Cnidaria, flatworms, and all higher animals.

Number of Embryonic Germ Layers

Germ layers are defined as the basic tissue layers in the early embryo which give rise developmentally to the organs and tissues of the adult (e.g., ectoderm, mesoderm, endoderm).

This is a concept that is applied only to organisms considered to have true tissues.

Two Germ Layers

Organisms with two germ layers are said to be diploblastic.

This is characteristic of Cnidarians.

Three Germ Layers

Such organisms are said to be triploblastic.

This is characteristic of flatworms and all higher organisms.

Four Germ Layers (?)

Some developmental biologists consider the neural crest tissue of vertebrate embryos to be a fourth germ layer.

Nature of the Main Body Cavity

Most triploblastic animals have a fluid-filled space somewhere between the body wall and the gut. Such a cavity can provide numerous functional advantages.

For example, peristalsis of the gut need not affect the body wall, and movements of the body wall during locomotion need not distort the internal organs.

We will consider three conditions with respect to the body cavity:

Acoelomates

These animals lack an enclosed body cavity; the only "body cavity" is the lumen of the digestive tube.

The space between the gut and the body wall is filled with a more or less solid mass of mesodermal tissue.

The major example of this is the phylum Platyhelminthes, the flatworms. Minor examples: Phylum Nemertea (Rhynchocoela) and Phylum Gnathostomulida (not responsible for these "minor" examples).

Pseudocoelomates

Pseudocoelomates have a fluid-filled cavity between the body wall and the gut, but it does not form within mesoderm, nor does it end up fully enclosed by mesoderm.

This cavity often is interpreted as being a developmental remnant of the blastocoel, the fluid-filled cavity of the blastula stage of the embryo.

To distinguish it from the next grade, this type of cavity is called a pseudocoelom.

Major examples of pseudocoelomates include the phyla Nematoda (round worms) and Rotifera (rotifers). Other phyla listed in the table that are considered to be pseudocoelomates are flagged with an asterisk. Note that recent molecular data in particular have challenged the "naturalness" of the pseudocoelomates as a possible taxon.

Coelomates

Coelomate animals also have a fluid-filled cavity between the body wall and the gut.

In this grade, however, the cavity is completely enclosed by mesoderm.

Major examples of coelomates include molluscs, arthropods, echinoderms, and chordates.

The protostome-deuterostome distinction

The distinction is based on several fundamental characteristics of early development.

Characteristic:

Protostomes
Deuterostomes

Determinate vs. indeterminate cleavage

Determinate
Indeterminate

Spiral vs. radial cleavage

Spiral
Radial

Fate of the blastopore

Mouth
Anus

Source of mesoderm

Lip of blastopore
Wall of archenteron

Formation of coelom

Schizocoely
Enterocoely

Phyla

Mollusca, Annelida, Arthropoda
Roundworms & flatworms, although non-coelomate, demonstrate other protostome features, and so are included in this group by some authorities.

Echinodermata, Hemichordata, Chordata

This text is based on notes very generously donated by Ralph Gibson, Ph.D. of the Cleveland State University.