Chapter 32: Introduction to Animal Evolution

Note: We will follow the traditional grouping of animals shown in Figure 32.4. Use this diagram for the quiz and in preparing for the exam. Do not use Figure 32.8.  [Obviously, these groupings are under review and will change with more information.]

Characteristics of all animals:

· all multicellular, heterotrophic eukaryotes

· must ingest organic molecules, rather than manufacture their own

· lack cell walls; have unique intercellular junctions

· unique tissues: muscle and nervous

· sexual reproduction in most

· Hox genes regulate development of embryo

Origins of multicellular animals from unicellular ancestor:

Colonial hypothesis , (Figs. 32.2 & 32.3):

· ·        heterotrophic colonial flagellates

· ·        extant choanoflagellates give support to this theory (cells with a collar and flagellum, form hollow ball)

· ·        so does sponge embryology

· ·        cell layers from cell proliferation, forming an interior space OR by invagination

Animals grouped in about 35 phyla, 15 of which are the major phyla

95% of the 1,000,000 spp. known are invertebrates

Majority aquatic or marine

Spend plenty of time with CD: Chapter 32 – Activity 32A and Figure 32.4

Both of these show that all phyla are divided into different groups based on four major evolutionary branches:

1^st – The first branch = lack of true tissues vs. possessing true tissues:

a. Parazoa (no true tissues)--phylum Porifera, the sponges

b. Eumetazoa (have a layered embryo which undergoes gastrulation Fig. 32.1)

2^nd – The second branch = symmetry and number of germ layers, (Figs. 32.5 & 32.6):

a. Radiata--radial symmetry, diploblastic (two germ layers in embryo)

ectoderm, endoderm

b. Bilateria--bilateral symmetry, triploblastic (three germ layers)

ectoderm, endoderm, mesoderm (gives rise to muscles)

3^rd – The third branch = type (or lack of) body cavity, (Fig. 32.6)

a. Acoelomates (having no coelom or body cavity)

b. Pseudocoelomates (having a coelom lined on one side by mesoderm, complete digestive tracts)

c. Coelomates (having a coelom lined on both sides by mesoderm)

4^th – The fourth branch = development of embryo, (Fig. 32.7)

a. Protostomes (schizocoelomates)

· ·        spiral and determinate cleavage

· ·        coelom derived from solid mass of mesoderm, splits to form coelom

· ·        mouth develops from blastopore

b. Deuterostomes (enterocoelomates) coelom derived from outpocketings of

archenteron; mouth forms opposite blastopore)

· ·        radial and indeterminate cleavage

· ·        coelom derived from folds of archenterons

· ·        anus develops from blastopore

c. Lophophorate phyla

· ·        phylogeny is unclear based on anatomy and embryology

Most animal phyla originated in a relatively brief span of geological time (according to fossil record)

• 565-543 million years ago—many Cnidarian fossils found from Ediacaran period (region of Ediacara Hills in Australia where fossils were first found) p. 595
• 565-525 myo - early all major animal body plans appeared during Cambrian explosion, ex. evolution of shell (Fig. 32.13)
• many more fossils found
• nearly all modern phyla represented

Three hypotheses for what sparked the Cambrian explosion:

• Ecological causes – predator/prey interactions
• Geologic causes – oxygen may have reached a level to support mobile animals
• Genetic causes – Different expressions of Hox genes. New field of “evo-devo”.