INTRODUCTION TO ANIMAL DIVERSITY
Reading: Chapter 18, p. 383 (homeotic genes); Chapter 21, pp. 457-459
(homeoboxes and Hox genes); Chapter 25, pp. 535-536; Chapter 32
A. WHAT ARE ANIMALS?
1. Animals are multicellular, heterotrophic eukaryotes.
2. Animals ingest other organisms or organic debris.
3. Animal cells are not surrounded by cell walls.
4. Animals have intercellular junctions found in no other group of
5. Animals have nervous tissue and muscle tissue.
6. Animals have common features of early development.
Fig. 32.2 Note the evident polarity in the gastrula stage – one end is different from
the other. How might the polarity in the multicellular gastrula have its origin in the
unicellular zygote? Introduction to Animal Diversity - 2
7. Almost all animals have Hox genes, which contain homeoboxes and
which are homeotic in function. Hox genes function during development to
specify features of the animal’s body plan.
Hox genes encode regulators of the transcription of other genes, thereby
turning on or off genes whose products build body parts during
Variation in Hox gene activity can therefore lead to variation in animal
body plan. Hox genes are of great evolutionary significance when we try
to understand animal diversification at the levels of the phylum and the
B. FUNDAMENTAL DIFFERENCES in ANIMAL FORM and DEVELOPMENT
1. Some of these differences define clades, while others define grades.
A grade is a group of organisms that share an important adaptation, but
not through common descent.
2. Presence of tissues – integrated groups of cells that perform specialized
functions Introduction to Animal Diversity - 3
no tissues – Parazoa (sponges, placozoans)
tissues – Eumetazoa
Fig. 32.11 Animal phylogeny based on molecular data. You should compare this
phylogeny with the one in Figure 32.10 (which is based on morphological
arguments) and note the similarities and differences.
Why, generally speaking, might a morphological tree differ from a molecular tree? Introduction to Animal Diversity - 4
3. Body symmetry and the number of germ layers in the embryo
radial symmetry and two germ layers (ectoderm and endoderm;
polarity of the adult animal
Radially symmetric animals are often sessile (attached to a substrate) or
bilateral symmetry and three germ layers (ectoderm, mesoderm and
polarities of the adult animal
Bilateral symmetry is associated with cephalization – the anterior
concentration of neural processing power, sensory apparatus, and
4. Early development: protostomes and deuterostomes
determinate cleavage – early cleavage products lose the ability to form
complete embryos by themselves
indeterminate cleavage – early cleavage products can form complete
embryos by themselves Introduction to Animal Diversity - 5
Fig. 32.9 Consider the associations of traits pictured here. Do you think these traits
are associated for functional reasons or for historical reasons?
5. Presence of a body cavity (a coelom) between the gut and the body wall.
coelom is lined with mesoderm only
pseudocoelom is lined with endoderm and mesoderm Introduction to Animal Diversity - 6
no coelom; mesoderm forms a solid mass of cells
Fig. 32.8 Where in your
own body is your coelom?
functions of the coelom Introduction to Animal Diversity - 7
C. ORIGINS of ANIMALS and EARLY ANIMAL EVOLUTION
1. The animal kingdom is monophyletic, with the divergence of animals
from a colonial protist occurring long before any fossil traces of animals
2. Animal near-relations alive today: choanoflagellates
3. Early animal evolution might have been stimulated by the end of the
Snowball Earth phase (Cryogenian Period).
4. The Doushantuo fossils (approx. 600 – 580 million years ago) look like
5. The Ediacarans (585 – 542 million years ago) – the first large multicellular
organisms. Introduction to Animal Diversity - 8
6. The Cambrian explosion (542 – 525 million years ago) – half of extant
animal phyla were represented.
Many basic body plans appeared and many diversified during the
key fossil formation: the Burgess shale
the Cambrian explosion – why?
emergence of predator-prey relationships
rise in atmospheric O – could support more active metabolism
evolution of Hox genes, which control development in early embryos Introduction to Animal Diversity - 9
DIGRESSION: GENETIC VARIABLITY in GENOME EVOLUTION
The following aspects of genome evolution could apply to any type of
organism, but are given here because genome evolution in animals is a
topic of particular interest in evolutio