Ch 32: An Overview of Animal Diversity (Concept Questions) Flashcards
Concept Check 32.1
- Summarize the main stages of animal development. What
family of control genes plays a major role?
In most animals, the zygote undergoes cleavage, which leads to the formation
of a blastula. Next, in gastrulation, one end of the embryo folds inward, producing
layers of embryonic tissue. As the cells of these layers differentiate, a wide
variety of animal forms are produced. Despite the diversity of animal forms,
animal development is controlled by a similar set of Hox genes across a broad
range of taxa.
Concept Check 32.1
- WHAT IF? What animal characteristics would be needed by
an imaginary plant that could chase, capture, and digest its
prey—yet could also extract nutrients from soil and conduct
photosynthesis?
The imaginary plant would require tissues composed of cells
that were analogous to the muscle and nerve cells found in animals: “Muscle”
tissue would be necessary for the plant to chase prey, and “nerve” tissue would be
required for the plant to coordinate its movements when chasing prey. To digest
captured prey, the plant would need to either secrete enzymes into one or more
digestive cavities (which could be modified leaves, as in a Venus flytrap) or secrete
enzymes outside of its body and feed by absorption. To extract nutrients from
the soil—yet be able to chase prey—the plant would need something other than
fixed roots, perhaps retractable “roots” or a way to ingest soil. To conduct photosynthesis,
the plant would require chloroplasts. Overall, such an imaginary plant
would be very similar to an animal that had chloroplasts and retractable roots.
Concept Check 32.2
- Put the following milestones in animal evolution in order
from oldest to most recent: (a) origin of mammals, (b) earliest
evidence of terrestrial arthropods, (c) Ediacaran fauna,
(d) extinction of large, nonflying dinosaurs.
c, b, a, d
Concept Check 32.2
- VISUAL SKILLS Explain what is represented by the
red-colored portion of the branch leading to animals.
(See Figure 26.5, “Visualizing Phylogenetic Relationships,”
to review phylogenetic tree diagrams.)
The red-colored portion of the tree represents ancestors of animals
that lived between 1 billion years ago and 770 million years ago. Although
these ancestors are more closely related to animals than to fungi, they would
not be classified as animals. One example of an ancestor represented by the
red-colored portion of this tree is the most recent common ancestor shared by
choanoflagellates and animals (see Figure 32.3). That common ancestor was
not an animal (or a choanoflagellate), but it was a direct ancestor of the animals.
Concept Check 32.2
- MAKE CONNECTIONS Evaluate whether the origin of cellto-
cell attachment proteins in animals illustrates descent with
modification. (See Concept 22.2.)
In descent with modification, an organism shares characteristics with
its ancestors (due to their shared ancestry), yet it also differs from its ancestors
(because organisms accumulate differences over time as they adapt to their surroundings).
As an example, consider the evolution of animal cadherin proteins,
a key step in the origin of multicellular animals. These proteins illustrate both of these aspects of descent with modification: Animal cadherin proteins share
many protein domains with a cadherin-like protein found in their choanoflagellate
ancestors, yet they also have a unique “CCD” domain that is not found in
choanoflagellates.
Concept Check 32.3
- Compare three aspects of the early development of a snail
(a mollusc) and a human (a chordate).
A snail has a spiral and determinate cleavage pattern; a human has radial,
indeterminate cleavage. In a snail, the coelomic cavity is formed by splitting of
mesoderm masses; in a human, the coelom forms from folds of archenteron. In
a snail, the mouth forms from the blastopore; in a human, the anus develops
from the blastopore.
Concept Check 32.3
- Describe how animals that lack a body cavity exchange
materials without an internal transport system.
Animals that lack a body cavity tend to have thin, flat
bodies. Such animals don’t require an internal transport system: With bodies
that are only a few cells thick, the exchange of nutrients, gases, and wastes can
occur across the entire body surface.
Concept Check 32.3
- WHAT IF? Evaluate this claim: Ignoring the details of their
specific anatomy, worms, humans, and most other triploblasts
have a shape analogous to that of a doughnut.
Most triploblasts have two openings
to their digestive tract, a mouth and an anus. As such, their bodies have a structure
that is analogous to that of a doughnut: The digestive tract (the hole of the
doughnut) runs from the mouth to the anus and is surrounded by various tissues
(the solid part of the doughnut). The doughnut analogy is most obvious at
early stages of development (see Figure 32.10c).
Concept Check 32.4
- Describe the evidence that cnidarians share a more recent
common ancestor with other animals than with sponges.
Cnidarians possess tissues, while sponges do not. Also unlike sponges, cnidarians
exhibit body symmetry, though it is radial and not bilateral as in most other
animal phyla.
Concept Check 32.4
- WHAT IF? Suppose ctenophores are basal metazoans and
sponges are the sister group of all remaining animals. Under
this hypothesis, redraw Figure 32.11 and discuss whether
animals with tissues would form a clade.
Under the hypothesis that ctenophores are basal metazoans, sponges (which
lack tissues) would be nested within a clade whose other members all have tissues.
As a result, a group composed of animals with tissues would not form a
clade.
Concept Check 32.4
- MAKE CONNECTIONS Based on the phylogeny in
Figure 32.11 and the information in Figure 25.11, evaluate
this statement: “The Cambrian explosion actually consists
of three explosions, not one.”
The phylogeny in Figure 32.11 indicates that molluscs are members of
Lophotrochozoa, one of the three main groups of bilaterians (the others being
Deuterostomia and Ecdysozoa). As seen in Figure 25.11, the fossil record shows
that molluscs were present tens of millions of years before the Cambrian explosion.
Thus, long before the Cambrian explosion, the lophotrochozoan clade had
formed and was evolving independently of the evolutionary lineages leading to
Deuterostomia and Ecdysozoa. Based on the phylogeny in Figure 32.11,
we can also conclude that the lineages leading to Deuterostomia and Ecdysozoa
were independent of one another before the Cambrian explosion. Since the
lineages leading to the three main clades of bilaterians were evolving independently
of one another prior to the Cambrian explosion, that explosion could be
viewed as consisting of three “explosions,” not one.
Figure 32.3 Question:
Are the data described in 3 consistent with
predictions that could be made from the evidence in
1 and 2 ? Explain.
Figure 32.3 As described in (1) and (2) , choanoflagellates and a broad range of
animals have collar cells. Since collar cells have never been observed in plants,
fungi, or non-choanoflagellate protists, this suggests that choanoflagellates may
be more closely related to animals than to other eukaryotes. If choanoflagellates
are more closely related to animals than to any other group of eukaryotes,
choanoflagellates and animals should share other traits that are not found in
other eukaryotes. The data described in (3) are consistent with this prediction.
Figure 32.8 Question:
Figure 32.8 (1) Any imaginary slice through the central axis of a radial animal
divides its body into mirror images. As a result, a radial animal has no front and
back sides and no left and right sides.
Figure 32.11 Question:
Cnidaria is the sister phylum
in this tree.
Summary of Key Concept Questions:
32.1
Describe key ways that animals differ from plants and fungi.
32.1 Unlike animals, which are heterotrophs that ingest their food, plants are
autotrophs, and fungi are heterotrophs that grow on their food and feed by
absorption. Animals lack cell walls, which are found in both plants and fungi.
Animals also have muscle tissue and nerve tissue, which are not found in either
plants or fungi. In addition, the sperm and egg cells of animals are produced by
meiotic division, unlike what occurs in plants and fungi (where reproductive
cells such as sperm and eggs are produced by mitotic division). Finally, animals
regulate the development of body form with Hox genes, a unique group of genes
that is not found in either plants or fungi.
