8. Chapter 32: Animal Evolution

Question 1

How about how many years ago did most animal forms arose in Precambrian seas?

Answer 1

500-600 mya

Question 2

What were the appearance of earlier animals?

Answer 2

sponges and jellies

Question 3

What are the seven criteria define animals? (there are exceptions)

Answer 3

1. Animals are multicellular, heterotrophic eukaryotes. (Take in organic molecules through ingestion) 2. lack cell walls 3. nervous tissue and muscle tissue 4. reproduce sexually (the diploid stage dominates the life cycle – no alternation of generations, flagellated sperm fertilizes nonmotile egg) 5. the blastula (zygote undergoes cleavage leading to the formation of a hollow ball of cells ) 6. gastrula (During gastrulation, part of the embryo folds inward and forms a blind pouch - produces two tissue layers: the inner endoderm and the outer ectoderm) 7. Hox genes (transformation of a zygote to an animal depends on the controlled expression of this special regulatory genes)

Question 4

What does DNA sequences that are highly conserved mean?

Answer 4

Doesn’t have many mutations – hasn’t changed dramatically over time, genes that regulate other genes (i.e. Hox genes)

Question 5

What are homeobox genes?

Answer 5

encode transcription factors – these genes switch on cascades of other genes (eg. all the genes needed to make a leg).

Question 6

Some animals develop directly into adults, but others have distinct larval stages. What is the larva?

Answer 6

a sexually immature stage that is morphologically distinct from the adult, often exploring different foods and habitats. (Animal larvae eventually undergo metamorphosis.)

Question 7

If we could trace all the animals lineages back in time, what common ancestor would they have converged from prior to joining with other taxa?

Answer 7

likely a colonial flagellated protist, related to choanoflagellates (tiny, stalked organisms inhabiting aquatic environments)

Question 8

Why is one hypothesis for animal evolution?

Answer 8

That a colony of identical cells evolved into a hollow sphere. The cells then invaginated, creating two layers of cells and a “gut”.

Question 9

How many animal phyla are recognized by zoologist?

Answer 9

35

Question 10

Why was the animal phylogenetic tree remodelled?

Answer 10

Molecular evidence challenged hypotheses based on anatomical features and embryonic development.

Question 11

Draw the traditional phylogenetic tree.

Answer 11

Question 12

What term is used for organisms that lack true tissues and organisms with true tissues?

Answer 12

1. Parazoa (i.e. Sponges) 2. Eumetazoa

Question 13

What are the organisms with true tissues further divided into?

Answer 13

Body symmetry: Radial (i.e. Cnidaria) and Bilateral (dorsal, ventral, anterior, posterior, left, and right).

Question 14

What is linked with bilateral symmetry?

Answer 14

Cephalization: the concentration of sensory equipment on the anterior end.

Question 15

How does the basic organization of embryonic tissue differs between radiata and bilateria?

Answer 15

1. The radiata are said to be diploblastic because they have two germ layers. (endoderm and ectoderm) 2. The bilateria are triploblastic. (The third germ layer, the mesoderm lies between the endoderm and ectoderm.)

Question 16

What does the ectoderm give rise to?

Answer 16

the outer covering and the central nervous system.

Question 17

What does the endoderm give rise to?

Answer 17

endoderm lines the developing digestive tube, or archenteron, and gives rise to the lining of the digestive tract.

Question 18

What does the mesoderm develop into?

Answer 18

the muscle and many other organs between the digestive tube and the outer covering of the animal

Question 19

Label this diagram.

Answer 19

1. Zygote
2. Blastula
3. Gastrula
4. Ectoderm
5. Endoderm
6. Archenteron
7. Blastopore
8. Blastocoel

Question 20

How can bilateral be divided?

Answer 20

By the presence or absence of a body cavity. 1. Acoelomates (the phylum Platyhelminthes) have a solid body and lack a body cavity. 2. Pseudocoelom is a body cavity, but it is not completely lined by mesoderm. (phylum Nematoda - roundworms). 3. Coelomates are organisms with a true coelom, a fluid-filled body cavity completely lined by mesoderm.

Question 21

What is the advantage of a body cavity?

Answer 21

Cushions the internal organs, helping to prevent internal injury. (Fluid in the body cavity can also function as a hydrostatic skeleton.)

Question 22

What can the coelomate phyla be divided into?

Answer 22

Differences in their development: 1. protostomes (mollusks, annelids, and arthropods) and 2. deuterostomes (echinoderms and chordates)

Question 23

What is the kind of cleavage does a protostome undergo? What does it show?

Answer 23

1. undergo spiral cleavage. 2. show determinate cleavage where the fate of each embryonic cell is determined early in development.

Question 24

What is the kind of cleavage does a deuterostome undergo? What does it show?

Answer 24

1. undergo radial cleavage. 2. show indeterminate cleavage whereby each cell in the early embryo retains the capacity to develop into a complete embryo

Question 25

What is another difference of protostomes and deuterostomes?

Answer 25

The fate of the blastopore, the opening of the archenteron. 1. In protosomes, the blastopore develops into the mouth. 2. In deuterostomes, the blastopore develops into the anus. (The second opening becomes the mouth)

Question 26

What is the new phylogenetic tree based on?

Answer 26

nucleotide sequences for the small subunit of ribosomal RNA.

Question 27

What two protostome clade does the new molecular-based phylogenetic tree support?

Answer 27

Lophotrochozoa and Ecdysozoa (Doesn’t molt versus molt)

Question 28

Why did traditional analyses favour an annelid-arthropod clade? What similarity supported the annelid-mollusk lineage?

Answer 28

1. because both have segmented bodies. 2. share a similar larval stage, the trochophore larva (supported by molecular data)

Question 29

In the new tree, where is platyhelminthes placed? Why?

Answer 29

within the lophotrochozoan clade, thought to be before the formation of body cavities molecular data suggests that flatworms are not primitive “pre-coelomates” but have lost their coeloms

Question 30

Why was the pseudoceolomate Nematodes placed within the Ecdysozoa?

Answer 30

As these animals grow, they molt the old exoskeleton and secrete a new, larger one, a process called edysis

Question 31

In the traditional tree, what was problematic about the assignment of the three lophophorate groups? How has it been remodelled?

Answer 31

1. Share morphological characteristics with protostomes and deuterostomes. (The molecular data place them among the phyla with the trochophore larvae, hence the name lophotrochozoans.)

Question 32

Draw and label old tree versus new tree.

Answer 32

Question 33

What can be deducted from fossil record and molecular studies of animal diversification?

Answer 33

1. occurred rapidly, b/w 565 to 525 mya making difficult to sort out the sequence of branching in animal phylogeny 2. Some cambrian fossil in Burgess shale bizarre-looking (may represent extinct “experiments”), most Cambrian fossils still represented in modern fauna.

Question 34

What are the three main hypotheses for what caused the rapid diversification of animals?

Answer 34

1. Ecological Causes: Predator-prey relationships – diversity leads to diversity. 2. Geological Causes: Increased atmospheric oxygen and the melting of snowball earth. 3. Genetic Causes: a few mutations in Hox genes may have drastically increased variation in morphology during development. (In the last half-billion years, animal evolution has mainly generated new variations on old “designs”.)