8. Chapter 32: Animal Evolution Flashcards
How about how many years ago did most animal forms arose in Precambrian seas?
500-600 mya
What were the appearance of earlier animals?
sponges and jellies
What are the seven criteria define animals? (there are exceptions)
- 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)
What does DNA sequences that are highly conserved mean?
Doesn’t have many mutations – hasn’t changed dramatically over time, genes that regulate other genes (i.e. Hox genes)
What are homeobox genes?
encode transcription factors – these genes switch on cascades of other genes (eg. all the genes needed to make a leg).
Some animals develop directly into adults, but others have distinct larval stages. What is the larva?
a sexually immature stage that is morphologically distinct from the adult, often exploring different foods and habitats. (Animal larvae eventually undergo metamorphosis.)
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?
likely a colonial flagellated protist, related to choanoflagellates (tiny, stalked organisms inhabiting aquatic environments)
Why is one hypothesis for animal evolution?
That a colony of identical cells evolved into a hollow sphere. The cells then invaginated, creating two layers of cells and a “gut”.
How many animal phyla are recognized by zoologist?
35
Why was the animal phylogenetic tree remodelled?
Molecular evidence challenged hypotheses based on anatomical features and embryonic development.
Draw the traditional phylogenetic tree.
What term is used for organisms that lack true tissues and organisms with true tissues?
- Parazoa (i.e. Sponges) 2. Eumetazoa
What are the organisms with true tissues further divided into?
Body symmetry: Radial (i.e. Cnidaria) and Bilateral (dorsal, ventral, anterior, posterior, left, and right).
What is linked with bilateral symmetry?
Cephalization: the concentration of sensory equipment on the anterior end.
How does the basic organization of embryonic tissue differs between radiata and bilateria?
- 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.)
What does the ectoderm give rise to?
the outer covering and the central nervous system.
What does the endoderm give rise to?
endoderm lines the developing digestive tube, or archenteron, and gives rise to the lining of the digestive tract.
What does the mesoderm develop into?
the muscle and many other organs between the digestive tube and the outer covering of the animal
Label this diagram.
- Zygote
- Blastula
- Gastrula
- Ectoderm
- Endoderm
- Archenteron
- Blastopore
- Blastocoel
How can bilateral be divided?
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.
What is the advantage of a body cavity?
Cushions the internal organs, helping to prevent internal injury. (Fluid in the body cavity can also function as a hydrostatic skeleton.)
What can the coelomate phyla be divided into?
Differences in their development: 1. protostomes (mollusks, annelids, and arthropods) and 2. deuterostomes (echinoderms and chordates)
What is the kind of cleavage does a protostome undergo? What does it show?
- undergo spiral cleavage. 2. show determinate cleavage where the fate of each embryonic cell is determined early in development.
What is the kind of cleavage does a deuterostome undergo? What does it show?
- undergo radial cleavage. 2. show indeterminate cleavage whereby each cell in the early embryo retains the capacity to develop into a complete embryo
What is another difference of protostomes and deuterostomes?
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)
What is the new phylogenetic tree based on?
nucleotide sequences for the small subunit of ribosomal RNA.
What two protostome clade does the new molecular-based phylogenetic tree support?
Lophotrochozoa and Ecdysozoa (Doesn’t molt versus molt)
Why did traditional analyses favour an annelid-arthropod clade? What similarity supported the annelid-mollusk lineage?
- because both have segmented bodies. 2. share a similar larval stage, the trochophore larva (supported by molecular data)
In the new tree, where is platyhelminthes placed? Why?
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
Why was the pseudoceolomate Nematodes placed within the Ecdysozoa?
As these animals grow, they molt the old exoskeleton and secrete a new, larger one, a process called edysis
In the traditional tree, what was problematic about the assignment of the three lophophorate groups? How has it been remodelled?
- Share morphological characteristics with protostomes and deuterostomes. (The molecular data place them among the phyla with the trochophore larvae, hence the name lophotrochozoans.)
Draw and label old tree versus new tree.
What can be deducted from fossil record and molecular studies of animal diversification?
- 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.
What are the three main hypotheses for what caused the rapid diversification of animals?
- 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”.)
