Monday, December 10, 2007

Concept 26.6

Q1: Which kingdoms in Whittaker’s five- kingdom system include organisms now in the domain Eukarya?

A1: Protista, Plantae, Fungi, Animalia

Q2: Based on Figure 26.22, explain why the kingdom Monera is no longer considered a valid taxon?

A2: Monera included both bacteria and archaea, but archaea are more closely related to eukaryotes than to bacteria.

Concept 26.5

Q1: How does the division of function differ for single-celled and multicellular organisms?

A1: A single-celled organism must carry out all of the functions required to stay alive. Most multicellular organisms have many types of specialized cells, and life functions are divided among specific cell types.

Q2: In what way is “Cambrian explosion” a good description of the early part of the fossil record of animal history? What is meant by the metaphor of a “long fuse” for the Cambrian explosion?

A2: Fossils of most major animal phyla appear suddenly in the first 20 million years of the Cambrian period. Molecular clocks suggest that many animal phyla originated much earlier.

Concept 26.4

Q1: What evidence supports the hypothesis that mitochondria preceded plastids in the evolution of eukaryotic cells?

A1: All eukaryotes have mitochondria or genetic remnants of these organelles, but not all eukaryotes have plastids.

Q2: How is a eukaryotic cell like a chimera?

A2: The chimera of Greek mythology contained parts from different animals. Similarly, a eukaryotic cell contains parts form various prokaryotes; mitochondria from one type, and bacterium, plastids from another type, and a nuclear genome from parts of the genomes of these endosymbionts and at least one other cell.

Concept 26.3

Q1: What do fossilized stromatolites suggest about the evolution of prokaryotes?

A1: Prokaryotes must have existed at least 3.5 b.y.a, when the oldest fossilized stromatolites were formed.

Q2: The first appearance of free oxygen in the atmosphere must have triggered a massive wave of extinctions among the prokaryotes of the time. Why?

A2: Free oxygen attacks chemical bonds and can inhibit enzymes and damage cells. Some organisms were able to survive in anaerobic habitats, however.

Concept 26.2

Q1: Your measurements indicate that a fossilized skull you unearthed has a carbon-14/ carbon-12 ratio about ¼ that of the skulls of present-day animals. What is the approximate age of the fossilized skull?

A1: 22,920 years (four half-life reductions)

Q2: Based on Table 26.1, how long did prokaryotes inhabit Earth before eukaryotes evolved?

A2: About 1,300 million years, or 1.3 billion years.

Concept 26.1

Q1: What hypothesis did Miller and Urey test in their experiment?

A1: The hypothesis that conditions on the early Earth could have permitted the synthesis of organic molecules from inorganic ingredients.

Q2: Why was the appearance of protobionts surrounded by membranes likely a key step in the origin of life?

A2: In contrast to random mingling of molecules in an open solution, segregation of molecular systems by membranes could concentrate organic molecules, and electrical charge gradients across the membrane could assist biochemical reactions.

Q3: What is ribozyme?

A3: A ribozyme is an RNA molecule that catalyzes a chemical reaction

Concept 24.3

Q1: How can the Darwinian concept of descent with modification explain the evolution of such complex structures as the vertebrate eye or heart?

A1: Such complex structures do not evolve all at once, but in increments, with natural selection selecting for adaptive variants of the earlier versions.

Q2: Explain why the concept of exaptation does not mean that a structure evolves in anticipation of some future environmental change.

A2: Although an exaption is co-opted for new or additional functions in a new environment, it existed in the first place because it worked as an adaptation to the original environment.

Q3: How can heterochrony cause the evolution of different body forms?

A3: The timing of different development pathways in organisms can change in different ways (heterochrony). This can result in differential growth patterns, such as those producing different patterns of webbing in salamander feet.

Concept 24.2

Q1: Explain why allopatric speciation would be less likely to occur on an island close to a mainland than on a more isolated island of the same size.

A1: Continued gene flow between mainland populations and those on a nearby island reduces the chance that enough genetic divergence will take place for allopatric speciation to occur.

Q2: Normal watermelon plants are diploid (2n=22) but breeders have produce tetraploid (4n=44) watermelons. If tetraploid plants are hybridized with their diploid relatives, they produce triploid (3n=33) seeds. These offspring can produce triploid seedless watermelons and can be further propagated by cuttings. Are the diploid and tetraploid watermelon plants different species? Explain.

A2: The diploid and tetraploid watermelons are separate species. Their hybrids are triploid and as a result are sterile because of problems carrying out meiosis.

Q3: In the fossil record, transitional fossils linking newer species to older ones are relatively rare. Suggest an explanation for his observation.

A3: According to the model of punctuated equilibrium, in most cases the time during which speciation occurs is relatively short compared with the overall duration of the species' existence. Thus, on the vast geologic time scale of the fossil record, the transition of one species to another seems abrupt, and instances of gradual change in the fossil record are rare. Furthermore, some of the changes that transitional species underwent may not be apparent in fossils.

Concept 24.1

Q1: Two bird species in a forest are not known to interbreed. One species feeds and mates in the treetops and the other on the ground. But in captivity, the two species can interbreed and produce viable, fertile offspring. What type of reproductive barrier most likely keeps these species separate? Explain.

A1: Since the birds are known to breed successfully in captivity, the reproductive barrier in nature must be prezygotic. Given the species differences in habitat preference, the reproductive barrier is most likely to be habitat isolation.

Q2: a. Which species concept can be used for both asexual and sexual species?
b. Which can only be applied to sexual species?
c. Which would be most useful for identifying species in the field?

A2: a. All species concepts except the biological species concept can be applied to both asexual and sexual species because they define species on the basis of characteristics other than abitlity to reproduce.
b. The biological species concept can be applied only to extant sexual species.
c. The easiest species concept to apply in the field would be the morphological species concept because it is based only on the appearance of the organism. Additional information about its ecological habits, evolutionary history, and reproduction are not required.