Biology Now, 2e

Answers ■ A21

having these traits die and do not reproduce. Traits that are merely

useless and not harmful will persist in the organisms that have

them. They may diminish because these structures are no longer

needed and do not give organisms a selective advantage, but the

organisms survive and reproduce just as well with or without

them.

Figure 11.13

Q1: If a sequence from another species showed a 96 percent

sequence similarity to humans, would that species be more or less

closely related to humans than chimpanzees are?

A1: The hypothetical species would be less similar to humans than

chimpanzees are (96% similar versus 98.4% in chimps).

Q2: Should similarities in the DNA sequences of genes be

considered evolutionary homology? Explain.

A2: All living organisms use DNA as their genetic material,

suggesting that the first true ancestral cell used DNA as its

genetic material. DNA sequence similarity is a homologous trait

because all descendant cells use DNA; exactly the same four

nucleotide molecules, in different orders, make up the DNA in all

cells on Earth.

Q3: How is the increased similarity in the DNA sequences

of genes between more closely related organisms—and the

decreased similarity between less closely related organisms—

evidence for evolution? Use the example in this figure to support

your answer.

A3: DNA sequence similarity is the gold standard for determining

species relatedness because it is the genetic material in all cells

on Earth—the best homologous trait that exists. The changes in

DNA sequences in populations over time create the changes in

traits that drive evolution. We can map the changes that occur

in populations or species by looking at sequence similarity and

re-creating a family tree. All other evidence for evolution supports

the theory that all organisms derived from an ancestral cell that

used DNA as its genetic material. The more related a species

is to another species, the more similar the DNA sequences are.

Humans and chimps are primates, mammals, and vertebrates,

and they are more similar in DNA sequence than are humans and

mice, which are only both vertebrates and mammals, but not both

primates. Of the examples in this figure, chickens are the least

similar to humans because although they are birds, which are

vertebrates, they are not mammals or primates. A nonvertebrate

animal like a jellyfish or worm would be even less similar to

humans than the species named here, but it would likely still show

some similarity.

Figure 11.14

Q1: Why should we expect to find N. fosteri fossils all over the

world, given that it first evolved in Pangaea?

A1: We expect to find N. fosteri fossils all over the world because

these organisms existed before the breakup of the mass continent

Pangaea. During the breakup, these organisms traveled with their

continent to the current locations.

Q2: Can we use biogeographic evidence to support evolution

without using fossil evidence? Give examples.

A2: Yes. We can use the current locations of living organisms that

are related to support evolution by biogeography. For example,

members of the primate family are found on almost all the

continents on Earth, suggesting that their common ancestor lived

at the time of Pangaea.

Q3: How might we use DNA sequence similarities together with

biogeography as evidence for evolution?

A3: We can couple DNA sequencing with the locations of fossil

or living organisms to support evolution by common descent. We

can perform DNA sequence analysis on all the primates on Earth,

coupled with their current locations, to reenact the history of

primate evolution.

Figure 11.15

Q1: Why are the similarities among organisms during early

development evidence for evolution? Give an example.

A1: Similarities between organisms during early development

suggest that they have a common ancestor whose early development

occurred in the same or similar manner. All vertebrates go through

similar stages of development in the early embryo. Many invertebrate

organisms also share the same steps in embryonic development.

Q2: Are the similar structures among vertebrate species during

embryogenesis homologous structures? Explain.

A2: Yes. The similar structures are homologous traits because they

are shared with a common ancestor.

Q3: Why do embryonic structures still exist at points during

embryogenesis if they are not used after birth?

A3: These structures can be considered vestigial traits, since

they are now useless to the organism in which they still exist

embryonically. Remember, vestigial traits still exist because they

do not harm the organisms’ ability to survive and reproduce.

CHAPTER 12

END-OF-CHAPTER ANSWERS

1. genetic drift, establish a new distant population


2. b


3. b


4. c


5. b


6. b


7. An individual that survives well but is unattractive to poten-
   tial mates or is unable to compete for access to mates will not
   reproduce and will not pass on genes.


8. Gene flow is the most likely mechanism because mutations
   are random, natural selection would have caused populations
   in different environments to diverge, and genetic drift is most
   relevant for small populations.