AP Biology : 7.7 Common Ancestry – Exam Style questions with Answer- MCQ

Question

Which of the following would best determine whether two species of
birds have a recent common ancestor?
(A) DNA sequences
(B) fossil record
(C) habitat distribution
(D) mating behaviors

▶️Answer/Explanation

Ans:

(A) DNA sequences are generally considered stronger pieces of
evidence of evolution because environmental factors are far less likely
to change the DNA sequence of a fossil. Choice (B) is incorrect
because geological events can change the relative positions of fossils in
rock layers and make the original location of the fossil position less
certain. Birds can fly and move to new habitats, so choice (C) is
incorrect. Choice (D) is incorrect because mating behaviors of birds
may be influenced by their environments or by learning these behaviors
from other birds.

Question

Which of the following true statements best supports the claim that
organisms are linked by lines of descent from a common ancestor?
(A) All organisms have the capacity to grow and reproduce.
(B) All organisms use oxygen for cellular respiration in mitochondria,
an organelle that was once a free-living prokaryote.
(C) Different species have specific traits and adaptations that allow
them to succeed in diverse environments.
(D) All organisms share a genetic code that allows one organism to
express a gene from another organism.

▶️Answer/Explanation

Ans: D

The passing down of DNA from one generation to the next is the
mechanism by which all life is linked. Changes in DNA sequences
(mutations) produce new genes and alleles, which can be inherited by
offspring through reproduction. The mechanism by which genes are used
inside cells is so similar across diverse forms of life that when genes from
one organism are introduced into the genome of a different organism, they
may be expressed: for example, genes for spider silk expressed by goats.
Although all life-forms have the capacity for growth and reproduction, this
fact alone supports but is not evidence that all organisms are linked by lines
of common ancestry. That different species have specific traits and
adaptations is a result of evolution through natural selection, but again, this
fact alone is not direct evidence of common ancestry. The true statements of
choices A, B, and particularly C actually require the model of natural
selection to make sense.

Question

Refers to the following data table (from Bailey, et al., 1991).
Pseudogenes are DNA sequences that resemble functional genes but differ
in several base pairs and are not transcribed. They can accumulate
mutations that appear to have no effect on the organism.

Which of the following statements is the least accurate interpretation
of the data above?
(A) Pongo and Pan may have diverged the longest time ago.
(B) Pan and Pongo most likely diverged further back in time than
Pongo and Gorilla.
(C) Pan may be the oldest of the four genera represented.
(D) Pan and Homo likely share the most recent common ancestor.

▶️Answer/Explanation

Ans: B

Generally, the lower the percent divergence of DNA sequences
between two species, the more recently they diverged from their
common ancestor. (Or, the greater the number of differences between two
sequences, the further back in time they diverged from their common
ancestor.) Homo, the genus of humans, has a percent divergence of 0.38%,
which represents the percent difference between two human sequences (as
noted beneath the table, humans are the only extant species of the Homo
genus). This difference does not reflect the difference in the entire
genome, just the percent sequence difference between each individual’s
ψη-globin pseudogene. The difference in the percent divergence between
Pan and Pongo is 3.42%, and the difference in the percent divergence
between Pongo and Gorilla is 3.39%. That’s only 0.03% difference, less
than the difference between the two humans.
The concept of a molecular clock assumes a steady rate in the change
in DNA sequences over time. It provides at least a relative basis for
estimating the time of divergence between lineages. The assumption of
steady rate changes is only true under very specific circumstances. For
example, pseudogenes undergo a faster rate of change in DNA sequence
because the mutations have no effect on genotype. Some DNA
sequences, considered conserved, accumulate very few mutations because
they are almost always deleterious. See answers 67 and 94 for more
information about molecular clocks.
Different species and groups of organisms undergo mutations at
different rates because of differences in body size and generation time.
It takes a lot more time for a hundred generations of elephants to pass than
for a hundred generations of bacteria, so even if the same number of
mutations occur in a hundred generations, the molecular clock of the
elephant is much slower than that of the bacteria. The adjustment for generation time (which is correlated to body size) is one of several factors
that are required for calibration of the molecular clock.
See the following box for a strategy for answering questions asked in the
negative and using the process of elimination.

Question

Which of the following true statements is the best single piece of
evidence that nonhuman apes such as chimpanzees, bonobos, and gorillas are more closely related to humans than any other mammal?

(A) Apes and humans have similar anatomy. They both have nails instead of claws, lack a tail and full facial hair, and have forward- facing eyes.
(B) Apes and humans have very similar behaviors and social
structures, and they both have the ability to use tools.
(C) Apes have 24 pairs of chromosomes and humans have 23.
(D) Humans have a greater percentage of DNA sequences in common
with apes than with any other group of organisms.

▶️Answer/Explanation

Ans: D

Although all the choices are ways to compare, contrast, and attempt
to relate organisms by common ancestry, DNA sequence comparisons are
the “gold standard.” Convergent evolution can produce similarities in
anatomy and behavior (choices B and C). Convergent evolution only
makes the organisms being compared appear to be related (e.g., a shark
and dolphin look quite similar, but the shark is a cartilaginous fish and the
dolphin is a mammal). Importantly, organisms do not become more related
as time passes; they can only maintain their degree of relatedness or
through future generations become more distantly related through
divergence.

Question

Estimate the difference in the number of mutations (in seven proteins)
accumulated between non-primate mammals and primates in a 10-
million-year period.
(A) 2.5
(B) 4
(C) 6.25
(D) 7.5

▶️Answer/Explanation

Ans: B (4)


The calculations in the diagram on the previous page show how to calculate
the number of mutations (in seven proteins) per 10 million years. The slope
for non-primate mammals is straightforward,

The slope of the trend line connecting the dots for primates will likely
vary from person to person, but the values should be in the same ballpark of

about 2.5 mutations per 10 million years. Because the numbers on the x-
and y-axes are difficult to estimate due to small values and large dots, where

the trendline crosses the 30 million year mark gives a good estimate of the
number of mutations (in seven proteins) in 30 million years. That number
may be as large as 7.5 or as low as 5 depending on your trendline, which would make the mutation rate per 10 million years as high as 2.5 or as low
as 1.7.
The difference can then be calculated:
6.25 – 1.7 = 4.55 mutations per 10 million years; round up to 5
Or if you round first, 6 – 2 = 4.
6.25 – 2.5 = 3.75 mutations per 10 million years; round up to 4
Or if you round first, 6 – 3 = 3.

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