BIO 200A EVOLUTIONARY BIOLOGY
Exam I Part II Fall 2011
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B. Define, give examples and answer the follow-up questions. (4 pts each)
1. Define cross-over: In meiosis, the exchange of corresponding pieces of chromatid segments
between homologous chromosomes—produces genetic recombination between homologous
chromosomes.
Example: When the X and Y chromosomes lie next to each other during meiosis the tips of
the chromatids can swap pieces; sometimes this means that the SRY gene that determines
maleness moves over to the X chromosome and a female (XX) will be born with male genitalia.
What is the relevance of cross-over to evolution? The relevance is that cross-over
increases the genetic variability in the population because it breaks up linkage patterns.
2. Radioactive dating: This is a method of determining the age of rocks or fossils. It depends
on the fact that certain elements have isotopes that are radioactive and that they slowly release
energy and particles converting them into different isotopes or elements. By knowing the rate
that this occurs, and by measuring the amounts of the original isotope and the product, one can
calculate how long the material has been in place.
Example: Radioactive 14C14N has a half life of 5,730 years and can be used to determine
the age of organic material up to 75,000 years old. Other isotopes such as 238U with at half life
of 4.5 billion years are used for rocks to date old strata.
What is the most compelling reason that we believe that the results are accurate?
Because we can check the method against samples where we know the real age and we find them
to be accurate.
3. Central Dogma of Molecular Biology: DNARNAProtein
Example: All proteins (structural and enzymes) are coded by the nucleotide sequence in
DNA. A gene code is transcribed into messenger RNA, which then travels to the ribosomes where
the message is translated into a protein (e.g. hemoglobin).
Why is this principle incompatible with Lamarck’s view of evolution? Lamarck argued
that events in the life of an organism could affect the heredity of the organism in such a way that
the characteristic could be passed on to the offspring—Inheritance of Acquired Characteristics.
If this were true then it would mean that DNA could be directly affected by the environment
toward specific goals, and there is no evidence for that.
4. Homeotic mutations: These are mutations in master genes that control several other genes
during embryological development. Once the master gene is activated all its “slave genes” are
activated too. So mutations that turn on a master inappropriately, causes abnormalities.
Example: The antennapedia and the bithorax mutations in Drosophila.
Why are somatic mutations irrelevant in evolution? They are irrelevant because they
occur in the body cells and not in the gametes. Only the latter pass on their characteristics to the
next generation.
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C. Comment on whether the following statements are true or false and explain your
reasoning.
(3 pts each)
1. Science as a discipline takes no position on the existence of gods or a God.
True. Gods are supernatural beings often with unlimited powers and science does not deal with
supernatural entities. It confines its scope to the natural world.
2. Without mutation evolution would stop.
True. If you rule out mutation, you rule out any new variations that could be introduced into the
population. In the short run, variability could still occur by the shuffling of alleles because of
sexual reproduction and recombination so evolution could occur. But eventually, all of the
poor arrangements would have been eliminated by natural selection and effectively nothing
new could occur and the population would be stabilized, i.e. no evolution.
3. It is impossible for a person to have to have xy chromosomes and have female anatomy.
False. If a person had lost the SRY gene during cross-over during meiosis in the father, then it
could happen. It is the SRY gene that initiates the development of the male hormones at the
7th week of human development. If it is absent, female anatomy will result.
4. An addition point mutation is much more likely to lead to a non-functional gene than a
substitution.
True. A substitution mutation will only change one nucleotide unit for another (e.g. substituting
adenine for guanine), and so will normally will result in one amino acid being substituted for
another. (And even that might not occur if the codons are equivalent, coding for the same
amino acid). In contrast, the addition of a new nucleotide into a functioning gene will cause
a frame shift so that all of the subsequent codons are altered and this will cause the entire
protein to be nonsense, or non-functional because it is so drastically altered.
5. Stabilizing selection reduces genetic variability while disruptive selection increases it.
True. Stabilizing selection is the process whereby individuals with extreme variations are
eliminated from the population—e.g. the tallest and the shortest individuals. This means that
the variation in the population is reduced in each subsequent generation. In contrast,
disruptive selection means that the average individuals are selected against so that the
extreme individuals in the population are favored. This means the population is continuously
made more diverse, and if this continues, the population will slowly diverge into two different
populations—e.g. small individuals and short, since the middl- sized individuals are being
continuously eliminated.
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D. List Questions
1. List 3 statements that are true about prokaryotic but not true about eukaryotes.
(3 pts)
a) No nuclear membrane in prokaryotes
b) No mitochondria, chloroplasts, ER, Golgi, 9+2 flagella, lysosomes, vacuoles
c) No mitosis or meiosis
d) Single circular chromosome
e) Oldest fossils on earth
f) Smallest cells on earth
2. List 3 ways that meiosis is different from mitosis.
(3 pts)
a) Meiosis goes through two cell divisions and mitosis only one
b) Meiosis produces 4 haploid cells and mitosis results in two diploid cells
c) Meiotic division (meiosis I) has the two sister chromosomes line up next to one another where
cross-over occurs between the chromatids. During mitosis the sister chromosomes do not lie next
to one another; all chromosomes line up directly below one another in a single row.
3. List and explain three (3) major problems or criticisms that Darwin could not solve, and then
describe in detail how we have resolved these problems today.
(9 pts)
a) Darwin did not know the age of the earth and so could not answer the critics who claimed that
the earth was not old enough for evolution to occur the way that he described it.. Today, because
of radioactive dating we know the earth is 4.6 billion years old, plenty of time for evolution to
occur.
b) Fleeming Jenkin argued that if a better specialized organism happened to occur in the
population that rather than it surviving and spreading its good traits around into the next
generation, that it would have to breed with the average individuals. So that its good trait would
be progressively diluted and lost with each succeeding generation, because traits blended
together during breeding. Today, we know that genes do not blend together. First, shown by
Mendel, the blending hypothesis is wrong. Genes remain distinct and so that good traits can
indeed be selected by the environment (natural selection) and be passed onto the next
generation.
c) Critics said that Darwin did not know how variations are developed. Today we know that
DNA develops mutations during replication and that these are the fundamental source of
variation in the population.
d) Critics asked where are the intermediate fossils (“transition species”) that should occur if
organisms are changing over time? Today, we have hundreds of transition species that show the
very intermediate connections that we anticipate. Archaeopteryx is a famous example, but there
are many more including fossil whale intermediates and horse intermediates, and hundreds of
human intermediate fossils.
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E. Make a phenogram with labels indicating on what basis you made the separations for
the following “organisms”: bacterium, virus, goldfish, whale, apple tree, and a frog.
(4 pts)
Any number of possible diagrams can be constructed.
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