BY 123 SI Session #13
Chapters 23-27
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1) In a population of 200 horses, 98 are homozygous dominant for brown fur (BB), 84 are
heterozygous (Bb), and 18 are homozygous recessive for white fur (bb).
a. The genotype frequencies of this population are:
BB= 49%, Bb= 42%, bb= 9%
b. The allele frequencies of this population are:
0.7 B allele, 0.3 b allele
2) Hardy Weinberg equations:
a. The allele frequencies in a population are A= 0.6 and a=0.4. Predict the genotype
frequencies for the next generation.
AA= .36, Aa= .48, aa= .16
b. What would the allele frequencies be for the generation you predicted in part a?
A= 0.6, a= 0.4
c. Suppose the actual genotype frequencies in the population differed significantly from what
was predicted in part a. What would this mean?
The population was not in Hardy-Weinberg equilibrium and was evolving.
3) What conditions must be maintained in order for a population to be in Hardy-Weinberg
equilibrium?
No mutations, random mating, no natural selection, large population, no gene flow
4) Match the following terms with their definition:
a. Genetic drift
b. Founder effect
c. Bottleneck effect
d. Gene flow
e. Relative fitness
f. Directional selection
g. Disruptive selection
h. Stabilizing selection
i. Intrasexual selection
j. Intersexual selection
k. Balancing selection
l. Frequency-dependent selection
__k___ can maintain stable frequencies of two or more phenotypes in a population.
__d___ migration of individuals or the transfer of gametes between populations.
__i___ sexual selection in which individuals of the SAME sex compete for mates
__g___ occurs when environment favors individuals on both extremes of a phenotypic range
__l___ a phenotype’s reproductive success declines if it becomes too common in the population
__c___ occurs when some disaster or other factor reduces population size dramatically.
__a___ chance fluctuations in a population’s allele frequencies from one generation to the next.
__j___ sexual selection in which individuals of one sex discriminate in choosing a mate
__f___ occurs during periods of environmental change in which individuals of one phenotype
are favored.
__b___ genetic drift which occurs when only a few individuals colonize a new area.
__h___acts against extreme phenotypes and favors more intermediate forms, generally
reducing the phenotypic variation.
__e___ a measure of an individual’s contribution to the gene pool of the next generation.
5) Fill in the Chart Below:
Type of Isolation
Pre or Post
Example
Reduced Hybrid
Fertility
Post
Gametic Isolation
Pre
Two species of frogs mate in a
laboratory setup and produce
viable but sterile offspring.
Two species of sea urchins
release gametes at the same
time, but the sperm fail to
fuse with eggs of a different
species.
The genital openings of two
species of land snails cannot
line up because their shells
spiral in opposite directions.
Mechanical isolation
Pre
Temporal isolation
Pre
Hybrid breakdown
Post
Behavioral Isolation
Pre
Reduced Hybrid Viability
Post
Two species of short-lived
mayflies emerge during
different weeks in spring.
Two species of salamanders
mate and produce offspring,
but the hybrid’s offspring are
sterile.
Two similar species of birds
have different mating rituals.
Peepers breed in woodland
ponds; leopard frogs breed in
swamps.
6) Differentiate between allopatric and sympatric speciation. What reproductive barriers might
exist in both cases?
Allopatric species form while geographically isolated from the parent population. Habitat
isolation.
Sympatric speciation happens when a reproductive barrier isolates the gene pool of the same
geographical range. Reproductive barriers might arise due to genetic change. Examples include
genetic drift, or sexual selection.
7) Match the following words with their definition:
a. Cambrian explosion
b. Pangaea
c. Permian mass extinction
d. Adaptive radiations
e. Heterochrony
f. Paedomorphosis
g. Homeotic genes
__c___ occurred 251 mya, claimed 96% of marine animal species and many terrestrial.
Marked the boundary between Paleozoic and Mesozoic.
__d___ periods in which many new species evolve and fill various ecological niches.
Commonly occur after mass extinctions.
__b___ supercontinent formed about 250 mya which altered many habitates and caused
many species to become extinct.
__f___ the retention of juvenile characteristics in the adult form of an organism.
__e___ evolutionary change in the rate and timing of development.
__g___ control spatial arrangement of body parts.
___a__ occurred 542 mya and was marked by the appearance of many animal phyla.
8) Define the following terms from Chapter 26:
a. Homology: similarity due to shared ancestory
b. Analogy: similarity due to convergent evolution
c. Clade: a phylogenetic grouping the includes an ancestral species and all of its descendents.
d. Horizontal gene transfer: a process in which genes are transferred from one genome to
another through mechanisms such as exchange of transposable elements and plasmids.