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Genetics Exam #3
May 5th, 2006
(5) Define and distinguish between disruptive and directional selection.
disruptive selection causes the norm to be lost
directional selection causes one of the extremes to be lost
(5) Define and distinguish between cladogenesis and anagenesis.
cladogenesis is speciation that occurs when a portion of the population changes and the original
population continues to exist. anagenesis is when an entire population changes over time so that
the two populations became different species over time; they never coexist.
(3) Define allele frequency.
percentage of time that allele is found in a population
(3) Define gene pool.
complete collection of all the alleles of all the genes in a given population
(3) Define local population.
population in which most of the individuals will find a mate
(3) Define evolutionary clock.
slow, constant accumulation of mutations over time that can be used to make a phylogenetic tree
(3) Define heterozygote advantage.
when a heterozygous individual has a survival advantage over either homozygous phenotype
(3) Define maternal inheritance.
genetic material inherited only from the mother-- mitochondrial DNA
(3) Define parapatric speciation.
speciation caused by migration to a new area or habitat
(3) Define mean fitness of a population.
proportion of the population that can go on to reproduce after natural selection
(3) Define sympatric speciation.
speciation that occurs by developing polymorphisms that make interbreeding impossible
(3) Define random mating.
individuals within a population mate without regard to genotype
(9) List and describe the 3 different types of equilibria (ie. how do the equilibria behave).
stable equilibrium: equilibrium changes from an outside force but moves back to the original
location
unstable equilibrium: equilibrium changes from an outside force and continues to change even
after the outside force is no longer being applied
neutral equilibrium: equilibrium changes from an outside force and once changed, stays where it
went
(10) Given that spotted is dominant over solid in snail shells and these are in a Hardy-Weinberg
equilibrium, what are the allele frequencies if 36% of the snails in a forest have solid colored
shells?
q2 = 0.36
q= sqrt(0.36)
q= 0.6
p=1-q
p=1-06
p=0.4
(8) List 4 of the major assumptions made for the Hardy-Weinberg equilibrium.
no natural selection
no migration
no mutation
large population size
males and females inherit genetic information equally
must have random matings within a population
(10) There are two major theories as to when and how evolution occurs. List and describe them.
punctuated equilibrium: idea that changes in a population occur rapidly for some reason, and this
causes sqecies to change rapidly to survive. After the period of rapid change, there is a long
period of equilibrium.
pyletic gradualism: idea that change in a population occurs slowly and builds up over long
periods of time; species are constantly, albeit slowly drifting apart if they cannot interbreed
(10) In peas, purple flowers (F) are dominant over white flowers (f). In one isolated population,
F has an allele frequency of 80% while f has an allele frequency of 20%. Before breeding one
year, the nearly extinct 1-eyed, 1-horned, flying purple pea plant eater came through and ravaged
the field. 70% of the purple flowered pea plants were eaten. After this selection, what are the
new allele frequencies?
original genotype frequency: p2 = (.08)2 = 0.64 after selection: 0.64 (1-0.7) = 0.192
2pq=2*0.8*0.2=0.32
0.32 (1-0.7) = 0.096
q2 = (0.2)2 = 0.04
0.04 (1)
= 0.040
total fraction left =0.328(sum above)
genotype %'s after selection = 0.192/0.328 = 0.585
0.096/0.328 = 0.293
0.040/0.328 = 0.122
alleles after selection F: [2*(0.585)+0.293]/2 = 0.7315
f: [2*(0.122)+0.293]/2 = 0.2685
(5) Based solely on genetic arguments, why do people believe the earliest humans came from
Africa?
Human DNA can be followed unambiguously through maternal inheritance; ie. mitochondrial
DNA. When mitochondrial DNA from around the world was analyzed, by far the most diversity
was found only in Africa-- 3 of the 4 major lineages. All other areas were more or less related to
one of the lineages, spreading outward from their common origin.
(8) If natural selection removes deleterious (bad) mutations from the population, why do many
entirely deleterious recessive phenotypes continue to show up generation after generation even
after equilibrium has been reached?
natural selection does indeed remove deleterious mutations, even those found in heterozygotes,
by selecting against the homozygous recessive phenotype. However, mutations do occur that
cause the normal allele to mutate into the recessive one. At some point, the rate at which natural
selection removes a deleterious allele is the same as the rate at which mutation creates the
deleterious allele, thus resulting in no NET change in allele frequency.