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Genetics Exam #3
December 6th, 2006
(5) Define and distinguish between punctuated equilibrium and phyletic gradualism.
punctuated equilibrium is evolution in short bursts followed by long periods of equilibrium
phyletic gradualism is the slow constant genetic changes over time
(5) Define and distinguish between biological and morphological speciation.
biological speciation is the inability of different populations to interbreed
morphological speciation is speciation based on physical characteristics and morphology
(3) Define genetic fitness.
genetic fitness is the survivability of a particular phenotype in an environment
(3) Define frequency dependent selection.
selection depends upon the allele frequency in a population
(3) Define evolutionary clock.
slow, constant change of DNA over time used to estimate time since most recent ancestor
(3) Define gene pool.
complete collection of all alleles of all genes in the genome
(3) Define mean fitness of a population.
percentage of individuals that survive after natural selection
(3) Define genetic load.
# of defective individuals tolerated in a population to maintain heterozygosity
(3) Define reproductive isolation.
separation of individuals into populations that are unable to interbreed
(3) Define local population.
population in which most individuals will find a mate
(3) Define sympatric speciation.
speciation caused by polymorphism in a population
(3) Define heterozygous advantage.
heterozygotes have a survival advantage compared to either homozygote
(10) In snapdragons, red and white are co-dominant traits. One one island, you counted 750 red
snapdragons, 950 pink snapdragons, and 300 white snapdragons. What are the allelic
frequencies for these traits?
red 750*2 + 950 = 2450 alleles /4000 total = 0.6125
white 300*2 +950 = 1550 alleles/4000 total = 0.3875
(10) Guppies are a small tropical fish commonly in the lakes of central america. Careless
tourists introduced a large predatory fish into one of the lakes, and it preferentially eats the
largest guppies. How would this predator affect the guppy population in this lake, and what type
of natural selection is this (in terms of how it changes the distribution of guppies)?
this is an example of directional selection, forcing the population toward smaller guppies
As larger guppies are eaten by the predatory fish, the guppies that were smaller would have a
selective advantage compared to the large ones.
(10) In the south pacific, one island had a population of 250 dodos, with an allele frequency for
allele A is 0.7 and a= 0.3. A large raft carries 100 dodos from a distant island that has an allele
frequency for A of 0.2 and a=0.8. What is the new allele frequency for the combined
population?
p(A) = 250/350 * .7 + 100/350 * .2 = 0.5 + 0.057 = .557
p(a) = 250/350 *.3 + 100/350 * .8 = .2143 + .2286 = .443
(10) Natural selection can work upon organisms in any of 4 different ways, or levels. List and
describe the 4 different ways.
zygotic selection- certain individuals are unable to survive because of genetic defects
gametic selection- when an organism's gametes are defective and less able to form zygotes
fertility selection- when a certain phenotype produces more offspring than another
sexual selection- when a certain phenotype is able to mate more frequently
(10) In humans, one gene d has 4 alleles, with the following frequencies: d1=0.4, d2=0.3, d3=0.2
and d4=0.1. What is the probability of an individual being heterozygous for d2?
# d2 heterozygotes = 2*d1d2 + 2*d2d3 + 2*d2d4
2*.4*.3 + 2*.3*.2 + 2*.3*.1
.24
+ .12 + .06 = .42
(10) In peas, green pods are dominant over yellow pods. Under normal circumstances, the allele
frequency for green is 40% and the allele frequency for yellow is 60%. Greenbugs that were
introduced one year on a freighter preferentially eat green pea pods, giving a selection coefficient
of 0.40. After 1 round of selection, what is the new allele frequency for green and yellow pods?
genetic fitness
GG = 0.42 = 0.16
* (1- 0.4) = 0.096
Gg = 2*.6*.4 = 0.48 * (1-0.4) = 0.288
gg = 0.62 = 0.36
* 1
= 0.360 +
0.744
normalizing the genotypes to determine the percentages after selection gives:
GG = .096/.744 = 0.129
Gg = .288/.744 = 0.387
gg= .36/.744 = 0.484
allele frequency of G after selection = (2*.129 + .387)/2 = 0.323
allele frequency of g after selection = (2*.484 + .387)/2 = 0.677