MICROEVOLUTION 2
Materials: 3 colors of beans, totaling 4000 for each
group. Each group has 49% one color, 42%
the second color and 9% the 3rd color.
Purpose: To simulate the microevolution model with
populations of colored beans, illustrating random mating and the effects of
selection and genetic drift.
Background: Populations, not individuals, evolve by
gradual changes over time in the frequency of alleles that are found at genetic
loci. These changes result from
mutation, selection, migration, or genetic drift. Collectively, these processes comprise
microevolution. Mechanisms of
microevolution are well understood. In
fact, one of these mechanisms, selection, has been used for centuries to
increase the productivity of crops and livestock.
According to the
microevolution model, a population of organisms can be considered to be a gene
pool, which is composed of all the copies of every allele in the population at
a given moment. In diploid organisms,
the genes in the gene pool occur in pairs in each individual, and individuals
may be homozygous or heterozygous for a particular trait.
In 1908, G.H.
Hardy and G. Weinberg showed that no matter how many generations elapsed,
sexual reproduction in itself couldn’t change the frequencies of alleles in a
gene pool. Changes if they occur, must be due to the action of other factors---the real
agents of evolution.
Example….a
population of plants with red and white flowers. Red
flowers have the dominant allele and white flowers are found in individuals
with only the recessive alleles. A study of 10,000
plants indicated that 84% of the plants had red flowers and 16% had white
flowers. Of the red flowered plants, 36%
of the total plants were homozygous for the red allele and 48% were
heterozygous.
The genotypic
frequencies were:
36% AA Where A = red allele
48% Aa a = white allele
16% aa
100%
The frequencies of
alleles can be obtained by the following:
Every individual
with the genotype AA contributes two A alleles to the gene pool, and every
individual with the genotype Aa contributes one A allele so given 10000 individuals, the
frequency of the A allele in the gene
pool equals:
2 x 36% x 10,000
+ 48% x 10,000 =
12,000 A
alleles
In the same
population, the frequency
a in the gene pool can
be calculated by similar reasoning (aa individual
contributes two a, and Aa contributes 1 a. Therefore,
2 x 16% x 10,000 + 48%
x 10,000 = 8,000 a alleles
On a percentage
basis, 60% (12,000/12,000
+ 8,000) of the alleles of the gene pool are A and 40% (8,000/ 12,000 + 8,000) are a.
This information
describes the gene pool at the time of the study. But, what happens to the allele frequencies
when this population reproduces sexually?
The frequency of
the dominant allele is p and that of
the recessive allele is q, in any
population where there are only two alleles at one gene locus:
Sooo, a population in Hardy-Weinberg
equilibrium, the homozygote dominant will be found p2% of the time, the heterozygote 2 pq% of the
time and the homozyous recessive q2% of the time.
(p2% … 36% AA
) (q2% … 16% aa) (2 pq% … 48% Aa)
Lab: Your lab
population is 4000 beans in a coffee can.
There are 3 colors: 49% are one color, 42% another color and 9% a third
color. These percentages correspond to
genotypes in hypothetical populations in which there are two alleles at a given
locus. Indicate what colors correspond
to the genotypes below:
Genotype % Color
Number (% x 4000)
AA 49 .
Aa 42 .
aa 9 .
Determining
starting allele frequency-Accounting method
# of AA
individuals x 2 =3920
+# of Aa individuals x 1 =1680
# of A alleles in gene pool =5600
Do the same for
the a allele
Determining
starting allele frequency-Hardy-Weinberg Calculation method
To use this there is the
major assumption that the genotypes are in proportion to what is expected by
the Hardy-Weinberg equilibrium.
(which is that AA=p2 and aa=q2 )
Calculate the p
and q values for the population in the can and enter it below.
P=________ q=________
Remember p+q=1 Are the relative frequencies of A and
a the same by the accounting and calculation methods?_____
1. To simulate
random mating in your population, mix up the can and withdraw two beans.
These are your
breeding pair.
2. Repeat this 100
times...recording the colors drawn each time.
RETURN BEANS TO
CAN AFTER EACH DRAW! (Why?)
3. Record the
info on the table below and list the Genotypes of the new breeding group.
AA______
Aa_______
aa_______
Total 200
Are these
frequencies the same as the total population of the can?
Compare them
using p and q
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