Download Unit 1 Notes #8 Other Mechanisms of Evolution - Mr. Lesiuk

Unit 1 Notes #8 - Other Mechanisms Of Evolution
- The number one mechanism that drives
evolution is “NATURAL SELECTION”.
Mutation that creates new alleles is also
another significant driver of evolution.
- There are some other factors/mechanisms
that also disturb/change the allelic frequencies
of a gene pool.
1. Genetic Drift:
- Another force that drives evolution is Genetic
Drift.
- Genetic drift occurs when there is a random (by
chance) change in the frequency of an allele in a
gene pool.
- Occurs most easily in small populations.
- Chance mating, or luck can dramatically change
the frequency of alleles in this small gene pool.
Will take place more easily in the population on
the left (2:4)
Example: Imagine a bucket of 250 red marbles
and 750 blue marbles. A handful of 30 or so
marbles may or may not perfectly conform to the
3:1 ratio in the bucket.
In fact, the smaller the sample, the less
representative the sample will be of the normal
distribution.
Example: A handful of 12 marbles maybe 10 red :
2 blue, or another sample of 12 might be all blue!
Small Sample
Taken
-Mutation along with Genetic Drift can work
together to change small populations very quickly.
-If the changed group is reunited with the main
population, the two groups may now be too
different for interbreeding.
- If so, two new species have evolved from one.
For Example:
Imagine that you have a population of rabbits that
you sampled near Enterprise Way. As a
researcher you extracted DNA from 25 rabbits
you trapped. In an effort to study the allelic ratios
for fur coat colour you run DNA testing to
determine the two alleles that each rabbit
possesses.
For fur coat colour there is one gene but two
different alleles (forms) for this gene. One allele
codes for the making of a black protein pigment.
This allele is dominant and it is denoted as “B”.
The other allele for this gene does not code for the
production of this black pigment in the fur. This
allele for white fur is recessive and is denoted as
“b”.
When the DNA was analyzed the following
genotypes were identified:
BB
Bb
Bb
Bb
BB
Bb
BB
BB
Bb
Bb
Bb
Bb
Bb
Bb
BB
BB
BB
Bb
Bb
bb
bb
Bb
BB
BB
Bb
1. What is the phenotypic ratio/percent for each
phenotype in the sample?
Black : ___84%____
White : ___16%____
2. What is the genotypic ratio/percent for each
genotype in this sample?
BB : __36%__ Bb : __48%__ bb : __16%___
3. As each rabbit has two alleles (one inherited
from each parent), how many alleles are in the
sample of 25 rabbits?
Total number of alleles in sample : __50___
4. What is the frequency/percentage of the “B”
allele? _30/50 = 60%____
5. What is the frequency/percentage of the “b”
allele? _20/50 = 40%__
- Imagine that this sample of 25 rabbits is an
accurate sample of the entire population of rabbits
and that the actual population of rabbits numbers
200 rabbits.
- In a small population like this it is very easy to
get a random change to the allelic frequencies.
Imagine that for no particular reason a number of
black rabbits do not mate one season and a
number of white rabbits mate twice that same
season. The allelic frequency could totally flipflop in a very short time. This change in the gene
pool was not due to natural selection or due to
mutation but rather by chance.
- Another example of this would be the odds of
flipping a heads on a quarter versus the odds of
flipping a tails on a quarter.
If I give everyone a quarter and said that the
quarter represented your genotype and that
everyone has the following genotype: “Hh”
- Where “H” = heads and
“h” = tails
Hh
Hh
Hh
Hh
Hh
Hh
Hh
Hh
Hh
Hh
Hh
Hh
Hh
Hh
Hh
Hh
Hh
Hh
Hh
Hh
Hh
Hh
Hh
Hh
Hh
Hh
Hh
Hh
Hh
Hh
- 30 Students with two alleles each, gives us a total
of 60 alleles. 50% of the alleles are “H” while the
other 50% of the alleles are “h”.
- When each person flips their coin twice we can
compare that to each person making two gametes
(2 eggs or 2 sperm), if everything goes as expected
we should get 30 Heads and 30 tails. If this were
the case, the allelic frequency for the next
generation should remain the same: “H” = 0.5 or
50% and the allelic frequency for “h” = 0.5 or
50%.
- But there is a chance that we might get 42 Heads
and only 18 tails. If that were the case, the gene
pool of the next generation would quickly start to
change as shown below:
Hh
HH
HH
hh
Hh
HH
Hh
hh
HH
Hh
Hh
HH
HH
Hh
Hh
Hh
Hh
Hh
HH
Hh
HH
HH
HH
Hh
HH
Hh
HH
HH
HH
Hh
- Now the allelic frequency of “H” would now =
0.7 or 70% and the allelic frequency of “h” would
only be 0.3 or 30%. Eventually the allele for
“tails” may get weeded out all together, but for no
real good reason. It just happened by chance.
- In the above picture you can see that the allelic
frequency shifted from 60% White : 40% Green
all the way to 100% White : 0% Green.
2. Differential Migration
-If a certain phenotype moves out of an area. The
frequency of those alleles decreases.
Emigrate – Move out
Immigrate – Move in
Ones that moved away: Frequency DECREASES
Ones that stay: Frequency automatically
INCREASES
Example:
Long necked giraffes on the plains.
3. Population Isolation
-If a small group of individuals is separated from
the main group, they may have a different
frequency of alleles in their gene pool.
-As the population grows, this frequency may
become much different from the main group.
4. Gene Flow
- Sometimes a barrier (mountain, river etc)
between two different populations of a species has
maintained two different gene pools. If that
barrier is overcome a mixing of the two gene pools
may occur causing a big disruption in the allelic
frequencies that were found in the original gene
pools.