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• Entry 57: Evolution Notes 2
What you will learn today…
1. How do we measure genetic variation in
a population?
2. Why is genetic variation in a population
important?
3. What are the sources of genetic variation
in a population?
What determines a heritable trait?
DNA
(gene)
mRNA
protein
Observed
trait
Therefore, if traits vary in a population, then the
genes (alleles) must vary in the population!
A population with variation in traits…
Grey
White
Tall ears
Short ears
...is caused by variation in alleles
Grey allele = G
Tt
GG
TT
Gg
tt
gg
tt
Gg
tt
GG
tt
Gg
tt
Gg
White allele = g
Tall ear allele = T
Tt
gg
Short ear allele = t
How do we measure genetic
variation in a population?
Grey allele = G
8 / 16 = 50% G
Tt
GG
TT
Gg
tt
gg
tt
Gg
tt
Gg
tt
GG
tt
Gg
“Gene Pool”
Tt
gg
White allele = g
8 / 16 = 50% g
Tall ear allele = T
4 / 16 = 25% T
Short ear allele = t
12 / 16 = 75% t
How do we measure genetic
variation in a population?
Gene Pool- Total genetic information available
in a population (all the alleles that are
present).
Allele Frequency- The percentage of an allele
in the gene pool. Tells you whether a given
allele is common or rare.
Why is genetic variation in a
population important?
• A gene pool without much variation
limits a species’ ability to further evolve.
(Variation is one of the 4 factors required
for natural selection to lead to evolution)
• If populations do not adapt to their
environment, they may become extinct.
Sources of genetic variation
1) SEXUAL REPRODUCTION
A. Meiosis – one allele is passed on from
each parent (recall that sperm and eggs are
haploid cells, each containing half the
necessary genetic information).
B. Random fertilization – only one of the
millions of sperm involved in mating will
fertilize the egg.
The randomness of sexual reproduction
explains why siblings can look so different.
Sources of genetic variation
2) MUTATION
Mutation means a change in DNA sequence.
A new DNA sequence may = new allele of a gene.
– Some mutations produce genes that are harmful
(e.g. Huntington’s disease)
– Most mutations produce genes that are neutral
(neither helpful nor harmful)
– Very, very few mutations produce genes that are
advantageous
The only way brand new alleles can be created is
through mutation.
Natural Selection
• Individuals with advantageous genes
are more likely to survive to
reproduce and pass on these genes
to their offspring.
• Individuals without advantages genes
are less likely to survive to reproduce,
and these genes do not get passed
on in the population.
Conclusion
• Mutation does NOT cause evolution
– it is only a source of variation (just one of the
factors required for natural selection)
• Natural selection determines if the allele
frequency will change within a population.
• EVOLUTION= Change in allele frequency
(A change in how often a form of a gene is
present)
Next Task: Kerosene Karl
1. Read the explanation on the back side of
your Evolution Notes 2 sheet and answer
the Prelab Questions.
2. In my Pickup Work folder open the
Kerosene Karl activity. Follow the
directions on your worksheet to complete
the activity.
3. Whatever you don’t finish is
HOMEWORK!!