Transcript Genetics of Evolution - Ms. Chambers' Biology

1.
How do we measure genetic variation in a
population?
2.
What are the sources of genetic variation in a
population?
1.
Why is genetic variation in a population
important?
DNA
(gene)
mRNA
protein
Observed
trait
Therefore, if traits vary in a population, then the
genes (alleles) must vary in the population!
Gene Pool- Total genetic information available
in a population (all the alleles that are
present).
Allele (Relative) Frequency- The percentage
of an allele in the gene pool. Tells you
whether a given allele is common or rare (%)

Population – group of individuals of the
same species that interbreed

Gene Pool – all genes (including all
alleles) present in a population

Relative Frequency – number of times an
allele occurs in a gene pool
Grey
White
Tall ears
Short ears
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
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



A gene pool without much variation limits a
species’ ability to further evolve.
Evolution- change over time in the gene
pools of a species
If populations do not change (adapt) to their
environment, they may become extinct.
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.
http://www.sciencegeek.net/Biology/review/graphics/Unit3/meiosis.jpg
MUTATION
2)
A change in DNA sequence.
New DNA sequence = new allele of a gene.



Many mutations produce genes that are harmful
(e.g. Huntington’s disease)
Some mutations produce genes that are neutral
(neither helpful nor harmful)
Very, very few mutations produce genes that are
advantageous, beneficial

Mutations add new alleles to the gene pool.
That is, they increase the variety of alleles in
the population.
Deck is Gene Pool – It contains all
possible alleles for the next
generation.
Drawing cards picks the alleles that
are inherited by the next
generation.
Shuffling of the deck is sexual
reproduction.
Adding new cards to the deck is
mutation.
(Mutation is rare, but shuffling
happens each time a new
generation is produced)


Individuals with advantageous genes survive
to reproduce and pass on these genes to their
offspring.
Individuals without advantages genes do not
survive to reproduce, and these genes do not
get passed on in the population.
Warmup:
Allele Frequency example
Figure 16-2, pg. 394- work out the frequency of each allele.
Sample Population
48%
heterozygous
black, Bb
16%
homozygous
black, BB
36%
homozygous
brown, bb
Frequency of Alleles
allele for
brown fur, b
allele for
black fur, B

Mutations
 Mistakes in replication
 Radiation or chemicals in environment

Gene Shuffling
 Assortment of chromosomes
 Crossing over

Variation in single gene
traits lead to only two
distinct phenotypes
Frequency of
phenotype determined
by frequency of alleles
100
Frequency of Phenotype (%)

80
60
40
20
0
Widow’s peak
No widow’s peak
Phenotype
Trait controlled by 2 or
more genes
 Many possible
genotype and
phenotype possibilities
 Bell shaped curve
typical of polygenic
traits
Frequency of Phenotype

Phenotype (height)
1.
How do we measure genetic variation in a
population?
2.
What are the sources of genetic variation in a
population?
3.
Why is genetic variation important in a population?
Read section 16-1 in textbook (pages
393-396)
 Complete worksheet 16-1: Genes and
Variations

Directional Selection
Food becomes scarce
Low mortality,
high fitness
High mortality,
low fitness

Individuals at one end of curve have higher fitness

Range of phenotypes shifts

Individuals near center
of curve have highest
fitness
Stabilizing Selection
Low mortality,
high fitness

Keeps center of curve
at same position and
narrows graph
Percentage of Population
High mortality,
low fitness
Birth Weight
Disruptive Selection
High mortality,
Low fitness
Population splits
into two
subgroups
specializing in
different
seeds.
Number of Birds
in Population
Low mortality,
high fitness
Beak Size
Number of Birds
in Population
Largest and smallest seeds become more common
Beak Size

Individuals at upper and lower ends of the curve
have higher fitness than individuals in the middle

Selection acts strongly against individuals of the
intermediate type

Random change in allele frequency by chance
A

Occurs in small
populations
st
1 (26)
.62
a
.38
rd (28)
3
.30
.70 as
 Founder effect – allele frequencies
change
th (30)
a result of4migration
of subgroup
of .63
.37
populationth
5 (32)
.52
.48
6th (28)
.34
.66
Section 16-2
Sample of
Original Population
Descendants
Founding Population A
Founding Population B
Section 16-2
Sample of
Original Population
Descendants
Founding Population A
Founding Population B
Section 16-2
Sample of
Original Population
Descendants
Founding Population A
Founding Population B

States that allele frequencies remain
constant (genetic equilibrium) unless one or
more factors cause them to change

No change in allele frequency of population =
no evolution in population





Random genetic drift
Gene flow
Non-random mating
Mutation
Natural selection
Random Mating
1.
•
Equal chance of passing on
alleles to offspring
Large Populations
2.
•
Genetic drift less likely to
occur
No Movement In or Out of
Population
3.
•
New members might bring
new alleles
No Mutations
4.
•
New alleles may be introduced
No Natural Selection
5.
•
All genotypes must have equal chance of
survival and reproduction

Speciation = formation of new
species

Species = group of organisms that
breed with one another and produce
fertile offspring

As new species evolve, populations
become reproductively isolated from
each other

Behavioral Isolation
 Differences in courtship rituals or other reproductive
strategies

Geographic Isolation
 Two populations separated by geographic barrier such as
rivers, mountains or bodies of water

Temporal Isolation
 Two or more species reproduce at different times of the
day or year
Reproductive Isolation
results from
Isolating mechanisms
which include
Behavioral isolation
Geographic isolation
Temporal isolation
produced by
produced by
produced by
Behavioral differences
Physical separation
Different mating times
which result in
Independently
evolving populations
which result in
Formation of
new species






Founders Arrive
Separation of Populations
Changes in the Gene Pool
Reproductive Isolation
Ecological Competition
Continued Evolution

Hummingbird video



Explain the hypothesis presented by the scientists profiled
in this segment to explain the process of speciation in
hummingbirds and possibly other species.
How does this hypothesis differ from the traditional view
that speciation often requires geographic separation of
populations?
Why were the researchers collecting blood from the
populations they studied? Discuss at least two possible
analyses that could be performed on those samples and,
identify at least two different questions that might be
answered with sufficient data.