Transcript selection - s3.amazonaws.com

SELECTION
Why doesn’t selection work anymore
on running
speed?
A) There are probably diseases involved
B) Too much inbreeding producing genetic
defects
C) There is no more genetic variation in the stock
D) Epistatic effects are now in play
E) Artificial insemination programs inadvertently
select for defective (weaker) sperm.
In order for Evolution to Occur
there must be variation in the
population
Which is NOT a cause of allele
variation in a population?
A) Homology
B) Mutation
C) Genetic recombination (cross over)
D) Sexual recombination
E) Migration (gene flow)
SO
Selection & Genetic drift
working on variation in the population cause
How Effective Is Selection?
Consider Artificial Selection
In fact, all of
these plants
Come from wild
mustard !
Artificial Selection cont’.

Farm animals (e.g. chicken egg production)
1933
126 eggs/hen/year
Today 365 eggs/hen/year
Artificial Selection in Dogs
Dogs

All dog fossils from
10,000 years ago are
from one species of
Asian Wolf

The Romans (2,000
yrs ago) had 6 breeds

All other breeds have
been developed in the
last 400 years !!!!!
NATURAL SELECTION

Differences in mortality & reproduction lead
to differences in the proportion of alleles
passing to the next generation.
Positive selection= a selective advantage for
one allele and that favored allele increases in
the population.
 Negative selection= a selective disadvantage
for a allele and that tends to disappear from
the population.

NATURAL SELECTION
IN BACTERIA
Development of antibiotic
resistance in bacteria

e.g. NATURAL SELECTION IN
BACTERIA
Resistance to antibiotics
e.g. NATURAL SELECTION IN INSECTS
Resistance to pesticides

Foliate
insecticide
NATURAL SELECTION OF PLANTS TO
Herbicide
Year Deployed
Resistance Observed
NATURAL SELECTION IN PLANTS
FOR COPPER TOLERENCE

Please look at the video.
WHAT PRODUCES SELECTION?

Internal Environment
– e.g. genetic environment
– e.g. physiological/developmental environment

External Environment
– Physical Environment (water, temp., 02,)
– Biological Environment
 Competition
 Predation
 Parasitism
 Food supply
 Mate selection (Sexual Selection)
Success must be judged in 2 ways:

Absolute Terms= Will the genome work?

Relative Terms= Is the genome the best
available model?
Small Selective Advantages

Suppose that there were a selective advantage
to having a long nose and suppose there was
a trend that the average nose length increased
0.001/year
How many inches will the nose
grow in 100,000 years?
A) 1.0 B) 10 C) 100 D) 1000 E) None of
the above.

SELECTION IN 2 ALLELE TRAITS

Selection against a dominant trait
Genotypes:
AA
Aa
aa
Rapid elimination of the trait should occur.
Acondroplastic Dwarfism
Caused by dominant allele D
Fitness & Natural Selection
How to calculate fitness
Compare the success of the favored allele to
that of the unfavored allele.
e.g. Acondroplastic dwarfism (dominant allele)
W= number of children produced by 100 dwarfs

number of children produced by 100 normals
W= 20/100= 0.20
= fitness coefficient for dwarfs
Clicker Question

If W= 20/100= 0.20 is
the fitness coefficient
for dwarfs
What is the fitness
coefficient of the
normal individuals?
A) Zero
B) 0.2
C) 0.5
D) 1.0
E) None of the above
Answer
Remember that the fitness of the best
allele is always = 1.0
 Since the normals produce more offspring
than the dwarfs, they have a
W = 1.0

Dwarfism would rapidly be eliminated from
the population if it were not for its high
mutation rate!
Selection in two allele systems
Cont’.

Selection against the recessive
Genotypes:
AA
Aa
aa
Selection will be slow to remove the “a” allele
in complete dominance because it is hidden in
the heterozygous condition “Aa.” These are
carriers.
Cystic Fibrosis
Caused by recessive trait, cc
 Normal allele C, causes channels in the cell
membranes (lung, gut) to allow Cl- out of cell
H20 follows.

If both alleles are cc, then this doesn’t happen
& thick sticky mucus clogs lungs & gut & these
tissues are breeding ground for bacteria
Cystic Fibrosis
Most common genetic disorder in Caucasians;
affects 1 out of 2,500 white babies
 5% of Caucasians are carriers

CC is normal
 Cc is normal, but carrier
 cc has cystic fibrosis

Clicker Question

A)
B)
C)
D)
E)
If two carriers have children, what are
the chances that their child will have CF?
100%
75%
50%
25%
Zero %
Cystic Fibrosis

Cc x Cc = 25% of kids are cc & have Cystic Fib.

Why aren’t they eliminated from the pop?
– Because Cc have protection against diarrhea ?
Selection in two allele systems
Cont’.

Selection favoring heterozygotes “Aa”
Can only occur if incomplete or co-dominance
because the trait must be expressed.
e.g. Sickle-cell anemia
Sickle Cell Co-dominance
Normal
HbN HbN
Mild Anemia
HbN HbS
W = 1.0
W = 0.9
Severe Anemia
HbS HbS
In the
USA:
W = 0.14
In
W = 0.88
W = 1.0
W = 0.14
Heterozygotes are favored in Africa because they are
more resistant to malaria!
Africa:
Natural Selection in Polygene Traits
Directional Selection
Natural Selection in Polygene Traits
Stabilizing Selection
Natural Selection in Polygene Traits
Disruptive Selection
Natural Selection in Humans

A)
B)
C)
D)
E)
What kind of
selection is this?
Directional
Stabilizing
Disruptive
None of the above
No way to know