Transcript The Genetic Engine

The Genetic Engine; Darwin
and Natural Selection
Hardy Weinberg (link to Video
introduction)
Modern
Biology &
Genetics
• Gregor
Mendel’s
genetic theory
of inheritance
• The
connection
between traits
and heredity
explored
The connection between Mendel
and Hardy Weinberg
• Use phenotype to
identify frequency of
recessive trait
• Take the square root to
identify frequency of
the recessive allele
• Use this to get
frequency of the
dominant allele
Sample: 4 recessive
individuals out of 100
•
Frequency of the recessive phenotype will be equal to q2
–
•
Note, q = frequency of the recessive allele
–
•
if 4 out of 100 are recessive, q2 = .04
So, q = the square root of .04 = .2
Heterozygous dominant frequency can now be calculated
• P=1-q
• P =. 1-.2 = .8
•
Heterozygous dominant frequency can now be calculated
• 2pq = 2(.2)(.8) = .32
•
Check your work!
• p2 + 2pq= q2 = 1
• .64 + .32 +. 04 = 1.0
Coloration in the Scarlet tiger moth
• Coloration in this species had been previously shown
to behave as a single-locus, two-allele system with
incomplete dominance. Data for 1612 individuals are
given below:
–
–
–
–
White-spotted (AA) =1469
Intermediate (Aa) = 138
Little spotting (aa) =5
Calculate the following frequencies:
A=
a=
AA =
Aa =
aa =
– Link for answer
More Practice
• One in 1700 US Caucasian newborns have
cystic fibrous. (use C for normal which is
dominant over c for cystic fibrous)
– What percent of the above population have cystic
fibrous (note, this is cc = q2)
– What is the frequency of the recessive allele? (q)
– What is the frequency of the dominant allele? (p)
– Calculate or state, the frequency of the 3 phenotypes
– Link to answers
Yet more practice
If 9% of an African population is born with a severe
form of sickle-cell anemia (ss), what percentage
of the population will be more resistant to
malaria because they are heterozygous (Ss) for
the sickle-cell gene?
–
–
–
–
–
Calculate q2 (Phenotype frequency)
Calculate q (Recessive Allele frequency)
Calculate p
(dominant allele frequency)
Calculate 2pq and q2 & CHECK YOUR WORK
Link to answer
Conditions for Hardy Weinberg
Five conditions needed to insure gene pool
stability (null condition for “no evolution”)
1. A large breeding population
2. Random mating
3. No change in allelic frequency due to
mutation
4. No immigration or emigration
5. No natural selection
What about natural selection?
• video
• Multiple generations of
selection – natural or
artificial will change a
character substantially
• Cumulative selective
response (in one
generation)
• Cumulative selection
differential (sum of the
effect over all generations)
How might stabilizing selection work?
• Why shouldn’t evolution
favor really, really, really,
really really tall people?
• Mortality rates for very
small and very large infants

• Stabilizing selection favors
intermediate characteristics
Disruptive selection favors
extremes: very large or
small bills
• Red portion of bar graph represents young that did
not survive (Black- Bellied Seed cracker)
Disruptive selection can lead to
speciation
“Truisms” about natural selection
• Natural selection can “happen” if the trait undergoing
selective pressure is genetically determined
• Natural selection can only work toward traits' that
increase fitness for survival and reproduction
• Natural selection acts by changing the frequency of
alleles in the gene pool over time – thus phenotypes do
not “disappear” all at once.
• Evolutionary rates are linked to generation time
• For vertebrates like man with long generation times, ,
evolutionary change is slow, massive mutations tend to
be eliminated quickly
Evolution?
• Natural selection favors organisms that
have traits that are “good enough”; and
usually acts on just a few characteristics
– Extremes are rarely selected for
Many important life forms have
short generation times
• HIV: the ultimate evolver
• Resistance in bacteria