back Welcome to IB 150...
Channel 12, 8 PM Sept. 26 & 27 "he not busy being born is busy dying"
Lecture 13: Microevolution
Assigned Readings:
Putting genetic drift and mutation together allele gain and loss; infinite alleles model; "neutral evolution" Predicting genotype frequencies from allele frequencies - the Hardy-Weingberg equilibium random mating Forensic genetics microsatellites
Ch. 23 (rest of chapter)
IB 150 EXAM ONE INFORMATION-Fall 2005 WHEN: THURSDAY, September 29th , 7-9 PM LOCATIONS-You must go to your assigned room.
1 2 3 4 BRING TO EXAM 1 2
IF YOUR TA IS ROSE or DEVI = GO TO 314 ALTGELD HALL IF YOUR TA IS BEN or ADAM = GO TO 112 GREGORY HALL IF YOUR TA IS DOMINIC or LIZ= GO TO 228 NATURAL HISTORY IF YOUR TA IS MONI, MAHESHI, or BETH =GO TO 213 GREGORY HALL
Sharpened PENCILS NON-PROGARAMMABLE CALCULATOR
IT IS PAST THE DEADLINE TO SIGN UP FOR THE CONFLICT
A Hardy-Weinberg Equilibrium is just like a Punnett square except for a population rather than a single pair cross. Gamete frequencies are the same as the allele frequencies.
Sperm with allele A p p2 AA
Sperm with allele a q pq Aa q2 aa
Ova with allele A
p
Ova with allele a
q
pq Aa
Fig. 23.5 Note: exam questions only on general issues, calculations on next exam
If the genotype frequencies predicted from the allele frequencies turn out to be close to those observed in the population, the population is said to be in a Hardy-Weinberg Equilibrium. A perfect Hardy-Weinberg equilibrium is seen only if there is no genetic drift, no mutation, no subdivision of populations, no natural selection, etc., but it is in fact rather robust to violations of these conditions.
Using H-W to determine frequencies of carriers in a population (must assume at least approximately random mating)
Phenylketonuria = recessive Mendelian trait 1/10,000 children born in the US have the disease What proportion of the population are carriers? q2 = 0.0001 2pq = 0.0198 q = 0.01 p = 0.99 ~2% of the population are carriers
The Hardy-Weinberg Equilibrium is important because: 1. It helps us understand human genetic diseases - (e.g., why are carriers so much more common than affected people?) 2. It is robust to many assumptions. 3. It is basic to a lot of more complex population genetics. 4. It is used in forensic genetics
Forensic genetics uses variation such as microsatellite variation differing numbers of short DNA repeats such as AC or AG or ATG. The replication enzymes tend to make frequent mistakes once a few repeats are in place, adding more repeats. Deletions also occur from time to time, so microsat loci do not grow infinitely long. One strand of a DNA molecule shown repeats length
...AATGCGTTAGCACACACGCTAGTACCGCATAG... 3 32 ...AATGCGTTAGCACACACACGCTAGTACCGCATAG... 4 34 ...AATGCGTTAGCACACACACACGCTAGTACCGCATAG... 5 36
These are 3 "alleles" of a microsatellite locus.
VoceroAkbani et al. 1996. Mapping human telomere regions with YAC and P1 clones: Chromosome specific markers for 27 telomeres including 149 STSs and 24 polymorphisms for 14 proterminal regions. Genomics 36:492506 .
Microsatellite variation - differing numbers of short DNA repeats an actual example with a pedigree. The DNA is run in an electrophoresis gel (bottom towards top), and the bands represent DNA fragments of a given size like 150 base pairs, etc.)
Examples of calculations of genotype frequencies using microsatellite variation - the more alleles, the more genotypes, and the lower the frequency will be for each. Note: allele frequencies are all the same in each case just to simplify calculations! Try the Punnett square yourself! Forensics uses Hardy-Weinberg Equilibrium (HWE)!
(Alleles) All. p HWE genotype frequencies 11 12 22 2 0.5 0.25 0.50 0.25 11 12 13 22 23 33 3 0.33 0.11 0.22 0.22 0.11 0.22 0.11 11 12 13 14 22 23 24 33 44 4 0.25 0.06 0.13 0.13 0.13 0.06 0.13 0.13 0.13 0.06
Frequencies become even lower when multiple loci (genes) are considered. If the genes are unlinked, you can determine the genotype frequencies for the two-locus genotypes by just multiplying the single locus HWE genotypes. Here the allele frequencies for the A gene are freq. A1 = p = 0.5, A2 = q = 0.5, and for the B gene are B1 = r = 0.4, B2 = s = 0.6.
Thus, with enough alleles and enough loci, the frequency (probability) of any genotype is very small - so we are all unique!
Lecture 13: Microevolution
Assigned Readings:
Putting genetic drift and mutation together allele gain and loss; infinite alleles model; "neutral evolution" Predicting genotype frequencies from allele frequencies - the Hardy-Weingberg equilibium random mating Forensic genetics microsatellites
Ch. 23 (rest of chapter)
Lecture 13: Natural selection
Assigned Readings:
Natural selection fitness, discrete traits, continuous traits Patterns of natural selection directional, balancing, stabilizing, disruptive, diversifying Sexual selection secondary sexual characteristics, intersexual, intrasexual selection Guppies natural versus cultivated strains, field observations of effects of predators on male color, experiments on balance of natural and sexual selection, field experiments
Ch. 23
Natural selection and the influenza virus influenza; surface proteins; immune response; 1918 flue epidemic; antigenic "drift"; antigenic shifts
Natural selection - examples and details
Anytime there is unequal survival and/or unequal reproduction of genotypes, there is selection. It results in changes in the frequencies of alleles. Fitness = a quantitative measure of an individual organisms ability to survive and reproduce.
There are 3 basic patterns of natural selection (same 3 occur with artificial selection also):
They are basically the same whether one is studying selection on discrete characters - e.g., white flowers vs purple flowers or quantitative characters - (polygenic or multifactorial characters) - e.g., height, weight, etc.
A useful term - polymorphism "A population is said to be polymorphic for a character if two or more distinct morphs are each represented in high enough frequency to be readily noticeable". (p 453)
Selection on discrete traits
fitness AA
Aa
aa
AA
Aa
aa
AA
Aa
aa
Directional selection: AA has higher fitness than Aa and aa Outcome: allele A replaces a
Diversifying or disruptive selection: Aa has a lower fitness than AA and aa Outcome: alleles A and B can be maintained (not fixed by drift). Special conditions.
Balancing or stabilizing selection: Aa has higher fitness than AA and aa Outcome: alleles A and B can be maintained (not fixed by drift)
Assuming independent assortment there are 23 = 8 gamete type (ABC, Abc, AbC, Abc, aBC, aBc, abC, abc) which can be combined in 64 ways, to produce 27 genotypes, and in this example, 7 phenotypes.
Selection on continuous traits - look back at normal curve. Fig. 14.12
aabbcc
Aabbcc - 2 ways aaBbcc - 2 ways aabbCc - 2 ways
Fig. 23.11 - a "common garden" experiment tells us if the continuous variation we see has a genetic basis.
Fig. 23.12 patterns of selection on a continuous trait.
Dark individuals have high fitness
Dark and light individuals have high fitness
Intermediate individuals have high fitness
Figure 23.16x1 Sexual selection and the evolution male of appearance
Figure 23.16x2 Male peacock
Another type of selection is called sexual selection. This often involves secondary sexual characteristicts. In one common type of sexual selection (intersexual selection), females choose males that have the most conspicuous "ornaments". In another (intrasexual selection), males fight for access to females.
In intersexual selection, natural selection is still really occurring - males that can survive when burdened with lots of ornaments must have good genes for running ability, flight ability, etc.
Examples of natural and sexual selection: Guppies (Poecillia reticulata) are small tropical American fish that have become good examples of microevoluton because 1. Spectacular success in artificial selection. 2. Observations on the interaction of natural and artificial selection "in the field" (nature). 3. Experimental replication of natural and artificial selection in the lab. 4. Experimental studies on the interaction of natural and artificial selection "in the field" (nature).
From Dr. K. Hughes web site at UIUC. The three guppies in the middle are females, which show no pronounced color variation. The males on either side are representative of the enormous variation in this species - effectively all male have a different appearance.
Domesticated guppies
Above falls, predators only of baby guppies (killifish).
Below falls, predators of adult guppies (pike cichlid).
Field observation: male size and coloration differs even in the same stream, and seems to be related to the presence or absence of predators of adult guppies. Can this observation be acted upon with experiments?
Yes. Lab experiments (in above ground swimming pools in a greenhouse at Princeton) showed that the amount of color on male guppies increases due to sexual selection females prefer to mate with brightly colored males. Adding killifish (prey only on baby guppies) does not affect the increase in male brightness. But adding a pike cichlid to a pool results in the fish evolving back to the appearance of "below-falls" fish dul colored males. Can this experiment be done "in the field"?
Yes, in one direction (brighter males). Small streams can be found (in Trinidad) which do not have guppies or pike cichlids (although even the smallest streams have killifish) Moving guppies from a "below-falls" population (dull males) to an "empty stream" results in males evolving to be brighter. So both lab and field experiments have corroborated the field observations about the balance of sexual and natural selection in guppies.
Fig. 22.12 Other features of guppies have also been found to be subject to experiments on natural selection.
Natural selection and medicine
An example using the influenza virus.
The influenza virus
Causes the "flu" symptoms we all know, but is not the only virus to cause runny noses, fevers, and all the rest of the symptoms. Is an RNA virus that replicates itself in animal cells. Like all viruses, it is a complete parasite, and cannot replicate unaided.
The influenza virus in the electron microscope. Note the projections on the outside, and in the photo on the left, indications of structures inside the capsule.
The projections are two types of proteins, haemaglutinin and neuramindase, which are antigens - molecular sites to which the human body can make antibodies.
The other player in this story is the human immune system. It is capable of making antibodies to various chemicals that are introduced into the body. Such chemicals are called antigens. Viral coat proteins are important antigens. A key fact is that it takes about 2 weeks for humans to make a new antibody. Once a human has made an antibody, it can be made more rapidly the second time (antibody "memory").
Influenza coat protein evolution:
The human immune system usually (given 2 weeks) destroys a virus infection (but often not before it is transmitted to another person). Eventually, almost all of the people who get the virus become immune and second infections fail. But the influenza virus is constantly giving rise to new mutations, with different amino acid sequences in coat proteins. Such a new sequence (surviving in a few people) can re-infect many people the next flu season because antibodies cannot be made rapidly against it. Thus there is continual evolution of new viral sequences. This process is called, con...