1. Evolution of Populations
(Ch. 23)
Doonesbury - Sunday February 8, 2004

2. One species, two populations
MAP
AREA
ALASKA
CANADA
Beaufort Sea
Porcupine
herd range
NORTHWEST
TERRITORIES
Fairbanks
Fortymile
herd range
Whitehorse
ALASKA
YUKON

3. Presence of lactate dehydrogenase
Populations evolve
Natural selection acts on individuals
differential survival
differential reproductive success
Populations evolve
genetic makeup of population changes over time
favorable traits (greater fitness) become more common
Presence of lactate dehydrogenase
The Mummichog (Fundulus heteroclitus heteroclitus) is a small killifish found in the eastern United States. It is capable of tolerating highly variable salinity and temperatures, and is found in estuaries and saltmarshes as well as less salty waters.
A year-round resident of tidal creeks and wetlands, this brownish-green saltwater minnow may reach a maximum length of 5 inches. Its Indian name means "they go in great numbers." It is also known as the common killifish.
A hardy fish, the mummichog is an important food source for larger fish and is often used as bait. The mummichog also has been used as a natural method of mosquito control in marsh ponds and ditches. It has been reported that one mummichog can eat as many as 2,000 mosquito larvae ("wrigglers") a day. The mummichog also feeds on other insects, small fish, crustaceans, and plant material.
Because of the extreme hardiness of the species, it is sometimes the only species found in severely polluted and oxygen-deprived streams, such as the Hackensack River and the Arthur Kill in New Jersey during the height of the water pollution problem in the United States. In 1973 the Mummichog became the first fish in space when carried on Skylab 3 as part of the biological experiments package later space missions by the U.S., such as Bion 3, have also carried Mummichog.
Mummichog

4. Changes in populations
Bent Grass on toxic mine site
Pocket Mice in desert lava flows
Pesticide molecule
Insect cell membrane
Target site
Resistant target site
Bent Grass growing on mine tailings; only individuals tolerant of toxic heavy metals will grow from the seeds blown in from nearby field
Target site
Decreased number of target sites
Insecticide resistance

5. Individuals DON’T evolve…
Individuals survive or don’t survive…
Individuals DON’T evolve…
Individuals are selected
Populations evolve
Individuals reproduce or don’t…
Populations evolve!

6. Fitness Survival & Reproductive success
Body size & egg laying in water striders
Survival & Reproductive success
individuals with one phenotype leave more surviving offspring

7. Variation & natural selection
Variation is the raw material for natural selection
there have to be differences within population
some individuals must be more fit than others

8. Where does Variation come from?
Mean beak depth of parents (mm)
Medium ground finch 8 9 10 11
1977
1980
1982
1984
Dry year
Wet year
Beak depth
Beak depth of
offspring (mm)
Mutation
random changes to DNA
errors in gamete production
environmental damage
Sex
mixing of alleles
recombination of alleles
new arrangements in every offspring
new combinations of traits
spreads variation
offspring inherit traits from parent

9. 5 Agents of evolutionary change
Mutation
Gene Flow
Non-random mating
Genetic Drift
Selection

10. Not every mutation has a visible effect.
1. Mutation & Variation
Mutation creates variation
new mutations are constantly appearing
Mutation changes DNA sequence
changes amino acid sequence?
changes protein?
changes structure?
changes function?
changes in protein may change phenotype & therefore change fitness
Every individual has hundreds of mutations
1 in 100,000 bases copied
3 billion bases in human genome
But most happen in introns, spacers, junk of various kind
Not every mutation has a visible effect.
Some effects on subtle.
May just affect rate of expression of a gene.

11. 2. Gene Flow Movement of individuals & alleles in & out of populations
seed & pollen distribution by wind & insect
migration of animals
causes genetic mixing across regions
reduce differences between populations

12. Are we moving towards a blended world?
Human evolution today
Gene flow in human populations is increasing today
transferring alleles between populations
Are we moving towards a blended world?

13. 3. Non-random mating
Sexual selection

14. 4. Genetic drift Effect of chance events founder effect Bottleneck
1 family has a lot of children & grandchildren therefore has a greater impact on the genes in the population than other families
Genghis Khan tracked through Y chromosome.
Warbler
finch
Tree finches
Ground finches

15. Conservation issues
Peregrine Falcon
Bottlenecking is an important concept in conservation biology of endangered species
loss of alleles from gene pool
reduces variation
reduces adaptability
Breeding programs must consciously outcross
Golden Lion Tamarin

16. 5. Natural selection
Differential survival & reproduction due to changing environmental conditions
climate change
food source availability
predators, parasites, diseases
toxins
combinations of alleles that provide “fitness” increase in the population
adaptive evolutionary change

17. Any Questions??

18. Measuring Evolution of Populations
(aka “Enter Math”)
Ch. 23

19. 5 Agents of evolutionary change
Mutation
Gene Flow
Non-random mating
Genetic Drift
Selection

20. Brief Terminology Review
Gene – determines a trait (ex. eye color) Allele – A variant of a gene (ex. brown eyes vs. blue eyes) All sexually reproducing organisms have 2 alleles for any trait. Dominant – An allele that will show a trait, regardless of the other other allele (ex. brown eyes) Recessive – An allele that will only show a trait if both alleles are recessive (ex. blue eyes)

21. New Terminology
Homozygous – any individual who has 2 copies of the same allele. Can be homozygous dominant or homozygous recessive. Heterozygous- any individual who has one copy of a dominant allele and one copy of a recessive allele. WILL SHOW THE DOMINANT TRAIT!

22. Populations & gene pools
Concepts
a population is a localized group of interbreeding individuals
gene pool is collection of alleles in the population
remember difference between alleles & genes!
allele frequency is how common is that allele in the population
how many A vs. a in whole population

23. Evolution of populations
Evolution = change in allele frequencies in a population
hypothetical: what conditions would cause allele frequencies to not change?
non-evolving population
REMOVE all agents of evolutionary change
very large population size (no genetic drift)
no migration (no gene flow in or out)
no mutation (no genetic change)
random mating (no sexual selection)
no natural selection (everyone is equally fit)

24. Hardy-Weinberg equilibrium
Hypothetical, non-evolving population
preserves allele frequencies
Serves as a model (null hypothesis)
natural populations rarely in H-W equilibrium
useful model to measure if forces are acting on a population
G.H. Hardy (the English mathematician) and W. Weinberg (the German physician) independently worked out the mathematical basis of population genetics in Their formula predicts the expected genotype frequencies using the allele frequencies in a diploid Mendelian population. They were concerned with questions like "what happens to the frequencies of alleles in a population over time?" and "would you expect to see alleles disappear or become more frequent over time?"
G.H. Hardy
mathematician
W. Weinberg
physician

25. Hardy-Weinberg theorem
Counting Alleles
assume 2 alleles = B, b
frequency of dominant allele (B) = p
frequency of recessive allele (b) = q
frequencies must add to 1 (100%), so:
p + q = 1 BB Bb bb

26. Hardy-Weinberg theorem
Counting Individuals
frequency of homozygous dominant: p x p = p2
frequency of homozygous recessive: q x q = q2
frequency of heterozygotes: (p x q) + (q x p) = 2pq
frequencies of all individuals must add to 1 (100%), so:
p2 + 2pq + q2 = 1 BB Bb bb

27. H-W formulas Alleles: p + q = 1 Individuals: p2 + 2pq + q2 = 1 B b BB

28. Using Hardy-Weinberg equation
population: 100 cats
84 black, 16 white
How many of each genotype?
q2 (bb): 16/100 = .16
q (b): √.16 = 0.4
p (B): = 0.6
p2=.36
2pq=.48
q2=.16 BB Bb bb
Must assume population is in H-W equilibrium!
What are the genotype frequencies?

29. Using Hardy-Weinberg equation
p2=.36
2pq=.48
q2=.16
Assuming H-W equilibrium BB Bb bb
Null hypothesis
p2=.20
p2=.74
2pq=.10
2pq=.64
q2=.16
q2=.16
Sampled data 1:
Hybrids are in some way weaker.
Immigration in from an external population that is predomiantly homozygous B
Non-random mating... white cats tend to mate with white cats and black cats tend to mate with black cats.
Sampled data 2:
Heterozygote advantage.
What’s preventing this population from being in equilibrium. bb Bb BB
Sampled data
How do you explain the data?
How do you explain the data?

30. Application of H-W principle
Sickle cell anemia
inherit a mutation in gene coding for hemoglobin
oxygen-carrying blood protein
recessive allele = HsHs
normal allele = Hb
low oxygen levels causes RBC to sickle
breakdown of RBC
clogging small blood vessels
damage to organs
often lethal

31. Sickle cell frequency High frequency of heterozygotes
1 in 5 in Central Africans = HbHs
unusual for allele with severe detrimental effects in homozygotes
1 in 100 = HsHs
usually die before reproductive age
Sickle Cell:
In tropical Africa, where malaria is common, the sickle-cell allele is both an advantage & disadvantage. Reduces infection by malaria parasite.
Cystic fibrosis:
Cystic fibrosis carriers are thought to be more resistant to cholera:
1:25, or 4% of Caucasians are carriers Cc
Why is the Hs allele maintained at such high levels in African populations?
Suggests some selective advantage of being heterozygous…

32. Malaria
Single-celled eukaryote parasite (Plasmodium) spends part of its life cycle in red blood cells 1 2 3

33. Heterozygote Advantage
In tropical Africa, where malaria is common:
homozygous dominant (normal)
die or reduced reproduction from malaria: HbHb
homozygous recessive
die or reduced reproduction from sickle cell anemia: HsHs
heterozygote carriers are relatively free of both: HbHs
survive & reproduce more, more common in population
Hypothesis:
In malaria-infected cells, the O2 level is lowered enough to cause sickling which kills the cell & destroys the parasite.
Frequency of sickle cell allele & distribution of malaria

34. Any Questions??

35. Review Questions

36. 1. In a random sample of a population of shorthorn cattle, 73 animals were red (CRCR), 63 were roan – a mixture of red and white (CRCr) – and 13 were white (CrCr). Estimate the allele frequencies of CR and Cr, and determine whether the population is in Hardy-Weinberg equilibrium.
CR , Cr ; because the population is large and a random sample was chosen, the population is in equilibrium.
CR 5 0.7, Cr 5 0.3; the genotype ratio is not what would be predicted from these frequencies and the population is not in equilibrium.
CR 5 0.7, Cr 5 0.3; the genotype ratio is what would be predicted from these frequencies and the population is in equilibrium.
CR , Cr ; the allele frequencies add up to greater than 1 and the population is not in equilibrium.
You cannot estimate allele frequency from this information.
Answer: c
Source: Taylor - Student Study Guide for Biology, Seventh Edition, Test Your Knowledge Question #12

37. 2. Genetic analysis of a large population of mink inhabiting an island in Michigan revealed an unusual number of loci where one allele was fixed. Which of the following is the most probable explanation for this genetic homogeneity? *
The population exhibited nonrandom mating, producing homozygous genotypes.
The gene pool of this population never experienced mutation or gene flow.
A very small number of mink may have colonized this island, and this founder effect and subsequent genetic drift could have fixed many alleles.
Natural selection has selected for and fixed the best adapted alleles at these loci.
The colonizing population may have had much more genetic diversity, but genetic drift in the last year or two may have fixed these alleles by chance.
Answer: c
Source: Taylor - Student Study Guide for Biology, Seventh Edition, Test Your Knowledge Question #20
Discussion Notes for the Instructor
The main discussion revolves around the causes of gene fixation. While many of the choices are possible, several are very unlikely. It is also a good question for pointing out that selection rarely leads to allele fixation in a large population
• Choice A: while this is possible the non-random mating would have to be very extreme to produce this result.
• Choice B: this by itself will not produce the results listed
• Choice C: given this is an island in Lake Michigan, this is the best choice
• Choice D: selection generally will not result in fixation.
• Choice E: this assumes that the initial population was large and then became smaller. Given the island location this is not the best explanation.

38. Which of the following statements is NOT true about genetic mutations?
Genetic mutations are always harmful
Mutations can occur when DNA molecules are copied
Mutations are the ultimate source of all variations in a population
Mutations that occur in the skin cells of parents can be passed to offspring
Mutations are the raw material that drives evolution.

39. Which of the following is NOT a component of Darwin’s theory of natural selection?
Mutations cause a significant amount of genetic variation
Evolution is a slow process that occurs over a long period of time
Variations among organisms are the basis by which organisms will or will not reproduce
Organisms who posses the most favorable variations have a higher comparative level of fitness
More individuals are born than can survive

40. Base your answers to the following questions on the choices below:
A. Founder effect
B. Adaptive Radiation
C. Gene Flow
D. Genetic Drift
E. Hardy-Weinberg Equilibrium
Occurs when a population undergoes a dramatic decrease in size.
Describes the introduction or removal of alleles when individuals enter or leave a population.
The term used to describe a theoretical, non evolving population.