1. Evolution of Populations
Chapter 23.
Evolution of Populations

2. Bent Grass on toxic mine site
Populations evolve
Natural selection acts on individuals
differential survival
“survival of the fittest”
differential reproductive success
who bears more offspring
Populations evolve
genetic makeup of population changes over time
favorable traits (greater fitness) become more common
Bent Grass growing on mine tailings; only individuals tolerant of toxic heavy metals will grow from the seeds blown in from nearby field
Bent Grass on toxic mine site

3. Individuals DON’T evolve!!!

4. Not every mutation has a visible effect.
Mutation & Variation
Mutation creates variation
new mutations are constantly appearing
Mutation changes DNA sequence
changes amino acid sequence?
changes protein?
change structure?
change 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.

5. Sex & Variation Sex spreads variation
one ancestor can have many descendants
sex causes recombination
offspring have new combinations of traits = new phenotypes
Sexual reproduction recombines alleles into new arrangements in every offspring

6. Variation impacts natural selection
Natural selection requires a source of variation within the population
there have to be differences
some individuals must be more fit than others

7. Changes in populations
Evolution of populations is really measuring changes in allele frequency
all the genes & alleles in a population = gene pool
Factors that alter allele frequencies in a population
natural selection
genetic drift
founder effect
bottleneck effect
gene flow

8. Natural selection
Natural selection adapts a population to its environment
a changing environment
climate change
food source availability
new predators or diseases
combinations of alleles that provide “fitness” increase in the population

9. Genetic drift Effect of chance events founder effect bottleneck
small group splinters off & starts a new colony
bottleneck
some factor (disaster) reduces population to small number & then population recovers & expands again
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.

10. Founder effect
When a new population is started by only a few individuals
some rare alleles may be at high frequency; others may be missing
skew the gene pool of new population
human populations that started from small group of colonists
example: white people colonizing New World
Small founder group, less genetic diversity than Africans
All white people around the world are descended from a small group of ancestors
100,000 years ago
(Chinese are white people!)

11. Distribution of blood types
Distribution of the O type blood allele in native populations of the world reflects original settlement
South & Central American Indians were nearly 100% type O for the ABO blood system. Since nothing in nature seems to strongly select for or against this trait, it is likely that most of these people are descendants of a small band of closely related "founders" who also shared this blood type

12. Distribution of blood types
Distribution of the B type blood allele in native populations of the world reflects original migration
The global frequency patterns of the type B blood allele: Note that it is highest in central Asia and lowest in the Americas and Australia. However, there are relatively high frequency pockets in Africa as well. Overall in the world, B is the rarest ABO blood allele.

13. Out of Africa Likely migration paths of humans out of Africa
According to the "Out of Africa" theory, modern humans appeared as a single African species nearly 100,000 years ago, then spread throughout the world (K.Wong, Is Out of Africa Going Out the Door?, Scientific American 281(2), August 1999).
Many patterns of human traits reflect this migration

14. Bottleneck effect
When large population is drastically reduced by a disaster
famine, natural disaster, loss of habitat…
loss of variation by chance
alleles lost from gene pool
narrows the gene pool

15. Cheetahs All cheetahs share a small number of alleles 2 bottlenecks
less than 1% diversity
as if all cheetahs are identical twins
2 bottlenecks
10,000 years ago
Ice Age
last 100 years
poaching & loss of habitat

16. Conservation issues
Bottlenecking is an important concept in conservation biology of endangered species
loss of alleles from gene pool
reduces variation
reduces ability to adapt
at risk populations

17. Gene flow Population spread over large area
migrations = individuals move from one area to another
sub-populations may have different allele frequencies
Migrations cause genetic mixing across regions = gene flow
new alleles are moving into gene pool
reduce differences between populations

18. 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?

19. Any Questions??

20. Measuring Evolution of Populations
Chapter 23.
Measuring Evolution of Populations

21. 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

22. Evolution of populations
Evolution = change in allele frequencies in a population
hypothetical: what would it be like if allele frequencies didn’t change?
non-evolving population
very large population size (no genetic drift)
no migration (movement in or out)
no mutation (no genetic change)
random mating (no sexual selection)
no natural selection (no selection)

23. Hardy-Weinberg equilibrium
Hypothetical, non-evolving population
preserves allele frequencies
Serves as a model
natural populations rarely in H-W equilibrium
useful model to measure if forces are acting on a population
measuring evolutionary change
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

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

25. Hardy-Weinberg theorem
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 100%, so:
p2 + 2pq + q2 = 1 BB Bb bb

26. 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!

27. Using Hardy-Weinberg equation
p2=.36
2pq=.48
q2=.16
Assuming H-W equilibrium BB Bb bb
Null hypothesis
p2=.20
p2=.74
2pq=.64
2pq=.10
q2=.16
q2=.16
Sampled data 1:
Hybrids are in some way weaker.
Immigration in from an external population that is predomiantly homozygous B
Not 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?

28. How do allele frequencies change?
Think of all the factors that would keep a population out of H-W equilibrium!

29. Real world application of H-W
Frequency of allele in human population
Example:
What % of human population carries allele for PKU (phenylketonuria)
Should you screen prospective parents?
~ 1 in 10,000 babies born in the US is born with PKU
results in mental retardation, if untreated
disease is caused by a recessive allele
PKU = homozygous recessive (aa)
PKU (phenylketonuria) is a rare, inherited metabolic disease that results in mental retardation and other neurological problems when treatment is not started within the first few weeks of life. When a very strict diet is begun early and well-maintained, affected children can expect normal development and a normal life span.
The disease arises from the absence of a single enzyme (phenylalanine hydroxylase). This enzyme normally converts the essential amino acid, phenylalanine, to another amino acid, tyrosine. Failure of the conversion to take place results in a buildup of phenylalanine. Through a mechanism that is not well understood, the excess phenylalanine is toxic to the central nervous system and causes the severe problems normally associated with PKU.
PKU is carried through an autosomal recessive gene. The incidence of carriers in the general population is approximately one in fifty people, but the chance that two carriers will mate is only one in Carrier tests are available only through PKU treatment programs.

30. H-W & PKU disease
frequency of homozygous recessive individuals q2 (aa) = 1 in 10,000 =
frequency of recessive allele (q): q = √ = 0.01
frequency of dominant allele (p):
p (A) = 1 – 0.01 = 0.99
frequency of carriers, heterozygotes:
2pq = 2 x (0.99 x 0.01) = = ~2%
~2% of the US population carries the PKU allele 300,000,000 x .02 = 6,000,000 people

31. Any Questions??