MeasuringEvolution of Populations
Genetic variations in populationsGenetic variation and evolution are both studied in populations.
Because members of a population interbreed, they share a common group of genes called a gene pool.
Gene pools consist of all the genes, including the different alleles for each gene, that are present in a population
Allele Frequency – the number of times an allele occurs in a gene pool, compared to the total number of alleles in that pool for the same gene
EVOLUTION, IN GENETIC TERMS, INVOLVES A CHANGE IN THE FREQUENCY OF ALLELES IN A POPULATION OVER TIME!!!
Genetic Drift
In small populations, individuals that carry a particular allele may leave more descendant than other individuals, just by chance.
Over time, a series of chance occurrences can cause an allele to become more or less common in a population.
This kid of random change in allele frequency is called genetic drift. Tends to reduce genetic variation Tends to take place in smaller populations
Two different models for genetic drift: bottleneck effect & founders effect
Figure 23.7
CRCR
CRCW
CRCR
CWCW CRCR
CRCW
CRCW
CRCWCRCR
CRCR
Only 5 of10 plants
leaveoffspring
CWCW CRCR
CRCW
CRCR CWCW
CRCW
CWCW CRCR
CRCW CRCW
Only 2 of10 plants
leaveoffspring
CRCR
CRCR CRCR
CRCRCRCR
CRCR
CRCR
CRCR
CRCRCRCR
Generation 2p = 0.5q = 0.5
Generation 3p = 1.0q = 0.0
Generation 1p (frequency of CR) = 0.7q (frequency of CW) = 0.3
The Bottleneck Effect A sudden change in the environment (ex: natural
disaster) may drastically reduce the size of a population Wipes out a random part of the population
The gene pool may no longer be reflective of the original population’s gene pool
Originalpopulation
Bottleneckingevent
SurvivingpopulationFigure 23.8 A
(a) Shaking just a few marbles through the narrow neck of a bottle is analogous to a
drastic reduction in the size of a population after some environmental disaster. By chance, blue marbles are over-represented in the new
population and gold marbles are absent.
The Founder Effect
The founder effect Occurs when a few individuals become
isolated from a larger population Migrate to another area
Can affect allele frequencies in a population
5 Agents of evolutionary changeMutation Gene Flow
Genetic Drift Selection
Non-random mating
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
1. very large population size (no genetic drift)
2. no migration (no gene flow in or out)
3. no mutation (no genetic change)
4. random mating (no sexual selection)
5. no natural selection (everyone is equally fit)
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 measuring evolutionary change
W. Weinbergphysician
G.H. Hardymathematician
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
bbBbBB
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
bbBbBB
H-W formulas Alleles: p + q = 1
Individuals: p2 + 2pq + q2 = 1
bbBbBB
BB
B b
Bb bb
What are the genotype frequencies?What are the genotype frequencies?
Using Hardy-Weinberg equation
q2 (bb): 16/100 = .16
q (b): √.16 = 0.40.4
p (B): 1 - 0.4 = 0.60.6
q2 (bb): 16/100 = .16
q (b): √.16 = 0.40.4
p (B): 1 - 0.4 = 0.60.6
population: 100 cats84 black, 16 whiteHow many of each genotype?
population: 100 cats84 black, 16 whiteHow many of each genotype?
bbBbBB
p2=.36p2=.36 2pq=.482pq=.48 q2=.16q2=.16
Must assume population is in H-W equilibrium!
Must assume population is in H-W equilibrium!
Using Hardy-Weinberg equation
bbBbBB
p2=.36p2=.36 2pq=.482pq=.48 q2=.16q2=.16
Assuming H-W equilibriumAssuming H-W equilibrium
Sampled data Sampled data bbBbBB
p2=.74p2=.74 2pq=.102pq=.10 q2=.16q2=.16
How do you explain the data? How do you explain the data?
p2=.20p2=.20 2pq=.642pq=.64 q2=.16q2=.16
How do you explain the data? How do you explain the data?
Null hypothesis Null hypothesis
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
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
Why is the Hs allele maintained at such high levels in African populations?Why is the Hs allele maintained at such high levels in African populations?
Suggests some selective advantage of being heterozygous…Suggests some selective advantage of being heterozygous…
Malaria Single-celled eukaryote parasite (Plasmodium) spends part of its life cycle in red blood cells
Single-celled eukaryote parasite (Plasmodium) spends part of its life cycle in red blood cells
1
2
3
Heterozygote Advantage In tropical Africa, where malaria is common:
homozygous dominant (normal) die of malaria: HbHb
homozygous recessive die of sickle cell anemia: HsHs
heterozygote carriers are relatively free of both: HbHs
survive 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.
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