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