Evolution of Populations
Genes and Variation
• Darwin and Mendel were
unaware of each others work. • Neither could fully develop • Neither could fully develop
their theories.
• In the 1930’s scientists
Variation and Gene Pools
• Genetic variation is studied in populations.
• Population: a group of the same species that can interbreed.
• A population shares a
Sample Population 48% heterozygous black Sample Population 48% heterozygous black Sample Population 48% heterozygous black • A population shares a
common group of alleles called a gene pool.
– A gene pool is all of the different genes that exist in a population.
Variation and Gene Pool, cont.
• The relative frequency of an allele is the number of times that a gene occurs in a gene pool.
– This is expressed as a percentage.
(40% Black, 60% Brown)
Frequency of Alleles
allele for brown fur
allele for black fur
Frequency of Alleles
allele for brown fur
allele for black fur
Frequency of Alleles
allele for brown fur
allele for black fur
(40% Black, 60% Brown)
• Evolution is caused by any
change in the relative frequency of alleles in a population.
– So evolution would be in action if the relative frequency
Sources of Variation
• Mutations: changes in the DNA sequence
– Caused by error in replication, radiation, chemicals in the environment.
– Only some mutations change the phenotype and affect fitness.
• Gene Shuffling: results from sexual
• Gene Shuffling: results from sexual reproduction (egg and sperm meet)
– 23 pairs of chromosomes can produce 8.4 million different gene combinations.
– Crossing over causes differences in genes. – Gene shuffling does not change the relative
Single Gene vs. Polygenic Traits
• The number of phenotypes produced for a single trait
depends on how many genes control the trait.
• Single Gene Trait: controlled F
re q u e n c y o f P h e n o ty p e (% ) 100 80 60 40 20 0 F re q u e n c y o f P h e n o ty p e (% ) 100 80 60 40 20 0 F re q u e n c y o f P h e n o ty p e (% ) 100 80 60 40 20 0
• Single Gene Trait: controlled by a single gene (2 alleles)
– Expressed in a bar graph • Polygenic Trait: traits
controlled by two or more
genes- offers a great deal of variation.
– Expressed in a bell shaped curve
F re q u e n c y o f P h e n o ty p e Phenotype (height) 0
Widow’s peak No widow’s peak
Phenotype
0
Widow’s peak No widow’s peak
Phenotype
0
Widow’s peak No widow’s peak
Natural Selection on a
Single Gene Trait
• Natural selection can act on a single gene trait that can change the allele frequency in a
population and lead to evolution.
Natural Selection on
Polygenic Traits
•
Natural selection can affect the
distribution of phenotypes in 3 ways:
1. Stabilizing Selection
• Individuals at the center have the greatest
fitness.
Key
Low mortality, high fitness
Stabilizing Selection
Birth Weight
Selection against both extremes keep
curve narrow and in same
place.
2. Disruptive Selection
• Both ends have greater fitness than the
middle.
Disruptive Selection Disruptive Selection
Largest and smallest seeds become more common.
3. Directional Selection
• Individuals at one end have greater fitness
than the middle or other end.
Directional Selection
Key
Low mortality,
Directional Selection
Food becomes scarce.
Low mortality, high fitness
Genetic Drift
•
Genetic Drift: the random change in allele
frequency.
– Occurs in small populations that break away from a larger group.
– Caused by individuals entering and exiting the – Caused by individuals entering and exiting the
populations.
• This adjusts the allele frequencies in the gene pool and results in a change from the original group’s gene pool.
– Ex. Founder effect: When the allele frequency changes as a result of the migration of a small group.
Evolution vs. Genetic Equilibrium
•
Hardy-Weinberg principle
– States that the allele frequency will stay in genetic equilibrium (no evolution is
occurring) if certain conditions are met.
1. Random mating 1. Random mating
2. Large population (so no genetic drift) 3. No movement in and out of the
population 4. No mutation
Speciation
•
Speciation:
The formation of a new
species.
– Species: a group of organisms that breed
together and produce fertile offspring. together and produce fertile offspring.
Reproductive Isolation
results from
which include
Reproductive Isolation
Isolating mechanisms
Behavioral isolation Geographic isolation Temporal isolation
produced by produced by produced by
which result in
which result in
Behavioral isolation Geographic isolation Temporal isolation
Behavioral differences Physical separation Different mating times
Independently evolving populations
