Lecture 19 : Natural Selection Flashcards
What do Evolutionary processes bring about changes in allele frequencies in populations
what do evolutionary processes include?
- Natural Selection
- Mutation
- Migration
- Genetic Drift
Explain NATURAL SELECTION: 3
- Takes place when there is DIFFERENTIAL REPRODUCTION OF GENOTYPES
- Alleles associated with greater survival or reproductive success will increase in frequency the next generation
- Unique among evolutionary forces in that it promotes ADAPTATION
What are adaptations?
Properties of living things that enable them to survive and reproduce in nature.
Response to selection is determined by variation in fitness,
What does fitness mean?
Fitness (W) = the relative reproductive success of a genotype. Range 0 to 1.
Selection Coefficient
- a related variable is the SELECTION COEFFICIENT (s), which is the relative selective intensity against a genotype.
- it is equal to 1- W
look at the examples
The general selection model
look at the model and the example
Types of Natural Selection and its Results
look at the table and the example
Rates of change due to natural selection depend on: (3) ?
- THE INTENSITY OF SELECTION
- THE DOMINANCE RELATIONS AMONGST THE GENOTYPES - under directional selection, dominant alleles will increase much more rapidly than recessive alleles
- THE ALLELIC FREQUENCIES - when the frequency of a deleterious recessive allele is high, the change in allele frequency each generation is high.
What is The mutation-selection balance? (2)
What are the equations for
1. For selection acting on the recessive allele, the frequency equilibrium is
- For selection acting on a dominant allele, the frequency at equilibrium is — q = u/s
- Mutation and selection act as opposing forces on detrimental alleles.
- Mutation and natural selection will eventually reach an EQUILIBRIUM (gain by mutation = loss by selection)
—> For selection acting on the recessive allele, the frequency equilibrium is — q = square root u/s
—> For selection acting on a dominant allele, the frequency at equilibrium is — q = u/s
Classic Examples of Natural selection acting on a single locus? (3)
- Industrial melanism in the pepper moth
- sickle cell anemia in humans
- shell polymorphisms in Cepaea nemoralis
Explain Industrial melanism in the peppered moth (4)
- before 1850 dark (melanic) forms of the peppered moth (Biston bistularia) were very rare in Great Britain ( approx 1 percent or less)
- By 1900, the frequency of the dark form was 90 percent or more
- since 1950 (after air pollution controls were implemented) the frequency of the dark forms have decreased
- These patterns have been observed in several species in the UK and North America.
Explain THE OBSERVATION UNDERSTANDINGS OF Industrial melanism in the peppered moth (3)
- before 1850 the LIGHTY form was favoured by natural selection because they could camouflage themselves on trees and avoid bird predation.
2, after the industrial revolution, soot covered the lichens on the tree trunks, creating a selective advantage for the dark form.
- after pollution controls were initiated lichens against covered trees and the Light form was favoured again.
Explain Sickle cell anaemia (4)
- Individuals with sickle cell anemia have abnormally shaped red blood cells and are unable to transport O2 efficiently (die young)
- Disease is caused by a recessive allele called S. The normal allele is Designated A. Heterozygotes do not have the disease, but their red blood cells are sickle-shaped under low oxygen conditions.
- expect strong natural selection against the S allele.
- frequency of S allele up to 0.17 in central Africa
Explain THE OBSERVATION UNDERSTANDINGS OF Sickle cell anemia: HOW CAN THE HIGH FREQUENICES OF THE S ALELLE BE EXPLAINED? (3)
Calculations?
Heterozygote Superiority ?
- Children with sickle cell trait (SA heterozygotes) have significantly fewer infections of the malaria parasite than normal (AA) children.
- When the parasite enters a red blood cell of an AS individual the cell dies, killing the parasite
- Heterozygote superiority: the heterozygote has a fitness advantage over homozygotes in environments where malaria is common.
Genotype AA AS. SS
Phenotype 1- S(1) 1. 1 - S(2)
What/Explain Shell polymorphisms in Cepaea nemoralis (3).
- Populations are highly polymorphic for shell colour and banding
- Each trait determined by a single locus
- relative frequencies of morphs vary between sites
- association of shell morphs with habitat
eg. wooded areas have more dark and unbanded shells (most cryptic)
