Number of Individuals
in the Population with
Number of Allele A Con-
tributed to The Gene Pool by that Genotype
Number of Allele a Contributed to the Gene Pool by that Genotype
Let the letter p stand for the frequency of allele A. Let the letter q stand for the frequency of allele a. We can calculate
p and q as follows:
• p = number of A alleles/total number of alleles = 140/200 = 0.7
• q = number of a alleles/total number of alleles = 60/200 = 0.3
Notice that p + q = 1.
The Hardy-Weinberg Theorem
It shows that allele frequencies do not change if Certain characteristics are met.The conditions for equilibrium are:
- No new mutations are Occurring, therefore no new alleles being created.
- There is no migration.
- The population is very Large.
- Mating is random, this Means that individuals do not choose mates based on genotypes.
- There is no natural Selection.
When the genotype remains constant, genotype Frequencies can be expressed in terms of allele frequencies
Forces of Evolution
There are 4 factors that cause allele frequencies to Change: mutation, gene flow, genetic drift, and natural selection.
●Mutation: It created new Genetic variation in a gene pool, it is how all new alleles arise. The Mutations that matter occurs in gametes, these can be passed to the offspring. Mutations alone do not have much effect on allele frequencies, but they provide The genetic variation needed for other forces to act.
●Genetic drift: Is a random Change that occurs in a small population. They are 2 conditions which genetic Drift occurs:
●Bottleneck effect: It occurs When a population suddenly gets smaller. By chance allele frequencies of the Survivors may be different from the original population.
●Founder effect: It is when a Few individuals start, or found, a new population. By chance, allele Frequencies of the founders may be different from allele frequencies of the Population they left.
●Natural selection: It occurs When there are difference in fitness among members of a population. As a result Some individuals pass more genes to the next generation. This causes allele Frequencies to change.
100% normal hemoglobin
Somewhat reduced fitness because of No resistance to malaria
Enough normal hemoglobin to prevent Sickle-cell anemia
Highest fitness because of Resistance to malaria
100% abdominal hemoglobin, causing Sickle-cell anemia
Greatly reduced fitness because of Sickle-cell anemia.
This is how natural selection can keep a harmful Allele in a gene pool.
●The allele (S) for Sickle-cell anemia is a harmful autosomal recessive. It is caused by a mutation In the normal allele (A) for hemoglobin (a protein on red blood cells).
●Malaria is a deadly tropical Disease.
●Heterozygotes (AS), with the Sickle-cell allele are resistant to malaria. Therefore, they are more likely to Survive and reproduce. This keeps the S allele in the gene pool.
This example shows that fitness depends on phenotypes And also on the environment.
There are 3 ways natural selection can affect the Phenotype
●Stabilizing selection: Occurs When phenotypes at both extremes of the phenotypic are selected against.
●Directional selection: Occurs When one of the two extremes is selected.
●Disruptive selection: Occurs When phenotypes in the middle of the range are selected against.