A change in allele frequency

Q: How do scientists know when this occurs?  A: They compare it to a non-changing population  = Ideal population (like a “perfect” car… it only exists in a showroom)

5 Characteristics of an “Ideal Population”  Population is very large  Large depends on the population, but thousands or hundreds are assumed  Mating is random  No phenotype has a greater chance of mating

 No mutation of alleles  No immigration or emigration  No movement into or out of population  No selection occurs  No survival advantage goes to one phenotype over another

If these 5 conditions are met…  There is no change in allele/genotypic frequencies…  and no evolution  When these rules are broken, microevolution is said to be occurring.

For Hardy-Weinberg Equilibrium:  p stands for the frequency of the dominant allele Ex. “A”  q stands for the frequency of the recessive allele. Ex. “a”  In H-W, or 100% of the alleles p + q = 1

 p 2 = frequency of homozygous dominant genotype Ex. “AA”  q 2 = frequency of homozygous recessive genotype Ex. “aa”  2pq = frequency of heterozygous genotype Ex. “Aa” p 2 + 2pq + q 2 = 1, or 100%

Example 1:  Approximately 9% of Americans of African descent suffer from sickle cell anemia, which is inherited as a recessive trait.  What is the frequency of the sickle cell allele?  Approximately what percentage of this sub- population carries the sickle cell allele?

Example 2:  Cystic fibrosis is known to occur as a recessive trait in human populations. In a genetic study, the frequency of the recessive allele for a population was found to be 2.0%.  What percentage of the population would be expected to exhibit Cystic Fibrosis?  What percentage of the population would be normal, but carry a CF allele?

What Hardy-Weinberg taught us…  1. We can track changes in genes  2. If genes change, a population is evolving

How does this happen? Natural Selection  Types:  A. Stabilizing Selection  B. Directional Selection

 Disruptive selection

 Not all changes in genes are due to natural selection, though…

Other ways to change gene frequencies…

Mutation a) Point b) Duplication

Sexual recombination

Genetic Drift  Change in allele frequencies that is due to chance  More common in small populations  Several types of genetic drift…

A. Founder effect  Small sample of a population settles in a new location (migrates)

B. Bottleneck Effect  Population declines to a very low number due to “event” then rebounds…  New “composition” to the population

2. Gene Flow  Genes “move” into or out of a population…