Evolutionary Change in Populations

1. Evolutionary Change in Populations Chapter 18

2. Frequencies • Gene pool – all the alleles in a population • Genotype frequency – total = 1.0 (100%) • homozygous dominant • heterozygous • homozygous recessive • Phenotype frequency – total – 1.0 (100%) • dominant • Recessive • Allele frequency • A or a • Each individual has 2: AA, Aa, or aa

3. Hardy-Weinberg Principle A way of measuring if a population is evolving Non-evolving populations ‘fit’ the principle – they are in genetic equilibrium Five conditions must be met: • random mating • no net mutations • large population size • no migration • no natural selection In other words: no evolutionary change

4. Hardy-Weinberg Principle… • Shows that the process of inheritance by itself does not cause changes in allele frequencies • Explains why dominant alleles are not necessarily more common than recessive ones • Seldom occurs in the natural world but instead provides us with a model for understanding evolution in sexually reproducing populations

5. Hardy-Weinberg Principle… p = frequency of dominant allele q = frequency of recessive allele Therefore: p + q = 1 From this we know that: p = 1 – q (frequency of dominant allele) q = 1 – p (frequency of recessive allele)

6. Hardy-Weinberg Principle… Each individual has 2 alleles, therefore: p2 + 2 pq + q2 = 1 {AA + 2Aa + aa = 1}remember, there are 2 different ways an individual can inherit Aa What the heck does this mean??

7. Problem #1 • You have sampled a population in which you know that the percentage of the homozygous recessive genotype (aa) is 36%. Using that, calculate the following: • The frequency of the aa genotype • The frequency of the a allele • The frequency of the A allele • The frequencies of the genotypes of AA and Aa • The frequencies of the two possible phenotypes if A is completely dominant over a

8. Problem #2 • Sickle-cell anemia is an interesting genetic disease. Normal homozygous individuals (SS) are more susceptible to malarial infection. Those with the sickle-cell trait (ss) often die due to the condition. Heterozygous individuals (Ss) tend to survive better than either of the homozygous conditions. If 9% of an African population is born with a severe form of sickle-cell anemia (ss), what percentage of the population will be more resistant to malaria because they are heterozygous (Ss) for the sickle-cell gene?

9. Problem #3 • There are 100 students in a class. Ninety-six did well in the course whereas four blew it totally and received a grade of F. In the highly unlikely event that these traits are genetic rather than environmental, if these traits involve dominant and recessive alleles, and if the 4% represent the frequency of the homozygous recessive condition, calculate the following: • The frequency of the recessive allele • The frequency of the dominant allele • The frequency of heterozygous individuals

10. Problem #4 • Within a population of butterflies, the color brown (B) is dominant over the color white. And 40% of the butterflies are white. Given this simple information, calculate the following: • The percentage of butterflies in the population that are heterozygous • The frequency of homozygous dominant individuals

11. Problem #5 • A rather large population of Biology instructors have 396 red-sided individuals and 557 tan-sided individuals. Assume that red is totally recessive. Calculate the following: • The allele frequencies of each allele • The expected genotype frequencies • The number of heterozygous individuals that you would predict in this population • The expected phenotype frequencies • Conditions happen to be really good this year for breeding and next year there are 1,245 young Biology instructors. Assuming all of the H-W conditions are met, how many of these would you expect to be red-sided and how many tan-sided?