1. The Evolution of Populations
Chapter 23

2. YOU MUST KNOW
How mutation and sexual reproduction each produce genetic variation
The conditions for Hardy-Weinberg equilibrium
How to use the Hardy-Weinberg equation to calculate allele frequencies to test whether a population is evolving
What effects genetic drift, migration or selection may have on a population and analyze data to justify your predictions

3. Discuss as a table:
What is a mutation?

4. Genetic variation makes evolution possible
A. Phenotypic variation reflects genetic variation
B. Mutations are the ONLY source of new genes and new alleles
1. Only mutations in cell lines that produce gametes can be passed on to offspring

5. Two major types of mutations
Gene mutations – change in the DNA sequence
Chromosomal mutations – usually from errors in crossing over
a. Often harmful, but can result in an expanded genome with new genes that may accumulate mutations over generations and take on new functions

6. Most genetic variations in a population are due to sexual recombination of genes that already exist
1. Crossing over
2. Independent assortment (223 possibilities in humans)
3. Fertilization

7. Hardy-Weinberg
A. Population genetics – study of how populations change over time
1. Population – a group of individuals of the same species that live in the same area and interbreed and produce fertile offspring

8. Gene Pool – all of the alleles in a population
1. If all individuals in a population are homozygous for the same allele, that allele is fixed
a. Ex. Fruit flies – 11,780 out of 13,700 genes are fixed – variation in the species comes from the other 1,920 genes
b. The more fixed genes in a population, the less diversity

9. Hardy-Weinberg Equilibrium
1. Used to describe a population that is NOT evolving
2. States that the frequencies of alleles in a population’s gene pool will remain constant over the course of generations unless they are acted upon by forces other than Mendelian segregation and recombination – this is very unlikely

10. 5 Conditions must be met
1. No natural selection
2. No mutations
3. No gene flow (emigration, immigration)
4. Population must be extremely large
5. Random mating

11. Whether or not a population is at equilibrium can be tested using the Hardy-Weinberg equation:
p2 + 2pq + q2 = 1
p = the frequency of A (dominant allele)
q = the frequency of a (recessive allele)
p2 = the frequency of AA (homozygous dominants)
pq = the frequency of Aa (heterozygotes)
q2 = the frequency of aa (homozygous recessives)
p + q = 1

12. Practice Problem:
In a plant population, red flowers (R) are dominant over white flowers (r). In a population of 500 individuals, 25% show the recessive phenotype. How many individuals would you expect to be homozygous dominant and heterozygous for this trait?

13. Practice Problem:
In a plant population, red flowers (R) are dominant over white flowers (r). In a population of 500 individuals, 25% show the recessive phenotype. How many individuals would you expect to be homozygous dominant and heterozygous for this trait?
You have been given q2 = .25
Solve for q

14. Practice Problem:
In a plant population, red flowers (R) are dominant over white flowers (r). In a population of 500 individuals, 25% show the recessive phenotype. How many individuals would you expect to be homozygous dominant and heterozygous for this trait?
You have been given q2 = .25
Solve for q: q = .5
Now that you have q, solve for p
p + .5 = 1

15. Practice Problem:
In a plant population, red flowers (R) are dominant over white flowers (r). In a population of 500 individuals, 25% show the recessive phenotype. How many individuals would you expect to be homozygous dominant and heterozygous for this trait?
You have been given q2 = .25
Solve for q: q = .5
Now that you have q, solve for p
p + .5 = 1
p = .5

16. Practice Problem: You have been given q2 = .25 Solve for q: q = .5
Now that you have q, solve for p
p + .5 = 1
p = .5
Now put use you p & q in Hardy-Weinberg
Homozygous dominant = p2 so (.5) 2
Heterozygous = 2pq so 2(.5)(.5) 2

17. Practice Problem: Now put use you p & q in Hardy-Weinberg
Homozygous dominant = p2 so (.5) 2
Heterozygous = 2pq so 2(.5)(.5)
p2 = .25
2pq = .5
Now multiply you frequencies by the size of the population 2

18. Practice Problem: p2 = .25 2pq = .5
Now multiply you frequencies by the size of the population
(.25)(500) = 125 individuals are Homozygous dominant
(.5)(500) = 250 individuals are Heterozygous
*Rule to remember – always find q first! 2

19. Hardy-Weinberg conditions are rarely met
A. Mutations – rare, but important
Natural selection – results in alleles being passed on to the next generation in proportions that are different than the frequencies of the present generation – remember – individuals with variations better suited to their environment are more likely to survive and reproduce (Hardy-Weinberg equilibrium would require that all alleles are equally advantageous)

20. C. Genetic drift – unpredictable fluctuation in allele frequencies between generations 1. Smaller the population, more likely genetic drift 2. Can cause a loss of genetic diversity

21. 3. Founder Effect – a few individuals become isolated from a larger population and establish a new population whose gene pool is not reflective of the source population

22. 4. Bottleneck effect – A sudden change in environment drastically reduces the size of the population. The few survivors have a gene pool that does not reflect the original population’s gene pool.
California condor – population reduced to 22 in 1982, 14 breeders

23. Gene flow – population gains or loses alleles by addition or subtraction from the population
1. Immigration, emigration, movement of pollen between populations
2. Can increase or decrease diversity within a population
3. Tends to decrease differences between populations

24. B. Natural selection acts on phenotypes, NOT genotypes
Natural selection is the only mechanism that consistently causes adaptive evolution
A. Relative fitness – contribution an organism makes to the next generation’s gene pool
B. Natural selection acts on phenotypes, NOT genotypes
Just an example, don’t need to know equation

25. 3 Major types of Natural Selection
Consider this original population

26. 1. Directional Selection – favors one extreme OR the other, frequency shifts towards one extreme OR the other

27. 2. Disruptive Selection – favors both extremes in the population resulting in higher frequencies of the extremes and few of the intermediates

28. 3. Stabilizing selection – favors the intermediates and decreases the frequency of individuals with the extreme phenotypes
Baby birth weights

29. Sexual Selection – certain traits make individuals more likely to attract mates than others
1. Sexual dimorphism – differences between the two sexes in secondary sex characteristics such as size, color, ornamentation, or behavior

30. Preservation of genetic variation
1. Diploidy – allows for recessive alleles to be hidden from selection
2. Heterozygote advantage – heterozygotes for certain traits are more likely to survive (ex. sickle-cell trait protects from malaria)