AP BIOLOGY Big Idea #1 – Part A – Part 2

1. AP BIOLOGYBig Idea #1 – Part A – Part 2 Natural Selection Acts on Phenotypes

2. Concept 23.2: The Hardy-Weinberg equation can be used to test whether a population is evolving • The first step in testing whether evolution is occurring in a population is to clarify what we mean by a population

3. Gene Pools and Allele Frequencies • A populationis a localized group of individuals capable of interbreeding and producing fertile offspring • A gene poolconsists of all the alleles for all loci in a population • A locus is fixed if all individuals in a population are homozygous for the same allele

4. Fig. 23-5 Porcupine herd MAP AREA CANADA ALASKA Beaufort Sea NORTHWEST TERRITORIES Porcupine herd range Fortymile herd range YUKON ALASKA Fortymile herd

5. Fig. 23-5a MAP AREA CANADA ALASKA Beaufort Sea NORTHWEST TERRITORIES Porcupine herd range Fortymile herd range YUKON ALASKA

6. The frequency of an allele in a population can be calculated: • For diploid organisms, the total number of alleles at a locus is the total number of individuals x 2 • The total number of dominant alleles at a locus is 2 alleles for each homozygous dominant individual plus 1 allele for each heterozygous individual; the same logic applies for recessive alleles

7. By convention, if there are 2 alleles at a locus, p and q are used to represent their frequencies • The frequency of all alleles in a population will add up to 1 • For example, p + q = 1

8. The Hardy-Weinberg Principle • The Hardy-Weinberg principle describes a population that is not evolving • If a population does not meet the criteria of the Hardy-Weinberg principle, it can be concluded that the population is evolving

9. Hardy-Weinberg Equilibrium • The Hardy-Weinberg principle states that frequencies of alleles and genotypes in a population remain constant from generation to generation • In a given population where gametes contribute to the next generation randomly, allele frequencies will not change • Mendelian inheritance preserves genetic variation in a population

10. Fig. 23-6 Alleles in the population Frequencies of alleles Gametes produced p = frequency of Each egg: Each sperm: CR allele = 0.8 q = frequency of 80% chance 80% chance 20% chance 20% chance CW allele = 0.2

11. Hardy-Weinberg equilibriumdescribes the constant frequency of alleles in such a gene pool • If p and qrepresent the relative frequencies of the only two possible alleles in a population at a particular locus, then • p2 + 2pq + q2 = 1 • where p2 and q2 represent the frequencies of the homozygous genotypes and 2pq represents the frequency of the heterozygous genotype

12. Fig. 23-7-1 80% CR(p = 0.8) 20% CW(q = 0.2) Sperm CW (20%) CR (80%) CR (80%) Eggs 16% (pq) CRCW 64% (p2) CRCR 4% (q2) CW CW 16% (qp) CRCW CW (20%)

13. Fig. 23-7-2 64% CRCR,32% CRCW, and4% CWCW Gametes of this generation: 64% CR+16% CR=80% CR = 0.8 = p 4% CW+16% CW=20% CW= 0.2 = q

14. Fig. 23-7-3 64% CRCR,32% CRCW, and4% CWCW Gametes of this generation: 64% CR+16% CR=80% CR = 0.8 = p 4% CW+16% CW=20% CW= 0.2 = q Genotypes in the next generation: 64% CRCR,32% CRCW, and4% CWCWplants

15. Fig. 23-7-4 20% CW(q = 0.2) 80% CR(p = 0.8) Sperm CW (20%) CR (80%) CR (80%) Eggs 16% (pq) CR CW 64% (p2) CR CR 4% (q2) CW CW 16% (qp) CR CW CW (20%) 64% CR CR,32% CR CW, and4% CW CW Gametes of this generation: 64% CR +16% CR=   80% CR = 0.8 = p 4% CW+16% CW=20% CW= 0.2 = q Genotypes in the next generation: 64% CR CR,32% CR CW, and4% CW CW plants

16. Conditions for Hardy-Weinberg Equilibrium • The Hardy-Weinberg theorem describes a hypothetical population • In real populations, allele and genotype frequencies do change over time

17. The five conditions for non-evolving populations are rarely met in nature: • No mutations • Random mating • No natural selection • Extremely large population size • No gene flow * Natural populations can evolve at some loci, while being in Hardy-Weinberg equilibrium at other loci

18. Applying the Hardy-Weinberg Principle: • We can assume the locus that causes phenylketonuria (PKU) is in Hardy-Weinberg equilibrium given that: • The PKU gene mutation rate is low • Mate selection is random with respect to whether or not an individual is a carrier for the PKU allele • Natural selection can only act on rare homozygous individuals who do not follow dietary restrictions • The population is large • Migration has no effect as many other populations have similar allele frequencies

19. The occurrence of PKU is 1 per 10,000 births • q2 = 0.0001 • q = 0.01 • The frequency of normal alleles is • p = 1 – q = 1 – 0.01 = 0.99 • The frequency of carriers is • 2pq = 2 x 0.99 x 0.01 = 0.0198 • or approximately 2% of the U.S. population

20. Concept 23.1: Mutation and sexual reproduction produce the genetic variation that makes evolution possible • Two processes, mutation and sexual reproduction, produce the variation in gene pools that contributes to differences among individuals

21. Genetic Variation • Variation in individual genotype leads to variation in individual phenotype • Not all phenotypic variation is heritable • Natural selection can only act on variation with a genetic component

22. Fig. 23-2 (b) (a)

23. Fig. 23-2a (a)

24. Fig. 23-2b (b)

25. Variation Within a Population • Both discrete and quantitative characters contribute to variation within a population • Discrete characterscan be classified on an either-or basis • Quantitative charactersvary along a continuum within a population

26. Population geneticists measure polymorphisms in a population by determining the amount of heterozygosity at the gene and molecular levels • Average heterozygositymeasures the average percent of loci that are heterozygous in a population • Nucleotide variabilityis measured by comparing the DNA sequences of pairs of individuals

27. Variation Between Populations • Most species exhibit geographic variation,differences between gene pools of separate populations or population subgroups

28. Fig. 23-3 1 3.14 5.18 2.4 6 7.15 8.11 9.12 10.16 13.17 19 XX 6.7 1 2.19 3.8 4.16 5.14 9.10 11.12 13.17 15.18 XX

29. Some examples of geographic variation occur as a cline, which is a graded change in a trait along a geographic axis

30. Fig. 23-4 1.0 0.8 0.6 Ldh-Bb allele frequency 0.4 0.2 0 44 42 40 34 46 38 36 32 30 Latitude (°N) Maine Cold (6°C) Georgia Warm (21°C)

31. Mutation • Mutationsare changes in the nucleotide sequence of DNA • Mutations cause new genes and alleles to arise • Only mutations in cells that produce gametes can be passed to offspring Animation: Genetic Variation from Sexual Recombination

32. Point Mutations • A point mutation is a change in one base in a gene • The effects of point mutations can vary: • Mutations in noncoding regions of DNA are often harmless • Mutations in a gene might not affect protein production because of redundancy in the genetic code

33. Mutations That Alter Gene Number or Sequence • Chromosomal mutations that delete, disrupt, or rearrange many loci are typically harmful • Duplication of large chromosome segments is usually harmful • Duplication of small pieces of DNA is sometimes less harmful and increases the genome size • Duplicated genes can take on new functions by further mutation

34. Mutation Rates • Mutation rates are low in animals and plants • The average is about one mutation in every 100,000 genes per generation • Mutations rates are often lower in prokaryotes and higher in viruses

35. Sexual Reproduction • Sexual reproduction can shuffle existing alleles into new combinations • In organisms that reproduce sexually, recombination of alleles is more important than mutation in producing the genetic differences that make adaptation possible