CH. 22/23 WARM-UP. 1. List 5 different pieces of evidence for evolution.

Transcription

1 CH. 22/23 WARM-UP 1. List 5 different pieces of evidence for evolution. 2. (Review) What are the 3 ways that sexual reproduction produces genetic diversity? 3. What is 1 thing you are grateful for today?

2 CH. 23 WARM-UP 1. In a population of 200 mice, 98 are homozygous dominant for brown coat color (BB), 84 are heterozygous (Bb), and 18 are homozygous (bb). a) The allele frequencies of this population are: B allele: b allele: b) The genotype frequencies are: BB: Bb: bb: 2. Use the above info to determine the genotype frequencies of the next generation: B (p): BB (p 2 ): bb (q 2 ): b (q): Bb (2pq):

3 THE EVOLUTION OF POPULATIONS C H A P T E R 2 3

4 WHAT YOU MUST KNOW: How mutation and sexual reproduction each produce genetic variation. The conditions for Hardy-Weinberg equilibrium. How to use the Hardy-Weinburg equation to calculate allelic frequencies and to test whether a population is evolving.

5 SMALLEST UNIT OF EVOLUTION Microevolution: change in the allele frequencies of a population over generations

6 Darwin did not know how organisms passed traits to offspring Mendel published his paper on genetics Mendelian genetics supports Darwin s theory Evolution is based on genetic variation

7 SOURCES OF GENETIC VARIATION Point mutations: changes in one base (eg. sickle cell) Chromosomal mutations: delete, duplicate, disrupt, rearrange usually harmful Sexual recombination: contributes to most of genetic variation in a population 1. Crossing Over (Meiosis Prophase I) 2. Independent Assortment of Chromosomes (during meiosis) 3. Random Fertilization (sperm + egg)

8 Population genetics: study of how populations change genetically over time Population: group of individuals that live in the same area and interbreed, producing fertile offspring

9 Gene pool: all of the alleles for all genes in all the members of the population Diploid species: 2 alleles for a gene (homozygous/heterozygous) Fixed allele: all members of a population only have 1 allele for a particular trait The more fixed alleles a population has, the LOWER the species diversity

10 HARDY-WEINBERG PRINCIPLE Hardy-Weinberg Principle: The allele and genotype frequencies of a population will remain constant from generation to generation UNLESS they are acted upon by forces other than Mendelian segregation and recombination of alleles Equilibrium = allele and genotype frequencies remain constant

11 CONDITIONS FOR HARDY-WEINBERG EQUILIBRIUM 1. No mutations. 2. Random mating. 3. No natural selection. 4. Extremely large population size. 5. No gene flow. If at least one of these conditions is NOT met, then the population is EVOLVING!

12 Hardy-Weinberg Principle Allele Frequencies: Gene with 2 alleles : p, q p = frequency of dominant allele (A) q = frequency of recessive allele (a) p + q = 1 Note: 1 p = q 1 q = p

13 Hardy-Weinberg Equation Genotypic Frequencies: 3 genotypes (AA, Aa, aa) p 2 + 2pq + q 2 = 1 p 2 = AA (homozygous dominant) 2pq = Aa (heterozygous) q 2 = aa (homozygous recessive)

14 ALLELE FREQUENCIES

15 GENOTYPIC FREQUENCIES

16 STRATEGIES FOR SOLVING H-W PROBLEMS: 1. If you are given the genotypes (AA, Aa, aa), calculate p and q by adding up the total # of A and a alleles. 2. If you know phenotypes, then use aa to find q 2, and then q. (p = 1-q) 3. Use p 2 + 2pq + q 2 to find genotype frequencies. 4. If p and q are not constant from generation to generation, then the POPULATION IS EVOLVING!

17 HARDY-WEINBERG PRACTICE PROBLEM #1 The scarlet tiger moth has the following genotypes. Calculate the allele and genotype frequencies (%) for a population of 1612 moths. AA = 1469 Aa = 138 aa = 5 Allele Frequencies: A = a = Genotypic Frequencies: AA = Aa = aa =

18 HARDY-WEINBERG PRACTICE PROBLEM #2: PTC TASTERS Taster = AA or Aa Tasters = q 2 = q = Nontaster = aa Nontasters = p + q = 1 p = 1 q = p 2 + 2pq + q 2 = 1

19 CAUSES OF EVOLUTION

20 CONDITIONS FOR HARDY-WEINBERG EQUILIBRIUM 1. No mutations. 2. Random mating. 3. No natural selection. 4. Extremely large population size. 5. No gene flow. If at least one of these conditions is NOT met, then the population is EVOLVING!

21 Minor Causes of Evolution: #1 - Mutations Rare, very small changes in allele frequencies #2 - Nonrandom mating Affect genotypes, but not allele frequencies Major Causes of Evolution: Natural selection, genetic drift, gene flow (#3-5)

22 MAJOR CAUSES OF EVOLUTION #3 Natural Selection Individuals with variations better suited to environment pass more alleles to next generation

23 MAJOR CAUSES OF EVOLUTION #4 Genetic Drift Small populations have greater chance of fluctuations in allele frequencies from one generation to another Examples: Founder Effect Bottleneck Effect

24 Genetic Drift

25 FOUNDER EFFECT A few individuals isolated from larger population Certain alleles under/over represented Polydactyly in Amish population

26 BOTTLENECK EFFECT Sudden change in environment drastically reduces population size Northern elephant seals hunted nearly to extinction in California

27 MAJOR CAUSES OF EVOLUTION #5 Gene Flow Movement of fertile individuals between populations Gain/lose alleles Reduce genetic differences between populations

28 HOW DOES NATURAL SELECTION BRING ABOUT ADAPTIVE EVOLUTION?

29 Fitness : the contribution an individual makes to the gene pool of the next generation Natural selection can alter frequency distribution of heritable traits in 3 ways: 1. Directional selection 2. Disruptive (diversifying) selection 3. Stabilizing selection

30 Directional Selection: eg. larger black bears survive extreme cold better than small ones Disruptive Selection: eg. small beaks for small seeds; large beaks for large seeds Stabilizing Selection: eg. narrow range of human birth weight

31 SEXUAL SELECTION Form of natural selection certain individuals more likely to obtain mates Sexual dimorphism: difference between 2 sexes Size, color, ornamentation, behavior

32 SEXUAL SELECTION Intrasexual selection within same sex (eg. M compete with other M) Intersexual mate choice (eg. F choose showy M)

33 PRESERVING GENETIC VARIATION Diploidy: hide recessive alleles that are less favorable Heterozygote advantage: greater fitness than homozygotes eg. Sickle cell disease

34 HHMI VIDEO: NATURAL SELECTION IN HUMANS R U N N I N G T I M E : 1 4 : 0 3 M I N

35 NATURAL SELECTION CANNOT FASHION PERFECT ORGANISMS. 1. Selection can act only on existing variations. 2. Evolution is limited by historical constraints. 3. Adaptations are often compromises. 4. Chance, natural selection, and the environment interact.

36 SAMPLE PROBLEM Define the following examples as directional, disruptive, or stabilizing selection: a) Tiger cubs usually weigh 2-3 lbs. at birth b) Butterflies in 2 different colors each represent a species distasteful to birds c) Brightly colored birds mate more frequently than drab birds of same species d) Fossil evidence of horse size increasing over time