The Evolution of Population: The Mechanisms of Microevolution I. Evolution (What actually changes?) A. Gene frequency: the amount of a particular allele as found in the gene pool B. Gene Pool: sum total of all the alleles in a population Gene Pool A Time Gene Pool C Gene Pool B Microevolution: The change in gene frequency in a population over time

Gene Pool A 100 Organisms Gene Pool B 100 Organisms # # DD 10 DD 25 Phenotype Genotype # Phenotype Genotype Dark DD 10 Dark DD 25 Time Dd 50 Dd 30 Medium Medium dd Light 60 Light dd 25 GF of D = .5 GF of d = .5 GF of D = .25 GF of d = .75 allele Gene Frequency = total alleles (10 x 2) + 30 (25 x 2) + 50 = .25 Gene Frequency D = Gene Frequency D = = .5 200 200 (60 x 2) + 30 (25 x 2) + 50 Gene Frequency d = Gene Frequency d = = .5 = .75 200 200 Slide 4

The Hardy – Weinberg Theory II. The Summary A. States that gene frequencies will not change in a population only due to sexual reproduction (Skip to III. Math Theory) B. Gene frequencies will not change (stay in Hardy-Weinberg equilibrium) unless one or more of the following is taking place: 1. Genetic drift (and/or small population size) 2. Migration of genes from other populations 3. Mutation 4. Selective mating 5. Natural selection Implication: The Hardy-Weinberg mathematically proves that microevolution will not take place unless one or more of the above is occurring

Assumption: Genes in gene pool interact independently IV. The Equations Equation #2 Equation #1 Assumption: Genes in gene pool interact independently Assumption: Genes found as pairs and interact within organism 2 2 p + q = 1 p + 2pq + q = 1 2 p = gene frequency of dominant allele p = % homozygous dominant q = gene frequency of recessive allele 2 q = % homozygous recessive 1 = total genes in gene pool (100%) 2pq = % heterozygous 1 = 100% of individuals Taste Lab and Application Worksheet

V. Concepts Relating to the 5 Hardy-Weinberg Conditions 1. Genetic drift (and/or small population size) 2. Migration of genes from other populations Large Populations vs Small Populations (Genetic Drift) 3. Mutation 4. Selective mating 5. Natural selection Small populations are more susceptible to “genetic drift” (random events that change gene frequencies due to sample size) a) Bottleneck effect A random “bottlenecking event” reduces the population number that result in new gene frequencies b) Founder effect When a new habitat is colonized, the genotypes of the original colonist will influence the gene frequencies as the population grows

B. Migration (Gene Flow) 1. Tends to reduce differences between populations Extensive gene flow will cause 2 populations to interact as 1 population C. Mutations Can immediately affect gene pool by substituting one allele for another 2. Rare; usually harmful 3. Becomes the source of new variations D. Natural Selection interactions on genetic variations (Phenotypes) 1. Types of genetic variations a) Polymorphism (morphs) Two or more phenotypes found in a population. Allows for natural selection to “pick” the most fit

b) Geographic Variation Regional differences in gene frequencies in isolated populations. Example: Island mice c) Clines Graded variations within a population along a geographic axis. Environmental gradient may lead to genetic variations Example: Yarrow d) Balanced Polymorphism Natural selection stabilizes gene frequencies of 2 or more phenotypes 1) Heterozygous Advantage Natural selection selects against the homozygous dominant and homozygous recessive. Example Sickle cell anemia 2) Frequency Dependent Selection The Survival and reproduction of a particular phenotype declines as the phenotype becomes more common. Example: parasite/host relationships 3) Neutral Variation Variations of no apparent selective advantage. Most variations are probably neutral

2. What is Meant by “Fitness” a) Darwinian Fitness The contribution an individual makes to the gene pool of the next generation as compared to other individuals A “super” phenotype in a sterile organism has no fitness value Evolutionary impact of a gene is only measured by the continued success of offspring b) Relative Fitness A quantitative value that compares the Darwinian fitness of phenotypes found in a population Example Frogs (page 1)

Selection favors heterozygous “Heterozygous advantage” 3. Patterns (Modes) of Natural Selection # of mice Phenotypes Lightest Darkest Directional Selection Diversifying Selection Stabilizing Selection Selection favors heterozygous “Heterozygous advantage” Selection favors one extreme Selection favors both extremes

E. Non-Random Mating 1. The disadvantage of sexual reproduction Asexual reproduction produces more offspring more efficiently (less energy) then sexual reproduction. Is sexual reproduction an unfit phenotype? Males do not directly produce offspring. Is maleness an unfit phenotype? 2. Sexual Dimorphism Phenotypic differences between males and females resulting from non-random mating. Example: peacocks and peahens a. Intrasexual Selection “within the same sex” Males compete with their own sex for mates. Males defeat other males for possession of females. b. Intersexual Selection “mate choice” One sex (female) chooses over individuals of the other sex Examples Although grebes compete for mates why are the males and females so similar?

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III. The Math Theory 1. What possible genotypes can results with two random frogs producing offspring from this gene pool? Gene Pool A D D d d d D d Frog #1 Chance of picking D = .5 Chance Of picking d = .5 d D D D D d d d D D D d d Frog #2 Chance of picking D = .5 Chance Of picking d = .5 Gene Frequency D = .5 Gene Frequency d = .5 Probability of genotypes of Possible Offspring .5 .5 As sexual reproduction takes place over time, will the gene frequencies ever change? DD .25 Dd .25 NO! .5 Dd .25 dd .25 .5 But… Gene Frequencies do change…. WHY? Slide 3

Genetic Drift due to Small Population Size Slide 6

An Example of a Bottleneck Event “earthquake hits flower island” Gene Frequency p = 1 q = 0 Slide 5 Gene Frequency p = .7 q = .3 Gene Frequency p =.83 q = .17

Flower Island “The Sequel” Founder Effect Flower Island Guano Island Colonization Gene Frequency P = .7 q = .3 Gene Frequency P = .5 q = .5 Slide 6

Morphological Differences in Yarrow at Different Altitudes Slide 7

Density Dependent Selection Slide 7

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Polymorphic Expressions of the Common Garter Snake Slide 6

Normal cells Sickle cells The distribution of the sickle cell gene and the distribution of malaria parasite Slide 7

1. Which is the most fit gene? Gene Pool B 100 Organisms 1. Which is the most fit gene? Phenotype Genotype # “d” is the most fit Dark DD 10 2. Which is the most “fit” phenotype? Light colored frog Dd 30 Medium 3. What is the “relative fitness” of the light colored phenotype? dd Light 60 Most abundant phenotype is always set at a relative fitness of “1” GF of d = .75 GF of D = .25 4. What is the relative fitness of the medium color phenotype? The medium colored frogs produce ½ the amount of surviving offspring ( 30/60) so its relative fitness is .5 5. What is the relative fitness of the dark phenotype? The dark phenotype produces 1/6 the amount of surviving offspring (10/60). Its relative fitness is .17 Slide 9

Directional Selection Thicker beaks in the dry years are more common due to the abundance of hard seeds and the lack of soft seeds Slide 9

Black Bellied Seed Crackers Slide 9 Black Bellied Seed Crackers Medium beaked birds have a hard time feeding on either hard or soft seed Smaller beaks feed on soft seeds best Larger beaks feed on hard seeds best

The “Unfit” Nature of Sexual Reproduction Male Male Male Many offspring; more fitness Fewer offspring; less fitness Slide 10

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Intrasexual selection or Intersexual selection? Pheasant on bottom-left Slide 10