Ch. 23 The Evolution of Populations

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1 Ch. 23 The Evolution of Populations 1

2 Essential question: Do populations evolve? 2

3 Mutation and Sexual reproduction produce genetic variation that makes evolution possible What is the smallest unit of evolution? genetic variations come from microevolution = change in allele frequencies in a population over generations Three main mechanisms that cause allele frequency: 1. natural selection* 2. genetic drift (chance events that alter allele frequencies) 3. gene flow (transfer of alleles between populations) * natural selection is the only mechanism that improves the match between organisms and their environment 3

4 When looking at genetic variation, need to look at: discrete characters classified as either/or Ex. black or white many determined by a single gene locus with different alleles at that locus (think dominant/recessive) Quantitative characters characteristics that vary along a continuum within a population; 2 or more genes involved Examples?? Calico cats 4

5 Scientists measure variation by looking at: 1. whole gene level (gene variability) 2. molecular level of DNA (nucleotide variation) Gene variability can be quantified by Average heterozygosity = the average % of loci that are heterozygous Heterozygous = two different alleles for a given locus Ex. Fruit flies 13,700 genes, heterozygous for 1,920 loci (14%) Nucleotide variability measured by comparing DNA sequences of 2 individuals in a population and then average data of many comparisons. What is the difference between you and me? 5

6 Variation between populations geographic variation differences between gene pools of separate populations or population subgroups Ex. mice in Madeira patterns of fused chromosomes differ from one population to the other. 6

7 clines a graded change in a trait along a geographic axis 7

8 What factors do you think affect clines? clines suggest natural selection because otherwise "there would be no reason to expect a close relationship between the environmental variable and the frequency of the allele" 8

9 Ultimate source of new alleles is Mutations mutation = any change in the nucleotide sequence of an organism's DNA Types of mutations: 1. point mutation change of one base in a gene as long as the amino acid that a section codes for is not changed, then it will have no effect. but can be detrimental 2. translocation movement of one part of chromosome to another chromosome or within itself. beneficial if it links DNA segments to have a beneficial effect 9

10 Gene duplication when a gene segment is duplicated due to errors in meiosis, slippage during DNA replication or pieces moving if are not harmful, can lead to mutations and natural selection Sexual reproduction leads to genetic variation and makes evolution possible Mechanisms that make this possible: 1. crossing over during meiosis 2. independent assortment of chromosomes 3. fertilization 10

11 Darwin couldn't explain how inherited variations were maintained in populations over time Gregor Mendel had a model, but his manuscripts weren't really looked at until early 20th century geneticists put Darwin's views and Mendel's view together = population genetics the study of how populations change over time mid 20th century modern synthesis a comprehensive theory of evolution that integrated ideas from many fields Fisher ( ) demonstrated rules of Mendelian inheritance Haldane ( ) studied rules of natural selection 11

12 Population = a localized group of individuals that are capable of breeding and producing fertile offspring some populations of species are isolated so rarely exchange genetic material Ex. Blue people of Kentucky Gene pool = all of the alleles at all gene loci in all individuals of the population fixed allele= one allele that exists at a particular locus in a population (all individuals are homozygous for that allele) Each allele has a frequency in the population 12

13 R Ex. 500 wildflower plants with 2 alleles for color C = W red, C = white absence of red pigment (total = 1000 alleles) R R C C x W W C C red flowers white flowers C R W C pink flowers incomplete dominance In the population of 500 plants: 320 plants = red 160 plants = pink 20 plants = white R C allele = 800 of the genes (320 x2 = 640 for C C plants, + R W 160 x1 = 160 for C C plants) R R 13

14 W R W C allele = 200 of the genes (160 x 1 = 160 for C C plants, +, W W 20 x 2 for C C plants if have two alleles for a particular locus use p to represent frequency of one allele and q to represent frequency of other allele p = dominant allele frequency q = recessive allele frequency p + q = 1 14

15 15

16 R p = frequency of C allele in the gene pool of this population R C accounts for 800 of the genes ( ) R 800C alleles/ 1000 total alleles of population = 0.8 or 80% frequency W C accounts for 200 of the genes ( ) W 200 C alleles/ 1000 total alleles of population = 0.2 or 20 % frequency W q = frequency of the C allele in the gene pool of this population 16

17 the sum of all frequencies must equal 1 therefore p + q = 1 Even if you have more than two alleles, the sum of the frequencies must still = 1 17

18 Hardy Weinberg Principle (Theorem) 1908 by Hardy and Weinberg worked independently = the frequencies of alleles and genotypes in a population's gene pool remain constant from generation to generation, provided that only Mendelian segregation and recombination of alleles are at work. works if gene pools not evolving Hardy Weinberg Equilibrium = The condition describing a nonevolving population (one that is in genetic equilibrium) 18

19 Conditions for Hardy Weinberg equilibrium* 1. works for extremely large population size smaller the population, more chance for fluctuation in allele frequencies 2. No gene flow no transfer of alleles between populations 3. no mutations mutations could modify the gene pool 4. Random mating allele frequencies would be different if choose mates with specific traits 5. No natural selection allele frequencies could be changed if have different survival and reproductive success *changes in these conditions result in evolution 19

20 Hardy Weinberg Theorem 20

21 Hardy Weinberg equilibrium using previous example and rule of multiplication can calculate the frequencies of the three possible genotypes assuming random union of sperm and egg R probability of two C alleles will come together x 0.8 = p x p = p = % of the next generation will have genotype C R C R W The frequency of C C individuals x 0.2 = q =0.04 or 4% W 21

22 R W R The frequency of heterozygotes C C can be from a sperm with C W W R and egg with C or a sperm with C and an egg with C 0.8 x 0.2 = 0.16 or 16% Can summarize unions of gametes using algebraic equation 2 2 p + 2pq + q = 1 22

23 Hardy Weinberg can be used to estimate percentage of population carrying an allele for an inherited disease Ex. PKU phenylketonuria = metabolic disorder 1 in 10,000 people have this in U.S. because allele is recessive corresponds to q 2 frequency of the recessive allele: q = = 0.01 frequency of dominant allele: p = 1 q = = 0.99 frequency of carriers (heterozygous) 2pq = 2 x 0.99 x 0.01 = (approximately 2% of U.S. population 23

24 Other ways to alter a population's genetic comparison any change in the conditions that Hardy Weinberg is based on 1. Natural Selection individuals with variations in their heritable traits that are better suited to environmental conditions can produce more offspring 24

25 Genetic Drift unpredictable fluctuations in allele frequencies from one generation to the next because of a population's small size 25

26 Two situations of genetic drift a. bottleneck effect happens when a sudden change in environment (like a fire or flood) drastically reduces the population size, so only survivors can pass genes on certain alleles may be more present than others, some may be eliminated humans can cause other species to go through this endangered species 26

27 b. The Founder Effect when a few individuals become isolated from main population and establish a new gene pool not reflective of original population. accounts for some human genetic disorders Ex colonist brought recessive alleles for retinitis pigmentosa (form of blindness) to Tristan da Cunha islands 1960's 4 people had disease (10x higher than normal) 27

28 Gene Flow genetic additions or subtractions from a population resulting from the movement of fertile individuals or gametes tends to reduce differences between populations could eventually form a single large gene pool 28

29 Natural Selection is the primary mechanism of adaptive evolution accumulates and maintains favorable genotypes in a population 29

30 Evolutionary fitness Natural Selection fitness = adaptive advantage, the contribution that an individual makes to the gene pool of the next generation, relative to the contributions of other individuals relative fitness = contribution of a genotype to the next generation compared to the contributions of alternative genotypes for the same locus relative fitness = "0" if an animal or plant is sterile 30

31 (Disruptive) 31

32 Directional selection favors individuals that deviate from the average caused by environmental changes or migration of members to new environment with different environmental conditions than original habitat Disruptive selection both extremes of phenotypic range are favored Stabilizing selection against extreme phenotypes and favors intermediate variants selection favors heritable traits 32

33 Directional Selection for beak size in Galapagos population of medium ground finch 33

34 Sexual Selection a form of natural selection in which individuals with certain inherited characteristics are more likely than other individuals to obtain mates. 34

35 Sexual selection natural selection for mating success sexual dimorphism =marked differences between sexes in secondary sexual characteristics, not associated with reproduction differences can be size, color, ornamentation 35

36 intrasexual selection selection "within same sex" like a direct competition among individuals of same sex usually in male birds Bower bird builds a colorful nest to attract female peacocks fanning their feathers 36

37 intersexual selection = mate choice mates are choosy about mates of other sex female's choice depends on male's appearance or behavior 37

38 Preservation of genetic variation in a population 1. Diploidy most eukaryotic organisms are diploid (2 of each chromosome one from each parent) organisms have recessive alleles only shown when inherit both recessive genes frequency is very low the rarer the recessive allele, more protection from natural selection 2. Balancing selection when two or more forms exist in a population. 38

39 a. heterozygote advantage heterozygote individuals have a greater fitness than homozygotes. Ex. sickle cell allele most common in Africa; also provides protection against malaria 39

40 Why natural selection cannot make perfect organisms 1. Selection can edit only existing variations favors only fittest phenotypes among those currently in the population 2. evolution is limited by historical constraints Ex. ancestral anatomy can't be changed 3. Adaptations are often compromises Ex. humans have prehensile hands and flexible limbs, but prone to injury 4. Chance, natural selection and the environment interact storms blowing insects or plant seeds to new areas they might not be the most fit organisms 40

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