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Evolution of Populations. The Smallest Unit of Evolution Natural selection acts on individuals, but only populations evolve – Genetic variations contribute.

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Presentation on theme: "Evolution of Populations. The Smallest Unit of Evolution Natural selection acts on individuals, but only populations evolve – Genetic variations contribute."— Presentation transcript:

1 Evolution of Populations

2 The Smallest Unit of Evolution Natural selection acts on individuals, but only populations evolve – Genetic variations contribute to evolution

3 Population genetics – study of how populations change genetically over time Mendelian genetics with the Darwinian theory populations as units of evolution

4 Gene Pools and Allele Frequencies Population localized group of individuals capable of interbreeding and producing fertile offspring gene pool – total aggregate of genes in a population at any one time – all gene loci in all individuals of the population

5 The Hardy-Weinberg Theorem population that is not evolving 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 preservation of genetic variation in a population

6 Hardy-Weinberg Equilibrium The five conditions for non-evolving populations are rarely met in nature: – Extremely large population size – No gene flow – No mutations – Random mating – No natural selection

7 Hardy-Weinberg Equilibrium If p and q represent the relative frequencies of the only two possible alleles in a population at a particular locus, then – p 2 + 2pq + q 2 = 1 – And p 2 and q 2 represent the frequencies of the homozygous genotypes and 2pq represents the frequency of the heterozygous genotype


9 LE 23-4 Generation 3 25% C R C R Generation 4 50% C R C W 25% C W C W 50% C W gametes 50% C R come together at random 25% C R C R 50% C R C W 25% C W C W Alleles segregate, and subsequent generations also have three types of flowers in the same proportions gametes Generation 2 Generation 1 CRCRCRCR CWCWCWCW genotype Plants mate All C R C W (all pink flowers) 50% C R 50% C W gametes come together at random X

10 Evolutionary Change Three major factors alter allele frequencies and bring about most evolutionary change: – Mutations – Natural selection – Nonrandom Mating – Genetic drift – Gene flow

11 Variations that make Natural Selection Possible Mutation – changes in the nucleotide sequence of DNA – new genes and alleles to arise – Point Mutations change in one base in a gene usually harmless may impact on phenotype

12 Mutations Chromosomal mutations that delete, disrupt, or rearrange many loci are typically harmful Gene duplication is nearly always harmful

13 Natural Selection Differential success in reproduction results in certain alleles being passed to the next generation in greater proportions

14 3 conditions for natural selection to occur and to result in evolutionary change 1.Variation must exist among individuals in a population 2.Variation among individuals must result in differences in the number of offspring surviving in the next generation 3.Variation must be genetically inherited 14

15 Sexual Recombination far more important than mutation produces the genetic differences that make adaptation possible

16 Nonrandom mating – Assortative mating Phenotypically similar individuals mate Increases proportion of homozygous individuals – Disassortative mating Phenotypically different individuals mate Produces excess of heterozygotes

17 Genetic Drift The smaller a sample, the greater the chance of deviation from a predicted result allele frequencies fluctuate unpredictably from one generation to the next reduces genetic variation through losses of alleles

18 Genetic Drift The Bottleneck Effect – sudden change in the environment that may drastically reduce the size of a population – gene pool may no longer be reflective of the original population’s gene pool

19 Genetic Drift The Founder Effect – a few individuals become isolated from a larger population – affects allele frequencies

20 Gene Flow genetic additions or subtractions from a population, resulting from movement of fertile individuals or gametes gain or loss of alleles reduce differences between populations over time

21 A Closer Look at Natural Selection From the range of variations available in a population, natural selection increases frequencies of certain genotypes, fitting organisms to their environment over generations

22 Evolutionary Fitness Misleading – “struggle for existence” – “survival of the fittest” Fitness – contribution 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 with contributions of alternative genotypes for the same locus

23 Directional, Disruptive, and Stabilizing Selection Selection favors certain genotypes by acting on the phenotypes of certain organisms Three modes of selection: – Directional favors individuals at one end of the phenotypic range – Disruptive favors individuals at both extremes of the phenotypic range – Stabilizing favors intermediate variants and acts against extreme phenotypes


25 The Preservation of Genetic Variation Diploidy – maintains genetic variation in the form of hidden recessive alleles Balancing selection – natural selection maintains stable frequencies of two or more phenotypic forms

26 Heterozygote Advantage – Some individuals who are heterozygous at a particular locus have greater fitness than homozygotes – Natural selection will tend to maintain two or more alleles at that locus – Sickle cell and malaria

27 Sexual selection – natural selection for mating success – sexual dimorphism differences between the sexes in secondary sexual characteristics Intrasexual selection – competition among individuals of one sex for mates of the opposite sex Intersexual selection – individuals of one sex (usually females) are choosy in selecting their mates from individuals of the other sex

28 Why Natural Selection Cannot Fashion Perfect Organisms Evolution is limited by historical constraints Adaptations are often compromises Chance and natural selection interact Selection can only edit existing variations

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