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Evolution of Populations. The Smallest Unit of Evolution. Natural selection acts on individuals, but only populations evolve Genetic variations contribute to evolution. Population genetics. Population genetics study of how populations change genetically over time - PowerPoint PPT Presentation
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Evolution of Populations
The Smallest Unit of Evolution
• Natural selection acts on individuals, but only populations evolve– Genetic variations contribute to evolution
Population genetics
• Population genetics – study of how populations change genetically
over time• Mendelian genetics with the Darwinian
theory• populations as units of evolution
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
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
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
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– p2 + 2pq + q2 = 1– And p2 and q2 represent the frequencies of the
homozygous genotypes and 2pq represents the frequency of the heterozygous genotype
LE 23-4
Generation3 25% CRCR
Generation4
50% CRCW 25% CWCW
50% CW
gametes50% CR
come together at random
25% CRCR 50% CRCW 25% CWCW
Alleles segregate, and subsequentgenerations also have three typesof flowers in the same proportions
gametes
Generation2
Generation1
CRCR CWCW
genotypegenotypePlants mate
All CRCW
(all pink flowers)
50% CR 50% CW
gametes gametes
come together at random
X
Evolutionary Change
• Three major factors alter allele frequencies and bring about most evolutionary change:– Mutations– Natural selection– Nonrandom Mating– Genetic drift– Gene flow
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
Mutations
• Chromosomal mutations that delete, disrupt, or rearrange many loci are typically harmful
• Gene duplication is nearly always harmful
Natural Selection
• Differential success in reproduction results in certain alleles being passed to the next generation in greater proportions
• 3 conditions for natural selection to occur and to result in evolutionary change1. Variation must exist among individuals in a
population2. Variation among individuals must result in
differences in the number of offspring surviving in the next generation
3. Variation must be genetically inherited
14
Sexual Recombination
• far more important than mutation • produces the genetic differences that make
adaptation possible
• Nonrandom mating– Assortative mating
• Phenotypically similar individuals mate
• Increases proportion of homozygous individuals
– Disassortative mating• Phenotypically different
individuals mate• Produces excess of
heterozygotes
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
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
Genetic Drift
• The Founder Effect– a few individuals
become isolated from a larger population
– affects allele frequencies
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
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
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
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
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
• 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
• 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
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