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
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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