Unit of Evolution & Population Genetics

Unit of Evolution

• Population is the unit of evolution
• Accumulation of small changes leads to large changes like speciation
• Natural selection acts on individuals, but populations evolve
• Mendelian genetics with Darwinian theory of evolution by natural selection are integrated

Population Genetics

• Study of how populations change genetically over time
• Darwin’s and Mendel’s ideas are reconciled

Definitions

• Species are interbreeding group of organisms
• Species are reproductively isolated from other species
• Population is the local group of organisms of the same species
• Gene pool is the total aggregate of genes in a population
• Fixed allele means only one allele exists for that population, so all individuals are homozygous
• All individuals are either AA or aa when an allele is fixed
• Allele frequency is the relative occurrence of a specific allele at a gene locus
• All white in cats is an example with the genotype (WW)
• The allele frequency of white cats is about 3%
• The chance of getting WW is 0.1%
• Evolution is the change in allele frequency in a population’s gene pool

Hardy-Weinberg Principle

• The principle applies to population that is not evolving
• Genotype frequencies are calculated
• It can be used to compare to a real population to determine if there are any differences
• If no differences exist than the real population is not evolving

Hardy-Weinberg Equilibrium

• H-W equation: p² + 2pq + q² = 1
• p is the frequency of the dominant allele
• q is the frequency of the recessive allele
• p² and q² represent the frequencies of the homozygous genotypes
• 2pq represents the frequency of the heterozygous genotype
• In a wildflower population with pink and white flowers, 480 flowers are pink (dominant, AA or Aa)
• In a wildflower population with pink and white flowers, 20 flowers are white (recessive, aa)
• 500 plants each have 2 alleles in total

Conditions for Hardy-Weinberg Equilibrium

• A set of conditions under which evolution does not occur
• A population is in genetic equilibrium when all criteria are met
• Large population - no chance event to affect allele frequency
• Mutation does not occur or cancels itself out
• All genotypes have equal reproductive success, i.e. no natural selection
• No net flow of alleles in or out of gene pool
• All mating in population is random

Alterations to Allele Frequencies

• Any condition of Hardy-Weinberg that is violated can cause a shift in allele frequencies, which is termed microevolution
• Key mechanisms that alter frequencies directly and cause the most evolutionary change: natural selection, genetic drift, gene flow
• Violations to the Hardy-Weinberg (H-W) conditions may be caused by mutations and non-random mating which can lead to shifts in allele frequency

Allele Alterations from Natural Selection

• Differential success in reproduction
• Certain alleles are passed to the next generation in greater proportions

Allele Alterations from Genetic Drift

• Allele frequencies can fluctuate unpredictably from one generation to the next, especially in small populations
• Genetic variation is reduced

Genetic Drift: Bottleneck Effect

• Bottleneck effect can occur: drastic reduction in population size
• The bottleneck can lead to over or under-representation of alleles in remaining population

Genetic Drift: Founder Effect

• Founder effect can cause a few individuals to become isolated from a larger population
• Gene pool differs due to founder effect from source population
• Founder effect can lead to adaptive radiation

Alterations to Allele Frequencies from Gene Flow

• Transfer of alleles into or out of a population due to movement of fertile individuals or their gametes
• Differences are reduced between populations over time

Mutations and Non-Random Mating

• Raw material for natural selection: substitution of one allele for another, gene duplication events can be mutations
• Shifting of genotypes happens with selective mating, due to non-random mating
• Inbreeding – increases in frequency of homozygous phenotypes
• Assortative mating – individuals mate with partners that are similar

Natural Selection

• Natural selection favors some alleles over others, genetic variations and reassortment increases
• Alleles that provide reproductive advantage increase in frequencies
• Adaptation evolves
• Relative fitness means that the contribution an individual makes to the gene pool of the next generation compared to other individuals
• Selection acts on the phenotype directly & genotype indirectly

Modes of Natural Selection

• Stabilizing
• Directional
• Disruptive

Stabilizing Selection

• Intermediate form exhibits favorability
• Extreme phenotypes are opposed, e.g., human birth weights

Directional Selection

• One phenotype is favored or at one end of the phenotypic range
• In Galapagos, during dry years large beaks in the ground finch are favored because they can handle the hard seeds

Disruptive Selection

• Individuals at both extremes of the phenotypic range are favored
• Black-bellied seed crackers

Limitations to Natural Selection

• Selection can act only on existing variations
• Evolution is limited by historical constraints
• Adaptations are often compromises
• Chance, natural selection, and the environment interact

Genetic Variation in Populations

• Diploidy occurs
• There are 2 alleles at each gene locus
• Genetic variation is maintained in the form of hidden recessive alleles
• Genetic variability expression is higher in diploidy compared to haploid organisms

Preservation of Genetic Variation in Populations

• Balanced polymorphism exist
• There is selection to preserve variation
• Frequency-dependent selection occurs
• Heterozygote advantage