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The Evolution of Populations

This overview of population genetics explores the mechanisms of genetic variation within populations. It defines key concepts such as populations, species, gene pools, and microevolution while delineating the factors that affect allele frequencies. The Hardy-Weinberg theorem underlines conditions for genetic equilibrium, while genetic drift, bottleneck effects, founder effects, gene flow, mutations, and natural selection demonstrate how gene pools evolve. Additionally, the significance of sexual selection and population variation highlights the complexity of evolutionary adaptation.

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The Evolution of Populations

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  1. The Evolution of Populations

  2. Population genetics • Population: • a localized group of individuals belonging to the same species • Species: • a group of populations whose individuals have the potential to interbreed and produce fertile offspring • Gene pool: • the total aggregate of genes in a population at any one time • Population genetics: • the study of genetic changes in populations • Modern synthesis/neo-Darwinism • Individuals are selected, but populations evolve.

  3. Allele Frequencies • A gene pool consists of all the genes at all loci within a population • Different alleles for each gene • Each allele has a frequency in the population • Allele frequencies can stabilize when a population is at equilibrium (evolution is not occurring) • Hardy-Weinburg

  4. Hardy-Weinberg Theorem • Serves as a model for the genetic structure of a nonevolving population (equilibrium) • 5 conditions: • Very large population size • No migration • No net mutations • Random mating • No natural selection

  5. Hardy-Weinberg Equation • p + q = 1.0 (p=1-q & q=1-p) • p=frequency of one allele (A) • q=frequency of the other allele (a) • p2 + 2pq + q2 = 1.0 • P2=frequency of AA genotype • 2pq=frequency of Aa plus aA genotype • q2=frequency of aa genotype

  6. Microevolution • A change in the gene pool of a population over a succession of generations • Causes: • Genetic Drift • The Bottleneck Effect • The Founder Effect • Gene Flow • Mutations • Nonrandom Mating • Natural Selection

  7. Genetic Drift • Genetic drift: changes in the gene pool of a small population due to chance (usually reduces genetic variability)

  8. The Bottleneck Effect • Type of genetic drift resulting from a reduction in population (natural disaster) such that the surviving population is no longer genetically representative of the original population

  9. Founder Effect • A cause of genetic drift attributable to colonization by a limited number of individuals from a parent population • Example: Pitcairn Island • Fletcher Christian and eight of the Bounty mutineers accompanied by twelve Tahitian women and six men made it their home in 1790

  10. Gene Flow • Genetic exchange due to the migration of fertile individuals or gametes between populations (reduces differences between populations)

  11. Mutations • A change in an organism’s DNA (gametes) • Accumulate over many generations • Original source of genetic variation (raw material for naturalselection)

  12. Nonrandom Mating • Inbreeding • Assortive Mating • Sexual Selection

  13. Sexual selection • Sexual dimorphism: • secondary sex characteristic distinction • Difference in appearance of males and females • Sexual selection: • Selection towards secondary sex characteristics that leads to sexual dimorphism • Females choose male with brightest colors, etc.

  14. Natural Selection • Differential success in reproduction • Only form of microevolution that adapts a population to its environment

  15. Selection in Action • Fitness: • contribution an individual makes to the gene pool of the next generation • 3 Types of Selection: • Directional • Diversifying (disruptive) • Stabilizing

  16. Population variation • Polymorphism: • coexistence of 2 or more distinct forms of individuals (morphs) within the same population • Geographical variation: • differences in genetic structure between populations (cline)

  17. Variation preservation • Prevention of natural selection’s reduction of variation • Diploidy • 2nd set of chromosomes hides variation in the heterozygote • Balanced polymorphism • heterozygote advantage (hybrid vigor; i.e., malaria/sickle-cell anemia) • frequency dependent selection (survival & reproduction of any 1 morph declines if it becomes too common; i.e., parasite/host)

  18. The Disadvantages of Sex • Asexual reproduction is much more efficient than sexual • However, most eukaryotes reproduce sexually… why? • Genetic Variation!

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