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Understanding Microevolutionary Causes and Hardy-Weinberg Criteria

Explore genetic drift, gene flow, mutations, nonrandom mating, and natural selection as causes of microevolution within populations. Learn how the Hardy-Weinberg equations help measure the degree of microevolution occurring. Discover the impact of sexual selection and neutral variation on population dynamics.

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Understanding Microevolutionary Causes and Hardy-Weinberg Criteria

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  1. CHAPTER 23 HARDY WEINBERG CRITERIA & POPULATION EVOLUTION (PART 3) Ms.Gaynor AP Biology

  2. A change in the gene pool of a population over a succession of generations We use 5 criteria for non-evolution to determine causes of microevolution The Hardy Weinberg equations are used to determine the degree of microevolution that is occurring for a given allele Causes of Microevolution

  3. Cause #1: Microevolution • Genetic drift: changes in the gene pool of a SMALL population due to chance (usually reduces genetic variability) • http://highered.mcgraw-hill.com/sites/0072835125/student_view0/animations.html# • 2nd to last video

  4. Genetic Drift Example #1 The Bottleneck Effect • Results from a reduction in population (natural disaster) such that the surviving population is no longer genetically representative of the original population

  5. Ex: NORTHERN ELEPHANT SEAL

  6. Genetic Drift Example #2 Founder Effect:a cause of genetic drift due to the colonization by a limited number of individuals from a parent population http://bcs.whfreeman.com/thelifewire/content/chp24/2402002.html

  7. Founder Effect in Amish Causes dwarfism and polydactyly http://www.mhhe.com/biosci/esp/2001_gbio/folder_structure/ev/m2/s4/evm2s4_7.htm

  8. Cause #2: Microevolution Gene Flow: genetic exchange due to the migration of fertile individuals or gametes between populations (reduces differences between populations)

  9. Cause #3: Microevolution Mutations: • a change in an organism’s DNA • original source of genetic variation (raw material for natural selection) • Mutations can immediately alter p and q in a population • Individual mutations are rare in a population, but there may be cumulative mutations that have an effect  cause evolution

  10. Cause #4: Microevolution Nonrandom mating • “picky” mating • Unequal chances of each egg getting fertilized • Assortative mating - choosing individuals more like self

  11. Sexual selection is natural selection for mating success It can result in sexual dimorphism, marked differences between the sexes in secondary sexual characteristics Sexual Selection

  12. Males are usually more colorful and larger Does NOT help cope with environment DOES lead to reproductive success Can be termed “Female Selection” Sexual Dimorphism

  13. Cause #5: Microevolution Natural Selection • differential reproductive success in reproduction due to variation • only form of microevolution that adapts a population to its environment • Phenotypes are selected FOR or AGAINST (not genotypes) • In a population at equilibrium, no phenotypes (therefore, alleles) are selected over other alleles • http://www.mhhe.com/biosci/esp/2001_gbio/folder_structure/ev/m2/s1/evm2s1_6.htm

  14. Animations to Review Hardy Weinberg and its Criteria • http://zoology.okstate.edu/zoo_lrc/biol1114/tutorials/Flash/life4e_15-6-OSU.swf

  15. 3 Modes of Natural selection • Directional selection favors individuals at one end of the phenotypic range • Disruptive selection favors individuals at both extremes of the phenotypic range • Stabilizing selection favors intermediate variants and acts against extreme phenotypes • http://wps.pearsoncustom.com/wps/media/objects/3014/3087289/Web_Tutorials/17_A02.swf

  16. LIGHT/DARK ROCK HABITAT DARK ROCK HABITAT MEDIUM COLOR ROCK HABITAT

  17. Mutations At first the mutation may not be beneficial to the organism (resistance to antibiotics) Once antibiotics are introduced the mutation is beneficial 2. Sexual recombinationincreases variation Sources of Variations

  18. Population variation SPRING Polymorphismcoexistence of 2 or more distinct forms of individuals (morphs) within the same population Geographical variation differences in genetic structure between populations (cline) SUMMER

  19. Evolutionary Fitness • “Struggle for existence”/“Survival of the fittest” • commonly used but can be misleading • Reproductive success is more subtle and depends on many factors • Fitness is determined by REPRODUCTIVE AND SURVIVAL success of individual • Variations & natural selection affect fitness • Sterile individuals even with a relatively fit allele will not be considered fit overall (=0) • Fitness is not determined by one trait but by the totality of traits in the organism

  20. Neutral variation is genetic variation that appears to favor NO selective advantage Frequencies can change but no advantage/ disadvantage for population A 4th Type of Natural Selection: Neutral Variation

  21. Sexual reproduction produces FEWER reproductive offspring than asexual reproduction a so-called “reproductive handicap” Sexual reproduction produces genetic variation Asexual vs. Sexual Reproduction

  22. Asexual reproduction Sexual reproduction Generation 1 Female Female LE 23-16 Generation 2 Male Generation 3 Generation 4

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