EVOLUTION OF POPULATIONS CHAPTER 23

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1 EVOLUTION OF POPULATIONS CHAPTER 23

2 Population Genetics Individuals DO NOT evolve, populations evolve Microevolution change in genetic makeup of a population from generation to generation Modern synthesis comprehensive theory of evolution that integrates ideas from many fields (genetics, statistics, biogeography, paleontology) Population Genetics study of how populations change genetically over time Population localized group of individuals that are capable of interbreeding and producing fertile offspring

3 Gene Pool total aggregate of genes in a population at any one time Made up of all the alleles at all loci in all members of the population Fixed if only one allele exists at a particular loci ; all individuals homozygous If 2 or more alleles for a particular loci then individuals homozygous or heterozygous

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6 Hardy-Weinberg Theorem Describes populations that are NOT evolving Frequencies of alleles & genes in a population s gene pool remain constant over generations unless they are acted upon by forces other than Mendelian segregation & recombination of alleles

7 Hardy-Weinberg Equilibrium p 2 + 2pq + q 2 = 1 Where p 2 = frequency of homozygous dominant population Where q 2 = frequency of homozygous recessive population Where 2qp = frequency of heterozygous population

8 Population Genetics & Health Science Use Hardy - Weinberg equation can be used to estimate the percentage of the human population that carries an allele Ex. PKU - recessive allele that occurs in 1 in every 10,000 births Recessive allele = q2 q2 =.0001 and q =.01 (take square root) p = 1 - q so =.99 2pq = 2 x.99 x.01 =.0198 Thus approx. 2% of population carries allele for PKU

9 Assumptions of Hardy-Weinberg Equilibrium - must meet 5 conditions 1. Very Large Population size - populations of finite size, genetic drift, which is chance of fluctuation in gene pool, can cause genotype frequencies to change over time 2. No migration - gene flow, transfer of alleles between populations due to migration, can increase frequency of genotypes 3. No net mutations - changing one allele into another, mutations alter gene pool

10 4. Random mating - if individuals pick mates with certain genotypes then random mixing of gametes that is necessary does not occur 5. No natural selection - survival & reproductive success may cause deviation from predicted frequencies Natural population not expected to be in Hardy-Weinberg equilibrium; deviation from a stable gene pool leads to evolution

11 Causes of Microevolution Evolution is a generation to generation change in a population s frequencies of alleles Factors that alter allele frequencies are genetic drift, natural selection, gene flow and mutation (genetic drift & natural selection most important)

12 Genetic Drift Change in a population s allele frequencies due to chance Populations shrink due to bottleneck effect and founder effect Bottleneck effect - natural disasters can reduce population size; surviving population may not represent the original population s gene pool; certain alleles can be overrepresented, while some are underrepresented or eliminated Reduces genetic variation within a population Founder Effect- genetic drift that occurs when a larger population colonizes in an isolated area (island, lake, etc.)

13 Natural Selection Hardy-Weinberg equilibrium requires that all individuals in a population are equal in their ability to survive & reproduce - this is never met Natural populations have variant individuals with some leaving more offspring than others Results in alleles being passed on disproportionately Natural selection accumulates & maintains favorable genotypes in the population

14 Gene Flow Genetic exchange due to the migration of fertile individuals or gametes between populations Tends to reduce differences between populations

15 Mutation Change in an organism s DNA If transmitted to gametes the mutation immediately changes the gene pool by substituting one allele for another Since mutations are rare they do not have much of a quantitative effect on a single generation If a mutation increases the frequency of an allele then it is because individuals carrying the allele are producing a disproportionate number of offspring

16 Genetic Variation within Populations Quantitative characters - vary along a continuum within a population ; usually indicates polygenic inheritance (ex. Height) Discrete characters - either or basis ; determine by a single gene locus with different alleles (ex. Color) Morphs - 2 or more forms of a discrete character are represented in a population (ex. Red or white flowers) Polymorphic - population in which morphs are represented in high enough frequencies to be noticed; applies only to discrete characters

17 Genetic Variation Between Populations Geographic variation- differences in genetic structure between populations Cline - type of geographic variation in which there is a graded change in some trait along a geographic axis (ex. Plant height at different elevations)