Lecture 01 - Evolutionary Change in Populations PDF

Welcome to Biology 1120 Professor Feely  FYI Office Hours: Holt Hall 217 Tuesday 2-3:30 and Thursday 1-2 contact me for other times if needed! Syllabus To Do by THURSDAY: - Polleverywhere - Review 21.2 - Login to Pearson By 28th:...

Welcome to Biology 1120 Professor Feely  FYI Office Hours: Holt Hall 217 Tuesday 2-3:30 and Thursday 1-2 contact me for other times if needed! Syllabus To Do by THURSDAY: - Polleverywhere - Review 21.2 - Login to Pearson By 28th: Optional EC  Objectives! 1) demonstrate knowledge of different levels of biological organization and apply to differentiate among various taxonomic groups. 2) demonstrate knowledge of evolution and apply to various taxonomic groups. 3) demonstrate knowledge of ecology at different levels of biological organization. 21.2:The Evolution of Populations Learning Outcomes Population genetics Hardy-Weinberg principle 5 conditions required for genetic equilibrium Explain how key evolutionary forces alter allele frequencies Nature and extent of genetic variation  Previous ideas on evolution and diversity focused on the individual Understanding how these organisms interact with each other and the environment is key to our understanding of diversity  Populations Individuals of same species live in a particular place at the same time (potentially interbreeding) Vary in characters Biologists study variation in a particular character by taking measurements of that character in a population  Phenotypic variation often reflects genetic variation Genetic variation is differences in the composition of genes or other DNA sequences among individuals Is introduced into a population by mutations Sexual reproduction recombines mutations in new ways, which may be expressed as new phenotypes Rapid reproduction can quickly generate genetic variation (ex. Bacteria, viruses) The “needs” of a population do not determine what mutations will occur!!! Population Genetics The study of genetic variability within a population and of the evolutionary forces that act on it It is possible to estimate the amount of observed variation that is genetic, as represented by the number, frequency, and kinds of alleles in a population  Gene pool A population’s gene pool includes all the alleles for all the loci present Diploid organisms have two alleles at each genetic position or locus Each individual has a different subset of alleles in the gene pool Homozygous, Heterozygous  Gene pool Evolution of populations is described in terms of genotype, phenotype, and allele frequencies Genotype – combination of alleles in an individual Genotype Frequency The proportion of a particular genotype in a population, expressed as a decimal fraction The sum of all genotype frequencies = 1.0 Genotype Number Genotype Frequency AA 490 0.49 Aa 420 0.42 aa 90 0.09 Total 1000 1.00 Gene pool Evolution of populations is described in terms of genotype, phenotype, and allele frequencies Phenotype Frequency Phenotype – the physical or chemical expression of an organism’s genes The proportion of a particular phenotype in the population - If two alleles are dominant and recessive, the dominant phenotype is the sum of two genotypes (AA and Aa) Phenotype Number Phenotype Frequency Dominant 910 0.91 Recessive 90 0.09 Total 1000 1.00 Gene pool Evolution of populations is described in terms of genotype, phenotype, and allele frequencies Allele Frequency The proportion of a specific allele (A or a) in a particular population A population of 1000 individuals carries 2000 alleles Allele Number Allele Frequency A 1400 0.7 a 600 0.3 Total 2000 1.0 The Hardy–Weinberg Equilibrium Frequencies of alleles and genotypes in a population do not change from generation to generation unless influenced by outside factors Genetic equilibrium: a population with no net change in allele or genotype frequencies over time If allele frequencies do change over successive generations, evolution is occurring G.H. Hardy Wilhelm Weinberg 1877 - 1947 1862 - 1937 Allele Frequencies If only two alleles, A and a, exist at a locus, the sum of their frequencies in a population must equal 1 p+q=1 p = frequency of the dominant allele (A) q = frequency of the recessive allele (a) When we know the value of p or q, we can calculate the other value: p = 1 − q, and q = 1 − p The Hardy–Weinberg Principle (cont’d.) The Hardy–Weinberg principle states that genotypes in a population at genetic equilibrium occur in the frequency: p2 + 2pq + q2 = 1 p2 = frequency of AA 2pq = frequency of Aa q2 = frequency of aa  p2 + 2pq + q2 = 1 p2 = frequency of AA - p * p =.64 q2 = frequency of aa - q * q=.04 2pq = frequency of Aa - pq + qp = 2pq =.32  Conditions of Genetic Equilibrium The Hardy-Weinberg principle represents an ideal situation that seldom occurs in the natural world Used for evolution, medical applications Genetic equilibrium exists only when five conditions are met: Random mating, no net mutations, large population size, no migration, and no natural selection If any of the Hardy-Weinberg conditions are not met - > microevolution occurs Microevolution: changes in allele or genotype frequencies that occur within a population over generations  Practice In corn, purple kernels are dominant to yellow. Out of 100 kernels randomly taken from a population in Hardy-Weinberg equilibrium, 15 kernels are yellow and 85 are purple. What is the phenotypic frequency of the yellow allele in this population? What is the allele frequency of the recessive allele? Microevolution There are five microevolutionary processes, the opposites of conditions for genetic equilibrium: Nonrandom mating Mutation Genetic drift Gene flow Natural selection In Genetic Drift, Random Events Change Allele Frequencies Random evolutionary changes in small breeding populations results in changes in allele frequencies in a population Decreases genetic variation within a population, but increases genetic differences among different populations In a small population, an allele present at a low frequency could be completely lost by chance OOPS!! Genetic Bottleneck Occasionally, a population may rapidly and severely decrease due to disease, exploitation, or sudden environmental change As the population again increases in size, many allele frequencies may differ from those preceding the decline Florida Panthers descended from just 6 individuals Researchers compared DNA samples from museum species from 19th century to samples collected in 1980 Found that late 20th century cats had only a third of the genetic diversity of their 19th century ancestors Modeling suggest population reduced to six individuals (1 female or related females) Culver et al. (2018). Animal Conservation Founder Effect Genetic drift that results when a few individuals from a large population found a new colony The only alleles in the new population will be those of the colonizers – allele frequencies are usually quite different from those of the parent population Finnish heritage disease is a genetic disorder more common in Finnish people due to founder effect Gene Flow Generally Increases Variation Within a Population The migration of breeding individuals between populations, with a corresponding movement of alleles, increasing genetic variability in the recipient population Gene flow between two populations increases genetic similarity Counteracts the effects of natural selection and genetic drift Adaptive Evolutionary Change Natural selection enables populations to change, thereby adapting to different environments and different ways of life Preserves individuals with favorable phenotypes (and their genes) and eliminates those with unfavorable phenotypes Individuals that survive and produce fertile offspring have a selective advantage Hoekstra et al. (2005). Heredity  Natural Selection Acts on Phenotype Many plant and animal characteristics are controlled by more than one gene (polygenic control) No selection Stabilizing selection Directional selection Disruptive selection Number of individuals Phenotype (a) (b) (c) (d) Three kinds of selection cause changes in the normal distribution: Stabilizing, directional, and disruptive selection Balancing selection Balancing selection occurs when natural selection maintains stable frequencies of two or more phenotypic forms in a population Heterozygote advantage Frequency-dependent selection  Heterozygote Advantage Heterozygous carriers of the sickle cell allele are more resistant to malaria while homozygotes for sickle cell usually die early (a) (b) Where malaria is a problem, the heterozygote is more fit than either homozygote Practice Questions (come by office hours for answers) Which of the following is an example of the founder effect? A. A flock of finches is separated by strong winds and a small group randomly colonize a new island. B. Finches on an island gradually evolve larger beaks over 100 generations of eating larger seeds C. Green fish stop breeding with blue fish of the same species in the same lake D. Red foxes are larger due to an increase in rabbit populations Drought on an island in the Galapagos led to finches with long beaks and short beaks being able to find food. However, the beaks that were intermediate diminished in the populations. What kind of selection was this for the finches? A) artificial selection B) directional selection C) stabilizing selection D) disruptive selection A group of green and yellow-stemmed roses live by the ocean. Green is dominant to yellow. You calculate their allele frequencies and find p=0.6 and q=0.4. One day, a tsunami rolls through and afterwards you recalculate the alleles of the new population that is still alive and find that the alleles are p=0.3 and q=0.7 Which microevolutionary process is this an example of? Why?