Population Dynamics Humans and malaria Science (2010), v.328:841 Global Map of Malaria Malaria a far more urgent task: reducing the horrific toll of malaria in central Africa, where five countries account for 50% of all global deaths from the disease and elimination is not possible. Science (2010), v.328:849 Malaria life cycle Malaria is large threat to human health in equatorial regions of the Earth http://www.cdc.gov/malaria/about/index.html http://www.who.int/topics/malaria/en/ But some human populations show resistance to the disease. How?? 1
Natural Selection Overproduction Environmental pressure/competition Pre-existing individual variation Heritable traits Happens over generations (time) Happens in populations (not single individuals) Offspring must be viable and fertile Darwin s Inescapable Conclusion Fact 1: Potential for overproduction and competition for existence Fact 2: Individual variation Evidence for humans? The inescapable conclusion: Unequal reproductive success Darwin called this process natural selection. The result of natural selection is evolution when a population has adapted to its environment. Human Population Growth The History of Global Population Growth Darwin s Inescapable Conclusion Fact 1: Potential for overproduction and competition for existence Fact 2: Individual variation Evidence for humans? The inescapable conclusion: Unequal reproductive success Darwin called this process natural selection. The result of natural selection is evolution when a population has adapted to its environment. The Modern Synthesis: Darwinism Meets Genetics The modern synthesis is the fusion of genetics with evolutionary biology. Populations as the Units of Evolution A population Is a group of individuals of the same species living in the same place at the same time. Is the smallest biological unit that can evolve. Copyright 2007 Pearson Education Inc., publishing as Pearson Benjamin Cummings 2
Genetic Variation in Populations Population genetics Focuses on populations as the evolutionary units. Tracks the genetic makeup of populations over time. Individual variation abounds in populations. Not all of this variation is heritable. Only the genetic component of variation is relevant to natural selection. Sources of Genetic Variation in sexually reproducing organisms Mutations Are changes in the DNA of an organism. Sexual recombination Crossing over Independent assortment of chromosomes Random fertilization Hemophilia Is a blood-clotting disease. Intergenerational Mutation Rate By how many mutations does your genome differ from your parents genome? Roach et al (2010) found about 60 mutations, 30 from each parent, that occurred during the formation of egg and sperm (meiosis). 3
Sickle-cell allele: genotype, phenotype, and resistance to malaria Malaria infection causes cells to sickle. Sickled cells, along with parasite, are killed. Sickle-cell Allele from The Human Evolution Coloring Book, 2nd Ed. Biochemistry of sickle-cell disease Sources of Genetic Variation in sexually reproducing organisms Mutations Are changes in the DNA of an organism. Sexual recombination Crossing over Independent assortment of chromosomes Random fertilization from Steinberg, M.H., (2006), Trends Pharm. Sci., 27(4):204-210. Meiosis, The Basis of Sexual Reproduction Sexual reproduction depends on Production of gametes through Meiosis. Fertilization. Gametes and the Life Cycle of a Sexual Organism The life cycle of a multicellular organism is the sequence of stages leading from the adults of one generation to the adults of the next. Copyright 2007 Pearson Education, Inc. publishing as Pearson Benjamin Cummings 4
Fertilization Humans are diploid organisms. Is the fusion of sperm and egg. Creates a zygote, or fertilized egg. Their cells contain two sets of chromosomes Their gametes are haploid, having only one set of chromosomes Sexual life cycles involve an alternation of diploid and haploid stages. Figure 8.14 Eukaryotic Chromosomes Each eukaryotic chromosome contains one very long DNA molecule, Typically bearing thousands of genes. The number of chromosomes in a eukaryotic cell Depends on the species. Meiosis is the process where the two sets of homologous chromosomes are separated into different cells - egg or sperm. Homologous Chromosomes Homologous chromosomes are matching pairs of chromosomes. Humans have Two different sex chromosomes, X and Y. A female has XX A male has XY Twenty-two pairs of matching chromosomes, called autosomes. Meiosis is the process where the two sets of homologous chromosomes are separated into different cells - egg or sperm. Start with one cell with 2 sets of homologous chromosomes End with 4 cells with 1 set of homologous chromosomes How does this happen? 5
The Process of Meiosis In meiosis, Haploid daughter cells are produced in diploid organisms. Two consecutive divisions occur, meiosis I and meiosis II, preceded by interphase. Crossing over occurs. From movie folder play: MeiosisOverview.html Interphase.html ProphaseI.html MetaphaseI.html AnaphaseI.html TelophaseICytokin.html ProphaseII.html MetaphaseII.html AnaphaseII.html TelophaseIICytokin.html Figure 8.16.1 Figure 8.16.2 Figure 8.16.3 Review: Comparing Mitosis and Meiosis All the events unique to meiosis occur during meiosis I. Figure 8.17 6
The Origins of Genetic Variation Offspring of sexual reproduction are genetically different from their parents and from one another. Independent Assortment of Chromosomes In independent assortment, every chromosome pair orients independently of the others during meiosis. Independent assortment of chromosomes Random fertilization Crossing over Random mutations Random Fertilization The human egg cell is fertilized randomly by one sperm, leading to genetic variety in the zygote. Crossing Over In crossing over, Homologous chromosomes exchange genetic information. Genetic recombination occurs. From the movie folder play: - CrossingOver.html When Meiosis Goes Awry How Accidents During Meiosis Can Alter Chromosome Number What happens when errors occur in meiosis? In nondisjunction, The members of a chromosome pair fail to separate during anaphase. Gametes with an incorrect number of chromosomes are produced. Copyright 2007 Pearson Education, Inc. publishing as Pearson Benjamin Cummings 7
The result of nondisjunction Figure 8.20 Down Syndrome: An Extra Chromosome 21 The incidence of Down Syndrome increases with the age of the mother. Down Syndrome Is a condition where an individual has an extra chromosome 21. Is also called trisomy 21. Abnormal Numbers of Sex Chromosomes Nondisjunction Also affects the sex chromosomes. 8