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Population Dynamics Humans and malaria Science (2010), v.328:841 Global Map of 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 Malaria life cycle http://www.cdc.gov/malaria/about/index.html http://www.who.int/topics/malaria/en/ Malaria is large threat to human health in equatorial regions of the Earth But some human populations show resistance to the disease. How?? 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 – The inescapable conclusion: Unequal reproductive success Evidence for humans? • 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. Copyright © 2007 Pearson Education Inc., publishing as Pearson Benjamin Cummings 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. – Population genetics • Focuses on populations as the evolutionary units. • Tracks the genetic makeup of populations over time. Genetic Variation in Populations – 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). Sickle-cell allele: genotype, phenotype, and resistance to malaria Malaria infection causes cells to sickle. Sickled cells, along with parasite, are killed. from The Human Evolution Coloring Book, 2nd Ed. Sickle-cell Allele Biochemistry of sickle-cell disease from Steinberg, M.H., (2006), Trends Pharm. Sci., 27(4):204-210. 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 Meiosis, The Basis of Sexual Reproduction – Sexual reproduction depends on • Production of gametes through Meiosis. • Fertilization. Copyright © 2007 Pearson Education, Inc. publishing as Pearson Benjamin Cummings 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. – Humans are diploid organisms. • Their cells contain two sets of chromosomes • Their gametes are haploid, having only one set of chromosomes Figure 8.14 – Fertilization • Is the fusion of sperm and egg. • Creates a zygote, or fertilized egg. – Sexual life cycles involve an alternation of diploid and haploid stages. 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. 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? Meiosis is the process where the two sets of homologous chromosomes are separated into different cells - egg or sperm. 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 The Origins of Genetic Variation – Offspring of sexual reproduction are genetically different from their parents and from one another. – Independent assortment of chromosomes – Random fertilization – Crossing over – Random mutations Independent Assortment of Chromosomes – In independent assortment, every chromosome pair orients independently of the others during meiosis. 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 – What happens when errors occur in meiosis? How Accidents During Meiosis Can Alter Chromosome Number – 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 Figure 8.20 – The result of nondisjunction Down Syndrome: An Extra Chromosome 21 – Down Syndrome • Is a condition where an individual has an extra chromosome 21. • Is also called trisomy 21. – The incidence of Down Syndrome increases with the age of the mother. Abnormal Numbers of Sex Chromosomes – Nondisjunction • Also affects the sex chromosomes.