Chapter 4: Genes and Their Evolution: Population Genetics Demes, Reproductive Isolation, and Species Evolution is about groups of potentially reproducing organisms. Deme refers to members of a species that produce offspring. All the genetic material within a population is referred to as the gene pool. “Species” = populations and members capable of breeding producing fertile offspring. Species are defined on the basis of reproductive isolation. Population genetics studies change over time (or the lack of it) in gene pools. Microevolution Change in gene frequency over a few generations For example, Morgan’s fruit fly eye color changes Macroevolution Substantial change in characteristics over many generations Hardy-Weinberg Law: Testing the Conditions of Genetic Equilibrium It is a method for studying genetic change in populations If no change is occurring within the population, gene frequencies remain the same If change is occurring, evolution is happening within the population Mutation: The Only Source of New Alleles Mutation is the only source of new genetic information Mutation can be any heritable change in the structure or amount of genetic material Mutations are only important to populations if found in sex cells Two types of mutations: – Spontaneous (no known cause) – Induced (from human causes such as exposure to X-rays) Most mutations are harmless Mutations are important for populations ONLY if they result in some change that impacts (positive or negative) the organisms ability to fit in the environment Positive changes passed on to successive generations by natural selection. Negative changes result in extinctions Natural Selection in Animals: The Case of the Peppered Moth and Industrial Melanism The peppered moth is the best documented evidence of natural selection Found in England Two forms: light and dark As pollution covered trees, lighter peppered moths were more easily preyed on by birds and the darker form became more prevalent 1970s, stricter pollution laws again changed the moths’ habitat, and the darker form became the easier prey; the lighter form became more common Natural Selection in Humans: Abnormal Hemoglobins and Resistance to Malaria The Hemoglobin S gene causes sickle-cell anemia in humans Found in millions of humans, destroys red blood cells (thus anemia) Cells become sickle-shaped and rigid, and plug capillaries Life expectancy 42-48 yrs The Geography of Sickle-Cell Anemia and a Possible Association with Malaria 25 % of people living in equatorial Africa have the S gene (symptoms: blood/oxygen issues, death during strenuous events such as sports) Frequencies overlap areas where malaria is endemic (constantly present). (Turks, No. Africans, Iranians, Arabs) This gene has 2 alleles. If you have one, you are malaria resistant and not highly anemic. If you have both, you are sickle-cell anemic One inherited from each parent, thus the 1:4 ratio observed in local populations A relationship has been documented between possession of one S gene and higher survival when exposed to malaria Scientists were puzzled by such a high frequency of a bad adaptation (anemia) The Biology of Sickle-Cell Anemia and Malarial Infection It was discovered that people with one allele had lower oxygen levels in their blood (due to anemia), which is where the malaria parasite finishes its life cycle The parasite generally cannot survive and reproduce History of Sickle-Cell Anemia and Malaria Sickle-cell tied to spread of Bantu, who carried the S mutation into equatorial Africa The Bantu introduced agriculture into the region; large, cleared areas were ideal environments for mosquitoes carrying malaria Genetic Drift: Genetic Change Due to Chance Random change in allele frequency over time Can lead to one allele being lost and the other fixed in a population May occur in a group that is endogamous (reproducing only within the group) Gene Flow: Spread of Genes across Population Boundaries Gene flow often refers to migration, though influenced by culture, social structure Effects of gene flow (exchange of genes between populations) have increased over time All living humans are the same species, but can we all really interbreed? In theory yes, but what factors prevent this? Natural Causes for Breeding Isolates Geography – Land masses, islands, oceans, mountain ranges, canyons, rivers, etc. Ecology – Deserts, jungles, flora and fauna distribution, etc. Cultural Causes for Breeding Isolates Humans control breeding by cultural factors including endogamy and exogamy. – Endogamy is mating with others from inside the same group – Exogamy is mating with others from outside the group – Worldwide, there are many examples of both The Adaptive Significance of Human Variation Humans have a number of long and short term adaptations that allow them to live within diverse environments Next chapter, we will learn how variation within the human species has helped humans adapt to different environmental conditions including: – Differences in altitude – Differences in exposure to sun – Differences in environmental temperatures – Long-term exposure to infectious diseases