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Major Objectives 1. Understand the historical context for evolutionary theory, including Darwin's field research that helped frame his view of life 2. Define Darwinism and Darwin's main ideas from the Origin of Species 3. Discuss examples of natural selection and other evidence for evolution "Nothing in biology makes sense except in the light of evolution." Theodosius Dobzhansky What is Evolution?? Change in gene frequency in a population over time Evolution was resisted by Western culture -Greek philosophers: Aristole (scala naturae) -Natural theology (1700's) Ladder of Life Lamarkian Evolution Figure 22.3 Formation of sedimentary rock and deposition of fossils from different time periods Figure 22.2 Fossils of trilobites, animals that lived in the seas hundreds of millions of years ago Figure 22.4 Strata of sedimentary rock at the Grand Canyon Galapagos Island Finches: Beaks Differed According to Food Supply Darwin's Intellectual Revolution Millions of DNA Switches That Power Human Genome's Operating System Are Discovered ScienceDaily (Sep. 5, 2012) — The locations of millions of DNA 'switches' that dictate how, when, and where in the body different genes turn on and off have been identified by a research team led by the University of Washington in Seattle. Genes make up only 2 percent of the human genome and were easy to spot, but the on/off switches controlling those genes were encrypted within the remaining 98 percent of the genome. In the mid-1990s, researchers were surprised to discover that fruit flies, mice, and humans who were born missing eye structures had defects in the same gene. This gene, called Pax6 (or eyeless in flies), is required for normal eye development in all animals with bilateral symmetry. Even in eyes that look very different, Pax6 functions in much the same way. When placed in a fly, the mouse Pax6 gene activates all the genes necessary to form a normal, functional fly eye (not a mouse eye). What the Encode project tells us about the human genome and 'junk ENCODE, the Encyclopedia of DNA Elements, is the most ambitious human genetics project to date. It takes the 3 billion letters described by the Human Genome Project in 2000, and tries to explain them. http://www.youtube.com/watch?v=Y3 V2thsJ1Wc http://www.youtube.com/watch?v=UBQ 5a7mCpMs Darwinism has a dual meaning -Evolution as the explanation for life's unity and diversity (descent with modification) -Natural selection as the cause of adaptive evolution Figure 22.7 Descent with modification Darwin's main ideas from "The Origin of Species" Artificial Selection-selecting desired traits 1. Natural selection is differential success in reproduction Idea of overreproduction influenced by Thomas Malthus (1798) 1. Differential success in reproduction Not All Organisms Get To Mate Why is this important? 2. Natural selection occurs through an interaction between the environment and the variability inherent among the individual organisms making up a population Why is this important? 3. The product of natural selection is the adaptation of populations of organisms to their environment How does this result in population change? How does natural selection work? Peppered Moths: Real Natural Selection Example •Original population: white in color, blended into lichens on trees •During industrial revolution, lichens died & trees covered in soot •Lighter moths had higher predation rates, darker moths had high survival rates •Over time, population became dominated by dark moths Darwin’s Theory of Natural Selection: Favorable traits that are heritable become more common in successive generations of a population, and unfavorable traits that are heritable become less common If phenotypes have a genetic basis, phenotypes will increase or decrease in frequency Results in change in gene frequency over time Darwin’s Theory Lacked Satisfactory Theory of Heredity • Modern Synthesis (1920-1940): • Unification of Mendel’s theory of heredity and Darwinian evolution Evidence for Evolution Evidence for Evolution Fossil Record Natural selection Homologous structures Molecular Biology-DNA Islands Vestigial Organs Convergent Evolution Same skeletal elements, different functions 500 endemic species Vestigial Organs Convergent Evolution- The independent development of similarity between species as a result of their having similar ecological roles and selection pressures Major Objectives 1. Do populations or organisms evolve? 2. What are the major differences between microevolution and macroevolution? 3. What are the four main factors that can alter genetic diversity? 4. Identify the three main modes of natural selection. 5. Describe the four main reasons why natural selection cannot produce perfection. Populations Evolve, Natural Selection Occurs at the Level of Organisms Types of Evolution *Microevolution-A change in the gene pool of a population from generation to generation *Macroevolution-Evolutionary change on a grand scale, encompassing the origin of new taxonomic groups, evolutionary trends, adaptive radiation, and mass extinction Four Main Factors That Can Alter Genetic Diversity 1) Genetic Drift A change in a population’s genetic diversity due to chance Why are small populations so vulnerable to extinction? Extinction Vortex Disturbance lowers population size Reduced Genetic Variability Reduced ability to survive environmental stochasticity Founder Effect—example for higher risk of breast or ovarian cancer in women Four Main Factors That Can Alter Genetic Diversity 1) Genetic Drift 2) Natural Selection Differential success in reproduction Four Main Factors That Can Alter Genetic Diversity 1) Genetic Drift 2) Natural Selection 3) Gene Flow Genetic exchange due to the migration of fertile individuals between populations Four Main Factors That Can Alter Genetic Diversity 1) Genetic Drift 2) Natural Selection 3) Gene Flow 4) Mutation An accidental change in an organism's DNA Natural Selection Common Misconceptions: Not Goal-Oriented or Progressive • Change follows environmental conditions "Survival of the Fittest" -Misleading phrase: Does NOT mean competitiveness contest between individuals -Darwinian Fitness: the contribution an individual makes to the gene pool of the next generation relative to the contributions of other individuals Natural Selection Common Misconceptions: Is Specific to Environmental Conditions • Specific phenotype is only adaptive in a specific environment Natural selection does not produce perfection soft-shell turtle 1) Evolution is limited by historical constraints 2) Adaptations are often compromises 3) Not all evolution is adaptive 4) Selection can only edit existing variations All phenotypic variation is NOT heritable Phenotypes are the result of: 1) inherited genotype 2) environmental influences Reaction Norms: pattern of phenotypic expression of a single genotype across a range of environments Four Main Modes of Genetic Selection 1) Directional Selection 2) Diversifying/Disruptive Selection 3) Stabilizing Selection 4) Sexual Selection Directional Selection Diversifying Selection Stabilizing Selection Sexual Selection Three Main Types of Speciation 1) Allopatric Speciation 2) Sympatric Speciation 3) Parapatric Speciation Allopatric Speciation Sympatric Speciation Females generally choose to lay their eggs on the type of fruit they grew up in, and males tend to look for mates on the type of fruit they grew up in. So hawthorn flies generally end up mating with other hawthorn flies and apple flies generally end up mating with other apple flies. This host shift from hawthorns to apples may be the first step toward sympatric speciation—in fewer than 200 years, some genetic differences between these two groups of flies have evolved. Parapatric Speciation Some of these plants live near mines where the soil has become contaminated with heavy metals. The plants around the mines have experienced natural selection for genotypes that are tolerant of heavy metals. Meanwhile, neighboring plants that don't live in polluted soil have not undergone selection for this trait. The two types of plants are close enough that tolerant and non-tolerant individuals could potentially fertilize each other — so they seem to meet the first requirement of parapatric speciation, that of a continuous population. However, the two types of plants have evolved different flowering times. This change could be the first step in cutting off gene flow entirely between the two groups. Major Objectives 1. Define the biological species concept and its limitations 2. Understand prezygotic and postzygotic barriers that isolate gene pools of biological species. Biological Species Concept A species is a population or group of populations whose members have the potential to interbreed with each other in nature to produce viable, fertile offspring. Lynx rufus Lynx canadensis What about limitations? *Difficult to test reproductive isolation of morphologically similar fossils *Difficult to test reproductive isolation of morphologically living species *Doesn't address species that reproduce asexually (e.g., bacteria) *Doesn't address hybrids (e.g., plants, zoo animals) Habitat Isolation Land and water garter snakes Behavioral Isolation the eastern and western meadowlarks shown below are very similar birds whose habitats overlap Temporal Isolation Mechanical Isolation Gametic Isolation Sea urchins release gametes at same time and place but they don’t fuse Reduced Hybrid Viability In crosses between different species of irises, for example, the embryos die before seeds form. Reduced Hybrid Fertility Mules are sterile hybrids formed by mating a female horse with a male donkey. Hybrid Breakdown Mating between two F1 hybrids produces a second hybrid generation, this F2 generation may be unable to reproduce because of hybrid breakdown. The second-generation hybrids are defective in some way that prevents successful reproduction. Hybrid breakdown has been demonstrated in sunflower hybrids.