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SB5 Chapters 14, 15, & 16 Unit 4: Evolution
How Populations Evolve Chapter 15
Clown, Fool, or Simply Well Adapted • All organisms have evolutionary adaptations • Inherited characteristics that enhance their ability to survive and reproduce • Example:The blue-footed booby of the Galápagos Islands has features that help it succeed in its environment • Large, webbed feet help propel the bird throughwater at high speeds • A streamlined shape, large tail, and nostrils that close are useful for diving • Specialized salt-secreting glands manage salt intake while at sea
Evidence of Evolution • Aristotle and the Judeo-Christian culture believed that species were fixed. • Fossils suggested that life forms change. • Early 1800s Jean Baptiste Lamarck, a French invertebrate specialist, studied fossils to learn about different invertebrates. • He was surprised by the similarities between the existing animals that he studied. • He also noticed that fossils showed traits changing over time. For example, he noticed that giraffes’ necks were getting longer and longer from generation to generation. He theorized that this trait developed as giraffes stretched their necks for higher leaves to avoid competition with shorter animals.
Charles Darwin (1830s) • English naturalist who sailed on the HMS Beagle • Mission of 5-yr voyage – map the coast of South America • Darwin’s role – collect biological and geological samples • Inspirational reading – Charles Lyell’s book proposing that the earth was millions of years old
Darwin’s Observations • Similarities between living things and fossil organisms • Diversity of life on the Galapagos Islands, such as blue-footed boobies and giant tortoises • Visited 4 islands in the chain; noticed each island had slightly different varieties of animal • Discovered many new species that were not found in S America
Darwin’s Conclusions • Darwin became convinced that the Earth was old (as Lyell said in the book he was reading). • He suggested that the Earth is continually changing. • He concluded that living things also change or evolve over generations. • He also stated that living species descended from earlier life-forms: descent with modification.
Another Darwin Conclusion • He believed that new species could appear gradually through small changes in traits. • He was unsure how this process would work. • He turned to animal breeders for help. • Breeders selected desired traits in pigeons and bred them. (We called this selective breeding in the Genetics Unit.) Darwin called this practice artificial selection.
Artificial Selection • Darwin explained that if humans could change species by artificial selection, then perhaps nature could do the same thing over time. • He concluded that this is how new species develop naturally over time…naturalselective breeding.
Darwin’s Theory – Natural Selection • Darwin proposed that natural selection is the mechanism of evolution. • Natural selection – NATURE’s way of selective breeding • Four Basic Principles: • Organisms in a population show variations. • Variations can be passed down to offspring. • Organisms produce more offspring than resources can support. • Variations that increase survival also increase the likelihood of reproduction and inheritance.
Proposed Result of Natural Selection • The favorable characteristics are passed down to offspring who are likely to survive due to the favorable characteristic. • Population size increases due to favorable characteristics in offspring. • Over time, whole populations change to include the favorable characteristic. Enough changes can give an entirely new species.
Evidence Supporting Evolution Fossil Record, Biogeography, Comparative Anatomy, Comparative Embryology, and Molecular Biology
Fossil Record Studies • Fossils offer evidence for evolution: • Hominid skull • Petrified trees • Fossilized organic matter in a leaf • Scorpion in amber
Fossil Record: Link Old to New • Many fossils link early extinct species with species living today • These fossilized hind leg bones link living whales with their land-dwelling ancestors
Biogeography: Plate tectonics • Continental drift is the slow, incessant movement of the Earth’s crustal plates on the hot mantle. EurasianPlate NorthAmericanPlate AfricanPlate PacificPlate Splitdeveloping NazcaPlate SouthAmericanPlate Indo-AustralianPlate Antarctic Plate Edge of one plate being pushed over edge of neighboring plate (zones of violent geologic events)
Biogeography CENOZOIC Eurasia North America Africa Crustal plates have shifted over millions of years to significantly change the layout of the globe. India SouthAmerica Australia Antarctica Laurasia Millions of years ago MESOZOIC Gondwana Pangaea PALEOZOIC
Comparative Anatomy: Homologous Structures • Homologous structures: anatomically similar structures inherited from a common ancestor • Forelimbs of vertebrates are adapted for different uses, but they have the similar bones. Human Cat Whale Bat
Comparative Anatomy: Vestigial Structures • Vestigial structure: reduced forms (smaller) of functional structures in other organisms • Example: Human appendix; structure is important for digestion in many mammals but of limited use in humans and apes
Comparative Embryology • Vertebrate embryos exhibit homologous structures during certain phases of development. • Example: Pharyngeal pouches • Develop into gills in fish • Develop into ears, jaws, and throats in mammals, birds, and reptiles
Connection – Natural Selection in Action • Darwin’s idea of natural selection and the evidence of comparative anatomy has lead scientists to observe evolution in action. • Example: Evolutionary adaptations have been observed in many populations of birds and insects • Camouflage adaptations
Connection – Natural Selection in Action Example: Insecticide resistance Insecticideapplication Chromosome with geneconferring resistanceto insecticide Additionalapplications of thesame insecticide willbe less effective, andthe frequency ofresistant insects inthe populationwill grow Survivor
Modern Synthesis of Ideas • Modern synthesis of evolution ideas connects Darwin’s theory of natural selection with population genetics • So…what’s population genetics?
Population Genetics • Populations are the units of evolution. • A species is a group of populations whose individuals can interbreed and produce fertile offspring. • Human populations tend to concentrate locally, as this satellite photograph of North American shows
Population Genetics • Microevolution is change in a population’s gene pool over time. • A gene pool is the total collection of genes in a population at any one time. • Microevolution involves a change in the relative frequencies of alleles in a gene pool. • Example: Bug color ratio (frequency of alleles) What is the ratio of gray-brown to yellow-brown to dark brown bugs?
Population Genetics: Hardy-Weinberg Principle • The Hardy-Weinberg principle states that evolution (change in the frequency of alleles) will not occur unless acted upon by forces that cause change. • Connection to meiosis: the shuffling of genes (crossing over, etc.) during sexual reproduction does not alter the proportions of different alleles in a gene pool • When allelic frequency remains constant, a population is in Hardy-Weinberg equilibrium. • We can map alleles in a population to observe if Hardy-Weinberg equilibrium exists.
Hardy-Weinberg Conditions • The population is very large • The population is isolated • Mutations do not alter the gene pool • Mating is random • All individuals are equal in reproductive success
Hardy-Weinberg & Gold Fish Crackers • Hardy (English mathematician) and Weinberg (German physician) developed an equation to show mathematically that random mating will not cause changes in allele frequencies. • EQUATION: p2 + 2pq + q2 = 1 • frequency of dominant + frequency of recessive = 1 • p + q = 1 • p2 = pp= homozygous dominant genotype • pq = heterozygous genotype • q2 = qq = homozygous recessive genotype
Hardy-Weinberg & Gold Fish Crackers • EQUATION: p2 + 2pq + q2 = 1 • Remember: p +q = 1 • Assume you have the following genotypes in gold fish crackers: FF, Ff, and ff. • Mating is random. Any fish could mate with any other fish. • F and f are the only two alleles from fish color. • F = brown; FF and Ff are brown fish. • f = gold; ff = gold fish
Hardy-Weinberg & Gold Fish Crackers • EQUATION: p2 + 2pq + q2 = 1 • Remember: p +q = 1 • Example: Let’s say that you’ve calculated that you have 16% ff fish. We can use the equation to calculate the frequency of the homozygous dominant and the heterozygous alleles. • Solved on whiteboard in class! • Fishy Frequencies Lab in Class!
Causes of Microevolution • Microevolution: change in frequencies of alleles in a gene pool • Example: if blue eyes began to disappear in the population. • Remember, the Hardy-Weinberg principle says that reproduction and variations in meiosis won’t cause microevolution. • Microevolution only occurs when a force acts on the population to cause the changes.
Microevolution Cause: Genetic Drift • Genetic drift is a change in a gene pool due to chance. • In large populations, enough organisms are reproducing with random selection of alleles to prevent changes in the gene pool. • In smaller populations, random selections could reduce the diversity by mere chance. • Extreme genetic drift can cause the bottleneck effect.
Microevolution Cause: Genetic Drift • Bottleneck Effect: a population dwindles to a very low number and then rebounds • Small population gives little diversity. • Little diversity reproduces little diversity. • Example: African cheetahs – so genetically similar that they appear to be inbred Originalpopulation Bottleneckingevent Survivingpopulation
Microevolution Cause: Genetic Drift • Founder Effect: a small segment of the population moves to a separate area • Separate segment may have traits that are uncommon to the original population • Uncommon traits become common in offspring in the new population • Can result in large genetic variability from one segment of the population to another
Microevolution Cause: Natural Selection • Natural selection acts to select individuals who are the best adapted for survival and reproduction. • Those who lack these traits often die before reproducing. • Natural selection results in the accumulation of traits that adapt a population to its environment • If the environment should change, natural selection would favor traits adapted to the new conditions.
Microevolution Cause: Natural Selection • Phenotypic variation may be environmental or genetic in origin. • Only genetic changes will result in evolutionary adaptations as only genes are passed to offspring. • Example: Environment may cause a tan, but tans are not passed down to offspring. Population skin color will only become darker if darker skin color genes become more common in offspring.
Three Outcomes of Natural Selection Originalpopulation Frequency ofindividuals Phenotypes (fur color) Originalpopulation Evolvedpopulation Stabilizing selection Directional selection Diversifying selection
Microevolution Cause: Natural Selection • Directional Selection: extreme version of a trait makes the organism more fit for survival and reproduction • Example: Peppered moth – until mid-1850s moths were light colored in England. By early 1900s, nearly all were dark colored. The darker the moth, the better it blended into the sooty surroundings of the industrial pollution Luck y for the moth species that it had a darker allele to begin favoring.
Microevolution Cause: Natural Selection • Low genetic variability may reduce the capacity of a species to survive as humans continue to alter the environment. • Example: African cheetah – diversity was decreased by bottleneck effect which resulted in fewer alleles (as if they’d been inbred) • Fewer alleles = reduced capacity to adapt to environmental challenges
Microevolution Cause: Sexual Selection • Sexual selection leads to the evolution of secondary sexual characteristics • These may give individuals an advantage in mating.
Nobody is perfect…or ever will be. • Natural selection cannot fashion perfect organisms. • Historical constraints • Adaptive compromises • Chance events • Availability of variations
Connection: “Perfect” Bacteria • The excessive use of antibiotics is leading to the evolution of antibiotic-resistant bacteria • Example: Mycobacterium tuberculosis • They aren’t perfect, but the are perfectly annoying to the CDC.