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Announcements ● Tutoring Center SCI I, 407 M 12-3, 5:30-6:30; W 8-9, 5:30-6:30, Th 8-12, 6-7; F 8-9 ● MasteringBiology Assignment due Thursday 5/26 ● Parts of Chapters 13 and 14 Evolution Evolution – genetic change in a population or a species over generations. •An understanding of evolution informs every field of biology, for example: Agriculture Medicine Biotechnology Conservation biology Evolution Darwin made two main points in The Origin of Species: • Organisms inhabiting Earth today descended from ancestral species • Natural selection was the mechanism for descent with modification Observations Overproduction of offspring Individual variation Conclusion Natural selection: unequal reproductive success Observing Natural Selection There are many examples of natural selection in action. Galápagos finches change beak size depending upon the size and shape of available seeds. Natural Selection in Action Examples of natural selection include: • Pesticide-resistant insects • Antibiotic-resistant bacteria • Drug-resistant strains of HIV Insecticide application Chromosome with gene conferring resistance to pesticide Survivors Reproduction Evolution Natural selection leads to: • A population (a group of individuals of the same species living in the same place at the same time) changing over generations • Evolutionary adaptation A flower mantid in Malaysia A Trinidad tree mantid that mimics dead leaves A leaf mantid in Costa Rica Evidence of Evolution – Biological evolution leaves observable signs. – We will examine five of the many lines of evidence in support of evolution: • The fossil record • Biogeography • Comparative anatomy • Comparative embryology • Molecular biology Evidence of Evolution •The Fossil Record – Fossils are: • Imprints or remains of organisms that lived in the past • Often found in sedimentary rocks Evidence of Evolution •Biogeography – The study of the geographic distribution of species that first suggested to Darwin that today’s organisms evolved from ancestral forms. Common ringtail possum Australia Koala Common wombat Red kangaroo Evidence of Evolution •Comparative Anatomy • Is the comparison of body structure between different species • Confirms that evolution is a remodeling process Human Cat Whale Bat Evidence of Evolution •Comparative Embryology – Early stages of development in different animal species reveal additional homologous relationships. Pharyngeal pouches Post-anal tail Chicken embryo Human embryo Evidence of Evolution •Molecular Biology – The hereditary background of an organism is documented in: • Its DNA • The proteins encoded by the DNA – Evolutionary relationships among species can be determined by comparing: • Genes • Proteins of different organisms 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 • The smallest biological unit that can evolve – Gene pool - The total collection of alleles in a population at any one time. – Microevolution - When the relative frequency of alleles changes over a number of generations, (evolution is occurring on its smallest scale). Mechanisms of Evolution – The main causes of evolutionary change are: • Genetic drift • Gene flow • Natural selection Mechanisms of Evolution – Genetic drift is: • A change in the gene pool of a small population • Due to chance rr RR RR Only 5 of 10 plants leave offspring Rr RR rr Rr rr RR Rr rr Rr Rr Rr Only 2 of 10 plants leave offspring RR RR RR RR RR Rr RR RR RR RR RR RR RR Rr RR RR Mechanisms of Evolution – The bottleneck effect: • Is an example of genetic drift • Results from a drastic reduction in population size Original population Bottlenecking event Surviving population Mechanisms of Evolution – The founder effect is likely when a few individuals colonize an isolated habitat and represent genetic drift in a new colony. Tristan da Cunha South Africa America Mechanisms of Evolution – Gene flow: • Is genetic exchange with another population • Tends to reduce genetic differences between populations Outcomes of Natural Selection Evolved population Frequency of individuals Original population Phenotypes (fur color) Original population (a) Directional selection (b) Disruptive selection (c) Stabilizing selection Mechanisms of Evolution ●Sexual dimorphism is: A distinction in appearance between males and females Not directly associated with reproduction or survival ●Sexual selection is a form of natural selection in which inherited characteristics determine mating preferences. (a) Sexual dimorphism in a finch species (b) Competing for mates Evolution Macroevolution: •Encompasses the major biological changes evident in the fossil record •Includes the formation of new species – Speciation: • Is the focal point of macroevolution • May occur based on two contrasting patterns – In nonbranching evolution: • A population transforms but • Does not create a new species Evolution and Speciation Speciation: •Is the focal point of macroevolution •May occur based on two contrasting patterns In nonbranching evolution: •A population transforms but •Does not create a new species Nonbranching Evolution (no new species) Branching Evolution (results in speciation) Biological Species Concept – Species is a Latin word meaning: • “Kind” or • “Appearance.” – The biological species concept defines a species as • “A group of populations whose members have the potential to interbreed and produce fertile offspring” Reproductive Barriers between Species – Prezygotic barriers prevent mating or fertilization between species. – Postzygotic barriers operate if: • Interspecies mating occurs and • Hybrid zygotes form Zygote Gametes Prezygotic barriers • Temporal isolation • Habitat isolation • Behavioral isolation • Mechanical isolation • Gametic isolation Viable, Postzygotic barriers fertile • Reduced hybrid viability offspring • Reduced hybrid fertility • Hybrid breakdown Mechanisms of Speciation Species can form by: • Allopatric speciation, due to geographic isolation • Sympatric speciation, without geographic isolation Allopatric speciation Simpatric speciation Mechanisms of Speciation Populations become allopatric Populations become sympatric Populations interbreed Gene pools merge: No speciation Geographic barrier Time Populations cannot interbreed Reproductive isolation: Speciation has occurred