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Evolution Unit Earth’s History Evolution Viewpoint • Big Bang Theory • Earth is 4.6 billion years old • Life has changed over time. • Many species failed to survive and became extinct. Life’s History Life’s existed for at least 3.6 billion years, most of Earth’s history. For most of its history, life was exclusively unicellular. The first half of life’s history saw only prokaryotic organisms. Multicellular eukaryotes arose about 1 billion years ago. Life’s History Animals diversified in the seas about 600 million years ago. Plants colonized land about 440 million years ago and were followed shortly by land animals. Humans of any sort are a very recent evolutionary innovation (~ 7 million years before present). Phylogeny • Phylon = tribe, geny = genesis or origin • The evolutionary history of a species or a group of related species. • Traces life backward to common ancestors. • How did we know about past life on Earth? Phylogeny • Found in fossils and the fossil record. Fossils • Any preserved remnant or impression of a past organism. Fossils • Relics or impressions of organisms from the past. • Earliest life forms- 3.6 billion years old. • Earth - 4.6 billion years old. • Problem: – Fossil record is incomplete. – Show changes over time from simple to complex. – Many fossils don't have descendants. Evidence for Evolution – The Fossil Record Types of Fossils 1. Mineralized 2. Organic Matter 3. Trace 4. Amber Mineralized Fossils • Found in sedimentary rock. • Minerals (rock) replace cell contents. • Ex: bone, teeth, shells Organic Matter Fossils • Retain the original organic matter. • Ex: plant leaves trapped in shale. • Comment – can sometimes extract DNA from these fossils. Trace Fossils • Footprints and other impressions. No organic matter present. Amber • Fossil tree resin. • Preserve whole specimen. • Usually small insects etc. Fossils - Limitations • • • • Rare event. Hard to find . Fragmentary. Dating. Fossil Dating Methods 1. Relative Dating - by a fossil's position in the strata relative to index fossils. • Law of Superposition 2. Absolute Dating - approximate age on a scale of absolute time. • called Radiometric dating • Estimated from half-life products in the fossil. • Ex: Carbon - 14 Potassium - 40 What do fossils tell us? • That the geographical distribution of organisms has changed over time. • Reason? – The land formations of the earth have changed. Continental Drift • The movement of the earth's crustal plates over time. • Drift is correlated with events of mass extinctions and adaptive radiations of life. Pangaea • 250 million years ago. • One super continent. • Many life forms brought into contact with each other. Mesozoic era • Pangaea began to break up. • 180 million years ago. Results: • Geographical Isolation. • New environments formed. • Old environments lost. • As the environments changed, so did Life. For example: • Australian fauna and flora are unique. • Separated early and remained isolated for 50 million years. Mass Extinctions • At the end of each Era, the Earth experienced a mass extinction. • The sudden loss of many species in geologic time. • May be caused by asteroid hits or other disasters. Eamples of Mass Extinction: Permian Extinction Cretaceous Extinction Permian Extinction • 250 million years ago. • 90% of species lost. Cretaceous Extinction • 65 million years ago. • Loss of the dinosaurs. • Good evidence that this event was caused by an asteroid that hit in the Yucatan, causing a “nuclear winter”. • Earth has a layer of iridium in the crust. Iridium is only found in outer space. Result of Mass Extinctions • Areas (niches) are open for the surviving species to exploit. • Rapid period of speciation (adaptive radiation). • Many new species are formed in a very short period of time. Origins of Life Prokaryotes (1st Life forms) Fossil Modern Bacterial Mats/ Cyanobacteria Early Thoughts on Origins of Life Spontaneous Generation = the idea that nonliving material can produce life Based on observations of environments: Idea that decaying meat produced maggots Idea that mud produced fish Idea that grain produced mice Was life produced by spontaneous generation? NO, spontaneous generation disproved by experiments of Redi and Pasteur Francesco Redi’s Experiment • 1668 Italian physician • Disproved the common belief that decaying meat produced maggots with his controlled experiment. • Redi disproved the spontaneous generation of large organisms. Louis Pasteur’s Experiment In the mid-1800s, Pasteur designed an experiment that disproved the spontaneous generation of microorganisms. Biogenesis= the idea that living organisms come from other living organisms Experiments disproving spontaneous generation lead to the theory of biogenesis. Modern Theories of How Life Developed on Earth Theory of Chemical Evolution The evolution of life by abiogenesis under primitive Earth conditions. • Reducing atmosphere present. • Simple molecules – Ex: H2O, CH4, H2, NH3 Steps of Chemical Evolution 1. Monomer Formation (amino acids) 2. Polymer Formation (proteins) 3. Protobiont Formation 4. Origin of Heredity (DNA & RNA) Complex Molecule Formation • Requires energy sources: – UV radiation – Radioactivity – Heat – Lightning – These factors were present on primitive Earth before life started. Alexander Oparin 1930s • Hypothesized steps of chemical evolution from primitive earth conditions. • Earth’s early atmosphere contained no free oxygen. Instead, it was composed of water vapor, carbon dioxide, nitrogen, methane, and ammonia. • Oparin suggested that energy from the sun, lightning, and Earth’s heat triggered chemical reactions to produce small organic molecules from the substances present in the atmosphere. • Then, rain washed the molecules into the oceans to form a primordial soup that gave rise to life. Miller and Urey, 1953 • Tested Oparin’s hypothesis by simulating primitive earth conditions in the lab. • Organic monomers formed, including amino acids, sugars, lipids, nucleotides, and ATP. • Early earth conditions could have formed monomers for life's origins. Alternate View of Origin of Life Life developed in Volcanic Vents. • Could easily supply the energy and chemical precursors for chemical evolution. • Most primitive life forms are the prokaryotes found in or near these vents. Meteorite Hypothesis • Space seeds • Molecules necessary for life arrived here on meteorites, rocks from space that collide with Earth’s surface. • Many meteorites contain some organic matter. • These molecules, which are necessary for the formation of cells, might have arrived on Earth and entered its oceans. Hypothesis • Life is a natural outcome of chemical evolution. • Life could be possible on many planets in the universe. Modern Earth • • • • Oxidizing atmosphere (contains oxygen). Prevents new abiotic formation of life. Life is present. Biogenesis (life comes from life) Descent with Modification: A Darwinian View of Life Question? How did the diversity of life originate? Through the process of Evolution. What is Evolution? The kind we’re talking about is sometimes called organic evolution to distinguish it from non-biological changes over time. Definition: Evolution is the progressive change in organisms (populations, not individuals) over time. These processes have transformed life on earth from its beginnings to today's diversity. Evolution is the most pervasive principle in biology. Charles Darwin • Father of the modern theory of evolution. • Theory of Evolution by Natural Selection • Descent with Modification. Darwin's Background • Trained as a Naturalist (after trying religion and medicine). Darwin’s Voyage of Discovery A reconstruction of the HMS Beagle sailing off Patagonia. Voyage of the Beagle Result • Darwin's training and travel opportunities allowed him to formulate and support his ideas on Natural Selection. Galapagos Finches Evolution Nothing in biology makes sense except in the light of evolution. – Theodosius Dobzhansky Charles Darwin in later years Darwin’s Ideas Did Not Develop in a Vacuum Contributor’s to Darwin’s thinking included: Charles Lyell – uniformatarianism. 1797-1875 Georges Cuvier – species extinction. 1769-1832 Darwin’s Ideas Did Not Develop in a Vacuum Contributor’s to Darwin’s thinking included: Thomas Malthus – struggle for existence. 1766-1834 Jean Baptiste de Lamarck – evolution by acquired characteristics. 1744-1829 Alfred Russel Wallace Independently Drew the Same Conclusions as Darwin Papers from Wallace and Darwin were jointly presented (with little impact) to the Linnaean Society in 1858. Darwin’s Observations and Inferences Observation 1: Left unchecked, the number of organisms of each species will increase exponentially, generation to generation. Observation 2: In nature, populations tend to remain stable in size. Observation 3: Environmental resources are limited. Inference 1: Production of more individuals than can be supported by the environment leads to a struggle for existence among individuals, with only a fraction of offspring surviving in each generation. Darwin’s Observations and Inferences Inference 2: Survival in the struggle for existence is not random, but depends in part on the heritable characteristics of individuals. Individuals who inherit characteristics most fit for their environment are likely to leave more offspring than less fit individuals. Darwin’s Observations and Inferences Inference 3: The unequal ability of individuals to survive and reproduce leads to a gradual change in a population, with favorable characteristics accumulating over generations (natural selection). Taken together, these three inferences are a statement of Darwin’s Theory of Evolution. Darwinian View • History of life is like a tree with branches over time from a common source. • Current diversity of life is caused by the forks (or splits) from common ancestors. Example “The Origin of Species” • 1859 published The Origin of Species • Documented the occurrence of evolution. • Suggested that the mechanism for evolution was Natural Selection. Nature • Determines which characteristics are favorable. • Determines who survives. • Result - “Natural Selection” Artificial Selection • When man determines the characteristics that survive and reproduce. • Result - the various breeds of animals and plants we’ve developed. Ex - Mustard Plant Original Cultivars Evolution Success Measured By • • • • Survival Reproduction Variation within a population Whoever lives long enough and has kids is the “winner” in evolution. Comment • Acquired characteristics may allow a species to evolve "outside" of Natural Selection. • Ex: culture, learning Proof for Evolution 1. Biogeography 2. Fossil Record 3. Taxonomy 4. Comparative Anatomy Homologous features Analogous features Vestigial organs 5. Comparative Embryology 6. Comparative Biochemistry and Molecular Biology (DNA sequences) 7. Artificial Selection Biogeography • The geographical distribution of species. • Problem: – Species mixtures on islands – Marsupials in Australia Taxonomy • Science of Classification. Main Categories • • • • • • • Kingdom Phylum or Division Class Order Family Genus Species Evidence for Evolution - Comparative Morphology Why use the same skeletal plan for these very different appendages? Comparative Anatomy 1. Homologous Structures Common "building plan” with divergent functions. Mammal forelimbs Homologous structures It’s Critical (and often difficult) To Distinguish Homology from Analogy Homologous structures, like the bat wing and gorilla arm, are similar because they are derived by modification of a shared ancestral structure. Homology is the key to establishing phylogenies. Distinguishing Homology from Analogy Analogy is similarity due to convergent evolution. Analogy mistaken for homology confuses phylogenies. Another Set of Analogies Created by Convergent Evolution Ocotillo of the US southwest Allauidia of Madagascar Comparative Anatomy 2. Vestigial Organs Rudimentary structures of marginal, if any, use. Whale Legs Human Example of Vestigial Organ Comparative Embryology • Closely related organisms go through similar stages in their embryonic development. • Ex: Gill pouches in vertebrates Evidence for Evolution - Comparative Embryology Why do embryos of different animals pass through a similar developmental stage? Recent discoveries of the conservation of molecular mechanisms of development are even more compelling. Comparative Embryology Molecular Biology • Study of Evolution at the DNA or protein levels. • Related species have similar DNA sequences. • Related species share a common ancestral DNA. The closer the relationship, the more similar the DNA sequences should be. Comparative Biochemistry and Molecular Biology Evidence of Evolution – Conservation and Diversification at the Molecular Level Why should different organism possess related genes? Why does the degree of relationship of genes match their degree of relationship established by other methods? What Drives Evolution? There are 5 sources of genetic variation. Only natural selection makes a population better adapted (more fit) to its environment. Mutations Provide Raw Material For Evolution One type of mutation at the level of the gene. One type of mutation at the level of the chromosome. Mutations are usually neutral or harmful in their effects; only rarely are they beneficial. Mutations “Just Happen” Mutations occur at random without regard to whether they have a beneficial, neutral or harmful effect. For this reason, mutations are a randomly acting evolutionary force. Mutation Mutation is the only source of new alleles in a species. Mutation acting alone works too slowly to drive evolution. Loss of an allele due to mutation With an average mutation rate, it takes ~ 70,000 generations, far more than the number of generations of modern humans, to reduce allele frequency by 50%. Chromosome Sources of Genetic Variation • Mutations. • Recombination though sexual reproduction. – Crossing-over – Random fertilization Natural Selection Natural selection leads to adaptation – an increase in the fitness of a population in a particular environment. Natural selection works because some genotypes are more successful in a given environment than others. Successful (adaptive) genotypes become more common in subsequent generations, causing an alteration in allele frequency over time that leads to a consequent increase in fitness. It’s not natural – but this is one outcome of strong selection. Modes of Natural Selection 1. Stabilizing 2. Directional 3. Disruptive 4. Sexual Three Forms of Natural Selection Stabilizing Selection • Selection toward the average and against the extremes. • Ex: birth weight in humans Human Birth Weight Is Under Stabilizing Selection Modern medicine relaxes this and other forms of selection. Stabilizing Selection for the Sickle Cell Allele In heterozygous form, the sickle cell allele of -globin confers resistance to malaria. Therefore, the allele is maintained, even though it’s harmful in homozygous form. Directional Selection • Selection toward one extreme. • Ex: running speeds in race animals. • Ex. Galapagos Finch beak size and food source. A Galapagos Finch, the Subject of a Classic Study of Evolution in Action Peter and Mary Grant and their colleagues observed how beak depth, a significant trait for feeding success, varied in populations experiencing climactic variations. Beak Depth Changed in a Predictable Way in Response to Natural Selection Significantly, beak depth is a genetically determined trait. Directional Selection Evolution of pesticide resistance in response to selection. Disruptive Selection • Selection toward both extremes and against the norm. • Ex: bill size in birds • Disruptive/Diversifying Selection - can split a species into several new species if it continues for a long enough period of time and the populations don’t interbreed. Sexual Selection • May not be adaptive to the environment, but increases reproduction success of the individual. • Sexual dimorphism. • Secondary sexual features for attracting mates. Life’s History and Diversity The Big Picture One Hypothesis of Hominan Evolution Hominans are the group of species (that includes us) on the line of descent from the last common ancestor of modern humans and chimpanzees. Note that many species of hominans have existed but there is only one extant species – Homo sapiens. Not all details of hominan evolution (the branching pattern of the cladogram) are firmly established. Africa is Birthplace of Hominans Sahelanthropus tchadensis – Possibly the Earliest Hominan This fossil skull is from a 7 million year old hominan that lies at the root of the hominin lineage. The specimen, named Toumaï (hope of life in the local language) is that of a young boy. Artist’s Conception of Two Hominan Species Homo habilis stayed close to home (Africa) Homo ercetus traveled widely through Africa and Asia. The Ebb and Flow of Hominan Species Nature (2004) 248 (Oct 28):1043 Note that the roots of ancient and modern migrations (with the exception of H. floresienses) begin in Africa Genetic Evidence Indicates That Modern Humans Arose In Africa About 150,000 Years Ago Starting About 60,000 Years Ago, Modern Human Migrated From Africa Genetic similarities and differences among modern populations indicates that modern humans migrating from Africa displaced existing hominads. This is the Out-of-Africa hypothesis. Neandertals Disappeared Soon After Modern Human Arrived in Their Range Neandertal and modern human skulls Neandertals are not our ancestors. Neandertals may have disappeared because they were outcompeted by our own species. When Did The First Modern Humans Arise? Anatomically modern humans arose ~ 150,000 years ago. Culturally modern humans probably did not exist until ~ 100,000 years ago. 17,000 year old paintings from the Lascaux cave in France. http://www.nhm.ac.uk/natureonline/evolution/what-isevolution/natural-selection-game/theevolution-experience.html http://phet.colorado.edu/en/simulation /natural-selection The End