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Transcript
Evolution of Populations Ch 17 • Process of change over time • A change in the genes!!!!!!!! Population Genetics • The science of genetic change in population. • Population – all the members of a species that occupy a particular area at the same time • Gene Pool – all the genes in all the members of a population Genes and Variation • Genetics Joins Evolutionary Theory – Variation is the raw material for natural selection – Gene pool – consists of all the genes, including all the different alleles for each gene, that are present in a population – Relative frequency – the number of times that the allele occurs in a gene pool, compared with the number of times other alleles for the same gene occur – Therefore – evolution is any change in the relative frequency of alleles in the gene pool of a population over time • 3 Sources of Genetic Variation – Mutations – Genetic Recombination in Sexual Reproduction (Ind assortment and crossing over) – Lateral Gene Transfer (conjugation) • Single-Gene (2 pheno) vs. Polygenic Traits (many pheno/bell curve) ***Natural Selection acts directly on PHENOTYPES not actual alleles*** some phenotypes are better suited to an environment than others and they will survive, reproduce and pass on their genes. Evolution as Genetic Change in Populations How Natural Selection Works – 3 Types Stabilizing Selection Individuals with the average form of a trait have the highest fitness Represents the optimum for most traits Results in a similar morphology between most members of the species Directional Selection Individuals that display a more extreme form of a trait have greater fitness than individuals with an average form of the trait A shift in one direction Peppered moth Disruptive Selection Individuals with either extreme variation of a trait have greater fitness than individuals with the average form of the trait A shift in both direction, away from the center Shell color (dark rocks and light sand) Genetic Drift • Random changes in the frequency of a gene in the absence of natural selection occurs because of CHANCE • Occurs efficiently in small populations because small changes affect more members • Two examples: a. Bottleneck effect b. Founder effect Genetic Drift a. Bottleneck Effect • Genetic drift (reduction of alleles in a population) resulting from a disaster that drastically reduces population size. – Examples: 1. 2. Earthquakes Volcano’s b. Founder Effect • Genetic drift resulting from the colonization of a new location by a small number of individuals. • Results in random change of the gene pool. • Example: 1. Islands (first Darwin finch) Hardy-Weinberg Principle • Genetic Equilibrium – situation in which allele frequencies in the gene pool of a population remain constant • The concept that the shuffling of genes that occurs during sexual reproduction, by itself, cannot change the overall genetic makeup of a population. • Shows mathematically and theoretically that there are situations where evolution DOES NOT OCCUR – Seldom achieved in nature Hardy-Weinberg Principle • This principle will be maintained in nature only if ALL five of the following conditions are met: 1. Very large population 2. Isolation from other populations 3. No net mutations 4. Random mating 5. No natural selection Hardy-Weinberg Principle Species • A group of populations whose individuals have the potential to interbreed and produce viable offspring. Speciation • The evolution of new species. Species that occupy an otherwise unoccupied niche face no competition, they will therefore have a 100% success rate Reproductive Isolation • Any mechanism that impedes two species from producing fertile and/or viable hybrid offspring -factor necessary for the formation of a new species. • Barriers: 1. Geographic (rivers, mountains) 2. Behavorial - differences in courtship behavior 3. Temporal - fertile periods (time) Interpretations of Speciation • Two theories: 1. Gradualist Model (Neo-Darwinian): Slow changes in species overtime. 2. Punctuated Equilibrium: Evolution occurs in spurts of relatively rapid change. Macroevolution • The origin of taxonomic groups higher than the species level. Adaptive Radiation aka Divergent Evolution • Emergence of numerous species from a common ancestor introduced to new and diverse environments. • Example: Darwin’s Finches Five steps to the 14 finch species on the Galapagos Islands: 1) founders arrive 2) geographic isolation 3) changes in gene pools 4) behavioral isolation 5) competition and continued evolution Darwin’s Finches an example of Adaptive Radiation Convergent Evolution • Species from different evolutionary branches may come to resemble one another if they live in very similar environments. • Example: 1. Ostrich (Africa) and Emu (Australia). 2. Sidewinder (Mojave Desert) and Horned Viper (Middle East Desert) 3. Shark and Dolphin Coevolution • Evolutionary change, in which one species act as a selective force on a second species, inducing adaptations that in turn act as selective force on the first species. • Example: 1. Acacia ants and acacia trees 2. Humming birds and plants with flowers with long tubes The Age of the Earth and Fossils – Fossils • Trace the evolution of modern species from ancient/extinct ancestors Radiometric Dating • Relative dating vs. – When unstable nuclei release particles or • Absolute dating radiant energy until the – nuclei becomes stable Half-life = The length of time it takes for one-half of a sample to decay to stable form The Earth is born… Date Event 4.6 bya Earth was born 4 bya Cooling of Earth, 1st solid rocks formed on earth’s surface 4-3.8 bya Volcanic activity & meteorites release gases that produce earth’s atmosphere Contained H2O vapor, CO, CO2, H2, N2, NH3, CH4 It did NOT contain oxygen 3.8 bya Cooling continues, water appears, beginning of oceans Earth cool enough for liquid to stay on the ground 3.5 bya Age of first prokaryotic microfossils Heterotrophic – obtained nutrients from organic “soup” Anaerobic – able to live in oxygen-free environment 3.4 bya Appearance of 1st autotrophs Organic soup begins to run out Photosynthesis begins – using H2S instead of water 2.2 bya Introduction of oxygen into the atmosphere More modern form of photosynthesis appeared Used H2O instead of H2S Caused earth to cool as they converted CO2 O2 Led to aerobic respiration Ozone layer protection 1.6-1.1 bya 1st eukaryotic cells evolved Nucleus contains DNA, have membrane bound organelles, etc. Increased the speed of evolution Increased genetic variation 1700s Spontaneous Generation (abiogenesis) – idea that life comes from non-life Disproven through Redi, Spallanzani, & Pastuer 1953 Miller & Urey mix methane, water, ammonia, and hydrogen with energy (sun & lightning) Primordial Soup Amino acids & other organic compounds are produced as by-products Provide glimpse at how molecules (proteins) may 1st have formed on the Earth Sexual reproduction evolved Multicellular organisms arose Additional Info The Age of the Earth and Fossils – The Age of the Earth • Earth had to be old enough for these proposed changes to occur • Earth is ~4.5 byo (determined by radioactive dating) History of Earth (24hours) 12:00am – Earth is formed 5:00am – Prokaryotes appear 4:00pm – Eukaryotes appear 10:00pm – Invasion of land 11:59:30pm – Humans appear Miller/Urey Experiment and Primordial Soup • Oparin and Haldane: hypothesized that the early atmosphere was composed of: ammonia (NH3), hydrogen gas (H2), water vapor (H20), and compounds made of carbon and hydrogen, like methane (CH4). They thought that at high temperatures, simple organic compounds (like amino acids) could form. When earth cooled, and lakes and oceans formed, theses simple compounds could be found in the water and enter complex chemical reactions fueled by lightning and ultraviolet violet radiation resulting in macromolecules essential to life like proteins. • Miller and Urey: 1953 set up an apparatus to test Oparin’s hypotheses. Their experiment produced a variety of organic compounds, including amino acids EndosymbioticTheory • What is the theory of endosymbiosis? Theory that large prokaryotic, unicellular organisms engulfed (ate) smaller prokaryotic, unicellular organisms. Engulfed prokaryotes eventually gave rise to modern mitochondria and chloroplasts. • What evidence supports the hypothesis that mitochondria and chloroplasts were once free living prokaryotic cells? Replicate independently and replicate like prokaryotes (binary fission), have their own DNA and their own ribosomes (also similar to DNA and ribosomes of prokaryotes)