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Biol 121- 2010.03.12- Evolution- Natural Selection (Ch. 24).docx

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BIOL 121
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Biol 121 225 Freeman 491-500 Mar. 12, 10 Natural Selection, Ch. 24 Natural selection -credit for ‘natural selection’ is given to Darwin -first proposed by -Lamarck and others had already proposed evolution as a pattern in nature -used to explain before Darwin began his work but Darwin had the crucial insight of seeing natural selection that could be used to explain pattern of descent with modification -four simple postulates/steps 1) Variation in a pop- Individual organisms that make up a pop vary in traits they possess, like size or shape 2) Heritable traits involved - Some trait differences are heritable, so passed on to offspring genetically 3) Variable fitness - In each generation, many more offspring are produced than can possibly survive -therefore only some individuals in the pop survive long enough to produce offspring, and among individuals that produce offspring, some produce more than others 4) Fitness due to heritable traits - Subset of individuals that survive best and produce the most offspring is not a random sample of the pop -individuals with certain heritable traits are more likely to survive + reprod -natural selection occurs when indiv with certain char produce more offspring than do indiv w/o those characteristics The outcome of these four steps -Evolution – change in genetic char of a pop over time is the outcome of these four steps -variation among individuals is necessary for evolution -populations must change over time for evolution to occur Summing up the 4 steps in only 2 Evolution by natural selection occurs when 1) heritable variation leads to… 2) differential success in survival and reproduction Allele frequencies WRT evolution -evolution can be defined as a change in allele frequencies in a pop over time Example with light and dark winged -two alleles for a gene for wing colour, one producing dark and other light moths -predation by birds – they eat dark-winged individuals much more readily than they can find and eat light-winged individuals if have light-coloured bark -therefore, predation by birds causes natural selection on wing colour leading to differential success -therefore, the allele for light coloured wings increases in the pop Darwinian fitness -ability of an individual to produce offspring (relative to that ability in other indiv in the pop) – this is a measurable quantity. How? count how many offspring it produces relative to other individuals Adaptation -when some individuals have a heritable trait that increase their fitness in their specific environment, that trait is naturally selected -the pop evolves and adapts to the specific environment Acclimatization -short term change in an individual and not passed on to offspring e.g. wood frogs in North Am. -wood frogs exposed to extreme cold in winter -ice forms in their skin and their bodies produce some sort of natural antifreeze, protecting their tissues -this is change in individual’s phenotype that occur in response to changes in environmental conditions -this phenotypic change is not passed on to the offspring, therefore no evo. Penicillin and resistance (WRT -this is an example of antibiotic resistance evolution) -penicillin – antibiotic/drug that blocks cell wall formation in some bacteria -resistant strains have a gene for lactamase in a plasmid  this breaks down penicillin -therefore, this trait is naturally selected b/c it increases fitness, and as the 1 Biol 121 225 Freeman 491-500 Mar. 12, 10 pop evolves, the allele frequency for this trait increases over time -the population then becomes resistant, and can also transfer the plasmid Mycobacterium tuberculosis -this causes tuberculosis and has been doing so in humans for >1000 yrs, leading to the lung infection disease -in the 1950s, the prevalence of this disease decreased b/c new drugs, healthier people, etc -but in the late 1980s, the number of cases started to increase -Rifampin – antibiotic to treat TB by binding to RNA polymerase and thereby interfering with transcription -a point mutation occurred which change leucine to serine in the AA sequence of RNAP -this allows transcription even in the presence of Rifampin  drug resistance -bacteria become resistant fast because they divide fast Galapagos finches – medium ground
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