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Microevolution: Unique Gene Pools and Changing Allele Frequencies

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Microevolution: Unique Gene Pools and Changing Allele Frequencies UNIT 9 Evolution Part 1 Microevolution: Unique Gene Pools and Changing Allele Frequencies Adaptations and Fitness • An adaptation is a genetically controlled trait that is favored by natural selection and gives the organism a reproductive advantage ensuring the trait is passed on to its descendants. • This trait may also allow the individual to survive longer thus increasing the reproductive rate of that individual. 2 Adaptations and Fitness • The antelope hare lives in the desert, and the snowshoe hare lives in the mountains. • Explain how the differences in their traits enhance their ability to survive in their respective environments. • Evolutionary success or fitness refers to the contribution of genes to the gene pool and NOT how long an organism lives. 3 The Effect of Environmental Change • Earth’s environment is NOT STATIC, but rather ever changing. • As a consequence, traits or adaptations that were favorable may become unfavorable. • The peppered moth, Biston betularia is native to England and exists in two forms, one is dark and the other light with a “peppered” appearance. Birds are its main predator. • Prior to the industrial revolution, only 2% of the moths were dark. • The industrial revolution produced vast amounts of sulfur dioxide and soot from the burning of coal which altered the environment. • Fifty years later 95% of the moths were dark. • Propose an explanation! 4 Industrial Melanism England has since regulated the burning of coal and as a result, the trees are returning to their original state (A). Consequently, the coloring among the population of moths in Britain has shifted back so that the peppered moths are once again favored. 5 Evolution Defined • Evolution is defined as a change in the inherited characteristics of biological populations over successive generations. • Evolutionary processes give rise to diversity at every level of biological organization, from the molecular to the macroscopic. • As a result diversity is prevalent among molecules such as DNA as well as individual organisms and species of organisms. 6 Microevolution Microevolution is simply a change in gene frequency within a population. • Evolution at this scale can be observed over short periods of time such as from one generation to the next. • Example: The frequency of a gene for pesticide resistance in a population of crop pests increases. • Such a change might come about because – natural selection favored the gene – the population received new immigrants carrying the gene (gene flow) – nonresistant genes mutated into a resistant version of the gene – of random genetic drift from one generation to the next 7 Microevolution • A gene is a sequence of DNA nucleotides that specify a particular polypeptide chain. • Genes code for proteins. • An allele is a particular form of a gene. For example: B represents the allele for black coat color and b for white coat color. • Selection acts on phenotype because differential reproduction and survivorship depend on phenotype not genotype. • Natural selection acts on individuals, but only populations evolve. 8 Macroevolution Macroevolution is evolution on a scale of separated gene pools (not individuals). • Think of it as an accumulation of changes which result in speciation (forming a new species). • Macroevolutionary studies focus on change that occurs at or above the level of species, in contrast with microevolution, which refers to smaller evolutionary changes (typically described as changes in allele frequencies) within a species or population. • The process of speciation may fall within the purview of either, depending on the forces thought to drive it. 9 More Evolution Terms • Species-a group of interbreeding organisms that produce viable and fertile offspring in nature • Gene pool-sum total of all the genes in a given species • Allelic frequency-is the percent occurrence for a given allele 10 Sources of Genetic Variation How does variation in a population or gene pool arise? 1. Mutations, gene duplication and chromosome fusion provide the raw material for evolution. 2. Meiosis and sexual reproduction produce new recombinants of phenotypes upon which natural selection operates. The wisteria pictured on the right has a mutation causing it to produce white flowers instead of purple flowers. 11 Types of Mutations • MOST mutations are deleterious as well as recessive. • Obviously, mutations occurring in somatic cells do not affect future generations. • Only mutations occurring in gametes affect future generations. Mutations may cause a sheep to have a 5th leg. • Mutations can occur at either the But this is not evolution! gene or chromosomal level. 12 Neutral Mutations Naturally evolving proteins gradually accumulate mutations while continuing to fold into stable structures. This process of neutral evolution is an important mode of genetic change and forms the basis for the molecular clock. • Cytochrome c is a small protein found on the mitochondrial membrane. • Between mammals and reptiles there are 15 different amino acids or mutations. 13 Neutral Mutations • Mammals and reptiles diverged 265 million years ago. • That means on average cytochrome c mutated every 17 million years. • In comparing the evolution of other organisms and their cytochrome c one mutation every 17 million years holds true. 14 Cytochrome c Comparison Molecular homology of cytochrome c (see three-letter code of amino acids) 1 6 10 14 17 18 20 Human Gly Asp Val Glu Lys Gly Lys Lys Ile Phe Ile Met Lys Cys Ser Gln Cys His Thr Val Glu Lys Pig - - - - - - - - - - Val Gln - - Ala - - - - - - - Chicken - - Ile - - - - - - - Val Gln - - - - - - - - - - Dogfish - - - - - - - - Val - Val Gln - - Ala - - - - - - Asn Drosophila <<< - - - - - - - - Leu Val Gln Arg Ala - - - - - - Ala Wheat <<< - Asn Pro Asp Ala - Ala - - - Lys Thr - - Ala - - - - - Asp Ala Yeast <<< - Ser Ala Lys - - Ala Thr Leu - Lys Thr Arg - Glu Leu - - - - - - • A dash indicates that the amino acid is the same one found at that position in the human molecule. • All the vertebrate cytochromes (the first four) start with glycine (Gly). • The Drosophila, wheat, and yeast cytochromes have several amino acids that precede the sequence shown here (indicated by <<<). • In every case, the heme group of the cytochrome is attached to Cys-14 and Cys-17 (human numbering). 15 Hemoglobin Comparison 16 Human Impact on Gene Pools It is well documented that humans have had an impact on certain gene pools. For example, humans have selected for certain desirable traits within the mustard family and cultivated different agricultural products for human consumption. 17 Artificial Selection When humans manipulate a gene pool it is called artificial selection. There are often consequences involved in such manipulations. For example in agriculture, farmers try to increase crop production, which may lead to many farmers growing only one variety of a particular crop such as corn. This leads to a loss of genetic diversity. If a disease attacks that particular variety of corn, the farmers growing that variety lose their entire crop. 18 Antibiotics and Artificial Selection • When antibiotics are applied to a population of microorganisms to treat an infection, some of the microorganisms may be naturally immune to the drug. • Why? A random mutation occurred in the genetic code of the microorganism conferring its resistance. • These resistant microorganisms continue to flourish and cause disease. • The only remaining option a physician has is to treat the infection with a different antibiotic and hope that none of the surviving microorganisms possess a different random mutation that makes them resistant to the second antibiotic as well. 19 Antibiotics and Artificial Selection • The increase in antibiotic- resistant bacteria has caused doctors to reduce the number of prescriptions written for antibiotics in general. • About 70% of pathogenic bacteria are resistant to at least one antibiotic and are called “super bugs” or MDR bacteria. (multidrug resistant) 20 MRSA or Methicillin-resistant Staphylococcus aureus • MDR bacteria do not respond to “first line of defense” antibiotics. • These types of bacteria are most commonly found in hospitals. • Skin boils or similar lesions that do not heal often result. • MDR bacteria can attack internal organs upon gaining entry into the body. 21 Effect of Sexual Reproduction Sexual reproduction recombines genes in new ways. This results in unique offspring that differ from either parent or sibling. Humans make 223 different kinds of gametes. Fertilization means that the uniqueness of an individual is 223 223. Or the probability that two siblings will be genetically identical (excluding identical twins) is 446. Sexual reproduction is like shuffling a deck of cards and every time getting a new and unique hand dealt. It is the major driving force of evolution. 22 Genetic Drift Small populations can experience changes in allele frequencies more dramatically than large populations. In very large populations the effect can be insignificant. Also in small populations genes can be lost more easily. When there is only one allele left for a particular gene in a gene pool, that gene is said to be fixed , thus there is no genetic diversity. 23 Genetic Drift • Genetic drift can be most profound in populations that are dramatically reduced (bottle neck populations) usually due to some environmental catastrophe. • Also genetic drift occurs when a small population arrives at a new habitat such as an island. 24 Bottleneck Example In 1900, the population of prairie
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