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3.3, 3.4, 3.5 textbook notes

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BIOL 1001
Tamara Kelly

Microevolutionary Processes & HWE - Bio 1001 3.3, 3.4, 3.5 (p 48-58) 3.3 The Process of Evolution: How Does Natural Selection Work? • Lamarck and other researchers has already proposed evolution as a pattern in nature long before Darwin began his work • Darwin’s crucial insight lay in recognizing a process called natural selection Darwin’s Four Postulates Darwin broke the process of evolution by natural selection into 4 logical sequences • 1. The individual organisms that make up a population vary in the traits they posses, such as size and shape 2. Some of the trait differences are heritable, meaning that they are passed onto offspring genetically (height) 3. In each generation, many more offspring are produced than can possible survive. Thus only some individuals in a population live long enough to produce offspring and some will produce more than others 4. The subset of individuals that survive best and produce the most offspring is not a random sample of the population. Instead individuals with certain heritable traits are more likely to survive and reproduce • Natural selection occurs when individuals with certain characteristics produce more offspring than do individuals without those characteristics. The individuals are selected naturally by the environment • Evolution, which is any change in the genetic characteristics of a population over time, is simply the outcome of these 4 steps Biological Definitions of Fitness and Adaptation Biological fitness; the ability of an individual to produce surviving offspring, relative to that • ability in other individuals in the population • Fitness is a measurable quantity • The concept of fitness, in turn, provides a compact way of formally defining adaptation • In biology, an adaptation is a heritable trait that increases the fitness of an individual in a particular environment relative to individuals lacking the trait • Adaptations increase fitness - the ability to produce offspring Microevolutionary Processes & HWE - Bio 1001 3.3, 3.4, 3.5 (p 48-58) 3.4 Evolution in Action: Recent Research on Natural Selection Case Study 1: How did Mycobacterium tuberculosis Become Resistant to Antibiotics • Myobacterium tuberculosis, is the bacterium that causes TB • In Europe and the US, TB was once as great a public health issue as cancer is now It receded in importance during the early 190s, for 2 reasons: • 1. Advances in nutrition made people better able to fight off most M. tuberculosis infections quickly 2. The development of antibiotics such as rifampin allowed physicians to stop even advanced functions • In the late 1980s, rates of the MTB infection surged in many countries, and in 1993 the WHO declared TB a global health emergency • M. tuberculosis responsible for the increase were largely or completely resistant to rifampin and other antibiotics that were once extremely effective • How and why did the evolution of drug resistance occur? A Patient History • 2 months after the TB tests proved normal, however, the young man was readmitted to the hospital with a fever, severe cough, and laboured breathing • Despite being treated with a variety of antibiotics, including rifampin, he died of respiratory failure of 10 days later • Samples of material from his lungs showed that M. tuberculosis was again growing actively there • But this time, the bacterial cells were completely resistant to rifampin Drug-resistant bacteria had killed the patient • • Where did they come from? • After examining extensive stretches from each genome, the biologists were able to find only one difference: a point mutation in a gene called rpoB A mutation in Bacterial Gene confers Resistance • The rpoB gene codes for a component of the enzyme RNA polymerase • Recall, that RNA polymerase transcribes DNA to mRNA, and that a point mutation is a single base change in DNA • Rifampin, the drug that was being used to treat the patient, works by binding to the RNA polymerase of MTB When the drug enters an MTB cell and binds to RNA polymerase, it interferes with • transcription • If sufficient quantities of rifampin are present for long enough, and if the drugs bind tightly, bacterial cells don’t make proteins efficiently and they produce few offspring • But apparently the substitution of a leucine for a serine prevents rifampin from binding efficiently Consequently, bacterial cells with the C --> T mutation continue to produce offspring • efficiently, even in the presence of a drug Microevolutionary Processes & HWE - Bio 1001 3.3, 3.4, 3.5 (p 48-58) 1. Large population of MTB bacteria in patients lungs make him sick. By chance, a point mutation in the rpoB gene occurs in one or more bacteria cells 2. Drug therapy begins killing most MTB, patient seems cured and drug therapy is ended 3. The mutant cells proliferate, resulting in another major infection of the lungs. The patient becomes sick again 4. A second round of drug therapy begins but is ineffective on the drug resistant bacteria. The patient dies (pg 50) Testing Darwin’s Postulates Does the sequence of events illustrated mean that evolution by natural selection occurred? 1. Did variation exist in the population? Yes, Due to mutation, both resistant and nonresistant strains of TB were present prior to administration of the drug 2. Was this variation heritable? Yes. The researchers showed that the variation in the phenotypes of the two strains – from the drug susceptibility to drug resistance was due to variation in their genotypes 3. Was there variation in reproductive success? Yes. Only a tiny fraction of M.turbucolosis cells in the patient survived the first round of antibiotics long enough to reproduce 4. Did selection occur? Yes. When rifampin was present, certain cells – those with the drug- resistant allele – had higher reproductive success than cells with the normal allele • M. tuberculosis individuals with the mutant rpoB gene had higher fitness in an environment where rifampin was present • The mutant allele produces a protein that is an adaptation when the cell’s environment contains the antibiotic • This study verified all four postulates and confirmed that evolution by natural selection had occurred • The M. tuberculosis population evolved because the mutant rpoB allele increased in frequency • Natural selection acts on individuals because individuals experience differential reproductive success o Only populations evolve o Allele frequencies change in populations, not in individuals  When people hunt or fish, they may cause strong directional selection by harvesting the largest, oldest or fastest growing individuals in the population  If mortality due to hunting or fishing exceeds mortality from natural causes, the result may be undesirable changes in the harvested population, as we select against the very traits we value  Evolution by natural selection occurs when heritable variation leads to differential survival and reproductive success  As a result, human predation swamps the selective effect of other causes of mortality  Humans cause for more rapid phenotypic changes than other predators do  Human harvested systems examined in the study showed phenotypic changes that were 300 percent higher than those in natural systems Microevolutionary Processes & HWE - Bio 1001 3.3, 3.4, 3.5 (p 48-58) Case study 2: Why are beak size, beak shape, and body size changing in Galapagos Finches? • Peter and Rosemary Grnat (biologists) have been investigating chang
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