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Chapter 1

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Ben Evans

Chapter 1: Biology and the Tree of Life The Cell Theory  Robert Hooke used a crude microscope to examine the structure of cork (bark tissue) from an oak tree  In the cork, he observed small, pore-like compartments that were invisible to the naked eye, which came to be called cells  Anton van Leeuwenhoek developed a much more powerful microscope and made the first observations of single-celled organisms like Paramecium  Today, a cell is defined as a highly organized compartment that is bounded by a thin, flexible structure called a plasma membrane and that contains concentrated chemicals in an aqueous solution  The chemical reactions that sustain life take place inside cells  Where cells come from o Most scientific theories have two components  The first describes a pattern in the natural world  The second identifies a mechanism or process that is responsible for creating that pattern o The complete cell theory is  All organisms are made of cells and all cells come from pre-existing cells o At that time, most scientists believed in spontaneous generation, which was a hypothesis (proposed explanation) o The all-cells-from-cells hypothesis in contrast, maintained that cells do not spring to life spontaneously, but are produced only when pre-existing cells grow and divide o Louis Pasteur set out to test the validity of the all-cells-from-cells hypothesis o He created two treatment groups, a broth exposed to a source of pre-existing cells and a broth that wasn’t o The spontaneous generation hypothesis predicted that cells would appear in both treatments while the other predicted that cells would appear in only the treatment exposed to a source of pre-existing cells o He used two glass flasks filled with the same amount of the same nutrient broth o Both were boiled for the same amount of time to kill any existing organisms such as bacteria or fungi o However, one flask had a straight neck, while the other had a swan neck o The straight neck flask was exposed to pre-existing cells after sterilization (bacteria and fungi that cling to dust particles in the air)  They could drop into the nutrient broth because the neck of the flask was straight o In contrast, in the swan neck flask, water would condense in the crook of the neck after the boiling treatment o This water would trap any bacteria or fungi that entered on dust particles o The results showed that treatment exposed to pre-existing cells quickly filled with bacteria and fungi o This observation showed that the heat sterilization step had not altered the nutrient broth’s capacity to support growth and because it supported the hypothesis that growth started with pre-existing cells o However, the treatment in the swan neck flask remained sterile o The results showed that the all-cells-from-cells hypothesis was correct  Pasteur achieved this via an effective experimental setup o The results had an important implication  If all cells come from pre-existing cells, it follows that all individuals in a population of single-celled organisms are related by common ancestry  Similarly, in a multicellular individual, all of the cells present are descended from pre-existing cells, tracing back to a fertilized egg  In this way, all of the cells in a multicellular organism are connected by common ancestry The Theory of Evolution by Natural Selection  Darwin and Wallace claimed that o Species are related by common ancestry o Characteristics of species can be modified from generation to generation  Evolution means that species are not independent and unchanging entities, but are related to one another and can change through time  Natural selection explained how evolution occurs  Natural selection occurs whenever two conditions are met o Individuals within a population vary in characteristics that are heritable, meaning traits that can be passed on to offspring  A population is defined as a group of individuals of the same species living in the same area at the same time o In a particular environment, certain versions of these heritable traits help individuals survive better or reproduce more than do other versions  If certain heritable traits lead to increased success in producing offspring, then those traits become more common in the population over time  In this way, the population’s characteristics change as a result of natural selection acting on individuals  Natural selection acts on individuals, but evolutionary change affects only populations  Ex. If tall wheat plants are easily blown down by wind, then in windy environments shorter plants will tend to survive better and leave more offspring than tall plants do  Populations of wheat that grow windy environments tend to become shorter from generation to generation, but in any given generation, none of the individual wheat plants get taller or shorter as a result of natural selection  This short of change in characteristics of a population, over time, is evolution  Evolution occurs when heritable variation leads to differential success in reproduction  Fitness means the ability of an individual to survive and produce offspring  Individuals with high fitness produce many offspring  An adaptation is a trait that increases the fitness of an individual in a particular environment  Consider wheat, wheat plants with short stalks have higher fitness than do individuals with long stalks o Short stalks are an adaptation to windy environments  Artificial selection o Deliberate picking in certain characteristics to get offspring with desired traits o Ex. Broccoli was created from a wild plant with think stalks o Steps  Parental generation  Select individuals with largest and most compact flowering stalks and breed them  Generation 2  Of the offspring, select the individuals with largest and most compact flowering stalks and breed them  Generation 3  Of the offspring, select the individuals with largest and most compact flowering stalks and breed them  Repeat dozens of times  Descendants  After many generations, average flowering stalks are much larger and more compact  Together, the cell theory and theory of evolution has two unifying ideas o The cell is the fundamental structural unit in all organisms o All species are related by common ancestry and have changed over time in response to natural selection The Tree of Life  Over the past several decades, biologists have documented dozens of cases in which natural selection has caused populations of one species to diverge and form new species  This divergence process is called speciation  Linnaean Taxonomy o Taxonomy is the effort to name and classify organisms o The Linnaeus’ system contains two parts  The first part indicates the organism’s genus  A genus is made up of a closely related group of species  The second term identifies the organism’s species  A species is made up of individuals that regularly breed together or have characteristics that are distinct from those of other species  Taxonomic Levels o Linnaeus created a hierarchy of taxonomic groups o From most specific to least specific the levels are species, genus, family, order, class, phylum and kingdom  Problems with Linnaeus’ proposal o He proposed that species could be organized into two kingdoms, plants and animals  Organisms that do not move and produce their own food are plants  Organisms that move and acquire food by eating other organisms are animals  Not all organisms fall into these categories  Moulds, mushrooms and other fungi survive by absorbing nutrients from dead or living plants and animals  Even though they do not make their own food they were placed in Plantae because they do not move  Some bacteria can also move and many make their own food, but initially they were thought to be plants o In addition, advances in microscopy allowed biologists to study the content of cells in more detail  In plants, animals and other organisms, cells contain a prominent component called a nucleus  Bacteria lacked this structure  Organisms with a nucleus are called eukaryotes, while organisms wit
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