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Chapter 1

Biology Chapter 1.docx

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Jon Stone

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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
oMost 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
oThe complete cell theory is
All organisms are made of cells and all cells come from pre-existing
oAt that time, most scientists believed in spontaneous generation, which
was a hypothesis (proposed explanation)
oThe 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
oLouis Pasteur set out to test the validity of the all-cells-from-cells
oHe created two treatment groups, a broth exposed to a source of pre-
existing cells and a broth that wasn’t

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oThe 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
oHe used two glass flasks filled with the same amount of the same nutrient
oBoth were boiled for the same amount of time to kill any existing
organisms such as bacteria or fungi
oHowever, one flask had a straight neck, while the other had a swan neck
oThe 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
oIn contrast, in the swan neck flask, water would condense in the crook of
the neck after the boiling treatment
oThis water would trap any bacteria or fungi that entered on dust particles
oThe results showed that treatment exposed to pre-existing cells quickly
filled with bacteria and fungi
oThis 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
oHowever, the treatment in the swan neck flask remained sterile
oThe results showed that the all-cells-from-cells hypothesis was correct
Pasteur achieved this via an effective experimental setup
oThe 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
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

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oSpecies are related by common ancestry
oCharacteristics 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
oIndividuals 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
oIn 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
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
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
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