Download Honors Biology Midterm Review

Biology Midterm Review
Chapter 1 Introduction to Biology
Vocabulary:
biosphere
ecosystem
dependent variable
biodiversity
homeostasis
constant
species
evolution
theory
biology
adaptation
microscope
organism
observation
gene
cell
data
molecular genetics
metabolism
hypothesis
genomics
DNA
experiment
biotechnology
system
independent variable
transgenic
1.1 KEY CONCEPT Biologists study life in all its forms.
Biology is the scientific study of all forms of life. Living things are found almost
everywhere on Earth, from very hot environments to very cold
environments and from the dry deserts to the ocean floor. The types of
living things found in a particular
region depend on which can survive there. Those living things that can
survive in an environment can differ greatly in size and shape.
All of the living things on Earth, and all of the places in which they live, make up the
biosphere. The variety of living things in a certain area, or across the entire biosphere,
is called biodiversity. Biodiversity can be measured in terms of the number of species
in an area or across the biosphere. Although there are several definitions of the term
species, you can think of a species as a certain type of living things that
can reproduce by interbreeding.
At least two million species exist on Earth. Each individual living
thing, no matter the species, is an organism. Every organism, from
any species, shares certain characteristics of life.
• All organisms are made of one or more cells. A cell is the basic unit of life on Earth.
• All organisms need chemical energy to carry out all of their cell functions. Energy
is used for metabolism, which is all of the chemical processes that
build up or break down materials.
• All organisms respond to physical factors, or stimuli, in their environment.
• Members of a species must be able to reproduce so that the species will survive.
When organisms reproduce they pass on their genetic material, which is called
DNA, to their offspring.
1.2 KEY CONCEPT Unifying themes connect concepts from many fields of biology
Several major concepts run through all of biology. These underlying ideas, or unifying
themes, demonstrate the relationships among all organisms and help to connect one
f ield of biology to others.
• Systems: A system is a group of related parts that interact to form a whole.
Groups of molecules can interact to form the cellular machinery for a
particular process. Groups of cells can interact to form an organ, such
as the heart. Groups
of organisms can interact within an ecosystem. An ecosystem is a
certain area in which living and nonliving things interact.
• Structure and Function: The biological function of a part of an
organism is directly related to that part’s structure. This relationship is
found in molecules within cells, among different types of cells, and
across different species. Different organisms
have different specialized structures that perform functions
specialized to that species.
• Homeostasis: All organisms must keep their internal conditions stable
in order to stay alive. Homeostasis is the maintenance of these
conditions. Homeostasis is necessary because the cells of all organisms
function best within a particular range
of conditions. If conditions vary too far from the ideal set, cells will not
be able to function normally. When internal conditions change, a
negative feedback system often acts to return the condition to normal.
• Evolution: Evolution is the process by which the genetic makeup of a
population changes over time. One way in which evolution occurs is
through the natural selection of genetic traits that give an individual an
advantage in a particular environment. Individuals with these
advantageous traits, or adaptations, are more likely to survive and
reproduce.
1.3 KEY CONCEPT Science is a way of thinking, questioning, and gathering evidence.
Scientists do not use one scientific method, but all science is based on the same
principles: curiosity, critical and logical evaluation of evidence, and the
open and honest exchange of data. In any scientif ic inquiry, scientists
• Make observations: Scientists use their senses and various
measurement tools to collect information, or make observations, about
the world.
• Form hypotheses: Scientists propose answers to scientific questions, or form
hypotheses, based on observations they, or other scientists, made.
• Test hypotheses: Scientists devise methods of observing and
experimenting to test their predictions.
• Analyze data: Scientists use statistics to analyze data. This analysis
tells a scientist whether a hypothesis is supported or not supported by the
data.
• Evaluate results: Scientists evaluate both their own results and the
results from other scientists.
Scientists use experiments to test hypotheses. A scientific experiment
uses tightly controlled conditions to test a possible cause-and-effect
relationship between variables.
In an experiment, there are constants and two types of variables:
the independent variable and dependent variables.
• Independent variables: An independent variable is a condition that is manipulated
by a scientist to determine its effect on a dependent variable. An
independent variable is the “cause.”
• Dependent variables: A dependent variable is measured by a
scientist to study the effect of the independent variable. It is the
“effect” and depends on the independent variable.
• Constants: Factors that are controlled so that they do not change are constants.
A hypothesis is a proposed explanation for a single observation. A
scientific theory, however, is a proposed explanation for a wide range
of observations and experimental results, and is supported by a wide
range of evidence.
Chapter 2 CHEMISTRY OF LIFE
Vocabulary
atom
adhesion
carbohydrate
bond energy
element
solution
lipid
equilibrium
compound
solvent
fatty acid
activation energy
ion
solute
protein
exothermic
ionic bond
acid
amino acid
endothermic
covalent bond
base
nucleic acid
catalyst
molecule
pH
chemical reaction
enzyme
hydrogen bond
monomer
reactant
substrate
cohesion
polymer
product
2.1 KEY CONCEPT All living things are based on atoms and their interactions
All matter, whether living or nonliving, is made of the same tiny building blocks, called
atoms. An atom is the smallest basic unit of matter. All atoms have
the same basic structure, composed of three smaller particles.
• Protons: A proton is a positively charged particle in an atom’s
nucleus. The nucleus is the dense center of an atom.
• Neutrons: A neutron has no electrical charge, has about the same
mass as a proton, and is also found in an atom’s nucleus.
• Electrons: An electron is a negatively charged particle found outside the nucleus.
Electrons are much smaller than either protons or neutrons.
Different types of atoms are called elements, which cannot be broken
down by ordinary chemical means. Which element an atom is depends on
the number of protons in the atom’s nucleus. For example, all hydrogen
atoms have one proton, and all oxygen atoms have 16 protons. Only about
25 different elements are found in organisms. Atoms of different elements
can link, or bond, together to form compounds. Atoms form bonds
in two ways.
• Ionic bonds: An ion is an atom that has gained or lost one or more
electrons. Some atoms form positive ions, which happens when an atom
loses electrons. Other
atoms form negative ions, which happens when an atom gains electrons.
An ionic bond forms through the electrical force between oppositely
charged ions.
• Covalent bonds: A covalent bond forms when atoms share one or
more pairs of electrons. A molecule is two or more atoms that are held
together by covalent bonds.
2.2 KEY CONCEPT Water’s unique properties allow life to exist on Earth.
The structure of the water molecule gives water unique properties. Water is a polar
molecule, which means that it has a region with a slight negative charge
(the oxygen atom), and a region with a slight positive charge (the hydrogen
atoms). The oppositely charged regions of water molecules interact to
form hydrogen bonds. A hydrogen bond
is an attraction between a slightly positive hydrogen atom and a slightly
negative atom. Hydrogen bonds are responsible for several important
properties of water.
• High specific heat: Water resists changes in temperature; it must
absorb a large amount of heat energy to increase in temperature.
• Cohesion: The attraction among molecules of a substance is called cohesion.
Cohesion due to hydrogen bonds makes water molecules “stick” together.
• Adhesion: The attraction among molecules of different substances
is called adhesion. Water molecules “stick” to many other materials
because of hydrogen bonds.
Many compounds that are important for life dissolve in water. Water
is the largest component of cells’ interiors, and chemical reactions in
the cell take place in this
water. When one substance dissolves in another, a solution is formed.
The substance present in the greatest amount is called the solvent.
Substances that are present in
lower amounts and dissolve in the solvent are called solutes. Polar
solvents, such as water, dissolve polar molecules and ions.
When some substances dissolve in water they break up into ions. A
compound that releases a hydrogen ion (a proton) when it dissolves in
water is an acid. Bases are compounds that remove, or accept,
hydrogen ions. A solution’s acidity, or its hydrogen ion concentration, is
measured on the pH scale. An acid has a low pH (pH below 7)
and a high hydrogen ion concentration. A base has a high pH (pH above
7) and a low hydrogen ion concentration. Organisms must maintain a
stable pH. Even a small change
in pH can disrupt many biological processes.
2.3 KEY CONCEPT Carbon-based molecules are the foundation of life.
Carbon atoms are the basis of most molecules that make up living things. Many
carbon-based molecules are large molecules called polymers that are made of
many smaller, repeating molecules called monomers. There are four
main types of carbon-based molecules in living things.
• Carbohydrates include sugars and starches, and are often broken down as a source
of chemical energy for cells. Some carbohydrates are part of cell
structure, such as cellulose, which makes up plant cell walls.
• Lipids include fats and oils and, like carbohydrates, are often broken down as
a source of chemical energy for cells. One type of lipid, called a
phospholipid, makes up most of all cell membranes.
• Proteins have a large number of structures and functions. Some
proteins are needed for muscle movement; another protein, called
hemoglobin, transports oxygen in blood. Another type of proteins,
called enzymes, speed up chemical reactions in cells.
• Nucleic acids are molecules that store genetic information and build proteins.
DNA stores genetic information in cells, and RNA helps to build the
proteins for which DNA codes.
2.4 KEY CONCEPT Life depends on chemical reactions.
At the most fundamental level, every process that takes place in an organism depends
on chemical reactions. In a chemical reaction, substances are
changed into different substances by the breaking and forming of
chemical bonds. The substances that are present at the start of a
chemical reaction, and are changed by the reaction, are called
reactants. The substances that are formed by a chemical reaction
are the products.
Chemical bonds must be broken in the reactants and new ones must be
formed in the products. Energy must be added to break chemical bonds.
In contrast, energy is always released when new bonds form. The amount
of energy needed to break a bond, or the amount of energy released
when a bond forms, is called bond energy.
All chemical reactions require the input of at least a small amount of
energy in order for bonds to break in the reactants and for the
reaction to start. The energy needed to start a chemical reaction is
the activation energy. In general, there are two types of
energy changes that can occur during a chemical reaction.
• Exothermic reaction: An exothermic chemical reaction releases more energy than
it absorbs. The bonds that are broken in the reactants of an exothermic reaction
have a higher bond energy than the new bonds that form in the products.
Energy is usually released as heat or light.
• Endothermic reaction: An endothermic chemical reaction absorbs
more energy than it releases. The bonds that are broken in the
reactants of an endothermic reaction have a lower bond energy than the
new bonds that form in the products. The energy that is absorbed
makes up for the difference.
Chapter 3 CELL STRUCTURE AND FUNCTION
Vocabulary
cell theory
vacuole
concentration gradient
cytoplasm
lysosome
osmosis
organelle
centriole
isotonic
prokaryotic cell
cell wall
hypertonic
eukaryotic cell
chloroplast
hypotonic
cytoskeleton
cell membrane
facilitated diffusion
nucleus
phospholipid
active transport
endoplasmic reticulum
fluid mosaic model
endocytosis
ribosome
selective permeability
phagocytosis
Golgi apparatus
receptor
exocytosis
vesicle
passive transport
mitochondrion
diffusion
3.1 KEY CONCEPT Cells are the basic unit of life.
The invention of the microscope in the late 1500s revealed to early scientists a whole
new world of tiny cells. Most cells are so small that they cannot be seen
without a microscope. The discoveries of scientists from the 1600s
through the 1800s led to the cell theory, which is a unifying concept of
biology. The cell theory has three major principles:
• All organisms are made of cells.
• All existing cells are produced by other living cells.
• The cell is the most basic unit of life.
All cells can be divided into two major groups: prokaryotic cells or eukaryotic cells.
The main differences between the two kinds of cells are in their structure:
• Eukaryotic cells have a nucleus defined by a membrane, while
prokaryotic cells have no nucleus.
• In eukaryotic cells, the DNA, or genetic information, is found in the
nucleus. In prokaryotic cells, the DNA is found in the cytoplasm, the
jellylike substance
that fills both types of cells.
• Eukaryotic cells have organelles, structures that perform jobs for a
cell. Most organelles are surrounded by membranes. Prokaryotic cells do
not have organelles surrounded by membranes.
Prokaryotic cells make up organisms called prokaryotes. All
prokaryotes are tiny and consist of single cells. Bacteria are
prokaryotic cells. Eukaryotic cells make
up eukaryotes. You are a eukaryote, as are plants and some types
of single-celled organisms. All multicellular organisms, or organisms
that have many cells, are eukaryotes.
3.2 KEY CONCEPT Eukaryotic cells share many similarities
Plants, animals, and some single-celled organisms are eukaryotes. Eukaryotic cells have an organized
internal structure and organelles that are surrounded by membranes. Organelles look different from
each other and have different functions. Several have a specific job in making and processing proteins
so that a cell can live, function, and reproduce. Plant and animal cells have a lot of the same parts, but
a few of their parts are different.
The list below tells you what each cell part does.
Part
nucleus
endoplasmic
reticulum (ER)
ribosomes
Golgi apparatus
vesicles
mitochondria
centrioles
vacuoles
cell walls
chloroplasts
cytoplasm
cell membrane
lysosomes
Job and Description
double membrane layer that stores and protects DNA; includes the nucleolus, a
dense region where ribosomes are assembled.
network of thin folded membranes that help produce proteins and lipids; two kinds
of ER: smooth and rough
tiny round organelles that link amino acids together to form proteins; may be in the
cytoplasm or on the ER, which makes it look rough
stacked layers of membranes that sort, package, and deliver proteins
little sacs that carry different molecules where they’re needed; made and broken
down as needed by the cell
bean-shaped organelles that release energy from sugars for the cell
found in animal cells; organize microtubules to form cilia and flagella
sacs that store materials for the cell; the materials might be water, food molecules,
ions, and enzymes
strong layer that protects, supports, and gives shape to plant cells; not found in
animal cells
change energy from the sun into chemical energy for the plant; not found in animal
cells
jellylike substance that fills a cell
double-layer of phospholipids that forms a boundary between a cell and its
surrounding environment
membrane-bound organelles that contain enzymes
3.3 KEY CONCEPT The cell membrane is a barrier that separates a cell from the external
environment.
The cell membrane forms a boundary that separates the inside of a cell from the outside
environment. It plays an active role by controlling the passage of materials into and
out of a cell and by responding to signals. The membrane is made of
molecules called phospholipids, which consist of three parts: (1) a
charged phosphate group; (2) glycerol; (3) two fatty acid chains.
The structure of phospholipids gives them distinct chemical properties. The
phosphate group and glycerol form a polar “head.” The fatty acid chains form
a nonpolar “tail.” Cells are both surrounded by water and contain water. In
the cell membrane, phospholipids
form a double layer, or bilayer. In this way, the polar heads interact with
the polar water molecules outside and inside a cell. The nonpolar tails are
sandwiched together inside
the bilayer, away from the water.
The cell membrane also includes a variety of molecules that give the membrane properties
it would not otherwise have.
• Cholesterol molecules make the membrane stronger.
• Proteins help molecules and ions cross the membrane and can act as receptors,
proteins that detect a signal and respond by performing an action.
• Carbohydrates help cells distinguish one cell type from another.
The fluid mosaic model describes the characteristics and makeup of the
cell membrane. The phospholipids can slip past each other like a fluid.
The membrane is made up of
many different molecules, like a mosaic.
The cell membrane has a property called selective permeability,
which means that it allows some molecules to cross but blocks
others. Selective permeability helps a cell maintain homeostasis.
Cells have receptors both in the cell membrane and inside the cell.
Receptors help cells communicate with other cells and respond to the
environment.
• Membrane receptors bind to signals that cannot cross the cell membrane.
They cross the membrane and transmit a message inside the cell by
changing shape.
• Intracellular receptors are located inside a cell and bind to molecules
that can cross the cell membrane. They may interact with DNA to control
certain genes.
3.4 KEY CONCEPT Materials move across membranes because of concentration differences.
Cells are continuously exchanging materials with their environment across the cell
membrane. Passive transport is the movement of molecules across a
cell membrane that does not require energy input by the cell.
Diffusion, a type of passive transport, is
the movement of molecules from an area of higher concentration to an
area of lower concentration. This difference in concentration from one area
to another is called a concentration gradient. When a molecule diffuses,
it can be described as moving down
its concentration gradient.
Not all molecules can cross the cell membrane. Facilitated diffusion is
the diffusion of molecules across a membrane through transport proteins,
proteins that form channels across the membrane.
Diffusion is a result of the natural energy of molecules. When molecules are
in solution, they collide and scatter. Over time, these molecules will become
evenly spread throughout the solution, which means that the molecules
have reached dynamic equilibrium. The molecules continue to move, but
their concentration remains equal.
Water also moves from a higher water concentration to a lower water
concentration. The diffusion of water is called osmosis. The higher the
concentration of dissolved particles that are in a solution, the lower the
concentration of water molecules. The reverse is also true. That is, the
lower the concentration of dissolved particles that are in a solution, the
higher the concentration of water molecules.
Scientists have developed terms to compare the concentration of
solutions with some reference point. Here, our reference point is the
concentration of particles in a cell.
• An isotonic solution has the same concentration of dissolved particles as a cell. A
cell in an isotonic solution will not change.
• A hypertonic solution has a higher concentration of dissolved particles than a cell.
A cell in a hypertonic solution will shrivel.
• A hypotonic solution has a lower concentration of dissolved particles than a cell. A
cell in a hypotonic solution will swell.
3.5 KEY CONCEPT Cells use energy to transport materials that cannot diffuse across the
membrane.
Cells use active transport to obtain materials they need that they could not get by means
of diffusion or facilitated diffusion. Active transport is the movement of a
substance against its concentration gradient by the use of transport
proteins embedded in the cell membrane and chemical energy. The
transport proteins used in active transport are often called pumps. Most
often, the chemical energy that is used comes from breakdown of a
molecule called ATP. A cell may use this energy directly or indirectly.
•
The sodium-potassium pump directly uses energy from the
breakdown of ATP to pump two potassium ions into a cell for every
three sodium ions it removes from the cell.
• The proton pump indirectly uses energy from the breakdown of ATP to
remove hydrogen ions (protons) from a cell. This action creates a charge
gradient, which is a form of stored energy. This charge gradient can then be
used to drive other pumps
to transport molecules such as sucrose.
Some molecules are too large to be transported through proteins. These
molecules can be moved in vesicles, so they never actually have to cross
the membrane. The movement of these vesicles also requires energy
from a cell.
• Endocytosis is the process of taking liquids or large molecules into a
cell by engulfing them in a vesicle. During endocytosis, the cell membrane
makes a pocket around the material to be brought in. The pocket pinches
together around the
material and breaks off, forming a vesicle, inside the cell. This vesicle then joins
with a lysosome, which breaks down the contents if needed and recycles the vesicle.
Phagocytosis is a type of endocytosis and means “cell eating.”
• Exocytosis is the process of releasing materials from a cell by fusion of a vesicle
with the cell membrane. In this process, a vesicle forms around select
materials. The vesicle is moved to the cell surface, and it fuses with the cell
membrane, releasing
the contents. Exocytosis plays an important role in releasing hormones and
digestive enzymes and in transmitting nerve impulses.
Chapter 5 CELL GROWTH AND DIVISION
Vocabulary
cell cycle
prophase
metastasize
mitosis
metaphase
carcinogen
cytokinesis
anaphase
asexual reproduction
chromosome
telophase
binary fission
histone
growth factor
tissue
chromatin
apoptosis
organ
chromatid
cancer
organ system
centromere
benign
cell differentiation
telomere
malignant
stem cell
5.1 KEY CONCEPT Cells have distinct phases of growth, reproduction, and normal functions.
Cells have a regular pattern of growth, DNA duplication, and division that is called the
cell cycle. In eukaryotic cells, the cell cycle consists of four stages: gap 1 (G1), synthesis
(S), gap 2 (G2), and mitosis (M). G1, S, and G2 are collectively called interphase.
• During gap 1 (G1), a cell carries out its normal functions. Cells may also
increase in size and duplicate their organelles during this stage. Cells must
pass a checkpoint before they can progress to the S stage.
• During synthesis (S), cells duplicate their DNA. At the end of the S
stage, a cell contains two complete sets of DNA.
• During gap 2 (G2), a cell continues to grow and carry out its normal
functions. Cells must pass a checkpoint before they can progress to the M
stage.
• The mitosis (M) stage consists of two processes. Mitosis divides the
cell nucleus, creating two nuclei that each have a full set of DNA.
Cytokinesis divides the cytoplasm and organelles, resulting in two
separate cells.
Cells divide at different rates to accommodate the needs of an
organism. For example, cells that receive a lot of wear and tear, such
as the skin, have a life span of only a few days. Cells making up many
of the internal organs have a life span of many years.
Cells tend to stay within a certain size range. To maintain a suitable size
range, cell growth must be coordinated with cell division. Cell volume
increases much faster than cell surface area for most cells. All materials
that a cell takes in or secretes enter and
exit through the membrane. The cell’s surface area must be large
enough relative to its overall volume in order for the cell to get its
necessary materials. Therefore, most cells tend to be very small.
5.2 KEY CONCEPT Cells divide during mitosis and cytokinesis.
During interphase, a cell needs access to its DNA to make use of specific genes and to
copy the DNA. During mitosis, however, the DNA must be condensed and
organized so that it can be accurately divided between the two nuclei.
DNA is a long polymer made
of repeating subunits called nucleotides. Each long continuous thread of DNA is called
a chromosome, and each chromosome has many genes.
During interphase, DNA wraps around organizing proteins called
histones and is loosely organized as chromatin, which looks sort of like
spaghetti. As a cell prepares for mitosis, however, the DNA and histones
start to coil more and more tightly until
they form condensed chromosomes. Each half of the duplicated chromosome is called a
chromatid. Both chromatids together are called sister chromatids, which are attached at
a region called the centromere. The ends of DNA molecules form
telomeres, structural units that do not code for proteins. Telomeres help
prevent chromosomes from sticking
to each other.
Mitosis is a continuous process, but scientists have divided it into
phases for easier discussion.
• During prophase the chromatin condenses into chromosomes,
the nuclear envelope breaks down, and spindle fibers start to
assemble.
• During metaphase spindle fibers align the chromosomes along the
middle of the cell.
• During anaphase spindle fibers pull the sister chromatids away from
each other and toward opposite sides of the cell.
• During telophase, the nuclear membranes start to form around each
set of chromosomes, the chromosomes start to uncoil, and the spindle
fibers fall apart.
• Cytokinesis divides the cytoplasm into two separate cells. In animal
cells, the cell membrane pinches together. In plant cells, a cell plate
forms between the two
nuclei. It will eventually form new cell membranes for the cells and a new cell wall.
5.3 KEY CONCEPT Cell cycle regulation is necessary for healthy growth.
The cell cycle is regulated by both external and internal factors. External factors come
from outside the cell. These include cell–cell contact, which prevents
further growth of normal cells, and chemical signals called growth factors.
Growth factors stimulate
cells to divide. Most cells respond to a combination of growth factors, not just one.
Some growth factors affect many different types of cells. Others
specifically affect one cell type. Internal factors come from inside the cell.
Very often, an external factor triggers the activation of an internal factor.
A cyclin is a type of internal factor. It activates kinases, which in turn, add
a phosphate group to other molecules that help
drive the cell cycle forward.
Cells not only regulate growth, but also death. Apoptosis is
programmed cell death. Apoptosis plays important roles in
development and metamorphosis.
When a cell loses control over its cycle of growth and division, cancer
may result. Cancer cells can continue to divide despite cell–cell contact or
a lack of growth factors. Cancer cells form disorganized clumps of cells
called tumors. Benign tumors tend to remain clumped together and may
be cured by removal. Malignant tumors have cells
that break away, or metastasize, and spread to other parts of the body,
forming new tumors. Malignant tumors are more difficult to treat than
benign tumors. Radiation therapy and chemotherapy are common
treatments for cancer. However, both treatments kill healthy cells as well
as cancer cells.
Cancer cells can arise from normal cells that have experienced damage
to their genes involved in cell cycle regulation. Damage may arise from
inherited errors in genes,
from mutations carried by viruses, and from carcinogens. Carcinogens
are substances known to produce or promote the development of cancer.
These include substances such
as tobacco smoke and other air pollutants.
5.4 KEY CONCEPT Cells work together to carry out complex functions.
Your body began as a single fertilized egg. Since that time, your cells have not only
gone through millions of cell divisions, but those cells have also undergone the process
of cell differentiation by which unspecialized cells develop into their
mature form and function. Groups of cells that work together to
perform a similar function make
up tissues. Groups of tissues that work together to perform a similar function make up
organs. Groups of organs that carry out related functions make up organ systems.
The interaction of multiple organ systems working together helps
organisms maintain homeostasis.
An organism’s body plan is established in the very earliest stages of
embryonic development. In both animals and plants, a cell’s location
within the embryo helps determine how that cell will differentiate. In
animals, cells migrate to specific areas that will determine how they
specialize. Plant cells cannot readily migrate because of their
cell walls. However, the cells remain very adaptable throughout the life of the plant.
Stem cells are a unique type of body cell characterized by three features:
• They divide and renew themselves for long periods of time.
• They remain undifferentiated in form.
• They can develop into a variety of specialized cell types.
Because of their ability to develop into other types of cells, stem cells
offer great hope for curing damaged organs and currently untreatable
diseases. However, they also raise many ethical concerns. Stem cells
can be categorized by their developmental potential,
as totipotent, pluripotent, or multipotent. Stem cells can also be classified by origin,
as adult or embryonic.
Chapter 6
Vocabulary:
somatic cell
egg
genotype
gamete
polar body
phenotype
homologous chromosome
trait
dominant
autosome
genetics
recessive
sex chromosome
purebred
Punnett square
sexual reproduction
cross
monohybrid cross
fertilization
law of segregation
testcross
diploid
gene
dihybrid cross
haploid
allele
law of independent assortment
meiosis
homozygous
probability
gametogenesis
heterozygous
crossing over
sperm
genome
genetic linkage
6.1 KEY
CONCEPT
Gametes have half the number of chromosomes that body cells have.
Your body is made of two basic cell types. One basic type are somatic cells, also called
body cells, which make up almost all of your tissues and organs. The second basic
type are germ cells, which are located in your reproductive organs. They are the cells
that will undergo meiosis and form gametes. Gametes are sex cells. They
include eggs and sperm cells.
Each species has a characteristic number of chromosomes per cell. Body cells are diploid, which means that
each cell has two copies of each chromosome, one from each parent. Gametes are haploid, which means that
each cell has one copy of each chromosome. Gametes join together during fertilization, which is the actual
fusion of egg and sperm, and restores the diploid number.
The diploid chromosome number in humans is 46. Your cells need both copies of each chromosome to function
properly. Each pair of chromosomes is called homologous. Homologous chromosomes are a pair of
chromosomes that have the same overall appearance and carry the same genes. One comes from the mother, and
one comes from the father. Thus, one chromosome from a pair of homologous chromosomes might carry a gene
that codes for green eye color, while the other carries a gene that codes for brown eye color. For reference, each
pair of homologous chromosomes has been numbered, from largest to smallest. Chromosome pairs 1 through 22
are autosomes. Autosomes are chromosomes that contain genes for characteristics not directly related to sex.
The two other chromosomes are sex chromosomes, chromosomes that directly control the development of sexual
characteristics. In humans, a woman has two X chromosomes, and a man has an
X and a Y chromosome. The Y chromosome is very small and carries few genes.
Meiosis is a form of nuclear division that reduces chromosome number from diploid to haploid. Each haploid cell
produced by meiosis has 22 autosomes and 1 sex chromosome.
6.2 KEY
CONCEPT During meiosis, diploid cells undergo two cell divisions that result
in haploid cells.
Meiosis occurs after a cell has already duplicated its DNA. Cells go through two rounds of
cell division during meiosis. During the first round, meiosis I, homologous chromosomes
separate from each other. During the second round, meiosis II, sister chromatids separate
from each other. Meiosis produces genetically unique haploid cells that will go through
more steps to form mature gametes.
Meiosis is a continuous process, but scientists have divided it into phases.
• Prophase I: The nuclear membrane breaks down, and the spindle fibers assemble.
The duplicated chromosomes condense, and homologous chromosomes pair up.
The sex chromosomes also pair together.
• Metaphase I: The homologous chromosome pairs randomly line up along the middle
of the cell. Because this is random, there are a mixture of chromosomes from both
parents on each side of the cell equator.
• Anaphase I: The paired homologous chromosomes separate from each other and
move to opposite sides of the cell.
• Telophase I: The nuclear membrane forms in some species, the spindle fibers break
apart, and the cell undergoes cytokinesis. Each cell has 23 duplicated chromosomes.
• Prophase II: The nuclear membrane breaks down if necessary and the spindle fibers
assemble again.
• Metaphase II: The chromosomes line up along the middle of the cell.
• Anaphase II: The sister chromatids are pulled apart from each other and move to
opposite sides of the cell.
• Telophase II: The nuclear membranes form again, the spindle fibers break apart, and
the cell undergoes cytokinesis.
The haploid cells produced by meiosis are not capable of fertilization. They must undergo
additional steps to form mature gametes. During gametogenesis, sperm cells—the
male gametes—and eggs—the female gametes—become specialized to carry out their
functions. Sperm cells lose much of their cytoplasm and develop a tail. Eggs receive
almost all of the cytoplasm during the divisions in meiosis. This is necessary for an
embryo to have all the materials needed to begin life after fertilization. The smaller
cells produced by meiosis in the female are called polar bodies, and they are eventually
broken down.
6.3 KEY CONCEPT Mendel’s research showed that traits are inherited as discrete units.
Traits are inherited characteristics, and genetics is the study of the biological inheritance
of traits and variation. Gregor Mendel, an Austrian monk, first recognized that traits are
inherited as discrete units. We call these units genes. Mendel conducted his experiments
with pea plants, which were an excellent choice because they are easily manipulated,
produce large numbers of offspring, and have a short life cycle. Mendel made three
important decisions that helped him to see patterns in the resulting offspring.
• Use of purebred plants: Mendel used pea plants that had self-pollinated for so
long that they had become genetically uniform, or purebred. This meant that the
offspring looked like the parent plant. Because of this characteristic, Mendel knew
that any differences he observed in the offspring were the result of his experiments.
• Control over breeding: At the start of his experiments, Mendel removed the male
flower parts from the pea plants. He then pollinated the female flower part with
pollen from a plant of his choosing, which produced offspring referred to as the
F1 generation.
• Observation of “either-or” traits: Mendel studied seven traits that appeared in only
two forms. For example, flowers were white or purple; peas were wrinkled or round.
Mendel observed that when he mated, or crossed, a purple-flowered plant with a
white-flowered plant, for example, all of the F1 offspring had purple flowers. Mendel
next allowed the F1 offspring to self-pollinate; that is, the plant mated with itself. In the
resulting offspring, the F2 generation, approximately three-fourths of the flowers were
purple and one-fourth were white. Mendel continued to find this 3:1 ratio for each of his
crosses, regardless of the specific trait he was examining.
Based on his results, Mendel concluded that traits are inherited as discrete units. He
also developed what is known as Mendel’s first law, or the law of segregation. This
law states the following:
• Organisms inherit two copies of each unit (gene), one from each parent.
• The two copies separate, or segregate, during gamete formation. As a result,
organisms donate only one copy of each unit (gene) in their gametes.
6.4 KEY CONCEPT Genes encode proteins that produce a diverse range of traits.
A gene is a segment of DNA that tells the cell how to make a particular polypeptide. The
location of a gene on a chromosome is called a locus. A gene has the same locus on
both chromosomes in a pair of homologous chromosomes. In genetics, scientists often
focus on a single gene or set of genes. Genotype typically refers to the genetic makeup
of a particular set of genes. Phenotype refers to the physical characteristics resulting
from those genes.
An alternative form of a gene is an allele. The pea plants that Mendel worked with had
two alleles for each gene. For example, there was an allele for round peas and an allele for
wrinkled peas. Genes are not limited to two alleles, however. Some genes are found in
many different forms throughout a population.
Your cells have two alleles for each gene regardless of how many alleles are present in
a population. Suppose there were 64 alleles of a hair color gene present in the human
population. Your cells would only have two of those alleles, one from your mother and
one from your father. If the two alleles are the same, they are homozygous. If the two
alleles are different, they are heterozygous.
Some alleles are dominant over others.
• A dominant allele is expressed when two different alleles or two dominant alleles
are present. Therefore, both homozygous dominant and heterozygous genotypes
can produce the dominant phenotype.
• A recessive allele is expressed only when both alleles are recessive. Therefore, only
the homozygous recessive genotype can produce the recessive phenotype.
Alleles may be represented using letters. Uppercase letters represent dominant alleles.
Lowercase letters represent recessive alleles.
6.5 KEY CONCEPT The inheritance of traits follows the rules of probability.
The possible genotypes resulting from a cross can be predicted using a Punnett square.
A Punnett square is a grid. The axes are labeled with the alleles of each parent
organism. The grid boxes show all of the possible genotypes of the offspring resulting
from those two parents.
A monohybrid cross is used when studying only one trait. A cross between a
homozygous dominant organism and a homozygous recessive organism produces
offspring that are all heterozygous and have the dominant phenotype. A cross
between two heterozygous organisms results in a 3:1 phenotypic ratio in the offspring,
where three-fourths have the dominant phenotype and one-fourth have the recessive
phenotype. The genotypic ratio resulting from this cross is 1:2:1 of homozygous
dominant:heterozygous:homozygous recessive.
A testcross is a cross between an organism with an unknown genotype (dominant
phenotype) and an organism with the recessive phenotype. If the organism with the
unknown genotype is homozygous dominant, the offspring will all have the dominant
phenotype. If it is heterozygous, half the offspring will have the dominant phenotype,
and half will have the recessive phenotype.
A dihybrid cross is used when studying the inheritance of two traits. Mendel’s dihybrid
crosses helped him develop the law of independent assortment, which basically states
that different traits are inherited separately. When two organisms that are heterozygous
for both traits are crossed, the resulting phenotypic ratio is 9:3:3:1.
Probability is the likelihood that a particular event, such as the inheritance of a
particular allele, will happen. The events of meiosis and fertilization are random, so
hereditary patterns can be calculated with probability.
6.6 KEY CONCEPT Independent assortment and crossing over during meiosis result
in genetic diversity.
In organisms that reproduce sexually, the independent assortment of chromosomes during
meiosis and the random fertilization of gametes creates a lot of new genetic combinations.
In humans, for example, there are over 64 trillion different possible combinations of
chromosomes. Sexual reproduction creates genetically unique offspring that have a
combination of both parents’ traits. This uniqueness increases the likelihood that some
organisms will survive or even flourish in changing conditions.
Genetic diversity is further increased through crossing over. Crossing over is the
exchange of segments of chromosomes between homologous chromosomes. It happens
during prophase I of meiosis I when homologous chromosomes pair up with each other
and come into very close contact. At this stage, the chromosomes have already been
duplicated. Part of a chromatid from each homologous chromosome may break off and
reattach to the other chromosome.
Crossing over is more likely to occur between genes that are far apart from each other on
a chromosome. The likelihood that crossing over will happen is much less if two genes
are located close together. Thus, genes that are located close together on a chromosome
have a tendency to be inherited together, which is called genetic linkage. Most of the
traits that Mendel studied were located on separate chromosomes, and so they assorted
independently. When genes are on the same chromosome, however, their distance from
each other is a large factor in how they assort. If they are far apart, crossing over is likely
to occur between them and so they will assort independently. If they are close together,
they are unlikely to be separated by crossing over and so they will not assort independently.
Chapter 7
Vocabulary
carrier
incomplete dominance
sex-linked gene
codominance
pedigree
X chromosome inactivation
polygenic trait
karyotype
7.1 KEY CONCEPT The chromosomes on which genes are located can affect the
expression of traits.
There are two types of chromosomes: autosomes and sex chromosomes. Genes on the sex
chromosomes determine an organism’s sex. Autosomes are all of the other chromosomes,
and they do not directly affect sex determination. Gene expression can differ depending
on the type of chromosome on which a gene is located.
• Autosomal genes: There are two copies of each autosome, which means that there
are two copies of each autosomal gene. However, the two copies of a gene may be
different alleles. Both copies of a gene can affect phenotype. Much of what has
been learned about human genes comes from studies of genetic disorders. Many
genetic disorders are caused by recessive alleles on autosomes. People who have one
dominant allele and one recessive, disorder-causing allele, do not have the disorder,
but can pass it on because they are carriers of the disorder.
• Sex-linked genes: Genes on the sex-chromosomes (the X and Y chromosomes in
many species) are sex-linked genes. In mammals, including humans, and some other
animals, XX individuals are female and XY individuals are male. Because males
have only one copy of each sex chromosome, all of the genes on each chromosome
will be expressed. Expression of sex-linked genes in females is similar to the
expression of autosomal genes: two copies of each gene can affect phenotype.
However, one X chromosome in each cell is randomly turned off by a process called
X chromosome inactivation.
7.2 KEY CONCEPT Phenotype is affected by many different factors.
Although some genetic traits are produced by one gene with dominant and recessive
alleles, most genetic traits are the result of more complex relationships among genes
and alleles. In many cases phenotype comes from more than just one gene, and many
genes have more than just two alleles.
• Incomplete dominance: In incomplete dominance, neither of two alleles is
completely dominant or completely recessive. Instead, the alleles show incomplete
dominance, where the heterozygous phenotype is somewhere between the
homozygous dominant and homozygous recessive phenotypes. The heterozygous
phenotype is a third, distinct phenotype.
• Codominance: In codominance, two alleles of a gene are completely and
separately expressed, and both phenotypes are also completely expressed. Human
blood type is an example of both codominance and a multiple allele trait. The
alleles for blood types A and B are codominant, which can be expressed as an AB
blood type. The allele for type O blood is recessive to the other two alleles.
• Polygenic traits: Traits that are produced by two or more genes are polygenic
traits. Because many different gene interactions can occur with polygenic traits,
these traits often have a wide, continuous range of phenotypes.
• Epistasis: An epistatic gene is a gene that can affect the expression of all of the
other genes that affect a trait.
The environment can influence gene expression, which then affects phenotype.
Human height is a trait that is partially due to environment. Another example is how
temperature affects sex determination of sea turtles.
7.4 KEY CONCEPT A combination of methods is used to study human genetics.
The patterns of inheritance in humans are the same as the patterns of inheritance in
other sexually reproducing organisms. Phenotypes are often the result of varying
degrees of dominance, several genes, multiple alleles, or sex-linked genes.
Only females can be carriers of sex-linked disorders. Females, who have an XX
genotype for their sex chromosomes, must have two recessive alleles to show a recessive
phenotype, such as for a recessive sex-linked disorder. Males, on the other hand, have
an XY genotype. They will show all of the phenotypes from the genes on their X
chromosome, even the recessive alleles, because they cannot have a second, dominant
allele that could mask the recessive allele.
The potential for a genetic disorder to be passed on through a family can be studied
using pedigree analysis. A pedigree is a chart that is used to trace phenotypes and
genotypes within a family. It can help show whether someone in a family may have
recessive alleles that cause a genetic disorder.
In addition to pedigrees, other methods of studying human genetics are used.
Karyotypes, for example, are pictures of all of a person’s chromosomes that can show
any large changes in the chromosomes.
Chapter 8
Vocabulary
nucleotide
RNA polymerase
mutagen
messenger RNA (mRNA)
mutation
double helix
ribosomal RNA (rRNA)
point mutation
base pairing rules
transfer RNA (tRNA)
frameshift mutation
replication
translation
DNA polymerase
codon
central dogma
stop codon
RNA
start codon
transcription
anticodon
8.2 KEY CONCEPT DNA structure is the same in all organisms.
DNA is a chain of nucleotides. In DNA, each nucleotide is made of a phosphate group,
a sugar called deoxyribose, and one of four nitrogen-containing bases. These four bases
are cytosine (C), thymine (T), adenine (A), and guanine (G). Two of the bases, C and T,
have a single-ring structure. The other two bases, A and G, have a double-ring structure.
Although scientists had a good understanding of the chemical structure of DNA by the
1950s, they did not understand its three-dimensional structure. The contributions of
several scientists helped lead to this important discovery.
• Erwin Chargaff analyzed the DNA from many different organisms and realized that
the amount of A is equal to the amount of T, and the amount of C is equal to the
amount of G. This A = T and C = G relationship became known as Chargaff ’s rules.
• Rosalind Franklin and Maurice Wilkins studied DNA structure using x-ray
crystallography. Franklin’s data suggested that DNA is a helix consisting of two
strands that are a regular, consistent width apart.
James Watson and Francis Crick applied Franklin’s and Chargaff ’s data in building a
three-dimensional model of DNA. They confirmed that DNA is a double helix in
which two strands of DNA wind around each other like a twisted ladder. The sugar and
phosphate molecules form the outside strands of the helix, and the bases pair together in
the middle, forming hydrogen bonds that hold the two sides of the helix together. A
base with a double ring pairs with a base with a single ring. Thus, in accordance with
Chargaff ’s rules, they realized that A pairs with T, and C pairs with G. The bases always
pair this way, which is called the base pairing rules.
8.3 KEY CONCEPT DNA replication copies the genetic information of a cell.
Every cell needs its own complete set of DNA, and the discovery of the
three-dimensional structure of DNA immediately suggested a mechanism by which the
copying of DNA, or DNA replication, could occur. Because the DNA bases pair in
only one way, both strands of DNA act as templates that direct the production of a new,
complementary strand. DNA replication takes place during the S stage of the cell cycle.
The process of DNA replication is very similar in both eukaryotes and prokaryotes,
but we will focus on eukaryotes.
• During the S stage of the cell cycle, the DNA is loosely organized in the nucleus.
Certain enzymes start to unzip the double helix at places called origins of
replication. The double helix unzips in both directions along the strand. Eukaryotic
chromosomes are very long, so they have many origins of replication to help speed
the process. Other proteins hold the two strands apart.
• The unzipping exposes the bases on the DNA strands and enables free-floating
nucleotides to pair up with their complementary bases. DNA polymerases bond
the nucleotides together to form new strands that are complementary to the original
template strands.
• The result is two identical strands of DNA. DNA replication is described as
semiconservative because each DNA molecule has one new strand and one original
strand.
DNA polymerase not only bonds nucleotides together. It also has a proofreading
function. It can detect incorrectly paired nucleotides, clip them out, and replace them
with the correct nucleotides. Uncorrected errors are limited to about one per 1 billion
nucleotides.
8.4 KEY CONCEPT Transcription converts a gene into a single-stranded RNA molecule.
DNA provides the instructions needed by a cell to make proteins. But the instructions
are not made directly into proteins. First, a DNA message is converted into RNA in a
process called transcription. Then, the RNA message is converted into proteins in a
process called translation. The relationship between these molecules and processes is
summed up in the central dogma, which states that information flows in one direction,
from DNA to RNA to proteins.
Like DNA, RNA is a nucleic acid. It is made of nucleotides that consist of a phosphate
group, a sugar, and a nitrogen-containing base. However, RNA differs in important ways
from DNA: (1) RNA contains the sugar ribose, not deoxyribose; (2) RNA is made up of
the nucleotides A, C, G, and uracil, U, which forms base pairs with A; (3) RNA is usually
single-stranded. This single-stranded structure enables RNA to fold back on itself into
specific structures that can catalyze reactions, much like an enzyme.
During transcription, a gene is transferred into RNA. Specific DNA sequences and a
combination of accessory proteins help RNA polymerase recognize the start of a gene.
RNA polymerase is a large enzyme that bonds nucleotides together to make RNA.
RNA polymerase, in combination with the other proteins, forms a large transcription
complex that unwinds a segment of the DNA molecule. Using only one strand of DNA
as a template, RNA polymerase strings together a complementary RNA strand that has
U in place of T. The DNA strand zips back together as the transcription complex moves
forward along the gene.
Transcription makes three main types of RNA.
• Messenger RNA (mRNA) is the intermediate message between DNA and proteins.
It is the only type of RNA that will be translated to form a protein.
• Ribosomal RNA (rRNA) forms a significant part of ribosomes.
• Transfer RNA (tRNA) carries amino acids from the cytoplasm to the ribosome
during translation.
The DNA of a cell therefore has genes that code for proteins, as well as genes that code
for rRNA and tRNA.
8.5 KEY CONCEPT Translation converts an mRNA message into a polypeptide, or
protein.
Translation is the process that converts an mRNA message into a polypeptide, or protein.
An mRNA message is made up of combinations of four nucleotides, whereas proteins
are made up of twenty types of amino acids. The mRNA message is read as a series of
non-overlapping codons, a sequence of three nucleotides that code for an amino acid.
Many amino acids are coded for by more than one codon. In general, codons that code
for the same amino acid share the same first two nucleotides. Three codons, called stop
codons, signal the end of the polypeptide. There is also a start codon, which both signals
the start of translation and codes for the amino acid methionine. This genetic code is the
same in almost all organisms, so it is sometimes called the universal genetic code.
Although tRNA and rRNA are not translated into proteins, they play key roles in helping
cells translate mRNA into proteins. Each tRNA molecule folds up into a characteristic
L shape. One end has three nucleotides called an anticodon, which recognize and bind
to a codon on the mRNA strand. The other end of the tRNA molecule carries a specific
amino acid. A combination of rRNA and proteins make up the ribosome. Ribosomes
consist of a large and small subunit. The large subunit has binding sites for tRNA. The
small subunit binds to the mRNA strand.
At the start of translation, a small subunit binds to an mRNA strand. Then the large
subunit joins. A tRNA molecule binds to the start codon. Another tRNA molecule binds
to the next codon. The ribosome forms a bond between the two amino acids carried by the
tRNA molecules and pulls the mRNA strand by the length of one codon. This causes the
first tRNA molecule to be released and opens up a new codon for binding. This process
continues to be repeated until a stop codon is reached and the ribosome falls apart.
8.7 KEY CONCEPT Mutations are changes in DNA that may or may not affect phenotype.
A mutation is a change in an organism’s DNA. Although a cell has mechanisms to deal
with mutations, exposure to mutagens may cause mutations to happen more quickly than
the body can repair them. Mutagens are agents in the environment that can change
DNA. Some occur naturally, such as UV light from the Sun. Many other mutagens are
industrial chemicals.
Mutations may affect individual genes or an entire chromosome. Gene mutations include
point mutations and frameshift mutations.
• A point mutation is a substitution in a single nucleotide.
• A frameshift mutation involves the insertion or deletion of a nucleotide or
nucleotides. It throws off the reading frame of the codons that come after the
mutation.
Chromosomal mutations include gene duplications and translocations. Gene duplication
is the result of improper alignment during crossing over. It results in one chromosome
having two copies of certain genes, and the other chromosome having no copies of
those genes. Translocation is the movement of a piece of one chromosome to another,
nonhomologous chromosome.
Mutations may or may not affect phenotype. Chromosomal mutations affect many
genes and tend to have a large effect on an organism. They may also cause breaks in
the middle of a gene, causing that gene to no longer function or to make a hybrid with
a new function. The effect of a gene mutation can also vary widely. For example, a
point mutation may occur in the third nucleotide of a codon and have no effect on the
amino acid coded for. Or the mutation may occur in an intron and thus have no effect.
However, the mutation might result in the incorporation of an incorrect amino acid that
messes up protein folding and function. Or it might code for a premature stop codon.
Even mutations that occur in noncoding regions of DNA can have significant effects if
they disrupt a splice site or a DNA sequence involved in gene regulation. For a mutation
to affect offspring, it must occur in an organism’s germ cells.
Chapter 10
Vocabulary
evolution
variation
fitness
species
adaptation
biogeography
fossil
artificial selection
homologous structure
catastrophism
heritability
analogous structure
gradualism
natural selection
vestigial structure
uniformitarianism
population
10.1 KEY CONCEPT There were theories of biological and geologic change before
Darwin.
Evolution is the process of biological change by which descendants come to differ
from their ancestors. Charles Darwin was not the first scientist to share his ideas about
evolution and how it occurs.
• Carolus Linnaeus proposed that plant varieties, or species—a group of organisms so
similar to one another that they can reproduce and have fertile offspring—can be
crossed to create new species.
• Georges Buffon proposed that species shared ancestors instead of arising separately,
the common thought of the time.
• Erasmus Darwin, Charles Darwin’s grandfather, noted that more-complex forms of
life seemed to arise from less-complex forms.
• Jean-Baptiste Lamarck recognized that changes in physical characteristics could be
passed on to offspring and were driven by environmental changes over time.
Although Lamarck had ideas that influenced Darwin’s thinking, his explanation of how
organisms evolve was flawed. He thought, for example, that the long necks of giraffes
evolved as generations of giraffes reached for leaves higher in the trees. This idea, which
was later discredited, is known as the inheritance of acquired characteristics.
The field of geology also offered insights into evolution. Geologists noted that
fossils—traces of organisms that existed in the past—in deeper layers of rock were quite
different than those found in the upper layers. There were several ideas proposed to
explain how such changes occur.
• The theory of catastrophism states that natural disasters such as floods and volcanic
eruptions have happened often during Earth’s long history. These events shaped
landforms and caused species to become extinct in the process.
• The principle of gradualism states that changes in landforms result from slow
changes over a long period of time.
• The theory of uniformitarianism states that the geologic processes that shape Earth
are uniform through time. The theory of uniformitarianism, proposed by geologist
Charles Lyell, combines gradualism with the observation that changes on Earth
have occurred at a constant rate and are ongoing. The concept of uniformitarianism
greatly affected Darwin’s thinking.
10.2 KEY CONCEPT Darwin’s voyage provided insights into evolution.
Darwin traveled aboard the ship HMS Beagle to map the coast of South America and
the Pacific Islands in 1831. He observed variation—the difference in the physical traits
of an individual from those of other individuals in the same population—between island
species on his voyage. The differences were especially noticeable on the Galápagos
Islands off of South America. Some differences seemed well-suited to the animals’
environments and diets. He noticed that species have adaptations, or features that
allow them to better survive in their environments. Adaptations can lead to genetic
change in a population over time.
• Saddle-backed tortoises, which have long necks and legs, lived in areas with a lot
of tall plants. Domed tortoises, with their shorter necks and legs, lived in wet
areas rich in mosses and short plants.
• Finches with strong, thick beaks lived in areas with a lot of large, hard-shelled
nuts. Species of finch with more delicate beaks were found where insects or fruits
were widely available.
On his voyage, Darwin also saw fossil evidence of species changing over time.
• He found fossils of huge animals, such as Glyptodon, a giant armadillo. He
recognized that these fossils looked like living species, which suggested to him
that modern animals might have some relationship to fossil forms.
• He observed fossil shells of marine organisms high up in the mountains. Later,
Darwin experienced an earthquake and saw firsthand the result: land that had been
underwater was moved above sea level.
Darwin realized that over long periods of time, gradual geologic or biological processes
can add up to great change.
10.3 KEY CONCEPT Darwin proposed natural selection as a mechanism for evolution.
Darwin’s ideas about evolution were influenced by many different sources. One important
influence was the work of farmers and breeders. Artificial selection, the process by
which humans change a species by breeding it for certain traits, provided Darwin with
some important insights. He noticed that breeders could produce a great amount of
diversity through selection of certain traits. In order for artificial selection to occur, the
trait must be heritable. Heritability is the ability of a trait to be inherited, or passed down,
from one generation to the next.
Darwin extended the ideas he gained from studying artificial selection to his theory of
natural selection. Natural selection is a mechanism by which individuals that have
inherited beneficial adaptations produce more offspring on average than do other
individuals. Unlike artificial selection, where humans do the selecting of traits, in natural
selection the environment is the selective agent.
Natural selection is based upon four principles:
• Overproduction: producing more offspring than are likely to survive
• Variation: the heritable differences that exist in every population
• Adaptation: a certain characteristic that allows an individual to survive better than
other individuals it competes against for resources
• Descent with modification: the spread of an adaptation throughout new generations
Natural selection works on physical traits rather than genetic material itself. New traits
are not made by natural selection. Natural selection can act only on traits that already
exist in a population.
10.4 KEY CONCEPT Evidence of common ancestry among species comes from many
sources.
Darwin found evidence supporting evolution from a wide range of sources. The most
important and convincing support came from fossils, geography, embryology, and
anatomy.
• The fossil is a record of change in a species over time. Geologists found that fossil
organisms on the bottom, or older, layers were more primitive than those in the
upper, or newer, layers. These findings supported Darwin’s concept of descent
with modification.
• Biogeography, the study of the distribution of organisms around the world, reveals
a pattern of evolution of organisms. Darwin’s observations on the Galapagos
islands, for instance, demonstrated that species can adapt to different environments
and evolve into separate populations or species over time.
• Embryology, the study of embryo development, reveals that even organisms that
are very different from each other in their adult forms can have similar patterns
of development. Two species that exhibit similar traits during development are
likely to have a common ancestor.
• Anatomy also provides insight into evolution. Homologous structures are features
that are similar in structure but appear in different organisms and have different
functions. Vestigial structures are remnants of organs or structures that had a
function in an early ancestor. Both homologous structures and vestigial structures
point to a shared ancestry among organisms that share them.