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What is Life?
The Case Study of the Peppered Moth
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Biology may not be magic, but it does create some impressive illusions:
a leaf-shaped
katydid
lands on a branch and "disappears,"
becoming indistinguishable from the real leaves of the tree. A
tiny cactus growing close to the ground looks so much like a small
rock that it's called a
living stone.
Unseen by hungry
predators, the katydid and the cactus escape being eaten by
hiding in plain sight.
In addition to mimicing their environments, organisms sometimes mimic each other.
The Viceroy butterfly looks extremely similar to the noxious tasting (to bird
predators) Monarch butterfly. Once they have tried dining on a Monarch, birds usually don't try
again. Because Viceroys look like Monarchs, they often don't get eaten either-a nice trick for the Viceroy,
who gets all the advantages of looking like a noxious tasting tidbit, without
having to expend the energy to produce the complex molecules that confer bad taste. Click to see numerous examples of
mimicry.
In the 1800's in England,
a common moth, the white peppered moth, was also very
adept at hiding in plain sight.
All it had to do was land on
a tree trunk which at that time was covered in light-colored lichens,
to become virtually indistinguishable
from the background and almost invisible to the birds that ate
it.
Then disaster struck. Increasing amounts of industrial smoke gradually
changed the bright landing places of the white peppered moth to
dark, sooty surfaces. The white peppered moths were now easily
spotted by birds when they landed on their former sanctuaries
and their numbers began to drop. The white peppered moth was now
involved in a different kind of "disappearing" act that
almost lasted forever.
Then something unusual began to happen. A dark-colored form of the peppered
moth, rarely seen prior to the industrial revolution began to
grow in numbers. Soon the population of peppered moths was back
to its former prevalence, but the majority of them were dark and
perfectly camouflaged on the newly blackened trees of industrialized
England. Somehow the peppered moth had "switched" colors
and the species continued to survive.
(To see pictures of the colour phases of the peppered moth against its
light and dark backgrounds, click
here
.)
To understand how the moths changed colors in response to a change
in their environment, we first must discuss, what they did not
do. The moths were not chameleons: an individual moth could not
change it color. It was the species, not the individual
moths, that changed color so that, by the mid-1950's, virtually
all peppered moths in industrialized England were dark. But where
did the dark moths come from?
The answer is genetic variability. The general traits among
individuals in any species are similar. Most people have two eyes,
a nose, 10 fingers and hundreds of other features that are similar
to all people. Yet we look different from each other: our eyes
are different colors, our noses have different shapes. These differences
in a particular characteristic are what we mean by the term genetic
variability. The differences derive from differences in the
genes that control the particular trait.
Genes are coded bits of information that determine a trait. Copies
of these genes are passed on from parent to offspring, who thereby
inherit traits characteristic of that particular family. Therefore
you will look more like your parents and/or your siblings than
you will resemble a person who is not related to you.
Occasionally a spontaneous change, a mutation, will occur
in a gene, which results in the offspring inheriting a new version
of a trait. In the white peppered moth, for example, a few dark
colored offspring were likely always being produced by mutations
of a pigment gene. Before the industrial revolution, however,
few of these dark forms persisted because they were easily seen
against the light tree surfaces and snatched up by birds. But
when environmental conditions changed, the light colored moths
became the easy targets and the dark form of the moth became "invisible",
was protected from predation and lived to mate and produce more
dark colored offspring. If there had not been the potential for
genetic variation in the peppered moth prior to the environmental
change, there would have been no dark peppered moth and the species
would now be extinct: another victim of the industrial revolution.
There has been a surprising final chapter to this story. If you
go to industrialized England today, you will once again see the
white peppered moth. Modern pollution controls have cleaned up
the air (remember the London Fog incident of 1952?) and the surfaces
of trees are once again lightly colored. The lighter form of the
moth is once again the form that is protectively colored and the
dark form is once again only rarely found.
The case of the peppered moth illustrates how a species can change
over time. The process that has changed the population of peppered
moths is the same one responsible for generating the millions
of species produced by 3.5 billion years of biological evolution.
Thus the study of the peppered moth provides a vivid portrayal
of how changes in the environment can change the genetic composition
of a species: evolution in action!
Index
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Differentiating the living and the inanimate
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This term's material is primarily concerned with living things
and their responses to their environment. Ignoring anthropogenic
change for the moment, we all recognize that the Earth is constantly
changing. Mountains crumble, continents collide, climates change
and yet life on Earth has persisted for at least 3.5 billion years.
What then does it mean for something to be alive?
Most of us have little trouble identifying something as being
alive, yet how did we know? Many of the properties that we use
to categorise an object as living are not found in all organisms
or may even be exhibited by some nonliving things. Yet most characteristics
trees in England where generally covered with
light-colored lichens.
popularly associated with living things (growth, movement, reproduction,
food consumption) may also be properties of nonliving entities.
Clouds and mineral crystals grow; rivers, air and clouds move;
fire consumes food (or at least fuel) as it grows and reproduces.
Yet none of these things is alive.
As it turns out, developing a definition of "life" is not a trivial exercise. Life is a classical example of an emergent property.
Hence, biologists have tended to adopt a definition of life that is really a list of the descriptive properties characterising
living things, not one of which could, by itself, be considered the single criterion of what it means to be alive:
- Organization Living organisms maintain a high degree
of complexity and hierarchical order. Complexity is a measure
of the number of parts that make up an object and the precision
by which the parts are organized.
- Cells Living organisms are composed of one or more
cells. For unicellular organisms-those that consist of one cell-the
cell is the organism. Most organisms, however are multicellular
consisting of hundreds to trillions of cells.
- Energy Organisms acquire and use energy, either via
photosynthesis or respiration. The original source of this energy
is sunlight.
- Reproduction Organisms produce offspring similar to
themselves
-
Heredity Organisms contain a genetic blueprint that
dictates their characteristics and some behaviors. This genetic
blueprint is contained in a specific sequence of molecules called
DNA
or deoxyribonucleic acid.
By now we should remember that life is heirarchical,
so you won't be surprised to find that
molecules of DNA
are grouped into genes which are grouped into
chromosomes
within the nucleus of the organism.
- Growth and Development Organisms grow in size and change
in appearance and abilities.
- Responsiveness Organisms respond to changes in their
environment
- Metabolism Organisms carry out a variety of controlled
chemical reactions
- Homeostasis Organisms maintain a relatively constant
internal environment, despite fluctuations in their external surroundings
You hopefully remember that emergent properties are a profound and fascinating characteristic of hierarchical systems: as the system generates a higher order of structural organization, new properties synergistically emerge that eventually exceed the sum
of the parts used to form the structure. We used a book analogy: when individual letters combine to form a word, a new property emerges-the word's meaning.
No new materials were added; the new property is strictly a function of the more complex, higher level of organization. Words combined into sentences at the next level generates another new property-a statement.
Hierarchical systems such as these exhibit a profound and fascinating
characteristic: as the complexity of a system generates a higher
order of structural organization, new properties that synergistically
emerge, exceeding the sum of the parts used to form the structure.
Returning to the book analogy: when individual letters combine
to form a word, a new property emerges-the word's meaning. No
new materials were added; the new property is strictly a function
of the more complex, higher level of organization. Words combined
into sentences at the next level generates another new property-a
statement.
Life is such an emergent property. It first emerges at the level
of the cell
(as opposed to the level of the atom or molecule)
and exceeds the sum of the cell's parts. Just as the
sentences in a book ultimately combine to produce a story, a story
emerges from the diversity of organisms that have combined to
yield the biosphere. We are not very good at interpreting that
story. Furthermore, just as the meaning of a written story is
quickly lost as sentences are removed or as the words and sentences
become unintelligible as letters are removed, we should wonder
at the unknown chaos lurking as we remove genetic diversity and
entire species from the biosphere.
Index
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Cell Organization
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We have known about cells since the invention of the microscope
in the 1600's. We now recognize that all organisms are composed
of one of more cells and that the cell is the basic organizational unit of
life. Cells are made up of
a number of structures, the most prominent of which is the nucleus.
The nucleus contains the genetic material of the cell, deoxyribonucleic
acid or DNA. The DNA is organized into discrete packages called
chromosomes. Genes are specific sections on the chromosomes.
Index
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Cell Division: Mitosis and Meiosis
Introduction
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Each of us was once just a single cell-a fertilized egg-formed from
the union of gametes: a sperm from our fathers and an egg from
our mothers. From this rather inauspicious beginning, we have
grown into organisms consisting of trillions of cells. How did
this happen?
Cells reproduce-that is they generate more of themselves-by dividing
it two. For multicellular organisms like us, countless divisions
of the fertilized egg have resulted in an astonishingly complex
unit with a number of emergent properties. Furthermore, cell division
continues as long as we are alive. Biologists estimate that more
than 25 million cells are undergoing division in each of our bodies
every second!
This enormous output of cells is needed to replace those cells
that have aged or died. Old, worn blood cells, for example are
removed and replaced by new ones at the rate of about 100 million
per minute. Not surprisingly then, anything that blocks cell division,
such as exposure to heavy doses of radiation, can have tragic
effects. Many people who valiantly worked to seal off the damaged
nuclear reactor at Chernobyl in the former Soviet Union, subsequently
died because their bodies were unable to replace worn out blood
cells.
Each dividing cell is called a
mother cell. Its descendants
are appropriately called daughter cells. There is a good
reason for using this "familial" terminology. The mother
cell transmits copies of its hereditary information (the DNA or
deoxyribonucleic acid sequences of the genes) to its daughter
cells which represent the next generation. In turn, daughter cells
can become mother cells to their own daughter cells, passing along
the same genes they inherited from their mother to yet another
cellular generation. For this reason, cell division is often referred
to as cellular reproduction.
Cell division is more than just a means of reproducing more cells
within the same organism; it is also the basis for reproducing
more organisms. Cell division, therefore, forms the link between
a parent and its offspring; between living species and their extinct
ancestors; and between humans and the earliest, most primitive
cellular organism.
Index
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Meiosis and Mitosis: Types of Cell Division
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In spite of the great complexity of living organisms and the types
of cells they contain, there are only two basic types of cell
division: one type is called
meiosis
the other mitosis.
For all intents and purposes, cells undergo mitosis when they
are growing, i.e. when the purpose of cell division is
to reproduce more cells within the same organism, e.g. more blood
cells, more skin cells, more liver cells, more of anything, whether that
production is to facilitate growth or replace worn-out cells.
When the intent
of cell division is to produce a new individual,
e.g. production of gametes or eggs and sperm,
cells divide meiotically.
For purposes of our discussion, we will restrict ourselves to
the process of cell division in what are called
eucaryotic
organisms. The majority of organisms in the world-and all multicellular
organisms, humans among them-are eucaryotes: organisms whose genetic
material is contained within a true nucleus. Many single-celled
organisms, e.g. bacteria, viruses and prions, lack a true nucleus and are classified as
procaryotes.
While procaryotes-with the exception of prions-do have genetic material, the process of
cell division in non-nucleate organisms is not identical to that
of eucaryotes and our discussion of cell division will be restricted
to the process as it occurs in eucaryotes.
As we will see, meiosis and mitosis are differentiated by the
way in which the genetic material is partitioned between the daughter
cells. When an organism is simply growing or replacing worn out
cells, it wants cells that are exact duplicates of the mother
cell. The process of mitosis
yields daughter cells that are
reasonably
exact replicas of the mother cell's genetic material, chromosome
by chromosome. However when an organism is going to produce gametes
(eggs or sperm) that will ultimately join to produce a new individual,
there has to be a mechanism for cutting the genetic material of
the mother cell in half and partitioning half of the mother cells
chromosome to each daughter cell. Meiosis is cell division that
produces daughter cells with half the genetic complement (i.e.
half the chromosome number) of the mother cell.
Index
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Chromosome Number: Haploid and Diploid
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We know that eucaryotic cells package their genetic material into
discrete packages called chromosomes which are found in the nucleus
of the cell.
Humans have 46 chromosomes.
Other organisms have
as few as 2 or as many as a 1,000 chromosomes. Chromosomes usually
occur in pairs which are virtually identical in appearance. Chromosome
partners are referred to as homologs or homologous chromosomes
because each chromosome in the pair contains the same sequence
of genes. Hence humans have 23 pairs of
chromosomes.
When chromosomes
are indeed present in homologous pairs, the cell is referred to
as a diploid or 2N cell. With the exception of your gametes
(eggs or sperm), every cell in your body is diploid, i.e.
it contains 23 pairs of chromosomes (for a total of 46 chromosomes).
Furthermore, one homologue of each pair was derived from your
mother, while the other homologue was contributed by your father.
Your gametes differ from all the other cells in your body in that
their chromosomes do not occur in pairs. There are only 23 chromosomes
in total, 1 copy of each chromosome, i.e. half the genetic
content of the original mother cell. Cells with only 1 of each
chromosome are called haploid or 1N cells. Your gametes
are the only
haploid
cells in your body.
The diploid cells that formed your gametes received their chromosomes from each of your parents,
so you might wonder if there is any pattern to the way meiosis divides your maternal versus your paternal chromosomes among your gametes.
The answer is no! While some combinations are extremely unlikely,
the possible combinations of chromosomes in your gametes could
range from all 24 being maternal down through ratios of 23:1 maternal:paternal,
22:2 maternal paternal to a 50/50 split, through a 1:23 maternal:paternal
ratio to an extremely unlikely gamete comprised of all paternally
derived chromosomes.
Index
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Meiosis and Mitosis: A quick recap
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Mitosis then is a process of cell (and obviously nuclear) division
by which cells duplicate their chromosomes and then divide in
two, generating two daughter cells whose genetic potential is
identical both to each other and to their mother cell. Mitosis
maintains chromosome number and generates new cells that are used
for growth or replacement of cells within the organism. Mitosis
can take place in either haploid or diploid cells.
Meiosis is a process of cell division that only occurs in diploid
cells. It results in daughter cells with only a 1N or haploid
number of chromosomes. Meiosis is the process used for production
of gametes. The process of sexual reproduction combines two haploid
(1N) gametes (an egg and a sperm) to produce a new diploid (2N)
individual.
(Click
here
for a graphical comparison of the two processes.)
Index
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Cancer: Cell Division Out of Control
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Cancer is a disease in which cells continue to grow and divide
indefinitely. For some reason, the cells no longer respond to
the normal metabolic checks and balances that should limit and
co-ordinate their growth. To fuel this unbridled growth, cancerous
cells out-compete surrounding, non-cancerous cells for energy
and nutrients. As the cancerous cells invade and spread to other
tissues, many organs of the body can become adversely affected
and ultimately die.
Why would a perfectly normal cell begin dividing wildly? We don't
yet know the answer, but it is becoming increasingly apparent
that the genes that control cell division play an important role.
Normal cells transform into cancerous cells when cancer causing
agents, called carcinogens, convert specific cell division
control genes within the cell into oncogenes. Cigarette
smoke, ultraviolet radiation, X-rays and more than 1,000 chemicals
including pesticides, household products and food additives are
all known carcinogens capable of causing alterations in the DNA
of cells, turning genes into oncogenes and triggering cancer.
Professors J. Michael Bishop and Harold Varmus from the University
of California were awarded the Nobel Prize in 1989 for their discovery
of the first oncogene. Several dozen cell division control genes
capable of being converted into oncogenes are now known. A great
deal of research is currently focused on how the activity of a
single gene and the protein for which it codes could trigger such
a complex process as uncontrolled mitosis.
Index
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Heredity
Case Study: The Genetic Basis of
Schizophrenia
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The human mind remains one of the most intriguing frontiers in
the biological sciences. Consider a question pondered by social
and natural scientists alike: To what extent does a person's genetic
makeup determine whether s/he will develop a serious psychological
disorder, such as schizophrenia or manic depression? In a sense,
this question is part of the larger issue of how much of our personalities
are a result of our genes-nature-as opposed to influences from
our environment-nurture.
Researchers unravelling the mysteries of schizophrenia-a disorder
characterised by depression, delusions, hallucinations and confusion-typify
scientists confronting the "nature vs. nurture" debate.
Early in this century, investigators tried to determine if schizophrenia
ran in families, as would be expected if it were an inherited
condition. The results were unequivocal: The likelihood that a
person born to a schizophrenic parent would be diagnosed with
the disease was about one in ten, compared to about 1 in 100 in
the general population. If both parents were schizophrenic, nearly
half their offspring were likely to share the same fate.
Had these results been obtained in rats or mice, rather than in
humans, it is likely that only one conclusion would have been
drawn: Schizophrenia is genetically determined. But many psychiatrists
of the period argued vehemently that the data could just as well
be interpreted to mean that schizophrenic parents created an environment
that fostered the development of the disorder in their children.
The debate has important implications. One the one side, the argument
that schizophrenia has its roots in environmental influences opens
the door to changing parental behaviour to lessen the risk. On
the other side, if the disease is genetically determined, there
is a possibility that it results from a simple biochemical imbalance
and can be treated with more effective drugs or gene therapy. How might we
discriminate between these two interpretations--nature vs nuture?
In the 1960's, Professor Leonard Heston of the University of Oregon
and Professor Seymour Kety and his colleagues at Harvard simultaneously
hit on a new approach to the question-the study of adoptees. Adoption
provides a "natural experiment" in which the two variables,
heredity and environment, can be separated. The natural parents
provide the genes, while the adoptive parents (assuming they have
no knowledge of the adopted child's parental schizophrenia) provide
the environment. If the disease has a major genetic component,
adoptees born to schizophrenic parents should exhibit a higher incidence
of the illnes than is observed in the general population or in adoptees raised by non-schizophrenic parents.
If genes are not a factor, the incidence of schizophrenia in the
adoptees should be no higher than that in the general population.
The results of the researchers' studies were clear-cut: Adopted
children who had a schizophrenic parent, but who were
raised by nonschizophrenic parents, had as high an incidence
of schizophrenia as children raised in the home of their
natural (schizophrenic) parents. Conversely, those adoptees who were born to a
nonschizophrenic parent, but who were raised by a schizophrenic, showed no increased risk of the disease.
These results supported the hypothesis that there is a genetic link to
schizophrenia. But there are more data to consider. If one member
of a pair of identical twins is a diagnosed schizophrenic, there
is only a 50 percent likelihood that the other twin will also
suffer from the "full-blown" disorder
(although as many as 85 percent show some behaviors, characteristic of less severe schizophrenic tendencies).
Identical twins have
identical genes; if schizophrenia is strictly determined by a
person's genetic inheritance, both twins should have the identical
disorder. There must be more to the story than just which genes
are acquired from the biological parents. Most mental-health experts
believe that our genes provide a strong predisposition or vulnerability
to the development of psychotic disorders but that environmental
factors also affect the severity of the disease as well as whether
or not the disease manifests itself at all. In other words, nature
and nurture do work together,
but the relative influence of environment versus genetics
(not only in schizophrenia, but in numerous instances of genetically
linked characteristics)
remains unclear.
Current investigations among biologists are focused on the specific
genes that predispose a person to schizophrenia. In 1988, two
papers were published in an issue of the scientific journal Nature.
One report described the use of a newly available gene-locating
technique to discover a single gene on chromosome 5, which is
associated with schizophrenia in a number of Icelandic and British
families. The second paper reported that this particular gene
was not associated with schizophrenia in members of a large Swedish
family suffering from the disease. Is one of these reports mistaken?
Probably not. It is more likely the case that irregularities in several
distinct genes can bring on the same symptoms. In other words,
schizophrenia may result from a number of different biochemical
miscues that generate a common suite of symptoms. The identification
of these genes and the determination of the biochemical processes
they control are two important areas of interest in clinical molecular
biology.
Index
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Gregor Mendel and Modern Genetics
Gregor Mendel: Monk and Mathematician
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Gregor Mendel is now called the father of modern genetics. However
in 1865, when he stood up before the Natural History Society of
Brünn in Austria to present the results of an 8 year study
of heredity, the audience greeted his historic presentation with
nothing more than polite applause. Mendel was so far ahead of
his time that the audience simply did not get the significance
of what they heard. Chromosomes had not yet been discovered. People
knew nothing of genes, diploidity, or meiosis: all of which provide
the physical basis for understanding what heredity is all about.
Mendel's work was published in the Brünn society's journal
but it generated no interest until 1900 when three different European
botanists independently re-discovered Mendel's paper which had
been sitting on the shelves of libraries throughout Europe for
35 years.
Mendel grew up on small farm in Austria, where he learned about
plant breeding. He later became a monk and had an opportunity
to study natural science and mathematics at the University of
Vienna. Back at the monastery, he taught mathematics and worked
in his laboratory-a small garden plot where he began a remarkable
series of experiments on
garden peas.
At the time Mendel began his experiments, most people supported
the "blending" hypothesis of inheritance in which parents
supposedly produced "hereditary fluids" that mixed together
to form offspring possessing a mixture of the characteristics
of their parents. There were problems with this "blending"
hypothesis that people were becoming uncomfortable about. For
example, it failed to explain why some children were "chips
off the old block", that is why some children resembled one
parent to the exclusion of the other or why characteristics "skipped"
a generation, only to reappear in a couple's grandchildren.
We'll never know what sparked Mendel's imagination, but his notebooks
show that he had a clear experimental plan to mate (or cross)
pea plants having different characteristics and to determine the
pattern by which these characteristics were transmitted to the
pea plant offspring.
Mendel understood both the need for controls in his experiments
as well as the value of being able to quantify his results. Consequently,
he chose to focus on clearly defined pea plant characteristics
that he knew would
breed true.
For example, one of the
characteristics he chose was seed color. Peas can produce seeds
in one of two colors: yellow and green. Green seeded pea plants
that breed true will always produce green peas when mated with
another green seeded pea plant. Yellow seeded pea plants that
breed true will always produce yellow peas when mated with another
yellow seeded pea plant. If Mendel's experimental organism had
been an animal rather than a plant, he would have been working
with a "pure" bred animal.
Index
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Mendel's Monohybrid Crosses
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Once Mendel was satisfied that he had pea plant stocks that would
breed true, he was ready to experiment with matings between his
breeding stocks. In the beginning, he limited his observations
to single-trait experiments called
monohybrid crosses.
He would for example mate a true breeding green seeded plant with
a true breeding yellow seeded plant and examine their hybrid offspring.
Or he would try mating a so-called smooth seeded plant with one which
always produced
winkled
seeds.
Hybrids are the result of matings between two different true breeding
organisms. (In this experiment, all the other characteristics
of the parent pea plants, e.g. height, flower color, seed
shape, etc. were the same, the only difference between
the pea plant parents was the color of the plant's seeds or the wrinkled versus smooth nature of the seed coat. For
other experiments, Mendel used parental plants with the same seed
color and varied one of the characteristics.)
Just to keep things straight, Mendel called his original parents
the P generation (the
parental
generation). He called the
first generation of offspring the F1 generation (the first
filial
generation). The results of Mendel's first summer of experiments
was interesting. All the plants in the F1 generation that resulted
from crossing green seeded and yellow seeded parents had yellow
seeds! All the F1 plants resembled the yellow seeded true breeding
parent, none resembled the true breeding, green-seeded parent.
Had the green-seeded characteristic disappeared?
The next summer, Mendel mated plants from the F1 generation, which
were all yellow seeded, to each other. He called the offspring
of this mating the F2 generation (the second filial generation). These plants were essentially
the incestual "grandchildren" of his true-breeding P
generation plants. At the end of the summer Mendel harvested his
F2 generation and began to count the seeds. His notes record his
surprise and excitement: some of the seeds were green. A characteristic
that had completely disappeared in the F1 generation had returned in the F2!
Furthermore there appeared to be a pattern to the numbers of yellow
versus green seeds: approximately 75% of the F2 generation seeds
were yellow (the color exhibited by all of their F1 parents),
while 25% of the F2 generation had green seeds. This 3:1 pattern
in the F2 generation turned out to be remarkably consistent across
all the characteristics Mendel examined. He concluded that heredity
was not a result of any random "blending" of parental
characteristics but represented an orderly, predictable pattern.
Index
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Mendel's Hypotheses: Genes, Alleles, and
Dominance
|
In spite of the fact that Mendel's F1 generation always appeared
identical to one of the original parents and not the other, Mendel
concluded that it had to be different in some way. How else could
he explain the fact that the original parents with yellow seeds
produced only yellow seeded plants when mated with each other,
while the F1 plants which looked like the yellow seeded parents
produced both green and yellow seeded offspring when they were
mated together? Furthermore, how could the green seeded characteristic
disappear in the F1 and re-emerge in F2 generation?
Mendel decided that the potential to produce the green seed characteristic
never actually disappeared, but was only hidden. Hence the way
an organisms looked, its phenotype, could be different
from its underlying genetic makeup or its genotype. He
suggested that hereditary traits were contained in some kind of
unit of inheritance which remained intact from one generation
to the next (of course these units are what we now call
genes).
Furthermore, Mendel proposed that each characteristic or trait
in the plants was determined by two independent factors, one derived
from each parent: these factors (which we now call
alleles)
could be either identical or different.
For each trait that Mendel investigated, he found that one of
the two alleles for a particular trait was
dominant over
the other.
The non-dominant trait was termed the recessive.
In the case of seed color, the allele for yellow seed
color was dominant over the allele for green seed color. Mendel
proposed a bookkeeping system that is still in use today. The
dominant allele is represented with a capital letter, the recessive
allele is referred to by the same letter in lower case. In our
seed color example, the two alleles for color are denoted by the
letter Y (yellow) and y (green).
We can now re-describe the P and F1 generations of pea plants
in terms of their genotype: their underlying genetic makeup.
The original, true-breeding parent pea plants must have had identical
alleles for seed color. The parental plants with yellow seeds
can be described as YY; the parental plants with green seeds are
then described as yy. When both alleles of a trait are the same,
the individual is described as
homozygous. Homozygous individuals'
phenotypes reflect their genotype.
In contrast to the parental generation, the F1 plants had one
dominant allele, Y, and one recessive allele, y. Consequently
they are designated Yy. When the alleles of a trait are different,
the individual is described as
heterozygous. In heterozygotes,
the phenotype, the way the organism appears, is different from
the genotype. The seeds of the F1 plants were yellow in appearance
but the allele for green seed color was there all along; it simply
was not "expressed" due to the dominance of the allele
for yellow seed color.
Mendel finally concluded that gametes must carry only one allele
for a particular trait. He postulated that somehow, during the
process of gamete formation, alleles had to separate with one
allele going to one gamete while the other allele went to a different gamete.
He called this process
segregation. Therefore in the F1 generation, half the gametes would
carry the dominate Y allele and other half would carry the recessive
y allele. (Mendel was of course predicting the process we now
call meiosis, a process only discovered years after his death.)
Thus when F1 plants were mated to other F1 plants, random combinations
of Y-containing sperm with Y-eggs, Y-containing sperm and y-eggs
or y-sperm and y-eggs (or vice versa) would lead to three possible
genotypes in the F2 generation: 25% of the fertilized eggs (future
seeds) should be homozygous dominants (YY), 50% would be heterozygotes
(Yy) and another 25% would be homozygous recessives (yy). Of course
both the homozygous dominants and the heterozygotes would appear
to be yellow because of the dominant nature of the Y allele, thus
leading to the 75%-25% (3:1) ratio Mendel observed in the F2 generation.
Index
|
Predicting Inheritance: Punnett Squares
|
A simple way to visualize a monohybrid cross is through the use
of a Punnett square: a technique first used by R.C. Punnett, a
poultry geneticist in the early 1900's. Punnett squares predict
the possible genotypes and phenotypes (and the expected ratios)
when there is an equal chance of acquiring either of two alleles.
In the Punnett square representation, all possible male gametes
are listed along one side of the square, while all possible female
games are listed along a perpendicular side. The possible combinations
of alleles following fertilization are shown in the boxes of the
Punnett
square.
Index
|
Hereditary Patterns in Humans
|
The principles of dominance and segregation of alleles apply to
all sexually reproducing diploid organisms, not just garden peas.
Hence Mendel's work also explains hereditary patterns in
humans.
It will explain the ability to roll your tongue, whether or not
you will have dimples or a widow's peak in your hair, whether
you'll be born without fingerprints or your ability to taste PTC.
All of these traits are examples of traits determined primarily
by dominant alleles (although we will see that other factors do
play a role). Therefore a person with just one of these dominant
alleles in their genotype will develop the corresponding characteristic,
irrespective of the second allele. A child of parents who are
both heterozygous for one of these traits will have the same 3:1
probability of exhibiting the phenotype of her parents as did
Mendel's F2 garden peas.
This does not mean that individuals with dimples or widow's peaks
or the ability to taste PTC are three times more numerous than
people without a particular characteristic. In fact for some traits
the recessive allele is far more common than the dominant allele
and most people are homozygous recessive. Do you know anybody
who has no fingerprints?
Having no fingerprints is extremely rare. Yet it is the gene for the presence of fingerprints
that is the recessive allele.
This is just a case where the
dominant allele is incredibly rare. Of course where it occurs, most
members of an affected family will have no prints since the dominant
allele when present is always expressed.
Although we suspect that most human characteristics
have a genetic component, few are as
straight-forward as the ability to taste PTC or the presence of finger-prints.
Furthermore, these latter examples of genetically based human traits are somewhat
frivolous. It really doesn't matter whether you have fingerprints or not.
Other disorders that we suspect have at least some
genetic aspects
(e.g.
Alzheimer's disease,
some types of
colon,
breast or other
cancers,
or
diabetes) are
far from frivolous. Yet as we learn more
about the nature of these disorders, we enter an area of intense
moral and ethical
debate.
Just as was the case with schizophrenia, the mechanisms of inheritance for most
traits are likely not controlled by a single gene. Nor do we understand the interplay
between genes and environment.
Even when the
Human Genome Project,
HGP
completes the mapping of the human genetic sequence, we likely will have more
questions than anwers.
Index
|
|
Post-Mendel Understanding
Partial Dominance
|
Even though
Mendel's "laws"
have provided the foundation
on which modern genetics has been built, several exceptions to
Mendel's principles have been discovered. For example, not all
traits have two alternative forms of the simple dominant-recessive
relationships of Mendel's peas. Some traits actually appear to
follow the notion of "blending". When red snapdragons
are mated or crossed with white snapdragons, for example, all
of the F1 progeny are pink-just as if the traits had blended.
But if pink plants are mated with other pink plants, the resultant
F2 generation produces an interesting result: 25% of the F2 are
white, 50% are pink and 25% are red. How does this happen?
The answer is
incomplete or
partial dominance. The red color of
snapdragons derives from a red pigment which is controlled by
the red allele. The white allele cannot produce any pigment. When
only one red allele is present (the heterozygote), only enough
pigment is produced to produce a pale red, or pink, color. In
cases of partial dominance, the heterozygous genotype is distinguishable
from either of the homozygous conditions.
Index
|
Co-dominance
|
Heterozygous individuals are also distinguishable from homozygotes
in cases of
co-dominance,
but the heterozygous phenotype is not an intermediate (diluted) form.
The classic example of co-dominance are human blood types: A, B, AB and O.
A and B are co-dominant alleles. An individual with one A allele and one B allele expresses an intermediate blood type called AB.
But both allele A and allele B are dominant to allele O. Hence an individual with
one A allele and one O allele will express type A blood. An individual with
one B allele and one O allele will express type B blood.
The fact that there are more than two alleles for blood type illustrates
another example of post-Mendelian genetics: the existence
of multiple alleles.
Index
|
Multiple Alleles
|
While the human ABO blood type system is an example of a situation where
there are more than two alleles for a single trait
within the population, any particular
individual can only possess two alleles.
We can reiterate this situation using our new vocabulary:
- The human gene pool contains three blood-type alleles: A, B and O.
- While
A and B are co-dominant with respect to each other, both are
dominant with respect to the O allele.
- An individual
with A and O alleles will express type A blood.
- An individual
with B and O alleles will express type B blood.
- The only people
with type O blood are those whose genotype is homozygous recessive
for the O allele.
Index
|
Continuous Variation: Polygenic Inheritance
|
Numerous human traits show a continuous pattern of variation.
Human skin color or human height are good examples.
When really tall people and really short people mate, their offspring are often intermediate in height rather than tall or short.
People of different skin colour usually have children whose skin colour is intermediate rather than the colour of one parent or the other. This is what we mean by a continuous trait.
Seed color in peas is a discontinuous
characteristic. There are only two colors: yellow and green. If
human skin color were discontinuous, i.e. determined by
a simple dominance system, humans would only come in two colors,
black and white.
But human skin color or height, along with a number of other human characteristics, show continuous variation. Continuous traits are those that are determined by a number of different genes at different
loci.
There are at least 3 genes at three different loci that determine
the amount of pigment in human skin. At each locus, there are
two possible alleles of the pigment gene, one for pigment
and one for no pigment. The darkness of skin is determined by
the total number of alleles for dark pigment in the genotype.
The darkest phenotype would have all 6 dominant (i.e. pigment
producing) alleles. The lightest phenotype would have all 6 recessive
(i.e. non pigment producing) alleles.
Geneticists suspect that many of the more elusive human traits
are similarly polygenic. Examples are susceptibility to cardiovascular disease,
schizophrenia and athletic prowess. Polygenic inheritance poses
particular difficulties for genetic counselling since the outcome
of any particular mating is the sum of many genotypic variables.
Index
|
Pleiotropy
|
The product of one gene can have far-reaching secondary effects
on many characteristics. Cystic fibrosis results from a single
gene miscue which leads to the production of an abnormal membrane
protein. This protein causes thick mucous to be secreted in the
lungs, various digestive organs and the sweat glands. The presence
of the mucous can lead to improper digestion, salty sweat
and breathing can be difficult. Mucous in the lungs can trap bacteria
which can lead to pneumonia or other lung infections
which can lead to death in those with cystic fibrosis.
When the
action of a single gene leads to a number of effects, the gene
is termed pleiotropic.
Index
|
Combined Effects: Epistasis
|
Mendel's pea plant characteristics were all independent traits.
The color of the seeds had no effect on the shape of the seed
or the height of the plant. In many cases, however, the expression
of a pair of alleles will be epistatic, that is it will
be influenced by the genotype at other loci on the chromosome.
Specifically epistasis is a process whereby a pair of recessive
alleles at one locus masks the effects of genes at other loci.
Albinism is an example of epistasis. Human beings have separate
genes for hair, eye and skin color. Consider an individual whose
genotype dictates brown hair, brown eyes and a relatively dark
complexion. All of these phenotypes require the production of
a dark pigment (know as melanin) which, in turn requires the activity
of the gene that controls melanin production. If the genotype
for the melanin gene is homozygous recessive, no melanin will
be produced and the skin, hair and eyes of the individual will
be devoid of pigment, irrespective of the genotypes of the hair,
eye and skin alleles.
The individual described will be a so-called "albino."
Index
|
Environmental Influences
|
Nearly all phenotypes are subject to modification by the environment.
A genetically predisposed tall person may not achieve full height
if deprived of adequate nutrition. A classic example of environmental
modification of a single-gene trait is the color pattern of
Siamese
cats. These cats have only one gene for hair color. However the
siamese allele controls the production of a temperature sensitive
pigment. The pigment is dark on cool areas of the cats body (feet,
nose, ears and tip of tail) and light on the core areas of the
cats body which are warmer.
Index
|
A Summary of Inheritance
|
Gregor Mendel discovered the pattern by which inherited traits
are transmitted from parents to offspring. He discovered that
inherited traits were controlled by pairs of factors (genes).
The two factors for a given trait in an individual can be identical
(homozygous) or different (heterozygous). In heterozygotes, one
of the gene variants (alleles) may be dominant over the other,
recessive allele. Because of dominance, the appearance (phenotype)
of the heterozygote (genotype Xx) is identical to that of the
homozygote with two dominant alleles (XX). An individual must
possess two recessive alleles (xx) to exhibit the recessive phenotype.
Monohybrid crosses between heterozygous parents produce three
times as many offspring with the dominant phenotype as with the
recessive.
Not all inherited traits are transmitted according to Mendelian
patterns: (1) Some alleles show incomplete dominance, whereby
the pair of alleles "dilute" each other's effect in
the heterozygous individual. (2) In co-dominance, both alleles
are fully expressed in heterozygous individuals. (3) Multiple
alleles (more than two) for a particular trait can combine to
form more than the three genotypes and two phenotypes expected
in the two allele situation. (4) In epistasis, alleles present
at one locus can effect the expression of genes at a different
locus. (5) A single gene may be pleiotropic, meaning that it can
have many effects. (6) One trait may result from the effects of
multiple genes at several loci; this is known as polygenic inheritance.
(7) Tracking the fate of alleles from generation to generation
can be complicated by the effects of environment on phenotype.
Index
|
|
Genetic Diversity, Adaptation and Evolution
Adaptation and Evolution
|
In our discussion of the suite of characteristics that define
life, we mentioned the ability to respond to environmental change.
Adaptation and evolution explain why the biosphere is populated
by millions of species rather than being stuck back at the protobiont
stage of life. At the same time, they help us understand the unmistakable
unity that exists among even the most diverse organisms.
Evolution is simply the idea that species change over time. As
a result of evolutionary changes, new species of organisms emerge
while older forms of life may disappear. Evolution explains how
all forms of life that have ever existed are members of one extended,
genetically related "family" of organisms. We can easily see the similarities between humans and other primates,
but we are also remarkably similar to organisms that seem as unlike us as mushrooms because we share
the same ancestor far, far back in time. And both we and the mushroom retain
many of the same molecular, genetic and cellular endowments provided
by that ancestor. It is this common ancestry that accounts for
the unity of life on Earth.
Biological success for a particular type of organism depends on
how well suited the organism is to its environment. An organism
can only survive in an environment that can supply all its essential
needs. Even then, it can succeed only if it can survive any adverse
conditions that it might encounter in that environment. Adaptations
are traits that improve the suitability of an organism to its
environment. The original coloration of the peppered moths discussed
earlier enabled them to avoid detection by predators. When the
environment changed, the moths were able to adapt and thus continued
to avoid detection. Adaptations can become modified over evolutionary
time and may eventually lead to the emergence of new species.
Mendel's findings provided the critical link in our understanding
of evolution. A key tenet in the theory is that favorable adaptations
will increase the likelihood that an individual will survive to
reproductive age and that its offspring will exhibit these same
favorable characteristics. Mendel's demonstration that units of
inheritance pass from parents to offspring without being blended
revealed the means by which advantageous traits could be preserved
in a species over many generations.
Index
|
Darwin and the Voyage of the Beagle
|
What would you do if someone offered you a free trip around the
world to pursue your hobby? Such an opportunity presented itself
to 22-year old Charles Darwin when one of his university professors
recommended that he be appointed "naturalist" on a scientific
expedition that was headed around the world!
It was 1831 and Darwin had just graduated from Cambridge. He was
still not sure what he wanted to do with his life having rejected
following in the footsteps of his physician father and being somewhat
antipathetic about the only obvious alternative: becoming a clergyman
in the Church of England. Then he was offered a truly once in
a lifetime opportunity as the ship's naturalist. His job on the world
cruise of the HMS Beagle (a 25 metre, three-masted survey ship)
was to collect and organize the thousands of specimens the expedition
anticipated collecting.
You can access a web version of Darwin's account of his voyage
(taken from his book The Voyage of the Beagle)
here.
Darwin left England with the firm belief that all the world's
plants and animals had been created directly by the hand of God.
Lying in his hammock only a few days into the voyage, suffering
horribly from seasickness, Darwin began to read a newly published
book by Charles Lyell, Principles of Geology. Lyell's book
marshalled compelling evidence that the world was much older than
had been previously believed and that it had changed gradually
over long periods of time due to natural forces such as mountain
building, erosion, volcanic eruptions, floods and earthquakes.
A month into the voyage, the Beagle sailed into the harbor in
the Cape Verde Islands. Darwin saw a cliff rising 15 metres above
the sea. As he explored the cliff, Darwin was surprised to find
that the cliff rock was embedded with seashells. It seemed obvious
that the cliff had originally been part of the sea floor and had
been subsequently lifted into the cliff structure. Later in the
voyage, high in the Andes Mountains, Darwin found more marine
deposits. This time they were hundreds of kilometres from the
sea. Not long afterwards, the Beagle experienced a severe earthquake
off the coast of South America.
He was surprised to find that land he had visited just a
few days previously had
risen nearly a meter as a result of the quake.
Then the biological evidence began to accumulate. On the east
coast of Argentina, Darwin found a fossil bed containing a huge
hippopotamus-like animal and an armadillo much larger than any
known to be then alive. It was clear to Darwin that these were
the remnants of extinct creatures, yet they were clearly related
to forms that were currently alive. He began to wonder what had
caused these ancient animals to disappear.
The captain of the Beagle suggested that they were animals somehow
left off Noah's ark during the great flood. Darwin was toying
with another idea: maybe these organisms had been driven to extinction
by new animals that invaded South America from the north when
the land bridge he had read about in Lyell's book connected the
north and south American land masses.
But Darwin's most profound experience was yet to come, occurring
when he reached the Galapagos Islands, located almost 1000 kilometres
away from the closest land in Ecuador. Darwin had visited other
volcanic islands on the cruise and noted that the plants and animals
on those islands tended to be reasonably similar to those living
on the nearby mainland.
One of his most important observations on the Galapagos concerned
a group of dull-looking birds-the now famous Darwin's
finches.
He observed and collected 13 species of finches that turned out
to be found only on the Galapagos. Although all the species of
birds were similar to one another in body form, they differed
in the shape of their beaks. Some had strong, thick beaks adapted
for crushing seeds, while others had beaks that were especially
suited for feeding on flowers or insects. Among the insect eaters
was the so-called woodpecker finch, which, unlike a true woodpecker
that catches insects with its long tongue, dug insects out of
the tree bark using a cactus spine held in its beak. Darwin wondered
why animals with such different feeding habits looked so much
alike.
He began to wonder if living organisms rather than being final
and unchangeable might be like Lyell's geological formations,
subject to slow change over time. He finally decided that all
the finches on the Galapagos had descended from one mainland finch
that had somehow drifted hundreds of kilometres from South America
even though he wasn't at all clear as to how individuals of one
species were able to give rise to the various species now found
on the Galapagos.
Darwin returned to England in 1836. He spent the next few years
examining the specimens he had collected, he read voraciously
and he began to prepare journal articles on the discoveries resulting
from his voyage. One of the books he picked up was by Thomas Malthus
who pointed out that a single pair of humans had the reproductive
potential to produce billions of people in a relatively short
time. It was evident to Malthus that since there were not billions
of people on Earth (at least not then!), something must be holding
the human population in check. Malthus suggested that the factor
responsible for keeping the population below its theoretical maximum
was mortality brought about by famine, wars and disease.
Darwin quickly realized that the same population potential must
exist in all populations and yet population numbers remained reasonably
constant. Darwin concluded that there must be a "struggle
for existence" among animals such that only a small percentage
of those born actually lived to maturity. Darwin was not suggesting
that there was any kind of physical struggle between individuals
but rather there was competition between individuals within a
community and a struggle to survive periods of potentially adverse
conditions in the environment, e.g. predation, parasitism,
disease, cold or heat.
Assuming such a struggle, Darwin assumed that variability among
members of the population was the key to which individuals survived
and which were eliminated. As he pondered the situation, Darwin
decided that some members of the population likely possessed characteristics
that gave them an increased chance of survival relative to other
members of the population. The survivors might have a more efficient
approach to food gathering or a particularly high level of resistance
to a common parasite or could perhaps move more quickly. Darwin
considered these individuals better adapted to their environment
and thus more likely to survive.
It was evident to Darwin that survival itself was not the most
important consideration: rather, surviving to reproduce successfully
was the critical factor. Those organisms best suited to survive
would tend to produce a greater number of offspring. According
to Darwin, the environment then "selected" those organisms
that would survive and reproduce, while it eliminated those that
would not. Viewed this way, natural selection becomes equivalent
to differential reproductive success, the production of more offspring
by individuals that are better adapted to their environment.
Although biologists of Darwin's time were unaware of the mechanisms
of heredity (remember Mendel's work was still sitting-ignored-on the shelves of numerous libraries),
they had observed that offspring did tend to inherit
the characteristics of their parents. Consequently, the offspring
of survivors would be expected to have the same traits that facilitated
their parents' survival.
As a result of differential reproductive success, populations
would be expected to gradually accumulate genetic traits that
would make its members better and better adapted to their environment.
Individuals with traits less suited would be less likely to successfully
compete and would therefore leave fewer if any offspring. This
change in the genetic composition of a population from generation
to generation is the very essence of the evolutionary process.
Giraffes, for example, have long necks because individuals that
happened to have longer necks were more successful in obtaining
food than were their shorter-necked competitors. Longer-necked
members of the populations were thus more likely to survive and
have offspring which, like their parents would have longer necks.
Natural selection produces organisms that are adapted to their
environment. However, environments do not remain constant over
time: climates change, fires destroy vegetation, new parasites
or predators may move into an area. Consequently, organisms that
are successful in a particular habitat at one time, might be poorly
adapted at another time.
Although Darwin was continually discussing his ideas about evolution
and natural selection with his friends in the scientific community,
it took him until 1842 to set down his preliminary ideas in a
brief essay which he circulated but did not submit for publication.
By 1858, 20 years after the voyage of the Beagle, Darwin was still
writing, revising and talking about his ideas.
Some scholars have suggested that Darwin, knowing
how controversial the idea of species arising from natural as opposed to supernatural forces would be
in Victorian society, took as long as he did to make sure his
arguments were unassailable.
Others have suggested he was actually extremely ill and could
work for only limited periods of time.
Nevertheless
in June of 1858,
Darwin received a letter from Alfred Wallace, a naturalist working
in Malaysia. Wallace asked Darwin if he would forward an enclosed
manuscript on an idea Wallace had about natural selection to Charles
Lyell for presentation at the Linnaean Society meetings, one of
the most prestigious scientific societies of the day for biologists.
Darwin read the manuscript and was shocked! Wallace's ideas were
almost identical to those he had been working over for
20 years.
It became clear to Darwin that other people were thinking along the same lines as he was.
Darwin's friends became concerned that he would never get credit for his
ideas and his years of work.
In discussing
the situation with Lyell, the decision was made to have Darwin and Wallace
jointly present their work at the Society's annual meeting. Rather than
becoming enemies or competitors, Wallace and Darwin became lifelong
friends. Darwin finally got moving on his book and The Origin
of the Species was published in 1859. The response was thunderous.
All 1,250 copies of the book sold out in the first day. Discussions,
debates, protests and personal attacks followed-continuing to
this very
day.
Index
|
A Summary of Darwin's Theory of Evolution by Natural
Selection
|
The concept that organisms are related because of common descent
had been discussed by a number of philosophers and naturalists
before the nineteenth century. Darwin's contribution was the first
to provide a feasible mechanism-natural selection-that explained
how biological evolution might have occurred. Darwin arrived at
this mechanism by the following logic:
1. All species have the reproductive potential to over-populate
the Earth, yet populations of organisms remain relatively stable
over time.
2. Consequently, a large percentage of the members of a population
must die at an early age.
3. There is variation among the members of a populations such
that different individuals exhibit different traits.
4. It follows that the members of a population whose traits make
them better adapted to their environment at a particular time
will be more likely to survive to reproductive age and to produce
more offspring than will members of the population that are less
well adapted. This is the basis of natural selection.
5. Since offspring inherit the traits of their parents, those
that have successful parents tend to acquire successful traits.
Consequently, a population will change (evolve) over time, its
members acquiring new traits that make them better adapted to
a changing environment.
6. Given sufficient time, this process of evolution by natural
selection can account for the formation of new species and thus
the diversity of life on Earth, both past and present.
Index
|
Acquiring genetic variation: mutation, random
assortment and
cross-over
|
It should be clear that not all members of a population are identical.
Heritable differences among individuals are the raw materials
for evolution. Even minute differences between individuals may
confer a selective advantage in a particular environment or in
the face of an environmental change. But where do these differences
come from?
The most dramatic source of genetic variation is the creation
of new alleles by mutation. Simply shuffling existing genes around
through meiosis, a process referred to as random or independent
assortment, is capable of creating new combinations of genes-and
hence the possibilities for epistatic, pleiotropic or other generators
of diversity. The process of chromosome duplication can also create
diversity through a process of crossing over. We will come to
a discussion of both these latter processes shortly, but only
mutation seems a reasonable explanation for how all the genes
present in today's millions of species could have arisen from
the relatively small complement of genes that were present in
the common ancestor to all living things.
Index
|
Mutation
|
If genetic information is stored in the linear sequences of nucleotide
bases (adenine, thymine, cytocine and guanine) along the strands
of DNA molecules, then changes in nucleotide sequence will alter
the nature of that genetic information. This is the definition
of a mutation.
Mutations may be simple point mutations, the substitution of one
nucleotide for another or they may be more serious
frameshift
mutations in which nucleotides are added or deleted from the sequence.
When the genetic sequence of a gene is altered, a new allele is
created. Most mutant alleles are detrimental since it is much
more likely that a change in nucleotide sequence (which is likely
to change the chemical or other characteristic that the squence
codes for) will disrupt a well-ordered, smoothly functioning organism
than it will increase the organism's fit to its environment
i.e. fitness.
For example, changing the genetic sequence of a gene
might make it stop producing a chemical needed for a critical
life function.
Sickle cell anemia
is an example of the kind of
change that can result from a single point mutation. A change
in one nucleotide, substitutution of thymine for adenine, in the
genetic sequence that codes for haemoglobin leads to a distortion of the
molecule that impairs its ability to transport oxygen in the blood.
Occasionally, however, a mutation will create an advantageous
characteristic that increases the fitness of the offspring. Sickle
cell anemia is so interesting because as a deleterious mutation,
it should have disappeared, but being heterozygous for the sickle
cell allele actually confers an advantage to individuals under
certain environmental conditions, i.e. high incidences of malaria.
Biologists estimate that on average, several thousand nucleotides
are lost from the DNA of a cell every day, so why don't we see
more mutations? DNA is usually able to maintain its nucleotide
sequences in the face of such molecular punishment through the
action of DNA repair chemicals that constantly patrol the chromosomes
of cells, searching for alterations and distortions in the genetic
sequence that they can recognize and repair.
In addition to internal chemical reactions that can lead to changes
in nucleotide sequences, mutations can also arise from external
assaults. Any agent capable of causing a gene mutation is also
a potential carcinogen since the alteration of certain types of
genes, e.g. oncogenes, can cause a normal cell to transform
into a malignant, cancerous cell. Chemicals are not the only carcinogenic
agents. DNA is extremely susceptible to damage by radiation. In
fact the most common human mutagen is ultraviolet radiation.
Index
|
Cross-over and genetic recombination
|
In addition to mutation, genetic diversity can result from a phenonenon
called
crossing over.
Cross over
occurs in the process of chromosome
duplication prior to cell division. Recall that chromosomes in
diploid organisms occur in pairs. In the process of cell division,
homologous pairs of chromosomes line themselves up two by two
prior to actual cell division.
Sometimes in the process of untangeling themselves preparatory
to lining up, bits on one homologue may be transferred to the
other homolog. Remember that one of the chromsome homologs was
derived from the organism's mother and the other from the father.
If the alleles on the two homologs were different, then essentially
what was a gene or a gene fragment from the organism's father
will now be present on the chromosome derived from the organism's
mother: hence the genetic material of the organism is no longer
an exact copy of what was originally present. The genetic material
has been recombined.
Now when the homologous chromosomes separate during meiosis, the
resultant gamete will be novel. While it still resembles the genetic
sequences of its parents in many ways, it is not an exact copy.
Furthermore this new genetic combination may produce a characteristic
that better suits the offspring to its environment. Crossing over
and the genetic recombination that results can greatly increase
genetic diversity and hence variability in offspring by producing
gametes with novel combinations of characteristics.
Index
|
Independent Assortment
|
Recall that the process of gamete production via meiosis reduces
the number of chromosomes by half: homologous pairs are separated
and one of each pair of chromosomes ends up in a gamete. Returning
to the example of the human genome, we recall that humans have
23 pairs of chromosomes. The process of gamete production results
in sperm or egg cells with only 23 chromosomes that are no longer
paired. We raised the question earlier as to whether there is
any pattern to which chromosomes end up in which gamete. The answer
was that there was no pattern: chromosomes assort themselves independently
of each other. Another way of saying this is that just because
the maternal homolog of chromosome 1 ends up in a particular gamete
has nothing to do with whether the maternal homolog of chromosome
2 ends up in the same gamete.
If that is true, then how many possible gametes might each of
us produce? The answer is phenonomenal: Two (the number of chromosomes
in a pair) times 2 times 2 times 2 for a total of 23 times (the
total number of chromosome pairs). In other words 2 raised to
the power of 23. If you do the multiplication that yields 8, 388,
608 possible chromosomal combinations in any gamete. Now remember
that the egg or sperm has to be fertilized (the process of restoring
the diploid condition) before it can grow into a new individual.
Each sperm or egg (that one in 8, 388, 608 chance combination
of chromosomes) now meets another sperm or egg that itself is
a one in 8, 388, 608 chance combination. Any human is then a one
in 70, 368, 744, 177, 664 occurrence! We are just approaching a
world
population of 6 billion. No wonder each of us is a genetically
unique individual, yet we are all human.
Index
|
icon.