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UNIT 7 NOTES
CHAPTER 22 SECTIONS 2-3
I’m sure by now you have all heard of Charles Darwin and his theory of
evolution. Most teachers of this subject start out with a discussion of Darwin’s
life, his voyage on the Beagle, and how he finally published his theory. None of
that will be on your test. The brief coverage of this in the first seven slides will
be plenty. For 100+ years, Darwin was the only one given credit for the theory.
Lately, though, Alfred Russell Wallace has also been given credit since he had
the same idea at the same time. He had contacted Darwin, who was older and
better known at the time, to ask him what he thought. When Darwin realized
that Wallace had come up with basically the same theory, he panicked. The two
talked and agreed that Darwin would publish first and get credit. Today you
might hear it called the Darwin-Wallace theory of evolution. I mention this just
so you will know it is the same theory.
In Darwin’s words, how can evolution be defined?
What does that mean?
What is a fossil?
Where are older fossils found?
Darwin was greatly influenced by a geologist named Lyell. Why? What did Lyell
believe that Darwin thought must also be true of living organisms?
Compare the finches in the slide (#11, Fig. 22.6 pg.456). Each has a slightly
different type of beak that is suited to the particular food each species eats. This
is the trait that helps each species survive. Each one has a different allele for
beak type. Over time mutations in the beak gene would produce the alleles that
make the different beak types. These alleles are the adaptations or genetic
variations that are favored by natural selection and provide some type of
advantage to the individual’s survival.
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Darwin’s two main ideas were:
1) Descent with modification explains life’s unity and diversity.
This means that organisms that come from the same ancestors
(descendants) are a lot alike (unity) but different (diversity) at the same time
due to changes (modification) that occur along the way. Now we know that is
because of genes and mutations and alleles, but none of that was known in
Darwin’s time. Recent ancestors are also descendants of more ancient ancestors,
so it goes back a long way.
2) Natural selection is a cause of adaptive evolution.
This means that the individuals that have the best adaptations to survive
in a particular environment (nature) are more likely to survive and reproduce and
pass on their genes. Over time the best adaptations will increase in number and
adaptations that aren’t such a good fit will become less numerous. Thus, over
great periods of time, species change (evolve).
Darwin also noticed that people modified other species by selecting breeding
pairs. He called this artificial selection because people were actively involved
and interfering with the natural process. Just look at all the things we got from
wild mustard over the years (Fig. 22.9 pg. 458, slide 15). And they are good for
you too, so eat’em up!
The main reason Darwin was able to come up with the theory of evolution was
that he was a very observant guy. And very patient. He did a lot of looking and
thinking. Darwin made four important observations and from these drew two
inferences to explain his theory. Define each of them:
1. Variation
2. Heritability
3. Overproduction
4. Competition
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1. Fitness
2. Adaptation
According to Darwin’s theory of natural selection, competition for limited
resources results in differential survival. Individuals with more favorable
phenotypes are more likely to survive and produce more offspring, thus passing
traits to subsequent generations.
• Individuals with certain heritable characteristics survive and reproduce at
a higher rate than other individuals
• Natural selection increases the adaptation of organisms to their
environment over time
• If an environment changes over time, natural selection may result in
adaptation to these new conditions and may give rise to new species
INDIVIDUALS DO NOT EVOLVE. POPULATIONS EVOLVE OVER TIME.
Scientific evidence supports the idea that evolution has occurred in all species.
The fossil record, comparison of proteins and DNA, phylogeny
Scientific evidence supports the idea that evolution continues to occur.
These are all examples that we can look at and study today and we can almost
watch the changes as they happen.
• Chemical resistance (mutations for resistance to antibiotics, pesticides,
herbicides or chemotherapy drugs occur in the absence of the chemical)
• Emergent diseases – HIV, H1N1, West Nile, Lyme disease
New diseases are the result of changes in the disease organism
(evolution) that allow it to infect new hosts.
• Observed directional phenotypic change in a population
(Grants’ observations of Darwin’s finches in the Galapagos pg. 468)
• A eukaryotic example that describes evolution of a structure or process such as
heart chambers, limbs, the brain and the immune system.
The panda’s thumb is one example (you can google it) and there is a
comparison of heart chambers in different organisms on pg. 902.
Evolution is supported by an overwhelming amount of scientific evidence.
Your book provides two examples of direct observations of evolutionary change.
One, the evolution of drug-resistant HIV, is not only relevant but is
demonstrative of all bacteria and viruses due to their rapid rate of reproduction.
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Does natural selection produce new traits? Explain.
What determines which traits are selected?
How does the fossil record provide evidence of evolution?
Fig. 22.16 pg. 462, slide 33, shows a great example of what can be found in the
fossil record that demonstrates change. If you just had the first and last ones
you might not make the connection. But having the two in between shows how
whales evolved from terrestrial organisms. IT WAS NOT DE-EVOLUTION!! There
is no such thing. Yes, terrestrial life came from the oceans, but when it got
crowded on land, some took to the sea for food and shelter, and eventually
evolved enough to move back to the sea and fill a niche there. Today whales are
very successful. Their only predator is us.
The fossil record is very important because it allows us to look at organisms that
are no longer living. Some people believe that they aren’t real and are just a
hoax to discredit religion. The theory of evolution does not in any way say that
there was no creator. The theory states that organisms have changed over time
– descent with modification. Maybe the creator, if there was one, created the
very first life form and then evolution took over. This is why evolution is still a
theory and not a law. We don’t know, and probably never will know, for surescientifically. You are free to believe whatever you want. Maybe Adam and Eve
were two Archaebacteria? Who knows?
The following is an important concept and you will have to keep the three types
of structures straight. The prefix homo- means same, so homo- anything means
the same something, like homosexuals prefer people of the same sex. I’m sorry
if that makes you uncomfortable (it shouldn’t) but that is the best way to
remember it because everyone knows what that means. This should not be
confused with the homo in Homo sapiens because here, as a genus name in
Latin, it means man.
What is homology?
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Homologous structures represent features shared by common ancestry.
These structures are built in the same way, but the parts may have changed
slightly in shape and/or size. Examples of homologous structures are:
Take a good look at Fig. 22.17 pg. 463, slide 35, and compare the bones that are
the same color. Notice how they have changed in shape or size, but the overall
structure is the same in each one. We are all animals and much more alike than
most people realize.
More anatomical similarities can be found by studying embryos – all vertebrate
embryos have tails at some point in their development.
Vestigial structures are remnants of functional structures, which can be
compared to fossils and provide evidence for evolution.
Examples of vestigial structures are: (Hint: things you have but don’t use)
Biochemical and genetic similarities, in particular DNA nucleotide and protein
sequences, provide evidence for evolution and ancestry.
The more similar any of these sequences are between species, the more closely
related those species are. The genetic code, that use of A’s, C’s, G’s, and T’s, is
universal- every organism on Earth uses it. This is what allows us to manipulate
it by genetic engineering and cross breeding. There is no better evidence of
evolution than that.
Since everything does use this code we can put all organisms somewhere on an
evolutionary tree. Darwin envisioned all forms of life being related through
ancestry and descent such that they could all be put on branches of a tree
showing their relatedness, with the root being the first life form, and the major
branches the six kingdoms. We are still working on a ‘complete’ one, but many
smaller ones for thousands of species exist. It is difficult to complete because
only 1 percent of all species that ever lived are still alive today to provide DNA.
Many trees are made using anatomical data because DNA is not available, but
fossils are. Fig. 22.19 pg. 464, slide 40, is an example.
Convergent evolution produces analogous structures. This occurs when
organisms that are not related develop the same or similar features because they
live in the same environment. Remember, natural selection chooses the design
that works best. If it works best in one group of organisms, then chances are
pretty good it will work as well in another group. Your book gives a stupid
example so I’ll give you some that are easier to remember. Don’t get me wrong,
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I love flying squirrels and sugar gliders, but that’s just not what comes to mind
first on this subject. Birds and butterflies are a good example. Both live much of
their lives in the air and both can fly because both have wings. The bird wings
has bones and feathers, the butterfly wing has some sort of support (I really
don’t know what) and is covered in scales. They are not made in the same way
at all, and, thus, are not homologous. They evolved separately in these species
as a result of natural selection in the environment in which they live. Another
good example is penguins and dolphins or sharks. Both have similar shapes to
help them glide through the water. Again, it is a result of the environment in
which they live and NOT common ancestry.
CHAPTER 23 SECTIONS 1-4
Before we talk about the evolution of populations, you should know what a
population is. A population is a group of organisms of the same species
occupying a certain area. The gene pool is all of the alleles of all the genes that
all the individuals in the population have. A population is the smallest unit of
evolution. POPULATIONS EVOLVE NOT INDIVIDUALS!! Natural selection,
however, does act on individuals, which, over time, produces the changes in the
populations that are evolution.
For example, Individual A with characteristic Q, may be better suited to the
environment than Individual B with characteristic F. There are not many
individuals with Q, but those that have it are highly sought after. Therefore,
Individual A will find a better mate, possibly multiple mates, have more offspring,
and pass on the gene for Q to many individuals. Individual B is not so lucky. F is
not considered as desirable so he will either find an inferior mate, fewer mates,
or none at all. Either way fewer genes for F are passed to the next generation.
As this continues over time, a GREEEEAAAAT deal of time, what will happen to
the occurrences, or frequencies, of these alleles?
Correct! A million years later almost every individual in this species has Q!! Very
few, if any, will have F. The frequencies of the alleles have changed. This is
evolution.
Define microevolution –
What two processes produce the variation in gene pools that contributes to
differences among individuals?
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Where does a mutation have to occur for it to be inherited?
Genetic variation and mutation play roles in natural selection. A diverse gene
pool is important for the survival of a species in a changing environment.
The more variation there is in a population the better the chances are that the
species will continue to survive should there be environmental changes. Diversity
provides many choices for natural selection.
Mutations are the primary source of genetic variation. Changes in genotype can
result in changes in phenotype. Whether or not a mutation is detrimental,
beneficial or neutral depends on the environmental context. Genetic changes
that enhance survival and reproduction can be selected by environmental
conditions, for example antibiotic resistance mutations, pesticide resistance
mutations, and sickle cell disorder and heterozygote advantage.
Review what you learned about mutations, mRNA processing, and transposons in
Units 5 and 6. Generally, smaller mutations cause fewer problems, but it all
depends on exactly where the mutation has occurred. Point mutations often go
unnoticed due to wobble effect. Sickle cell disorder is an exception.
Chromosomal mutations that delete, disrupt, or rearrange many loci are typically
harmful. Duplicated genes can take on new functions by further mutation.
Changes in chromosome number often result in human disorders with
developmental limitations, including Trisomy 21 (Down syndrome) and XO
(Turner syndrome). Since these changes are detrimental to the individual they
would be selected against. These individuals are less fit in the evolutionary
sense. Animals born with conditions such as these usually do not survive at all,
and, therefore, do not reproduce and do not pass on these genetic changes.
Mutations are not always bad and they are the mechanism for change. Without
them we would not exist. Selection (of mutations) results in evolutionary change.
How does sexual reproduction contribute to genetic variation?
How did the different alleles originate?
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THE HARDY-WEINBERG PRINCIPLE
Define:
Population –
Gene pool –
Fixed locus –
Hardy-Weinberg is always used for populations of diploid organisms. I have
never seen an example that was not diploid. If you are confused here, I’m sure
you are not alone. If you are not confused by this, congratulations, but you
probably are alone. We will go over this part of the notes in class and many
example problems. The only way to truly learn this is by doing problems, so yes,
you will have homework. Because there is that grid-in section of the AP exam
that is mathematical in nature, I’m pretty sure you will see this on your exam,
and I know it is on your unit test, so mind your p’s and q’s. (Pun intended)
The frequency of an allele in a population can be calculated:
The total number of alleles at a locus =
The total number of dominant (or recessive) alleles =
By convention, if there are 2 alleles at a locus, p and q are used to represent
their frequencies. The frequency of all alleles in a population will add up to 1.
Write this equation:
The Hardy-Weinberg principle describes a population that is not evolving
(allele frequencies do not change).
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If a population does not meet the criteria of the Hardy-Weinberg principle, it can
be concluded that the population is evolving (allele frequencies will
change).
In a given population where gametes contribute to the next generation
randomly, allele frequencies will not change. Hardy-Weinberg equilibrium
describes the constant frequency of alleles in such a gene pool.
If p and q represent the relative frequencies of the only two possible alleles in a
population at a particular locus, then:
p2 + 2pq + q2 = 1
If the alleles are represented as B and b then:
p2 =
q2 =
2pq =
Conditions for a population or an allele to be in
Hardy-Weinberg equilibrium are: (1) a large
population size, (2) no gene flow - no migration (3)
no mutations, (4) random mating and (5) no natural
selection. These conditions are seldom met.
Organisms can be evolving at some loci but not at others. Not every gene an
organism has undergoes change at the same time. Mutations are actually few
and far between so even when there seems to be drastic changes in an organism
over time, the actual genetic change is probably very small.
Your book gives an example of how to use the H-W equation to estimate the
percentage of a population carrying the allele for an inherited disease (pg. 474,
slide 67). It can also be used to determine whether or not a population is
evolving. Try slide 68, Concept check question 3 pg. 475:
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Previously you were given 5 conditions for a population to be in Hardy-Weinberg
equilibrium. Any violation of any one of those conditions and the population is
not in H-W but is evolving. Any migration into or out of the population causes a
change in allele frequency because as an individual comes into or leaves so do
the alleles it has. If mating is not random, but choices are being made by
individuals based on some characteristic, then that is selection. Mutations
introduce new alleles so frequencies must change. Small populations are subject
to other conditions that can bring about drastic change almost overnight since
one individual represents a higher percentage than it would in a larger
population.
There are three major factors that alter allele frequencies and bring about most
evolutionary change:
–
–
–
1.
2.
3.
In natural selection choices are being made based on some phenotypic
characteristic. Selection is made FOR a specific characteristic so its allele
frequency would go up over time, like we saw with characteristic Q at the
beginning of this chapter.
Genetic drift describes the loss of alleles in a small population that can happen
randomly and just by chance. Slide 72, Fig. 23.8, pg. 476, shows an example,
which is the best way to explain this concept. If you have a small population and
only some of the individuals reproduce, alleles are lost from the gene pool. Again
using the flowers as an example, someone could pick all the red ones, before
they reproduce, because she likes those best, and that would remove all those
alleles from that population so that the following year they would be no more red
ones. This is something that happened to the flowers by chance. The flowers
made no choices or selections. Natural disasters are often the causes of genetic
drift and the population is left with whoever happened to survive.
The founder effect is another example of genetic drift. This time instead of
losing most of a population and being left with a few random individuals, the few
random individuals somehow become isolated from the original population (that
could be very large) and start their own new population. Since they were
selected at random, chances are that their gene frequencies are different from
those in the original population. Think of the pioneers that settled the old west in
the 1800s. They were founders of new towns with just a few alleles to begin
with. If you want a really great example, google HMS Bounty and Pitcairn Island.
This is a story about a sailing ship that went looking for breadfruit in Tahiti,
stayed too long so the men got attached to some women, they left, there was a
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mutiny on the ship and they through out the captain, they went back to Tahiti to
get their women, and then went looking for somewhere to hide- Pitcairn Island.
Today it is still relatively isolated and almost all the people there are descended
from the mutineers of the Bounty. It is a very small gene pool with little genetic
diversity. Other examples of founders include the Amish and the Mormons.
The bottleneck effect is a sudden reduction in population size due to a change
in the environment. A sudden change in the environment may cause selection for
a specific group leaving only certain individuals that may not be reflective of the
original diversity. Again, this could be caused by natural disaster, overhunting, or
habitat destruction by us. Understanding the bottleneck effect can increase
understanding of how human activity affects other species. (Case study of the
greater prairie chicken pg. 477)
Reduction of genetic variation within a given population can increase
the differences between populations of the same species. (Greater
prairie chicken)
Genetic drift can cause harmful alleles to become fixed. If the selected
individuals have a high frequency of a harmful allele then there is the chance of
it becoming fixed. This would mean that all individuals would be homozygous for
the harmful gene which could lead to extinction of the population.
Gene flow is the movement of alleles among populations. This would tend to
reduce the differences between populations rather than increase them the way
genetic drift does. This can be caused by migration of individuals or gametes
such as pollen that is carried by the wind. Gene flow is not necessarily a good
thing. Individuals that migrate into a population can bring bad alleles as well as
good ones. There are examples of each in the PowerPoint, slides 79-80.
While genetic drift and gene flow can cause evolutionary changes that are good
OR bad, natural selection always chooses the phenotypic traits that are the best
match for the environment. By choosing phenotypic traits, the alleles that
produce these traits are also selected for. Traits that are selected for will
increase within the population and traits selected against will decrease.
Natural selection acts on phenotypic variations in populations.
The level of variation in a population affects population dynamics.
A population’s ability to respond to changes in the environment is affected by
genetic diversity. Species and populations with little genetic diversity are at risk
for extinction because there are fewer alleles to choose from:
• California condors
• Black-footed ferrets
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•
•
•
•
Prairie chickens
Potato blight causing the potato famine
Corn rust affects on agricultural crops
Tasmanian devils and infectious cancer
How, though, does selection actually happen? Is there a little gnome out there in
the woods pointing at different individuals and saying “yeah” or “nay”? A fairy
that sprinkles pixie dust? A Red Queen who offs heads? Of course not. That
would be too easy. Selection is caused by the fact that individuals that have
traits that are better suited to a particular environment will get more food, hide
from predators easier, live longer, and have more offspring. The more offspring
you have the more genes you pass on. The more genes you pass on the more
you contribute to the gene pool of the next generation. If you pass on more than
the next guy, you are more fit. I just described what concept?
Environments can be more or less stable or fluctuating, and this affects
evolutionary rate and direction; different genetic variations can be selected in
each generation. Genetic diversity allows individuals in a population to respond
differently to the same changes in environmental conditions.
• Not all animals in a population stampede.
• Not all individuals in a population in a disease outbreak are equally affected;
some may not show symptoms, some may have mild symptoms, or some may be
naturally immune and resistant to the disease.
Some phenotypic variations significantly increase or decrease fitness of the
organism and the population. This may cause directional, disruptive, or
stabilizing selection. See Fig. 23.13 pg. 480, slide 85.
• Sickle cell anemia – heterozygote advantage
• Peppered moth
• DDT resistance in insects – insects with this mutation could survive when
others could not and thus, the frequency of this allele has increased in the fruit
fly population.
Define and sketch an example of each:
Directional selection –
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Disruptive selection –
Stabilizing selection –
When does adaptive evolution occur?
Environments change and act as a selective mechanism on populations.
The example of the peppered moth is a good one- the environment got darker
due to pollution and the darker moths became more prevalent. It is also a good
example of how humans can impact variation in other species, albeit indirectly.
Here are several other examples:
• Artificial selection
• Loss of genetic diversity within a crop species
• Overuse of antibiotics – leads to strains of drug resistant pathogens
Phenotypic variations are not directed by the environment but occur
through random changes in the DNA and through new gene
combinations.
These changes can then be selected for or against by the environment. Nature
can only select from the choices it is given – it does NOT create the choices.
Chapter 24 Sections 1-4
This chapter deals with the origin of species, as you can tell, obviously, from the
title. Darwin considered this to be the “mystery of mysteries” because, at the
time, there was no scientific explanation as to where different types of organisms
came from.
Speciation is the process by which one species splits into two or more species,
or the transformation of one species into a new species over time. This concept
explains both the unity and diversity of life on Earth. The diversity is obvious
because when speciation occurs you have new species, or different kinds of
organisms, which is the definition of diversity- lots of different kinds of things.
The unity part of it is more difficult for people to understand. Unity means to be
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the same or have consistency between groups. Organisms that have evolved
from previous organisms share many similarities which is where the unity comes
in. You share more than 98% of your DNA with a chimpanzee. There are obvious
differences yet many similarities as well since you both share a common
ancestor. Exactly how much difference in DNA is required to make a new species
is still up for debate, and it may also depend on the species involved. There may
be more changes required to produce a new species in some types of organisms
than in others.
Define these terms:
Microevolution –
Macroevolution –
Biologists compare many things when determining groups, or species, of
organisms. What are they?
According to the biological species concept, what is a species?
So, basically, a species is a bunch of organisms that are all the same and are
able to mate and make more of the same NATURALLY, and those offspring CAN
ALSO REPRODUCE. I emphasize this because somebody is going to say “What
about the ligers and tigons? There’s an example of two species (lions and tigers)
that mated and produced offspring!” Yup, they are, BUT: 1) ligers and tigons are
not capable of reproduction, they are sterile, and 2) lions live in Africa and tigers
live in Asia. They are not even on the same continent, let alone in the same
country, so they would not mate NATURALLY. The more we learn, the more we
are able to do in a lab and mess with stuff. Just because it happens in a lab
doesn’t mean that’s how it really works, and this is certainly NOT an example of
natural selection in any way. This is more of the ‘just because we can doesn’t
mean we should’ category.
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This definition also mentions populations. Let’s talk more about lions. They live in
groups called prides. You’ve all seen The Lion King, so you know how it works.
These prides are actually different populations that live in an area. They are
mainly females with young and the males kinda wander around on their own.
Which males mate with which females over the years can change as new males
wander in or grow up and challenge the older males. This creates gene flow,
and that is what keeps these different populations from becoming different
species. All the lions in Africa are free to roam about and mate with whomever
they want. Some males will go great distances and spend their lives far from
where they were born so lion genes are spread around amongst all the lions and
not just in one area, thus maintaining the lion species and not creating new
ones.
That’s how we maintain species. So, how do we get new ones? Pretend you are
Spock for a moment and think logically. If members of populations are free to
move about and migrate from one population to another and mate, and that
maintains species, what would happen if those same members were NOT
allowed to migrate and mate? What if the populations were isolated from each
other and there was no gene flow? Logically, one would have to assume that
such a situation would have the opposite effect and rather than maintain species,
new species would eventually emerge due to mutations, sexual reproduction,
and natural selection. Thank you, Mr. Spock.
New species arise from reproductive isolation over time, which can involve scales
of hundreds of thousands or even millions of years, or speciation can occur
rapidly through mechanisms such as polyploidy in plants. (We will discuss this
later.) Speciation may occur when two populations become reproductively
isolated from each other.
Reproductive isolation is the existence of biological factors (barriers) that
impede two species from producing viable, fertile offspring. See Fig. 24.4 pgs.
490-491, slides 96-101. Reproductive isolation can be classified by whether
factors act before or after fertilization.
Pre-zygotic barriers do what?
List them here with an example: (there’s more room on the next page)
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What do post-zygotic barriers do?
List them here with an example:
There are many other definitions of “species” and some are more useful than
others for different circumstances. We will only be using the biological species
concept, as does your text, with its focus on reproductive barriers.
Speciation results in diversity of life forms. Species can be physically separated
by a geographic barrier such as an ocean or a mountain range, or various preand post-zygotic mechanisms can maintain reproductive isolation and prevent
gene flow. Reproductive isolation keeps species apart. How do separate species
form in the first place? There are two main ways:
Allopatric speciation involves geographic isolation. Gene flow is interrupted
because a population is physically divided by some geographical feature such as
mountains or a highway. Over time reproductive barriers may develop.
What is the evidence of this?
Sympatric speciation takes place in geographically overlapping populations. It
is less common than allopatric speciation due the continued gene flow in the
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overlapping populations. It usually occurs due to polyploidy, habitat
differentiation, or sexual selection.
Polyploidy is
Draw an example: See Fig. 24.10 pg. 295, slide 111.
Changes in chromosome number often result in new phenotypes, including
sterility caused by triploidy and increased vigor of other polyploids. Polyploidy is
much more common in plants than in animals. Many important crop species such
as wheat, cotton, and tobacco are polyploids.
Habitat differentiation happens when a population becomes isolated due to a
switch in habitat or food source (a new niche). If some members of a species
start to eat a different food such as a different species of tree, they could, over a
long period of time, become a new species based on their new food source
which gives them a different niche than the rest of the original population.
Sexual selection is the result of some members choosing and mating with a
specific subset of the original population (a color preference, for example). This
is thought to be how the many species of cichlid fish in Lake Victoria evolved.
Here, it happens quite often apparently, because they are always finding new
species of those fish there.
In summary:
•
•
•
•
•
In allopatric speciation, geographic isolation restricts gene flow between
populations
Reproductive isolation may then arise by natural selection, genetic drift, or
sexual selection in the isolated populations
Even if contact is restored between populations, interbreeding is
prevented
In sympatric speciation, a reproductive barrier isolates a subset of a
population without geographic separation from the parent species
Sympatric speciation can result from polyploidy, natural selection, or
sexual selection
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As we saw with the ligers, sometimes it is possible for members of different
species to mate. This example doesn’t happen naturally, but mules do, and so do
others. Animals, or plants, such as these are called hybrids.
What is a hybrid zone?
Keep in mind that hybrids are only going to happen between closely related
species. Horses and donkeys are pretty close (same family), as are lions and
tigers. Frogs and rabbits are not so you won’t see a frabbit anytime soon. These
hybrid zones do give scientists an opportunity to study evolution in action
because they can study the factors that cause reproductive isolation.
In a hybrid zone reproductive barriers can be (1) strengthened, leading to
reinforcement of the two species and a decrease in hybrids, (2) weakened, leading
to fusion of the two species into one, or (3) continued formation of hybrids and
stabilization of the two species. See Fig. 24.14 pg. 499, slides 122-125.
There are still questions concerning how long it takes new species to form and
how many genes need to be different for organisms to be considered different
species. Scientists study the fossil record every day and many scientists have
many opinions as to what it says. Basically, there are two schools of thought as
to how fast evolution occurs.
The first theory is called gradualism. This theory states that change is very
slow and that species make many small changes over a long period of time to
become new species. See Fig. 24.17 (b) pg. 502, slide 129. This can be seen in
the fossil record for some species.
The second theory is called punctuated equilibrium and was first published by
Stephen J. Gould. He wrote many books on this and other subjects if you are
interested. This theory says that species stay the same for long periods of time
and then undergo rapid change to new species. (Fig. 24.17 (a))This can also be
seen in the fossil record for some species.
It is hard to distinguish between the two, though, because what looks like rapid
change in the fossil record could still be tens of thousands of years. Sediment
build up to cover remains and make fossils takes a very long time. There may be
changes that occur that never produce fossils making it seem rapid when it isn’t.
All species are different and change in different ways. Species with shorter life
cycles and that produce more offspring have the potential to evolve at a faster
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rate than species with longer life cycles that produce less offspring. This doesn’t
mean they do just that they could. It seems likely though, since the dinosaurs
are gone and mammals are still here. Just throwing that out there for you to
consider.
There is also the question of how much change is needed for a new species. For
some species it can be as little as one allele or it could be very many. Genomics
is a new field of study involving the identification and study of gene sequences in
different organisms and comparing them. By comparing the same genes in
different organisms letter by letter, the amount of change that has actually
occurred can be determined. From this type of data we can find out how much
change is needed to become a different species. There will be different answers
for different species depending on the gene that changes, I think. Some changes
will be subtle and not produce great change, while others may be more
dramatic. Right now, no one knows for sure.
Chapter 25 Sections 1-4
There are several hypotheses about the natural origin of life on Earth, each with
supporting scientific evidence. You do not have to learn about each hypothesis,
just be aware that we are still just making a guess as to what happened since
there wasn’t anybody there with their cell phone at the time to put it on
Facebook and Twitter. There have been experiments done to recreate the
conditions of early Earth to see if it was possible for life to form on its own from
the mixture of inorganic compounds thought to be present at the time. This may
have produced very simple cells through a series of stages:
1.
2.
3.
4.
Abiotic synthesis of small organic molecules
Joining of these small molecules into macromolecules
Packaging of molecules into “protobionts”
Origin of self-replicating molecules
Put simply, this is one version of how they think life began:
First there was this nasty atmosphere with lots of inorganic chemicals in it like
nitrogen (N2), hydrogen (H2), carbon dioxide (CO2), carbon monoxide (CO), and
water vapor (H2O). It was really, really hot too, like a sauna. This made these
chemicals all mix together and start to make new stuff like ammonia (NH3) and
methane (CH4). This was step 1. Now there are small organic molecules that can
start to get together to make bigger organic molecules (macromolecules) like
amino acids and lipids. (Step2.) Since lipids are hydrophobic they want to stay
with each other and keep out the water, so they start to form small round
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UNIT 7 NOTES
structures called protobionts, (Step 3.) a sort of type of cell without organelles,
or really much of anything in it. More like fat droplets that just might catch some
other stuff inside by accident when they form. (Remember when we made oil
drops in water in the very first lab we did?) As time went on, larger, more
complicated macromolecules like proteins and nucleic acids may have formed.
Once there was RNA and/or DNA with the ability to replicate (Step 4.), you now
have a crude form of reproduction, and the start of life. Keep in mind that this
may have taken over a billion years to complete, and, yes, it was all random and
by chance. That is the true miracle.
Abiotic synthesis of small organic molecules (Step 1.) has several possibilities:
• Evidence is provided by experiments of Miller and Urey. See Fig. 4.2 on
pg. 59. Be familiar with this experiment! This is what I outlined above and
is often referred to as the heterotroph hypothesis.
• It is also possible that life began underwater near hydrothermal vents
rather than in the atmosphere. These vents provide inorganic nutrients
that present day organisms use.
• Amino acids have also been found in meteorites.
The joining of these monomers produced polymers (macromolecules) with the
ability to replicate, store and transfer information. These complex reaction sets
could have occurred in solution (organic soup model) or as reactions on solid
reactive surfaces. Small organic molecules polymerize when they are
concentrated on hot sand, clay, or rock. Whether on land or in the sea, these
polymers could then spontaneously form protobionts.
What is a protobiont?
What are the two key properties of life that protobionts exhibit?
The RNA World hypothesis proposes that RNA could have been the earliest
genetic material.
What advantage did protobionts with self-replicating, catalyzing RNA have?
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UNIT 7 NOTES
What is the evolutionary significance of this?
The fossil record documents the history of life.
By looking at fossils scientists can see patterns of change. Using dating methods
to put the fossils in order chronologically the patterns of evolution emerge. Keep
in mind that not every organism that ever existed left behind a fossil.
What types of organisms tended to form fossils?
**You are NOT required to know how any of the dating methods work.
See Fig. 25.6 pg. 513, slide 145, for an example of how fossils can be used to
show change.
Key events in life’s history include the origins of single-celled and multi-celled
organisms and the colonization of land.
**You are NOT required to learn the names of the eons, eras, or epochs or any
of the dates.
The Earth formed approximately 4.6 billion years ago (bya), and the environment
was too hostile for life until 3.9 bya, while the earliest fossil evidence for life
dates to 3.5 bya (stromatolites). Taken together, this evidence provides a
plausible range of dates when the origin of life could have occurred.
What types of organisms were the only life forms for over 1 billion years and still
exist today?
As oxygen began to accumulate in the early atmosphere things began to change.
What is the likely source of this oxygen?
What opportunity did this “oxygen revolution” provide?
By what process?
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UNIT 7 NOTES
The hypothesis of endosymbiosis proposes that mitochondria and plastids
(chloroplasts and related organelles) were formerly small prokaryotes living
within larger host cells.
What is an endosymbiont?
Serial endosymbiosis supposes that mitochondria evolved before plastids
through a sequence of endosymbiotic events. This just means that the same host
cell (or, most likely, its ancestor) had an endosymbiont that became a
mitochondrion and then had a different endosymbiont that became a chloroplast.
Remember that these are evolutionary events that took place over LARGE
amounts of time so it is not the same host cell continually, but these
endosymbionts are being passed down to the next generation and eventually
replicating and forming new ones for the next generation. See Fig. 25-9 pg. 517,
slide 156.
What is some evidence that supports this theory?
After these events unicellular organisms could now be different. Some were still
prokaryotic. Some were now eukaryotic, and, of these, some had mitochondria
only, and some had plastids as well. All that was left now was for some of them
to get together and start living as an interdependent group, or what is commonly
called today, a multicellular organism. Further diversification then led to plants,
animals, and fungi.
The Cambrian explosion refers to the sudden appearance of fossils resembling
modern phyla in the Cambrian period around 530 million years ago.
Colonization of land began to occur around 500 million years ago.
Arthropods and tetrapods are the most widespread and diverse land animals.
What are they?
Arthropods are
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UNIT 7 NOTES
Tetrapods are
**The following section provides examples of evolution previously discussed and
that you should be familiar with.
Organisms share many conserved core processes and features that
evolved and are widely distributed among organisms today.
a. Structural and functional evidence supports the relatedness of all
domains.
1. DNA and RNA are carriers of genetic information through transcription,
translation and replication.
2. Major features of the genetic code are shared by all modern living systems.
All organisms use the same nucleotides to make DNA and RNA – A, T, C, G, U.
This can be evidenced by studying viruses which must use other types of
organisms in order to reproduce, and genetic engineering technology where
genes from one organism can be put into another and protein products are
produced.
3. Metabolic pathways are conserved across all currently recognized domains.
Glycolysis (splitting glucose as the first step in cellular respiration) occurs in most
organisms.
Cell communication processes such as G-protein coupled receptors; cAMP, etc.
are common among many organisms.
b. Structural evidence supports the relatedness of all eukaryotes.
These features are all common among all eukaryotes and similar in form and
function thus providing evidence of common ancestry:
• Cytoskeleton (a network of structural proteins that facilitate cell movement,
morphological integrity and organelle transport)
• Membrane-bound organelles (mitochondria and/or chloroplasts)
• Linear chromosomes
• Endomembrane systems, including the nuclear envelope
The rise and fall of dominant groups reflect continental drift, mass extinctions,
and adaptive radiations.
Describe continental drift:
What kind of speciation occurred after Pangea broke up?
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UNIT 7 NOTES
How do they know that the continents were once attached?
Species extinction rates are rapid at times of ecological stress.
Ecological stress can be caused by many things: climate change, extreme
volcanism, or meteorite impacts for instance. All of these have been known to
have happened. Nothing like a meteorite crashing to Earth to break up the
monotony!
• The fossil record shows that most species that have ever lived are now
extinct
• At times, the rate of extinction has increased dramatically and caused a
mass extinction
What is a mass extinction?
How many were there?
It is possible that we are currently undergoing another mass extinction. If we
are, this time we are the cause. By massive burning of fossil fuels we have let
loose tremendous amounts of CO2 and methane which are greenhouse gases.
This has caused an increase in the global temperature which in turn may have
dire consequences. The details of this will be further discussed in Unit 9. Due to
our tremendous ability to reproduce and need for space we have also destroyed
most of the habitats on Earth that other species need to survive. In evolutionary
time, these changes are extremely rapid so organisms do not have time to
adapt. This is what leads to extinction. Will we wake up and realize what we are
doing in time to change and stop a mass extinction? I doubt it. It would not be
economically feasible. What most people do not realize is that we can not live
here on Earth alone. We need the other species that share this world with us.
We need biodiversity. Without it we, as a species, will not survive. Then the
cockroaches will take over the world because they are indestructible. ¡¡Viva La
Cucaracha!!
What are some of the consequences of a mass extinction?
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UNIT 7 NOTES
What is adaptive radiation? Give an example.
Chapter 26 Sections 1-3, 6
Define:
Phylogeny –
Systematics –
Taxonomy –
What are the two parts of Linnaeus’ system that we still use today?
1.
2.
Make sure you know what the binomial, or two-part scientific name of a species
is made up of, and how it is written. Yours, for example, is Homo sapiens.
Write the taxonomic groups in order from largest to smallest. Circle the ones that
make up the two-part scientific name of a species.
Phylogenetic trees and cladograms are graphical representations
(models) of evolutionary history that can be tested.
Systematists depict evolutionary relationships in branching phylogenetic trees.
Basically, taxonomy is how we group and name species based on their
phylogeny- who is related to or descended from whom based on certain
characteristics they have and DNA. A phylogenetic tree represents a hypothesis
about evolutionary relationships. As new information becomes available
phylogenetic trees are redrawn to reflect that. Here is an example:
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UNIT 7 NOTES
Fig. 26-5
Branch point
(node)
Taxon A
Taxon B
Taxon C
ANCESTRAL
LINEAGE
Sister
taxa
Taxon D
Taxon E
Taxon F
Common ancestor of
taxa A–F
Polytomy
On the model above, define the circled terms.
Phylogenetic trees show only patterns of descent. They DO NOT indicate when
species evolved or how much genetic change there was. The lengths of the lines
do not indicate any specific length of time. (unless they have been specially
constructed or a timeline added. See pg. 544 Figs. 26.12 and 26.13)
Phylogenetic trees and cladograms can be constructed from morphological
(shape) similarities of living or fossil species, and from DNA and protein
sequence similarities, by employing computer programs that have sophisticated
ways of measuring and representing relatedness among organisms. (BLAST lab)
Phylogeny provides important information about similar characteristics in closely
related species. For examples of how scientists use and apply phylogenetic data
in the real world read “Applying Phylogenies” on pg. 539 and look at the inquiry
box. In both cases DNA data was used to construct trees that helped identify
organisms involved in criminal action.
How scientists infer (make) phylogenetic trees:
The term “infer” is used because these trees are always changing whenever new
data is available from new technologies. Infer implies that this is the best guess
they can make at this time.
First scientists gather data. They will look at organisms’ structures (morphology),
DNA, and proteins. Organisms with similar morphologies or DNA sequences are
likely to be more closely related than organisms with different structures or
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UNIT 7 NOTES
sequences. Scientists then have to then decide whether similarities are due to
homology or analogy. What is the difference?
Analogous structures are not helpful in determining phylogenies. These
homoplasies (analogous structures or molecular sequences that evolved
independently) can be identified by computer programs.
So once scientists have decided that a characteristic is homologous, that
characteristic can be used to infer phylogeny and scientists can construct a tree.
Shared characters are used to construct phylogenetic trees.
Cladistics
is grouping organisms by common descent. What is a clade?
Fig. 26-10
Define each term under the picture:
A
A
A
B
B
C
C
C
D
D
E
E
B
Group I
D
Group II
E
F
F
F
G
G
G
(a) Monophyletic group (clade)
(b) Paraphyletic group
Group III
(c) Polyphyletic group
As scientists look at organism’s characteristics they determine which
characteristics appeared first in ancestors and then which ones came later in
descendents. They often work backwards as well. Grouping them together in this
way leads to the formation of phylogenetic trees. What is a:
Shared ancestral character –
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UNIT 7 NOTES
Shared derived character –
When looking at several characteristics at a time and several organisms, scientists
often use a character table like the one shown in Fig. 26.11 pg. 543, slide 210. By
checking off who has what characters a pattern of relatedness forms from which a
phylogenetic tree can be made. Phylogenetic trees and cladograms can represent
traits that are either derived or lost due to evolution. For example:
• Number of heart chambers in animals – evidence shows evolution in higher
organisms from two chambered hearts to three and then four like ours. See pgs.
901-902.
• Opposable thumbs – a characteristic only of monkeys, apes, and humans. See pg.
723.
• Absence of legs in some sea mammals – whales and dolphins show evidence in
bone structure of having had legs at one time. These bones have mostly
disappeared leaving only remnants. Dolphin embryos show hind limbs during
development, but these disappear before birth.
An outgroup is a species or group of species closely related to the ingroup
(species being studied) to which scientists can compare to determine whether
characters are shared or derived. A character common to both the ingroup and
outgroup would be ancestral because everyone has it. As mentioned scientists can
use these trees to work backwards and predict features of ancestors like egg-laying
in dinosaurs.
New information continues to revise our understanding of the tree of life. Now that
scientists can look at DNA letter by letter, systematics has changed how organisms
are classified. Early taxonomists classified all species as either plants or animals.
Later, five kingdoms were recognized: Monera (prokaryotes), Protista, Plantae,
Fungi, and Animalia. We now have a three-domain system:
1.
2.
3.
The three-domain system is supported by data from many sequenced genomes,
largely rRNA genes that evolve slowly.
Horizontal gene transfer is
Why is this a problem?
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