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Evolution
There are millions of different kinds of living things on Earth. Each and every
kind of living thing is well suited to where it lives. For example, there are many
different kinds of cacti. In picture A to the right shows a close up view of a cactus
plant. Cactus plants have spines instead of leaves. The spines are an adaptation
that helps them to reduce water loss. Cactus plants are well suited for growing in
areas where there is little water available. How do scientists explain the great variety
of living things on the face of Earth? How do they explain the fact that living things
are well suited to where they live? We will learn the answer to these questions and
more in this unit.
Changes in living things
Living things are well suited to where they live. The reason for this is that all
living things in the world have certain kinds of adaptations. An adaptation is a trait
that makes a living thing able to survive in its surroundings. You first learned about
adaptations when we studied the traits of living things at the beginning of the year.
Look at the foot of the bird shown below on the left. The bird spends a great
deal of its time perched on branches. Note the bird’s long toes. The long toes curl
around small shoots and help the bird remain perched on the branch. The bird
shown to the right below spends a great deal of time in water. How does the
webbing between the toes help this bird to swim?
An Example of Survival
How do traits help organisms survive in their environments? What is the
outcome when an organism with a certain trait is able to survive? These questions
are answered in the following example.
A group of mice live in an area that has dark soil. Owls that eat mice also live
in this area. Because dark mice blend in well with the dark soil, owls cannot see them
easily. Thus, the dark mice are better protected because they blend with the soil
color. Their color is an adaptation, a trait that helps them to survive.
Dark mice do not always have offspring that are also dark in color. Every now
and then they have light-colored offspring. The light-colored offspring, in turn, have
other light-colored. Light colored mice are easy to spot against the dark soil. As a
result of their color, the light-colored mice are usually the first to be eaten by the owls.
Light-colored mice living on dark soil are poorly adapted to their surroundings. Few
light mice survive. As a result, few light mice reproduce. The number of light-colored
mice tends to remain low. The dark mice, however, survive and reproduce. The dark
mice will continue to outnumber the light mice.
Suppose chemical changes take place in the soil and cause it to change to a
lighter color. Now owls can spot the dark mice on the ground more easily than they
can spot the light mice that blend in with the soil. As a result, the owls eat more dark
colored mice. The light-colored mice now live to reproduce and begin to
outnumber the dark-colored mice.
Dark mice are no longer adapted to these surroundings. However, light mice
are adapted to the light-colored soil. Dark color has become an unfavorable trait.
The balance between the two mouse types begins to change. More light mice
survive and reproduce. Any dark mice that are born are more likely to be eaten.
Few of them get a chance to survive and reproduce.
Natural Selection
We can now ask an important question using the mouse and owl story. What
determined which mouse was better adapted to its surroundings? The owls
determined, by eating certain mice, which color was an adaptation for survival.
Only mice that are not eaten survive to reproduce. On the dark soil, more dark mice
survived because the owls did not see them. On the light soil, more light mice
escaped the owls. Natural selection is the process in which something in a living
thing’s surroundings determines if it will or will not survive to have offspring. In natural
selection, something in nature does the selecting. In our example, the owls did the
selecting. When the soil became lighter, the group of mice changed from mostly
dark to mostly light. The change was the result of natural selection.
Living things that are suited to their surroundings survive. They will be the ones
most likely to reproduce. Their traits will be passed on to their offspring. Living things
that are not suited to their surroundings won’t survive. They won’t reproduce, they
won’t have offspring, and their traits won’t be passed on.
Why wouldn’t a bright red frog survive in a muddy pond? Can you explain
why more mud-colored frogs are likely to survive in muddy ponds than red frogs?
How can web-toed frogs survive in water better than frogs with toe pads?
Mutations
Adaptations are traits that help living things survive in their environments.
Remember from your study of genetics that genes control traits. Thus, genes control
adaptations. What is the source for new traits that help living things survive? Many
new traits come from mutations. Remember, a mutation is a change in the DNA
code. Mutations may supply living things with sources of new traits. Thus, they may
supply new adaptations.
Are all mutations helpful for survival? No, some are harmful. For example, a
mutation that causes a change in the gene that controls fur color of deer. Inheriting
this gene causes the normal brown coat of deer to change to white. The deer no
longer blends in with its surroundings in the summer months. It can be seen more
easily by its enemies and may be eaten.
What if the deer had a mutation that gave it extra long legs? The long legs
might help it run faster from its enemies and escape being eaten. This new trait
would probably help the deer in its surroundings. The trait could be passed to the
offspring, thus increasing the number of individuals with that trait.
Mutations are natural events. Mutations appear in every living thing. These
changes in genes may be helpful, harmful, or have no effect at all.
Species Formation
A species is the smallest group of living things that can breeds with others of
the same species and produce fertile offspring. Fertile means being able to
reproduce by forming egg or sperm cells. For example, horses and donkeys look very
similar to each other but cannot produce fertile offspring if they are bred together.
They do produce offspring called mules but all mules are incapable of reproducing.
Now we have a problem. Only members of the same species can breed and
form offspring. Yet, new species are constantly appearing on Earth. How can new
species appear? Look
at the picture below
and follow along in it
as you read to find out
how this is possible.
Let’s start with a
group of rabbits living
one either side of a
shallow stream. The
animals at the top of
the picture are all of
the same species.
They can cross the
stream easily, but they
are unable to swim.
Due to a flood, the
stream becomes a
very wide river. It
remains that way. The animals become separated into tow groups. Being unable to
swim, they are unable to cross the river. They continue to live apart for thousands of
years. Note that the living conditions on each side of the river are different. During
the time of separation, natural selection has taken place in each group. In each
group, individuals lacking traits favorable for the new environment have died.
Individuals with the favorable traits have survived and reproduced. The two groups
gradually become different because their environments are different. In time, each
group may become a different species.
In the example just given, three events lead to the development of two new
species. First, a barrier is formed that separated members of a species. The barrier
could have been a river, ocean, new mountain, glacier, or a lava flow. Second, the
animals found themselves living in different environments. Third, the groups began to
show different traits as a result of natural selection. The two groups in time became
two different species. As a result, they would not be able to breed and form fertile
offspring if brought back together.
The finches that live on the Galapagos Islands are a well-known example of
the forming of a new
species. On the Galapagos
are several species of finch.
Each species has different
beak shape as seen in the
picture on the next page.
Some finches have thick
beaks and are adapted to
eating seeds. Some have
small beaks and are adapted to eating insects. How did the finches come to live on
the islands? The ancestor of the Galapagos finches probably flew to the islands from
the mainland of South America. New species began to evolve when the finches
began to spread out over the islands. The different groups of finches didn’t come
into contact with one another for a long time. Over time, the different groups
became adapted to their new environments. They also became less like one
another. A single finch ancestor had evolved into many different species.
Explanations for Evolution
How can we explain that life forms have changed with time? What is the
evidence that life forms had a common beginning?
Darwin’s work
Much of what we have stated so far about adaptations and natural selection
is not new. Charles Darwin said it over 100 years ago. When he was young, Darwin
made a voyage around the world on a ship. During the trip, he observed many kinds
of plants and animals and gathered examples of them. On some islands he
collected living things not found anywhere else on Earth. Darwin say that these living
things were similar to life present in other parts of the world. For 20 years after his trip,
Darwin studied the material he had collected. Finally in 1859, he wrote a book
explain evolution and his theory of natural selection. It was called On the Origin of
the Species.
Darwin made a number of important points in his book. We will summarize
several of his more important ideas.
1. Living things overproduce. More offspring are
produced than survive. A single maple tree forms
thousands of seeds. Frogs lay hundreds of eggs as
seen in the picture to the left.
2. There is variation among
offspring. A variation is a trait
that makes an individual
different from others in its species. Each living thing does not appear exactly
like all the others. Some of the differences between individuals are inherited.
The picture to the right shows variations in the color and pattern of monarch
butterflies.
3. There is a struggle to survive. There are more living things than there are
resources to go around. This results in competition. Competition is the struggle
among living things to get their needs for life. Young pine trees compete for
light, water, and soil nutrients. Rabbits compete with other rabbits for food,l
shelter, and mates.
4. Natural selection is always taking place. Individuals that have less desirable
traits are less fit. They reproduce fewer offspring. Individuals that have
desirable traits are more fit. They reproduce more offspring. The organisms
alive today are the ones that are better suited totheir surroundings. The traits,
or variations, that make them more fit are the ones they inherited ans will, in
turn, pass to their offspring.
Darwin realized that species of organism are always changing. He knew that the
changes in species do not occur quickly. Darwin’s studies led him to form the theor
of evolution by natural selection. Evolution is a change in the hereditary features of a
group of organisms over time. When a species changes throug time, it is said to have
evolved.
Fossil Evidence
What evidence supports evolution? Some evidence of evolution comes from
fossils. Fossils are the remains of once-living things from ages past. A fossil may be a
print of a leaf. It may be a footprint of an animal. It could even be a skeleton. A
fossil could be an animal trapped and frozen in ice. Or a fossil could be an insect
trapped in hardened plant sap. When living things from the past are compared to
living things today, we can see that change has occurred. An extinct life-form is one
that no longer exists.
Fossils are found in the Earth’s
crust. They are present in
sedimentary rocks.
Sedimentary rocks form from layers
of mud, sand, and other fine
particles. The mud, sand, and fine
particles are called sediments.
These sediments form at the bottom
of seas. Many animals and plants
die ans settle to the bottoms of
oceans, lakes, or ponds with the
sediments. These sediments change
into rock over millions of years.
Fossils form within these sediment
layers. Fossils give us a record of
what types of living things were on
the the Earth in the past. Sceintists
can tell how old fossils are by dating them. Being able to date fossils gives sceintist an
idea of the history of life on Earth.
The figure to the right is a side view of sedimentary rocks in the Earth’s crust.
Where is the oldest layer of rock located? Because it was the first layer to form, it is
on the bottom. Younger layers settle on top of the oldest layer. So, as you move up
the layers, the rocks get younger. This is similar to stacking newspapers. Suppose you
always pu the most recent newspaper on top of the stack. As long as the stack is left
alone, the oldest paper will always b e on the bottom. The newest paper will be on
top. Fossils found in the lower layers of rock are older than those found in the upper
layers.
Compare fossils in the bottom layers to those near the top. How many fossil
forms found toward the bottom and middle layers are still alive today? Not too
many. How many fossil forms found in the top layers are still alive today? Certainly
more than in the middle and bottom layers.
Other Evidence
Relationships among different species can be shown by comparing the origins
of body structures and comparing body chemistries. Each of these comparisions is
evidence of evolution.
Another comparision also provides evidence of evolution. The picture below
show five different animal embryos at different stages of development. As you can
see from the early
stages of
development,
salamanders, birds,
chimps and humans
look very similar. If
you look at the first 3
drawings in each row,
you can’t tell which
organism is which.
How is the similar
appearance of
embryos evidence of
evolution? All four
animals are
chordates (animals
with backbones)
They have a common
ancestry. The embryos share some of the same traits from their common ancestry.
These traits result inall of the embyros looking similar in early stages.
What other evidence is there of evolution? If later life-forms evolved from
earlier ones, wouldn’t the later forms have something in common with the earlier
forms? Early life-forms are made of cells. So are later life-forms. Early life-forms have
DNA as part of their chromosomes. So do later life-forms. The gene code in early life-
forms is made of nitrogen bases, A, T, C, and G. Later life-forms have the same kind
of gene code.
Have you ever wondered what that little pink lump is in the corner of your eye?
What does it do? What does you appendix do? Both of these body parts are called
vestigial structures. A vestigial strucutre is a body part that no longer has a function.
How is a vestigial body part evidence of evolution? Most of these body parts do
have jobs in other animals. For example, in many mammals the appendix helps
digest food. Rabbits are examples of animals with an appendix that still works to
digest food. This part of a rabbit intestine helps to break down plant material, the
rabbit’s chice source of food. The pink lump in your eye is all that is left of a third
eyelid. In other animals the third eyelid is usually very thin aqnd covers the entire eye.
Frogs and turtles are examples of animals with third eyelids. The third eyelid in these
animals protects the eye while the animal is under water. Birds, fish, and reptiles also
have third eyelids that protect the eye.
We may not have any use for our appendixes or what remains of our third
eyelid. These structures, however, are still useful in related animals. Thus, the
presence of vestigial structures is evidence of a common ancestor for us and related
animals. We still have the genes for appendixes and third eyelids, even though we
don’t use these structures. Animals that are related to us also have the genes for
these traits.