Download Chapter 7

Chapter 7
The Evolution of Living Things
Section 1
This chapter deals with animals and how they have changed over time. It will discuss similarities
and differences between organisms.
Differences Among Organisms
Some organisms have changed over time to help in their survival. An adaptation is a
characteristic that helps an organism survive and reproduce in its environment. These adaptations
may be structures on the organism, or behaviors that the organism has for surviving (locating
food, protecting it, avoiding predators, or even traveling).
Living things that share the same characteristics and adaptations may be members of the same
species. A species is a group of organisms that can mate with one another and produce fertile
offspring. Example: All White-tailed deer are the same species and are able to mate with one
another and produce fertile offspring. A white-tailed deer and elk are two different species, so
they can not mate and produce fertile offspring.
Change of Organisms Over Time, Does It Happen?
Scientists estimate the Earth to be 4.6 billion years old. Scientists have found fossils of organisms
and these fossil remains suggest organisms have changed through time. Scientists think that
newer species have descended from older species through the process of evolution. Evolution is
the process by which populations accumulate inherited changes over time. Because of evolution,
some scientists think that all organisms have a common ancestor.
Evidence: Fossils
The crust of the Earth is in layers with different types of soil and rocks layered on top of each
other. When particles of soil, sand, dust, and other components of the Earth’s crust are deposited
in an orderly fashion, the older layers are deposited first. This means that if you dig into the
Earth, the deeper you dig, the older the soil layers. Fossils are the solidified remains or imprints
of once living organisms and are found in the layers of sediment in the Earth. Fossils can be of
whole organisms or simply pieces. When an organism is covered with sediment, over long
periods of time the minerals in the sediment will “leach” or enter into the organism. Basically, the
organism is replaced by rock. Or the organism may decompose and leave a hollow area where it
was decomposing, which results in a mold.
The Fossil Record
Fossils provide a historical account of the sequence of life and this sequence is known as the
fossil record. Fossils found in the upper crust resemble modern day organisms and those deeper in
the crust may just show similarities and look less like modern day organisms.
Gaps in the Fossil Record
Scientists have found thousands of fossils over time, and they have noticed there appears to be
gaps in the fossil record. They believe these gaps are because the right conditions may not always
be present for fossil formation. In order to have well formed fossils, the organism must be buried
in fine sediment and in the absence of oxygen. Oxygen promoted the decay too rapidly. Some
scientists have found some evidence they believe fill the gaps and complete the fossil record.
Vestigal Structures
Vestigal structures are structures an organism has that are remnants of once useful structures.
Scientist Argument: Whales look like fish, but they breathe air and give birth to live young and
produce milk for the young, which makes them mammals. There are some bones near the tail of
whales that some scientists believe are the remnants of hind limbs and the whale is actually a
descendant of a doglike organism that walked on land.
More Evidence for Evolution
Skeletal Structures: When scientists compare the arm bones of a human, front leg of a cat, a
dolphin flipper, and a bat wing, they notice that each appendage has very similar bones and bone
structure. Scientists use this as arguments for species going through evolution and having one
common ancestor.
DNA of Different Organisms: Scientists have discovered that all organisms have the same
components in their nucleotides (phosphate, sugar, base) to make up their genetic material. The
difference is how the nucleotides are arranged in each species of organisms.
Embryonic Structures: Scientists have looked at embryos of different species (fish, chickens,
dogs, humans and other vertebrates) and during the early stages of development, they have very
similar features. These similarities are what make scientists believe all vertebrates have a
common ancestor.
SECTION 2
How Does Evolution Happen?
Charles Darwin: He was a scientist and served aboard the HMS Beagle. (HMS represents Her
Majesty’s Ship for the British Navy). While on the Beagle, he served as the naturalist (a scientist
who studies nature). He made observations that later became the foundation for his theory of
evolution by natural selection.
Darwin collected and analyzed thousands of species while on the voyage. One particular location
he visited was the Galapagos Islands (off the West Coast of South America). Darwin studied the
plant and animal communities on the islands and noticed they were similar to the species on the
mainland of South America, but not identical. One particular organism that caught his eye was
finches (small birds). There were three species he compared and he noticed that each one had a
different beak structure, which each used differently to obtain food. Why were the island finches
a little different from mainland finches and why were the island finches different from each
other? Darwin wondered.
He decided the differences were due to the birds on the island adapting over long periods of time
to specific ways of obtaining food. One finch had a strong beak for busting hard seeds, a second
finch had a tough beak that served well for eating cactus and their nectar, and the third finch had a
small pointed beak adapted to feeding and searching in narrow cracks and crevices for insects.
Learning from Farmers, Animal and Plant Breeders
During Darwin’s life, many farmers, plant and animal breeders produced a variety of organisms
by selecting specific organisms which had specific traits and breeding them. A trait is a
distinguishing characteristic, in this case large ears of corn. This procedure of choosing which
organisms to breed based on the characteristics desired is called selective breeding. We have
discussed selective breeding before during genetics chapters.
Darwin learned from geologists. Darwin read a book called Principles of Geology by Charles
Lyell. Darwin learned the Earth was much older than most people believed. This made more
sense to Darwin because he thought changes and adaptations (evolution) would take great lengths
of time to occur.
Darwin also learned from Thomas Malthus. Malthus wrote about populations having natural
populations control mechanisms. He discussed populations becoming too large and the result
being not enough resources for the organisms and starvation killing off many of the members.
Once the population was small enough, the food sources recovered and the remaining population
would rebound until it overshoots the food sources and many of the organisms starve until the
environment recovered and could support the smaller population. Malthus wrote about this
occurring in cycles.
Natural Selection
As Darwin sailed on the HMS Beagle, he theorized that evolution occurred through a process
called natural selection. Basically, it is the idea of “survival of the fittest”. Those organisms that
are best suited for surviving in a particular ecosystem will be the organisms to produce offspring
which are also better suited to survive. As generation after generation is produced, the organisms
that are “best fit” to survive will be the ones that go on and pass their characteristics on to new
offspring.
Darwin also had ideas that genetic changes occurred due to mutations. Mutations are changes in
the genetic make-up or genes of an organism. Some mutations do not have any effect on the
organism that has the mutation. Some mutations may cause birth defects or even death of the
organism in which the mutation occurs. Some mutations, on the other hand, can actually result in
benefits to an organism. The organism may become stronger, faster, and capable of capturing
prey easier and when these “beneficial” mutations occur, the organism may pass that trait on to its
offspring. This in turn makes the offspring more “fit” to survive within the given ecosystem.
Section 3 Chapter 7
Natural selection occurs over time in several ecological processes we can discuss. One example
occurs when bacteria become resistant to antibiotics we use in such great quantity. When bacteria
come into contact with the antibiotics and do not die they essentially build up a resistance to the
drug and it takes 1) either a higher dosage to get the same effect or 2) a stronger antibiotic.
Therefore, it is very important to follow the Dr. orders that come with any drug you are
prescribed, and take all of the prescription. Just because you feel better does not mean you should
stop taking the medicine. If you relapse back into the sickness, it will take more antibiotics or
stronger antibiotics to get well. This brings us to a term known as generation time which is the
period between the birth of one generation and the birth of the next generation.
Another example of natural selection is a classic case of the peppered moth. There are two forms
of the peppered moth. One is dark coloration and the other is light in color. When the light
colored moth landed on trees with dark bark, they “stood out” against the dark background and
birds would eat the moths. The dark moths on dark bark would not “stand out” against the dark
background so the birds would not feed on them as easily or as often. The trees often had dark
bark due to the pollution in the air.
As time passed, the dark colored moths survived more often and passed on the dark traits to later
generations. As more people became aware that pollution was harmful and laws were passed the
environment became less polluted. As less and less pollution filled the air, the bark on the trees
became light colored and the trend of moth colors changed. The light colored moths were able to
blend into the bark and were not preyed upon easily of as often. This meant the light colored
moths passed their genes on to future generations and light colored moths became more
numerous. The dark moths were fed upon more often because they did not blend into the light
colored tree bark.
New Species Being Formed
Some evidence suggests that “new species” were formed due to organisms being separated by
geographical features (rivers, mountains, deserts). This is known as separation evolution or
adaptation evolution or division evolution. An example of this evolution has occurred at the
Grand Canyon. Millions of years ago, before the canyon eroded so much there was a population
of squirrels. The population became divided as the canyon became wider and deeper. There ended
up one group on the North rim of the canyon and another group on the South rim of the canyon.
Conditions are so different on the two rims that the squirrels had to adapt to their location (North
or South rim). Today, the squirrels are slightly different in color and size and scientists classify
them as different sub-species.
This can occur also when species become introduced to new habitat. If a strong storm in the
ocean blows birds to new islands, the birds may have to adapt to the new environmental
conditions or die. If they adapt, they “evolve” to fit their new environment and conditions.
Know all the definitions and examples of these definitions. We will work on review questions in
class for this chapter.