Download How Does Evolution Happen?

How Does Evolution Happen?
trait
selective breeding
natural selection
mutation
• Describe the four steps of
Darwin's theory of evolution
natural selection.
• Explain how mutations
important to evolution.
are
by
The early 1800s was a time of great scientific discovery.
Geologists realized that the Earth is much older than anyone
had previously thought.
Evidence showed that gradual
processes had shaped the Earth's surface over millions of years.
Fossilized remains of bizarre organisms were found. Fossils of
familiar things were also found, but some of them were in
unusual places. For example, fish fossils and shells were found
on the tops of mountains. The Earth suddenly seemed to be
a place where great change was possible. Many people thought
that evolution occurs, but no one had been able to determine
how it happens-until
Charles Darwin .
Charles Darwin
In 1831, 21-year-old Charles Darwin, shown in Figure 11,
had just graduated from college. Like many young people
just out of college, Darwin didn't know what he wanted to
do with his life. His father wanted him to become a doctor.
However, Darwin was sickened by watching surgery. Although
he eventually earned a degree in theology, he was really interested in the study of plants and animals.
Darwin was able to talk his father into letting him sign
on for a S-year voyage around the world. He served as the naturalist (a scientist who studies nature) on a
British naval ship, the HMS Beagle. During this
voyage, Darwin made observations that later
became the foundation for his theory of evolution by natural selection.
Charles Darwin, shown
at far left, sailed around the world
on a ship very similar to this one.
Figure 11
Darwin's Excellent Adventure
As the HMS Beagle made its
way around the world, Darwin
collected thousands
of plant
and animal samples and kept
detailed notes of his observations. The Beagle's journey is
charted in Figure 12. During the
journey, the ship visited the
Galapagos Islands, shown below,
which are 965 km (600 mi) west
of Ecuador, a country in South
America.
Darwin's Finches
Darwin
observed that the animals and
plants on the Galapagos Islands
werevery similar, yet not identical,
to the animals and plants on the
nearby South American mainland. For example, he noted that the finches living on the Galapagos Islands
differed slightly from the finches in Ecuador. The finches on
the islands were different not only from the mainland finches
but also from each other. As you can see in Figure 13, the birds
differed from each other mainly in the shape of their beaks and
in the food they ate.
Figure 12 The course of the
HMS Beagle is noted by the
red line.
Figure 13 The beaks of these three species of finches are
adapted to the different ways the finches obtain food.
The large gro.und finch has a
heavy, strong beak adapted
for cracking big, hard seeds.
Thisfinch's beak works like
a nutcracker.
The cactus finch has a tough
beak that is good for eating
cactus and its nectar. It works
like a pair of needle-nosed
pliers.
The warbler finch's small,
pointed beak is adapted for
probing into cracks and crevices
to obtain small insects. This beak
works like a pair of tweezers.
Darwin Does Some Thinking
Have you ever heard
of a bank that has
no money, only
seeds? Read
about it on
page 172.
Darwin's observations raised questions that he couldn't easily
answer, such as, "Why are the finches on the islands similar
but not identical to the finches on the mainland?" and "Why
do the finches from different islands differ from oIle another?"
Darwin thought that perhaps all the finches on the Galapagos
Islands descended from finches on the South American mainland. The original population of finches may have been blown
from South America to the Galapagos Islands by a storm. Over
many generations, the finches that survived may have adapted
to various ways of living on the Galapagos Islands.
After Darwin returned to England, he spent many years
working on his theory of how evolution happens. During
this period, he gathered many ideas from a variety of sources.
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Figure 14 D~fJs are a good example of how selective breeding works. Over the past 72,000 years, dogs have been
selectively bred to produce more than 750 different breeds.
Darwin Learned from Farmers
and Animal and Plant Breeders
In Darwin's time, many varieties
of farm animals and plants had
been selectively produced. Farmers
chose certain traits, distinguishing
qualities such as plump corn kernels, and bred only the individuals that had the desired traits.
This procedure is called selective
breeding because humans, not
nature, select which traits will be
passed along to the next generation. Selective breeding in dogs,
shown in Figure 14, has exaggerated certain traits to produce
more than 150 different breeds.
In your studies of genetics and
heredity, you learned that a great
variety of traits exists among individuals in a species. Darwin was
impressed that farmers and breeders could direct and shape these
traits and make such dramatic
changes in animals and plants in
just a few short generations. He
thought that wild animals and
plants could change in a similar
way but that the process would
take much longer because variations would be due to chance.
Darwin Learned from Geologists
Geologists told Darwin that they
had evidence that the Earth
was much older than anyone
had imagined. He learned from
reading Principles of Geology, by
Charles Lyell, that Earth had
been formed by natural processes over a long period of
time. Lyell's data were important because Darwin thought
that populations
of brganisms
changed very slowly, requiring a
lot of time.
:Irwin Learned from the Work of Thomas Malthus In
s Essay on the Principle of Population, Malthus proposed that
unans have the potential to reproduce beyond the capacity
their food supplies. However, he also recognized that death
used by starvation, disease, and war affects the size of human
>pulations. Malthus's thoughts are represented in Figure 15.
Darwin realized that other animal species are also capable
producing too many offspring. For these animal species,
uvation, disease, and predators affect the size of their poputions. Only a limited number survive to reproduce. Thus,
ere must be something special about the survivors. What
lits make them better eqUipped to survive and reproduce?
uwin reasoned that the offspring of the survivors inherit
litS that help them survive in their environment.
Malthus's Principle of Population
Increase Versus Food Availability
Figure 15 Malthus
thought the human
population would
increase beyond
what the available
food supply could
support.
Could We Run out
of Food?
Malthus thought we could.
Do the following activity to
better understand Malthus's
hypothesis. Get 2 empty egg
cartons and a bag of rice.
Label one carton "Food supply" and the second carton
"Population growth:' In the
food supply carton, place one
grain of rice in the first cup.
Increase the amount by one
in each subsequent cup. Each
grain represents a unit of
food. In the population
growth carton, place one
grain of rice in the first cup,
and double the number of
grains of rice in each subsequent cup. This rice represents people.
1. How many "people" are
there in the last cup?
2. How many units of food
are there in the last cup?
3. What conclusion can you
draw?
Natural Selection
In 1858, about 20 years after he returned from his voyage (
the HMS Beagle, Darwin received a letter from a natural
named Alfred Russel Wallace. Wallace had independenl
arrived at the same theory of evolution that Darwin had bel
working on for so many years. Darwin and Wallace discuss,
their research and made plans to present their findings at
meeting later in the year. Then, in 1859, Darwin publish
his own results in his book called On the Origin of Species
Means of Natural Selection. Darwin theorized that evoluti(
occurs through a process he called natural selection. n
process, examined below, is divided into four parts.
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produces
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Overproduction
The individuals in a population
are slightly different from one another. Each
2 individual
has a unique combination of traits,
Genetic Variation
such as size, color, and the ability to find food.
Some traits increase the chances that the
individual will survive and reproduce. Other
traits decrease the chances of survival. These
variations are genetic and can be inherited.
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Natural S.el~.ction in Four Steps
Struggle to Survive A·natural environment
does not have enough food, water, and other
resources to support all the individuals born.
In addition, many individuals are killed by
other organisms. Only some of the individuals
in a population survive to adulthood.
Successful Reproduction Successful reproduction
is the key to natural selection. The individuals
that are well adapted to their environment, that
is, those that have better traits for living in their
envir,(~mment, are more likely to survive and
reproduce. The individuals that are not well
adapted to their environment are more likely
to die early or prodblce few offspring.
to any of the specific breeds. Your grandfather
says that over the past 50 years each generation has looked less like the generation that
preceded it.
By the time you visited the kennel, what may
have happened to make most of the dogs look
similar to each other but not to any specific
original breed? Base your
ans"l'er on what you've
learned about selective
breeding in this section.
Imagine that your grandfather has owned a
kennel for more than 50 years but has never
sold a dog. He cares for the dogs and keeps
them in one large pen. Originally there were
six labs, six terriers, and six pointers. There are
now 76 dogs, and you are surprised that only
a few look like pointers, labs, and terriers. The
other dogs look similar to each other but not
More Evidence of Evolution
One of the observations on which Darwin based his theory
of evolution by natural selection is that parents pass traits to
their offspring. But Darwin did not know how inheritance
occurs or why individuals vary within a population.
During the 1930s and 1940s, biologists combined the principlesof genetic inheritance with Darwin's theory of evolution
by natural selection. This combination of principles explained
that the variations Darwin observed within a species are caused
by mutation. or changes in a gene.
Since Darwin's time, new evidence has been collected from
many fields of science. Although scientists recognize that other
mechanisms may also playa part in the evolution of a species,
the theory of evolution by natural selection provides the most
thorough explanation for the diversity of life on Earth.
.
REVIEW
.
1.Why are some animals more likely to survive to adulthood than other animals?
2. Summarizing Data What did Darwin think happened
to the first small population of finches that reached the
Galapagos Islands from South America?
3. Doing Calculations A female cockroach can produce 80
offspring at a time. If half of the offspring were female,
and e~ch female produced 80 offspring, how many cockroaches would there be in 3 generations?
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TOPIC: The Galapagos Islands, Darwin
and Natural Selection
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