Download Natural Selection By Cindy Grigg 1 In 1831, Darwin was the ship`s

Natural Selection
By Cindy Grigg
In 1831, Darwin was the ship's naturalist aboard the
HMS Beagle for a five-year journey. The ship sailed to
South America and the Galapagos Islands. During those
five years, Darwin saw many unique animals. He wondered
how and why the animals seemed to be perfectly suited to
their environment. He began to develop his ideas about
natural selection. He knew his ideas would meet with
disapproval and opposition. He waited more than twenty
years to publish them!
1
In fact, Darwin would probably have never published his ideas if it had not
been for Alfred Russel Wallace. In 1858, Wallace published a paper about
evolution. A friend of Darwin's urged him to publish his own ideas on the subject.
In 1859, Darwin's book, The Origin of the Species was published. Darwin
explained the process of natural selection, the method by which individuals of a
species that are better adapted to their environment are more likely to thrive and
reproduce than other members of the species. Darwin also named some factors that
affect the process of natural selection. Among these were overproduction,
competition,, and variations.
2
Overproduction happens when species reproduce many more offspring than can
possibly survive. The world has limited resources. Many species create many more
offspring than there are resources like food, water, and living space to support
them. This creates a struggle to survive for the offspring. Those who are better able
to survive then pass on their genes to their offspring. This is the process of natural
selection. Darwin studied the reproduction of elephants, one of the slowest
breeding land mammals, and found that if a single female survived and reproduced
at the same rate, after 750 years there could be 19,000,000 descendants of this
single mother!
3
The competition that exists among offspring to survive limits survival of all
offspring. Competition for food, living space, and mates leads to adaptations of
individuals. Competition usually is not a direct confrontation between two
individuals. For example, Darwin's finches have specially selected beaks adapted
to their diets. These adaptations have allowed the birds to survive during dry
seasons or when the food supply is limited by other things. Their beaks help the
birds better compete with other birds and animals for the limited food supply.
4
Individuals of a species are different from others in many of their traits. This is
called a variation. We see variations in family members who share some traits but
also have traits that differ. In individuals in a species, there are variations of traits,
too. Individuals who are able to run faster, perhaps because of bigger leg muscles,
will have a better chance at survival than those who run slower. Those individuals
are better adapted to their environment and are more likely to survive and
reproduce. Their offspring may inherit the allele for bigger muscles. Those
offspring will then be more likely to survive and pass on the allele to their
offspring.
5
In this way, “natural selection” of traits that help individuals survive and
reproduce leads to evolution. Variations of helpful traits slowly build up in the
gene pool. Unhelpful traits, like the trait for smaller leg muscles, disappear over
time.
6
Without any variation, all members of a species would have the same traits.
Each would have an equal chance at survival. Evolution would not occur.
7
Darwin didn't have any knowledge of genes and mutations. He could not
explain how variations were passed down from parents to offspring. Now scientists
know that variations can be caused by the crossing-over of alleles during meiosis
to create new combinations of traits. Parents pass genes to offspring. Traits that are
controlled by genes are inherited. These traits can be changed by natural selection.
8
In 1977, a study of Galapagos finches showed that traits could change very
quickly by natural selection. Only two- and-one-half centimeters of rain fell that
year, much less than the yearly average of about thirteen centimeters. Many plants
died because of the drought. Fewer plant seeds were available for the finches to
eat. The finches had to eat seeds that were enclosed in tough, thorny seedpods. Not
surprisingly, the finches with bigger, stronger beaks were able to get more food.
Their beaks were strong enough to open the tough seedpods. Many of the finches
with smaller beaks died out. The next year, more finches on that island had larger,
stronger beaks. A climatic change had caused natural selection show changes in
this species in only one year. Peppered moths are another species that natural
selection has caused changes in a short amount of time.
9
Copyright © 2010 edHelper
Name _____________________________
Natural Selection
Date ___________________
1. Two British men proposed an
2. What is natural selection?
explanation of how evolution occurs.
Name them.
3. What is overproduction?
When individuals produce more
offspring than simply replacing
themselves
When individuals in a species
can't get enough food
When plants produce more food
than is needed
When a species produces many
more offspring than can survive
5. What is variation?
Any difference between species
The way a species might
change over time
The way an individual changes
throughout its lifetime
Any difference between
individuals of the same species
4. Competition between individuals is
usually:
Fierce
Necessary
Indirect
Direct
6. Why are variations important?
Some variations give
individuals an advantage that may
help them survive and reproduce.
Some variations may be passed
down to offspring by genes.
Variations over time may lead
to evolution.
All of the above
None of the above
Name _____________________________
Natural Selection
7. The only traits that can lead to
natural selection and evolution are
traits that:
Help a species survive
Don't cause mutations
Are controlled by genes that
can be inherited
All of the above
Date ___________________
Natural Selection - Answer Key
1 Charles Darwin and Alfred Russel Wallace
2 The process by which individuals that are better adapted to their environment are more likely
to survive and reproduce than other members of the same species
3
When a species produces many more offspring than can survive
4
Indirect
5
Any difference between individuals of the same species
6
All of the above
7
Are controlled by genes that can be inherited