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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!
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.
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!
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.
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.
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.
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.
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.
In 1977, a study of Galapagos finches showed that traits could change very quickly by natural selection. Only
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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 to 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.
Natural Selection
Questions
1. Two British men proposed an explanation of how evolution occurs. Name them.
2. What is natural selection?
3. What is overproduction?
A. when individuals produce more offspring than simply replacing themselves
B. when plants produce more food than is needed
C. when a species produces many more offspring than can survive
D. when individuals in a species can't get enough food
4. Competition between individuals is usually:
A. indirect
B. direct
C. fierce
D. necessary
5. What is variation?
A. the way an individual changes throughout its lifetime
B. any difference between individuals of the same species
C. any difference between species
D. the way a species might change over time
6. Why are variations important?
A. Some variations give individuals an advantage that may help them survive and reproduce.
B. Some variations may be passed down to offspring by genes.
C. Variations over time may lead to evolution.
D. All of the above
E. None of the above
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7. The only traits that can lead to natural selection and evolution are traits that:
A. help a species survive
B. don't cause mutations
C. are controlled by genes that can be inherited
D. all of the above
Summarize Darwin's main ideas about natural selection.
How do helpful variations increase in a species over time?