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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 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.