Evolution and Natural Selection
continued
Stabilizing Selection
• The most common form
of selection involves
favouring the average
condition.
• This type of selection
favours the species that
show very little
divergence from the
norm.
• An example of stabilizing
selection can be seen in
the Siberian Husky.
• The Siberian Husky is a medium sized dog that
weighs between 16-27kg.
• These dogs have strong pectoral and leg muscles,
allowing them to move quickly in dense snow.
• If the Siberian Husky had heavier muscles, it
would sink deeper into the snow.
• This would slow them down and cause them to
sink and get stuck in the snow.
• However, if the Siberian Husky had lighter
muscles, they would not be strong enough to pull
sleds and equipment and therefore have little
value as a working dog.
Directional Selection
• In this type of selection, average individuals are
not favoured but rather one of the extremes is
best adapted for survival.
• This occurs as a result of a change in the
environment.
• Antibiotic resistant bacteria developed when
antibiotics became widely used.
• Insect resistance to pesticides occurred when the
use of pesticides became popular to increase
food production.
• Insects that have built a resistance to pesticides
are able to survive and produce more insects that
have the same resistance.
• Insects that lack the
resistance are unable to
stay “fit” and therefore
cannot survive and
reproduce.
• Directional selection is
influential in the
evolution and formation
of new species.
Disruptive Selection
• This type of selection process favours the extreme
traits in a population.
• Changing environmental conditions or the presence of
a natural predator creates a change in the species that
favours the extremes and excludes the average.
• Darwin observed this type of selection in the finch
population that he studied in the Galapagos Islands.
• Darwin noted that the finches shared similarities that
indicated that they derived from a common ancestor.
• Each of the finches had a different beak size that
showed an adaptation to the type of seeds found on
the island.
• The big beaks were suited
to retrieve bigger seeds
and the small beaks were
able to utilize the small
seeds.
• Medium sized beaks were
too large to retrieve the
small seeds and the beaks
were not strong enough to
crush the large seeds.
• Disruptive selection is
influential in the evolution
and formation of new
species.
Review Question
• Determine and explain the type of selection that
appears in figure 1.
• Determine and explain the type of selection that
appears in figure 2.
• Determine and explain the type of selection that
appears in figure 3.
• Note in all the images, the red line represents before
selection and the blue line represents after selection.
Answers
• Figure 1 is an example of stabilizing selection. The average tail
length is beneficial for the survival and reproduction of
lizards. Lizards with small tails and long tails have a lower
survival rate.
• Figure 2 is an example of disruptive selection. Each of the
extreme colours of shells, white or brown, is selected over the
average yellow. This means that the yellow shell has a lower
survival and reproductive success as compared to the white
and brown shell.
• Figure 3 is an example of directional selection. Only one
extreme is selected. Elephants with the longer trunk have a
higher success of reproduction and survival compared with
the other elephants
Sexual Selection
• What features do people
possess that attract you? Is it
the colour of their eyes, their
hairstyle, or the fancy clothes
that they wear?
• What will you look for in a
mate?
• Believe it or not, most
organisms possess some
physical feature that is geared
to attract the opposite sex
partner for reproduction.
• Think about the bright fancy
feathers that some birds
possess.
• Even though plants cannot select their mate, they
do need the help of birds and insects for pollination.
• Therefore the fragrant odour and colourful petals
are purposely exposed to attract organisms to help
spread the pollen from one plant to another.
• Another feature of sexual selection pits males
against males in terms of their ability to overpower
and dominate their territory.
• By governing an area, a male can attract and attain a
female for a mate.
• These qualities only exist in males and therefore are
not affected by the environment conditions.
Artificial Selection
• All the previous types of
natural selection occurred
naturally in an undisturbed
fashion.
• Artificial selection, as the
name implies, is the
manipulation of species due
to the involvement of
humans.
• Even Charles Darwin noted
this as the domestication of • Horse and dog breeding is a
plants and animals became
perfect example of how humans
popular.
can choose to selectively breed
• Today, artificial selection is
certain species together to
much more prevalent and
sophisticated than in Darwin's create a desired product.
era.
• Scientists have even taken this one step further
with the use of recombinant DNA in genetic
engineering and also in the process of nuclear
cloning.
• Researchers in the field of genetic engineering
are able to manipulate the DNA of an organism so
as to alter the gene.
• When this is done, a desired phenotype is now
expressed in the organism.
• Artificial selection accelerates the rate of
evolution as it eliminates the randomness of
mating.
• Essentially, humans are the ones that choose
which species are the fittest and which species
will be eliminated from the population.
• A genetically modified
(GM) plant, like corn that
produces a gene that acts
as a pesticide, is an
example of artificial
selection using
recombinant DNA.
• The GM corn has an
advantage over corn that
does not produce the
pesticide and is therefore
less likely to be affected
by insects or birds.
• This will reduce the nonmodified corns' ability to
survive and reproduce.
Further Reading
• http://www.biologyonline.org/2/10_natural_selection.htm
• http://www.biologyonline.org/2/11_natural_selection.htm
• Natural Selection Activity (available on the
Student-Teacher share drive: Mr. White Biology /
Unit 5)
Homework
• There are many hypotheses about creation and evolution
including those proposed by Aristotle, Charles Lyell, and
Baron Georges Cuvier. Research the theory and explain the
contributions that each of the following individuals have
made to creation and/or evolution:
–
–
–
–
Thomas Malthus
Jean-Baptiste Lamarck
Stephen Jay Gould
Niles Eldredge
• Charles Darwin is known as the father of evolution for his
research while on the Galapagos Islands. Darwin made
several conclusions about the process of evolution. Using
research skills determine the main conclusions that Darwin
made about evolution.