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Theory of Evolution by Natural Selection
NATURAL SELECTION is the way in which nature favours the reproductive success of some
individuals within a population over others
PROCESS OF NATURAL SELECTION (variation , selection, inheritance, time)
1. VARIATION WITHIN POPULATIONS: Individuals within a population can contain many
different traits/adaptations. Genetic variation occurs by chance through genetic mutations
and recombination.
2. SELECTION: Through competition or change the environment can act upon individuals
differently depending on the traits they posses. Healthy populations produce far more
offspring than could survive and reproduce.
3. INHERITANCE: parents pass on their traits to their offspring genetically. As a result of
selective pressures, some individuals would have a better chance of survival
4. TIME: Since traits can be heritable, those that survive live long enough can reproduce and
pass on their traits. Over time the POPULATION CHANGES. The more advantageous trait,
which allows for survival and more offspring, will become more common in the population.
If this process continues, eventually, most individuals in the population will have the
advantageous trait
Darwin’s Observations: Finches
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An ancestral population of medium-size billed ground finches are amongst the first
seed eating birds to arrive on the
Galapagos Islands.
These birds had access to all types of
seeds ( mostly large and some medium)
and had little to no competition
Over time, the finch population grew to
it maximum carrying capacity. New born
finches born with variability (some
slightly lager bills and some slightly
smaller than average sized bills) were
exposed to competition within the
species
Most of the birds had to compete for
the medium sized seeds, while those with the larger bills were able to utilize the larger
seeds as a food source
Generation after generation, the average bill size continues to increase.
MODERN TAKES ON THE THEORY
Survival of the fittest is a term used to describe the process of natural selection (Herbert Spencer,
a contemporary of Darwin coined this term). An individual that is considered fit, has adaptations
that allow it to survive longer and favour reproduce success.
Adaptation is a result of a characteristic or feature of a species that makes it well suited for
survival or reproductive success in its environment
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Examples:
 A white coat of a polar bear provide an adaptation for camouflage
 The long snout of an anteater provides it with an adaptation to better access its food
source
 The thick blubber of a seal provides it with an adaptation to survive colder
temperatures of its habitat
 The size and shape of a ground-finch’s beak determines which food sources it can
access and provide an adaptation that makes them suitable for specific niches ( a
habitat suitable for its behaviour and food preferences)
TYPES OF SELECTION:
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Directional selection:
o Selection that occurs when selection favours individuals with a more EXTREME
variation of a trait. The result is a shift in the average condition
o Example: The beak size of the ground finches was a result of directional selection
toward the larger beak size.
o Example: Hummingbirds exposed to a new habitat with longer flowers. Individuals
with slightly longer bills will be more successful or “fit” in comparison to those with
small or medium bills as the environmental pressures favour long bills.
Stabilizing selection:
o Occurs when the AVERAGE phenotype within a population is favoured by the
environment. Selection against individuals exhibiting traits that deviate from the
current population average occurs.
o Example: Hummingbirds in an unchanging environment with medium flowers, the
most common medium-sized bills would be favoured as longer bills may require
more energy to grow and carry around and short bills limit access to food.
Disruptive selection:
o Selection that favours individuals with OPPOSITE EXTREMES of a trait over the
individuals with the intermediate variations.
o Example: If a habitat exists where flowers can be found with both small and large
flowers, then hummingbirds with either long or short bills. Neither plant is ideal for
the medium-sized bill, decreasing their survival and ability of reproduction
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Sexual selection:
Differential reproductive success caused by variation in the ability to obtain mates,
resulting in sexual dimorphism (males and females evolving distinct appearances and
behaviour) and mating and courtship behaviours.
Most common types of sexual selection are female selection and male vs. male
competition.
o Colouring: Male peacocks have brighter plumage than female counterparts in order
to attract them as a mate
o Body size: Male deer are larger than females so as to better compete during mating
rituals
o Physical attributes: the horns of male big-horn sheep are substantially larger than
the females. Males require big horns to compete head-to-head clashes.
o Mating behaviour: the male bower bird creates elaborate nesting areas or
“bowers” to attract a female. The most impressive bowers are those that are
largest and with the most trinkets (often blue). Disadvantages include decreased
longevity as a result of the brighter colours or risky behaviour
NATURAL SELECTION IN ACTION:
Directional selection in Tibetan population
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Directional selection in a human population. Tibetan people have inhabited the Himalayan
Mountains for thousands of years. Oxygen levels average at 40% of that found at sea level.
Short term adaptation of people from the lowlands results in increased red blood
production. This over time can be harmful as it makes the blood more viscous, causing
extra stress on the heart, reduced infertility and higher child mortality.
Tibetans do not exhibit high red blood cell counts, but do not have issues coping at these
high altitudes. Geneticists have discovered a genetic mutation that increases the oxygencarrying capacity in their blood, which has been selected for.
Antibiotic resistance in bacteria:
o Antibiotic resistance means that bacteria can survive in the presence of an
antibiotic
o Penicillin was used to treat many infections, but today it is rarely used do to the
fact that many bacteria have acquired resistance to it
o This resistance has been noted in several other instances with more newly
developed antibiotics. Some strains of tuberculosis are resistant to ALL known
antibiotics. Treating cholera has now been reduced to a single effective antibiotic.
o Resistance is thought to be a result of an existing mutation. This mutation allows
the mutated bacteria to survive and then reproduce, thus creating a population
resistant to the antibiotic
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Industrial Melanism: The Peppered moth
o Evolution of this moth (the onset of melanism, darker colour) is thought to be a
result of an environmental change.
o Industrial melanism is the darkening of several species of moths during the recent
period of industrialization in many
countries
o Before 1848, trees within their habitat
were covered in off-white lichens, which
allowed for the peppered moths to
camouflage.
o Due to coal mining in surrounding areas,
the trees became covered in soot, and the
peppered moths became easy targets for
prey. The black form became common.
Optional video: http://www.youtube.com/watch?v=JakdRczkmNo
QUESTIONS:
1. Name the 6 steps in the process of Natural Selection?
2. What is meant by survival of the fittest? Who coined this term?
3. What is an adaptation?
4. What are the different types of selection? Provide an example for each.
5. What are three examples of selection in action?
6. What is industrial melanism? Describe the mechanism of natural selection in the peppered
moth.