Download Natural Selection

Learning Objectives
• Be able to explain the 4 factors in Natural
Selection
• Be able to differentiate between Natural
Selection and Artificial Selection
Natural selection and Evolution
• Natural selection will lead to evolution
– when individuals with certain characteristics have a
greater survival or reproductive rate than other
individuals in a population
Natural selection =
Process by which traits
become more or less
common in a population
Natural Selection is…
• …..the logical result of four features of living
systems (VIST)
• Variation - individuals in a population vary from one
another
• Inheritance - parents pass on their traits to their
offspring genetically (overproduction)
• Selection - some variants reproduce more than
others (Survival of the fittest, competition,
overproduction)
• Time - successful variations accumulate over many
generations
V is for…..Variation
• Organisms within a population exhibit
individual variation in appearance and
behaviour
• Could be seen as “mutations”
– Ex: body size, hair color, voice properties
– Ex: webbed feet
I is for….Inheritance (of successful
variations)
• Organisms that survive and reproduce have the
chance to pass their traits to the next generation
The picture on the right
illustrates the crows
eating the green beetles
and not the brown ones.
Over time, the brown
beetles will be the ones
to survive and reproduce
passing on their traits to
the next generation
I is for….Inheritence
• Species produce far more eggs/seeds than
could be supported by ecosystem
• Necessary to ensure there would be enough
adults to continue the species
– Ex: female horseshoe crab lays 100, 000 eggs during
breeding season
• Only 1 in 130, 000 eggs survives
to adulthood
• Many eggs lost to predators
before hatching
S is for….Selection
• In order to have their traits passed down,
organisms have to struggle to survive
• Struggle for Existence (competition)
– Members of same species compete for food, water,
shelter, space and mates
• Those best suited for survival/competition had
advantage
– Ex: long and short neck
turtles competing for food
– Ex: feather train on male
peacocks to attract mate
S is for….Selection cont’d
• “Survival of the fittest”
– How well suited an organism is to its environment
• Fitness: ability of an individual to survive and
reproduce in its environment
• Adaptation: inherited characteristic that
increases an organism’s chance of survival
• Fitness is the result of adaptation
– ex: prey with some defense mechanism is more
“fit” than prey without a defense mechanism
T is for…. Time
• Overtime, the successful variations will
accumulate
Artificial Selection
• When humans choose traits and breed
organisms for that trait
• Also known as selective breeding
– Ex: dogs, plants
Natural Selection vs. Artificial Selection
• Natural Selection
– Dominated by environmental factors
– Where nature chooses organism with best traits
for survival
• Ex: tigers developed stripes to sneak up on prey
• Artificial Selection
– Controlled by humans, “goal-directed”
– Person chooses which traits of the organism they
want it to have
• Ex: dogs
• http://learn.genetics.utah.edu/content/variati
on/artificial/
Example…
• African elephants typically have large tusks. The ivory in the
tusks is highly valued by some people, so hunters have
hunted and killed elephants to tear out their tusks and sell
them (usually illegally) for decades.
• Some African elephants have a rare trait -- they never
develop tusks at all. In 1930, about 1 percent of all elephants
had no tusks. The ivory hunters didn't bother killing them
because there was no ivory to recover.
• Meanwhile, elephants with tusks were killed off by the
hundreds, many of them before they ever had a chance to
reproduce.
• The alleles for "no tusks" were passed along over just a few
generations. The result: As many as 38 percent of the
elephants in some modern populations have no tusks
• Would you say this is a case of Natural Selection or Artificial
Selection and why?
• The bollworm is a pest that eats and damages
cotton crops
• Some cotton crops have been genetically modified
to produce a toxin that's harmful to most
bollworms.
• A small number of bollworms had a mutation that
gave them immunity to the toxin. They ate the
cotton and lived, while all non-immune bollworms
died.
• The intense population pressure has produced
broad immunity to the toxin in the entire species
within the span of just a few years
• Describe what the 4 factors of natural selection are
in this example
• How is artificial selection dependent on
variation in nature?