Natural Selection
The unequal survival and reproduction of organisms due to
environmental forces, resulting in the preservation of
favorable adaptations.
It is a two step process:
1. The Production of variation in a population
2. Non-random aspects of survival and reproduction
Natural Selection is a two step process:
Step One: The Production of Variation. (Chance)
1.Mutations – not common, source of new alleles and genes
2.Meiosis - source of most variation (in sexual organisms), via new allele
combinations.
3.Random mate selection & fertilization
Why Sex? Advantages of Sexual Reproduction (Video)
Are there disadvantages to sexual reproduction?
Amazonian Frogs
Step Two: Non-random aspects of survival and
reproduction
1. Superior success of certain phenotypes when selection
pressure is applied
2. Nonrandom mate choice
Humans select traits for dogs, pigeons and other animals when
they breed them.
Bred Pigeons came from a single original species
Who selects the traits for wild plants & animals?
NOBODY!!
There is no agent involved in natural selection.
Natural selection is a process of elimination in response
to selection pressures like:
•Competition for food
• Competition for a mate
• Changes in the environment
• Predators
• Parasites
Main Types of Selection Pressures
• Directional Selection
– Occurs when selection favours a more extreme
variation of a trait
– often happens when environment changes in a
consistent way- - e.g. hummingbirds move into a
climate that has plants with longer flowers (normally
medium length bills are favoured, now long bills are
favoured).
Directional Selection
Other examples:
• Neck of Giraffe
• Antibiotic resistance of bacteria
• Moth color (melanin)
• Camouflage/Mimics
• Many sexually selected traits
Directional Selection: Mimicry
(mimic environment)
Pre-industrial revolution
Post-industrial revolution
Main Types of Selection Pressures
• Directional Selection
– Occurs when selection favours a more extreme
variation of a trait
• Stabilizing Selection
– occurs when the average phenotype is favoured by the
environment. Often happens in an environment that
doesn’t change.
– e.g. Medium-billed hummingbirds most common.
Longer bill requires more food & energy to grow &
carry around, shorter bill = reduced ability to reach
food within flowers.
Stabilizing Selection
• When the extremes of the
trait aren’t as well suited
Examples
• Human birth rates are
subject to stabilizing
selection
Main Types of Selection Pressures
• Directional Selection
– Occurs when selection favours a more extreme variation of a trait
• Stabilizing Selection
– occurs when the average phenotype is favoured by the
environment. Often happens in an environment that doesn’t
change.
• Disruptive Selection
– both (opposite) extremes of a trait are favoured over individuals
with the intermediate variations.
– e. g. 2 species of plants with different-sized flowers may be
available for hummingbirds – based on flower size, not well suited
for medium-length bill, so
__________________________________
• Sexual Selection
Disruptive Selection
• Causes divergence within the
species
• Occurs when two different
types of resources in one
area
• Results in specialization for
each branched group
• May lead to formation of
new species
• E.g. Darwin’s Finches
Main Types of Selection Pressures
• Directional Selection
– Occurs when selection favours a more extreme variation of a trait
• Stabilizing Selection
– occurs when the average phenotype is favoured by the environment. Often
happens in an environment that doesn’t change.
• Disruptive Selection
– both (opposite) extremes of a trait are favoured over individuals with the
intermediate variations.
• Sexual Selection
– the favouring of any trait that specifically enhances the mating
success of an individual. Often leads to males & females of a
species evolving different appearances and behaviours. E.g.
colouration and behaviours -> downfall!! Avoiding predators !
Evolution through sexual selection
• Why does the peacock have
such a beautiful tail?
Darwin's theory of sexual
selection - nature's
extravagances (like peacock's
tail) are advantages in the
mating game.
• peahens choose their mates
by the size and shape of his
tail.
• Makes sense in evolutionary
terms - largest tail would
indicate healthy bird and
better chance for healthy
offspring.
Sexual Selection
What do we select for in a mate and
how does that affect evolution?
Sweaty T-Shirt Experiment
Sexual
Selection:
decreased
survivorship
What type of selection is it?
• A Mutation Story - Sickle Cell Anemia:
This segment from the PBS series Secret of
Life: "Accidents of Creation.", describes the
role of the sickle cell gene in natural
selection.
Co-evolution
• Two species can evolve together, each
selecting for traits in the other (try to “oneup” each other)
• Futurama Arms Race Video
• An evolutionary arms race can occur (drug
resistant microbes)
• Evolution driven by another species video
• Organisms can evolve together in symbiosis
– Leaf cutter ants video
Evolutionary Change without Selection
• Genetic drift – the random shifting of the
genetic makeup of the next generation
• Genetic bottlenecks – result in a loss in
genetic diversity following an extreme
reduction in the size of the population
(following a natural disaster, over-hunting,
etc)
• Founder effect – occurs when individuals
establish a new population (the finches
moving to the Galapagos islands)
Hardy-Weinberg Principle
• In large populations, where only random
chance is at work, allele frequencies are
expected to REMAIN THE SAME
• As soon as a different factor other than
chance is introduced, evolutionary change
will occur
• What are the different factors that could be
introduced to cause evolutionary change?
Use text handout to explain
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Natural selection
Small population size
Mutation
Immigration/emmigration
Horizontal gene transfer
• Activity: Modeling Genetic drift