powerpoint presentation - natural...
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Natural Selection
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Natural Selection
Natural selection is the mechanism by which
evolution happens.
Thus, evolution and natural selection are inextricably
woven together.
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Natural SelectionThe 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
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Natural Selection is a two step process:
Step One: Production of variation (Chance)
Mutations
- review 1 gene = 1 protein protein synthesis
- form can’t be separated from function
Meiosis
- recombination due to crossing-over in 1st division
- random movement of chromosomes in both
Step Two: Non-random aspects of survival and
reproduction
Superior success of certain phenotypes
Nonrandom mate choice
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Recall the four basic aspects of natural selection:
1.Populations produce more offspring than the
environment can support.
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Recall the four basic aspects of natural selection:
2.Overpopulation leads to competition for limited
resources.
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Recall the four basic aspects of natural selection:
3.There is (genetic) variation within a population.
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Recall the four basic aspects of natural selection:
4.Those most able to secure resources (fittest)
disproportionately pass on their adaptive traits to the
next generation (survive to have more offspring.)
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Remember to Forget…
All your common misconceptions about evolution.
• evolution has a goal or end in mind
• evolution leads to perfection
• a single individual can evolve
• fittest means biggest, strongest, fastest
• it’s only about survival
• humans came from apes
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Selection pressure: force that tends to increase allele frequencies
associated with adaptive phenotypes.
• Competition for food
• Competition for a mate
• Changes in the environment
• Predators
• Parasites
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Normal Distribution
When graphed using a
histogram, data sets can
have different
distributions.
When data has no left
or right bias and is not
jumbled, it is said to
have a “normal
distribution” or bell
curve.
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Normal DistributionsThey aren’t always “perfect”
but you can see the trend.
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Main Types of Selection Pressures• Directional Selection
– Natural selection favors one extreme of the population for that trait
– often happens when environment changes in a consistent way- e.g.
climate gets colder.
• Disruptive Selection
– Natural selection favors both extremes selected
– Causes species to diverge
• Stabilizing Selection
– Natural selection favors the average for population selected
• Sexual Selection
– Mate choice
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Directional Selection• Neck of Giraffe
• Antibiotic resistance of bacteria
• Moth color (melanin)
• Camouflage/Mimics
• Many sexually selected traits
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Directional Selection
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Stabilizing Selection• When the extremes of the
trait aren’t as well suited
Examples
• Bird clutch size
• Elk Antlers size
• Human birth weight
• Tail length in birds
– Example of exception?
Explain!
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Stabilizing Selection
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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
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Disruptive Selection
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Every species’ ultimate fate…
Adapt to changing conditions and evolve into new species.
OR
Extinction without leaving decedents.