evolution, natural selection & selective pressures
TRANSCRIPT
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Evolution, Natural Selection
& Selective & Selective Pressures
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Page 1: Concept MapEvolution!Definition:
Natural Natural SelectionDefinition:
Genetic Variation:
Selective pressure:
Many Generations:
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Page 2: Categories of Natural Selection
Directional selection:
Stabilizing selection:
Disruptive selection:
What is it?
Example graph with the change
What is it?
Example graph with the change
What is it?
Example graph with the change
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What is Evolution?
Evolution = Change in allele frequency in a population over time (generations)over time (generations)
Ex: If any inherited trait is increasing (or decreasing) in frequency, the population is evolving
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How does Evolution Occur?
• Evolution occurs through a process called natural selectionselection
• Natural selection occurs when certain individuals are better able to survive and reproduce due to their phenotypes (traits)
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Theory of Evolution by Natural
Selection
• Charles Darwin was the first person to propose the idea of evolution viato propose the idea of evolution vianatural selection in 1859
• Many people before him had hypothesized about evolution, but he was the first person to propose a valid mechanism for how it works
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What is Needed for Nat. Selection to Occur?
1. A population containing genetic variation
2. Selective pressures in the 2. Selective pressures in the environment that cause certain individuals to have greater fitness(survival and reproduction)
3. MANY reproductive generations (time)
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What is a Selective Pressure?
• Selective pressures are any factors in the environmentthat affect the survival &/or that affect the survival &/or reproduction of individuals in a population
• Can be biotic (living) or abiotic (non-living)
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What is a Selective Pressure?
• Examples of bioticselective pressures:– Predators, – Predators,
competitors (organisms that compete for the same resources), pathogens….
Predation is
a biotic
selective
pressure
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What is a Selective Pressure?
• Examples of abioticselective pressures:– Temperature, – Temperature,
sunlight, humidity, rainfall, snow, wind, soil minerals, salinity, fire… Fire is an
abiotic
selective
pressure
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Monogenic Traits
• Some traits are monogenic –coded for by only one gene
• Ex: Widow’s peak, blood type, • Ex: Widow’s peak, blood type, earlobe attachment, many genetic disorders
• Monogenic traits appear as distinct categories, e.g. blood type A, B, AB or O
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Monogenic Traits
• Phenotype frequencies for monogenic traits are shown on simple bar graphsare shown on simple bar graphs
• If evolution is occurring, the phenotype frequencies will change over generations
3 phenotypes: brown OR black
OR red
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In itia l P opulation G eneration 10
G eneration 20 G eneration 30
Fre
quen
c y o
f
phenoty
pe
What do you notice in this population of lizards with three skin color
phenotypes?
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Initial Population Generation 10
Generation 20 Generation 30
Fre
que n
cy o
f ph
e not y
pe
• Evolution is occurring!occurring!
• What selective pressure could cause this selection for black lizards over red and brown?
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Polygenic Traits
• Most traits are polygenic – coded for by more than one gene
• Polygenic traits show a wide range of phenotypesphenotypes
• Polygenic traits are continuous,and NOT separate categories
• Ex: Height, eye color, skin color
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Polygenic Traits
2 5
3 0
3 5The fact that there is such a wide range of lengths indicates this trait is polygenic
0
5
1 0
1 5
2 0
2 5
2 0 m m 3 0 m m 4 0 m m
Length
Fre
qu
ency
lengths indicates this trait is polygenic
Polygenic traits generally have frequencies that follow a bell-shaped curve
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• What do you notice about the wingspan phenotypesafter 250
S e a G u l l P o p u la t io n - G e n e r a t io n 1
0
5
1 0
1 5
2 0
8 1 8 2 8 3 8 4 8 5 8 6 8 7 8 8 8 9 9 0 9 1 9 2 9 3 9 4 9 5
W i n g s p a n ( c m )
Fre
qu
en
cy
phenotypesafter 250 generations of sea gulls?
W i n g s p a n ( c m )
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• There has been selection for longerwingspans
S e a G u l l P o p u la t io n - G e n e r a t io n 1
0
5
1 0
1 5
2 0
8 1 8 2 8 3 8 4 8 5 8 6 8 7 8 8 8 9 9 0 9 1 9 2 9 3 9 4 9 5
W i n g s p a n ( c m )
Fre
qu
en
cy
W i n g s p a n ( c m )
0
5
10
15
20
25
30
81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97
Fre
qu
en
cy
Wingspan (cm)
Sea Gull Population - Generation 250
What selective pressure could cause this?
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Three Categories Of Natural Selection
• Directional (common)
• Stabilizing (common)
• Disruptive (uncommon)
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Directional Selection
•
Hawk vision ratings in population
before selection
Vision ratings after many generations –selection for better vision has occurred
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Stabilizing Selection
• When phenotypes in the middlegive the organism an advantage
• Ex: Human birth weight• Ex: Human birth weight• Low birth weight = infant mortality
• High birth weight = delivery complications
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Stabilizing Selection
Peak weight does NOT change over time since weights at the ends are selected against
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Disruptive Selection
• Selection in favor of phenotypes on both ends of the curve
• Ex: Short and long beaks when two sources of food are present
• Ex: Short and long beaks when two sources of food are present
• Short-beaked birds can eat the seeds more easily, long beaked birds can eat the insects more easily
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Disruptive Selection
Over many generations, birds with short and long beaks become more common, while
those in the middle die out