Evolution, Natural Selection & Selective & Selective Pressures
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Evolution, Natural Selection
& Selective & Selective Pressures
Page 1: Concept MapEvolution!Definition:
Natural Natural SelectionDefinition:
Genetic Variation:
Selective pressure:
Many Generations:
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
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
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)
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
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)
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)
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
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
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
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
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?
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?
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
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
• 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 )
• 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?
Three Categories Of Natural Selection
• Directional (common)
• Stabilizing (common)
• Disruptive (uncommon)
Directional Selection
•
Hawk vision ratings in population
before selection
Vision ratings after many generations –selection for better vision has occurred
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
Stabilizing Selection
Peak weight does NOT change over time since weights at the ends are selected against
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
Disruptive Selection
Over many generations, birds with short and long beaks become more common, while
those in the middle die out
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