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Evolution:
Change in the heritable (genetic)
traits of a population over time
Note: when we discuss evolution, we are talking about
populations changing, not individuals
Natural selection:
The main mechanism by
which evolution occurs
4 Conditions for Natural Selection:
1. Variation: Individuals in a population
are not identical to each other.
2. Inheritance: Traits are passed to
offspring; traits have a genetic basis
3. Environmental population limits:
Environmental limiting factors prevent
all individuals from surviving to
reproduce; some die young.
4. Environmental selection:
• Individuals in the population with more
favorable (advantageous) traits are the
ones that survive to reproduce.
• Individuals without advantageous traits
die before reproducing.
These factors result in a change in the
average trait of the population…
Biologists call this EVOLUTION!
1. VARIATION
Members of a population have traits similar to
the average trait of the entire population, but
they are not identical.
YOUR TURN:
Using height as
an example,
sketch a graph to
represent the
statement above.
Mean (average) height
Frequency
•
Height (cm)
2. INHERITANCE
• DNA determines the traits of individuals
DNA
mRNA
protein
trait
• Individuals inherit DNA from their parents
• This causes the traits of the offspring to
resemble the traits of the parents
3. ENVIRONMENTAL
POPULATION LIMITS
Population
A. For all species, if every individual born into a
population were to reproduce, the population
would grow exponentially
Time
3. ENVIRONMENTAL
POPULATION LIMITS
Population
B. Environmental factors (limiting factors) prevent
the majority of individuals from surviving to
reproduce
Time
4. SELECTION
•
Individuals without advantageous traits
die before reproducing.
Individuals with advantageous traits
survive to reproduce.
Frequency
•
These individuals
survive to reproduce
Characteristic
Populations change, not individuals
Frequency
• The “average” characteristic or other measure of
the population changes over generations
These individuals
survive to reproduce
Characteristic
The environment is the selective
force behind evolution
• The environment determines what
characteristics are “favorable”
• Because the environment changes over
time, the characteristic that is more
favorable for a population changes
• Therefore, characteristics of the population
change, or evolution occurs
Evolution of Peppered Moths
Dark form
Peppered form
Explain the evolution of color in
the peppered moth population.
1. Describe what trait evolved.
2. Describe how each of the 4 conditions for
natural selection exists in the moth example:
1.
2.
3.
4.
Variation
Inheritance
Population limits
Selection
• The population of peppered moths became
darker in color near industrial areas.
• Originally, some moths were dark, some moths
were peppered (light).
• Color is determined by genes inherited from
parents.
• Not all moths survive to reproduce- some are
eaten by birds.
• Pollution killed lichen and made the environment
darker, so darker moths were better
camouflaged and not eaten as much as
peppered (light) moths.
• This caused the average color of the moth
population to become darker