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Unit 7: Natural Selection
7.1 Introduction to Natural Selection
ESSENTIAL KNOWLEDGE
Describe the causes of natural selection
❖ Natural selection is a major mechanism of evolution
❖ According to Darwin’s theory of natural selection, competition for limited resources ->
differential survival
➢ Individuals with more favorable phenotypes are more likely to survive and produce
more offspring, thus, passing traits to subsequent generations
Explain how natural selection affects populations
❖ Evolutionary fitness is measured by reproductive success
❖ Biotic and abiotic environments can be more or less stable/fluctuating
➢ Affects the rate and direction of evolution
➢ Different genetic variations can be selected in each generation
DAILY VIDEOS
Natural selection is a major mechanism of evolution
● Evolution - change in the genetic makeup of a population over time
● Natural selection - process by which organisms, having adaptations suited for a particular
environment, have a greater chance of survival and reproduction, thereby passing the
adaptations to subsequent generations
○ Charles Darwin is mainly credited with the development of the theory of natural
selection
○ There are several conditions that must be met for evolution by natural selection
○ Influenced by environmental stability, genetic variation, adaptations, and fitness
○ Result in increase or decrease of specific phenotypes within a population over time
There is a competition for limited resources
● Not all organisms in a population have the same chance of survival
● Competition - when organisms struggle with other organisms to get limited resources
○ Organisms will compete for resources (e.g. space, food, mates, nutrients, light)
○ Differences in phenotypes will determine how competitive an organism is
■ Can lead to differential survival
Favorable phenotypes improve chances of survival
● Variation - genetic differences among organisms within a population
○ Mutations and sexual reproduction INCREASE variation within populations
● Adaptations - traits that provide an ADVANTAGE in a particular environment
○ Individuals with adaptations have a greater chance of survival and reproduction
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UNIT 7and
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GUIDE.pdf Explanations
KEY: Standard, Extra Notes, Essential Knowledge, AP Videos (not in essential knowledge), Questions
○ Number of individuals within a population having adaptations can increase over time
Evolutionary fitness is measured by reproductive success
● Fitness - ability of an organism to survive and produce fertile offspring
● Reproductive success - production of offspring
○ Reproductive success over several generations is a component of evolutionary
fitness
● Heritability - ability to pass on adaptations to successive generations
○ Due to selection, the traits of individuals that have more reproductive success
become more common in the population
○ Reproductive success and heritability of adaptations contribute to evolutionary
fitness
Ecosystem stability determines the rate and direction of evolution
● Biotic and abiotic environments can remain more or less stable
○ Environments can experience major disruptions and change quickly or change slowly
over time
○ Populations are less likely to evolve in environments that remain stable over long
periods of time
○ More unstable the environment -> faster rate of evolution
EXTRA NOTES
● Evolution - change ot gene pool over time (sustained change – never going to return to its
normal population gene pool)
○ Driven by BOTH random occurrences AND natural selection
○ Only 1 mechanism needs to occur
● Equilibrium: when the population’s gene pool is not seeing a sustained change
○ To maintain, all 5 conditions must be met
● Reducing variation in 2 different populations -> cause populations to be more different from
each other
5 Fingers of Evolution
1. Pinky: Genetic Drift – small population is more likely to undergo changes in its gene pool due
to random events (1 individual represents a large portion)
2. Ring: Nonrandom Mating – sexual selection changes populations by allowig some individuals to
apss on their genes more foten to create the next generation than others
3. Middle: Mutations – random changes to DNA of an individual in a population can result in a
new trait that is passed on in new generations
a. Impossible to not have mutations
b. Evolution or equilibrium: depends if mutation can be sustained or is lethal
4. Pointer: Gene Flow/Migration – individuals entering or leaving a population can end up
changing the percept of people with a certain trait
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Page 3? of
Clear,
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AP BIO
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Easy-to-Understand
Coverage
7 STUDYof GUIDE.pdf
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ExplanationsAP
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UNIT 7and
STUDY
Easy-to-Understand
GUIDE.pdf Explanations
KEY: Standard, Extra Notes, Essential Knowledge, AP Videos (not in essential knowledge), Questions
5. Thumb: Natural Selection – unfavorable traits are removed leaving the best adapted
individuals to reproduce and pass on their traits
a. Only mechanism that can create adaptations -> help with survival & reproductive
fitness of organism
★ Nonrandom: Natural selection, nonrandom mating
★ Random: Genetic drift, gene flow, mutations
5 Components of Darwin’s Theory of Evolution
1. Genetic HERITABILITY variations exist within individuals of a population
a. Mutations
b. Sexual Reproduction: random mating, random fertilization, gamete formation
i. Gamete formation: crossing over, independent assortment
2. All species undergo a struggle to exist
a. Biotic (living): predators, diseases
b. Abiotic (nonliving): natural disasters
c. Human influences: hunting, war
3. Environmental factors will eliminate individuals with the least favorable variations
(Natural Selection)
a. Cause: something in nature is eliminating individuals with unfavorable traits
4. Those individuals that are best adapted are more likely to reproduce more offspring for
the next generation because they survive longer (Survival of the Fittest)
a. Fitness is determined by # of offspring that survive to reproductive age
5. After many generations, the population will be much more adapted in its environment
(until the environment changes again) (Descent with modification)
a. Small generational improvements add up overtime to create a population that is very
different from its ancestors and can create different species if:
i. Natural selection removed unfavorable traits
ii. Less competition and struggle so they survive longer & reproduce more
iii. Descendants have some variations that were better adapted continue
7.2 Natural Selection
ESSENTIAL KNOWLEDGE
Describe the importance of phenotypic variation in a population
❖ Natural selection acts on phenotypic variations in populations
❖ Environments change and apply selective pressures to populations
❖ Some phenotypic variations significantly increase or decrease fitness of the organism in
particular environments
DAILY VIDEOS
Natural selection acts on phenotypic variations in populations
? Exam-Focused Answers ?for
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Page 1? of
Clear,
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Comprehensive
AP BIO
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Easy-to-Understand
Coverage
7 STUDYof GUIDE.pdf
All Key
ExplanationsAP
Topics ? Clear,
BIOConcise,
UNIT 7and
STUDY
Easy-to-Understand
GUIDE.pdf Explanations
KEY: Standard, Extra Notes, Essential Knowledge, AP Videos (not in essential knowledge), Questions
Unit 7: Natural Selection
7.1 Introduction to Natural Selection
ESSENTIAL KNOWLEDGE
Describe the causes of natural selection
❖ Natural selection is a major mechanism of evolution
❖ According to Darwin’s theory of natural selection, competition for limited resources ->
differential survival
➢ Individuals with more favorable phenotypes are more likely to survive and produce
more offspring, thus, passing traits to subsequent generations
Explain how natural selection affects populations
❖ Evolutionary fitness is measured by reproductive success
❖ Biotic and abiotic environments can be more or less stable/fluctuating
➢ Affects the rate and direction of evolution
➢ Different genetic variations can be selected in each generation
DAILY VIDEOS
Natural selection is a major mechanism of evolution
● Evolution - change in the genetic makeup of a population over time
● Natural selection - process by which organisms, having adaptations suited for a particular
environment, have a greater chance of survival and reproduction, thereby passing the
adaptations to subsequent generations
○ Charles Darwin is mainly credited with the development of the theory of natural
selection
○ There are several conditions that must be met for evolution by natural selection
○ Influenced by environmental stability, genetic variation, adaptations, and fitness
○ Result in increase or decrease of specific phenotypes within a population over time
There is a competition for limited resources
● Not all organisms in a population have the same chance of survival
● Competition - when organisms struggle with other organisms to get limited resources
○ Organisms will compete for resources (e.g. space, food, mates, nutrients, light)
○ Differences in phenotypes will determine how competitive an organism is
■ Can lead to differential survival
Favorable phenotypes improve chances of survival
● Variation - genetic differences among organisms within a population
○ Mutations and sexual reproduction INCREASE variation within populations
● Adaptations - traits that provide an ADVANTAGE in a particular environment
○ Individuals with adaptations have a greater chance of survival and reproduction
? Exam-Focused Answers ?for
Comprehensive
Maximum Marks
Coverage
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Key Topics ? Students
Clear, Concise,
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and Easy-to-Understand Explanations
,? Comprehensive Coverage of All Key Topics
Page 2? of
Clear,
20? Concise,
Comprehensive
AP BIO
and UNIT
Easy-to-Understand
Coverage
7 STUDYof GUIDE.pdf
All Key
ExplanationsAP
Topics ? Clear,
BIOConcise,
UNIT 7and
STUDY
Easy-to-Understand
GUIDE.pdf Explanations
KEY: Standard, Extra Notes, Essential Knowledge, AP Videos (not in essential knowledge), Questions
○ Number of individuals within a population having adaptations can increase over time
Evolutionary fitness is measured by reproductive success
● Fitness - ability of an organism to survive and produce fertile offspring
● Reproductive success - production of offspring
○ Reproductive success over several generations is a component of evolutionary
fitness
● Heritability - ability to pass on adaptations to successive generations
○ Due to selection, the traits of individuals that have more reproductive success
become more common in the population
○ Reproductive success and heritability of adaptations contribute to evolutionary
fitness
Ecosystem stability determines the rate and direction of evolution
● Biotic and abiotic environments can remain more or less stable
○ Environments can experience major disruptions and change quickly or change slowly
over time
○ Populations are less likely to evolve in environments that remain stable over long
periods of time
○ More unstable the environment -> faster rate of evolution
EXTRA NOTES
● Evolution - change ot gene pool over time (sustained change – never going to return to its
normal population gene pool)
○ Driven by BOTH random occurrences AND natural selection
○ Only 1 mechanism needs to occur
● Equilibrium: when the population’s gene pool is not seeing a sustained change
○ To maintain, all 5 conditions must be met
● Reducing variation in 2 different populations -> cause populations to be more different from
each other
5 Fingers of Evolution
1. Pinky: Genetic Drift – small population is more likely to undergo changes in its gene pool due
to random events (1 individual represents a large portion)
2. Ring: Nonrandom Mating – sexual selection changes populations by allowig some individuals to
apss on their genes more foten to create the next generation than others
3. Middle: Mutations – random changes to DNA of an individual in a population can result in a
new trait that is passed on in new generations
a. Impossible to not have mutations
b. Evolution or equilibrium: depends if mutation can be sustained or is lethal
4. Pointer: Gene Flow/Migration – individuals entering or leaving a population can end up
changing the percept of people with a certain trait
? Exam-Focused Answers ?for
Comprehensive
Maximum Marks
Coverage
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Key Topics ? Students
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, ? Comprehensive Coverage of All Key Topics
Page 3? of
Clear,
20? Concise,
Comprehensive
AP BIO
and UNIT
Easy-to-Understand
Coverage
7 STUDYof GUIDE.pdf
All Key
ExplanationsAP
Topics ? Clear,
BIOConcise,
UNIT 7and
STUDY
Easy-to-Understand
GUIDE.pdf Explanations
KEY: Standard, Extra Notes, Essential Knowledge, AP Videos (not in essential knowledge), Questions
5. Thumb: Natural Selection – unfavorable traits are removed leaving the best adapted
individuals to reproduce and pass on their traits
a. Only mechanism that can create adaptations -> help with survival & reproductive
fitness of organism
★ Nonrandom: Natural selection, nonrandom mating
★ Random: Genetic drift, gene flow, mutations
5 Components of Darwin’s Theory of Evolution
1. Genetic HERITABILITY variations exist within individuals of a population
a. Mutations
b. Sexual Reproduction: random mating, random fertilization, gamete formation
i. Gamete formation: crossing over, independent assortment
2. All species undergo a struggle to exist
a. Biotic (living): predators, diseases
b. Abiotic (nonliving): natural disasters
c. Human influences: hunting, war
3. Environmental factors will eliminate individuals with the least favorable variations
(Natural Selection)
a. Cause: something in nature is eliminating individuals with unfavorable traits
4. Those individuals that are best adapted are more likely to reproduce more offspring for
the next generation because they survive longer (Survival of the Fittest)
a. Fitness is determined by # of offspring that survive to reproductive age
5. After many generations, the population will be much more adapted in its environment
(until the environment changes again) (Descent with modification)
a. Small generational improvements add up overtime to create a population that is very
different from its ancestors and can create different species if:
i. Natural selection removed unfavorable traits
ii. Less competition and struggle so they survive longer & reproduce more
iii. Descendants have some variations that were better adapted continue
7.2 Natural Selection
ESSENTIAL KNOWLEDGE
Describe the importance of phenotypic variation in a population
❖ Natural selection acts on phenotypic variations in populations
❖ Environments change and apply selective pressures to populations
❖ Some phenotypic variations significantly increase or decrease fitness of the organism in
particular environments
DAILY VIDEOS
Natural selection acts on phenotypic variations in populations
? Exam-Focused Answers ?for
Comprehensive
Maximum Marks
Coverage
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