EVO 2.2 Selective Mechanisms- Pre AP Biology Study Notes - New Syllabus.

EVO 2.2 Selective Mechanisms- Pre AP Biology Study Notes

EVO 2.2 Selective Mechanisms- Pre AP Biology Study Notes – New Syllabus.

LEARNING OBJECTIVE

EVO 2.2(a) Describe how selective pressures in the environment can affect an organism’s fitness.
EVO 2.2(b) Explain how selective pressures in the environment could cause shifts in phenotypic and/or allele frequencies.
EVO 2.2(c) Use data to describe how changes in the environment affect phenotypes in a population.
EVO 2.2(d) Predict how allelic frequencies in a population shift in response to a change in the environment.

Key Concepts:

EVO 2.2.1 Darwin’s theory of natural selection is that a selective mechanism in biological evolution may lead to adaptations.
a. Abiotic ecosystem components (e.g., nutrients) and biotic ecosystem components (e.g., predators) act as selective pressures.
b. Favorable traits in a given environment lead to differential reproductive success, or fitness, and over time can produce changes in phenotypic and/or allele frequencies.
c. Heritable traits that increase an organism’s fitness are called adaptations.
d. Over time, the relative frequency of adaptations in a population’s gene pool can increase.
e. Patterns of natural selection can include phenomena such as coevolution, artificial selection, and sexual selection.
EVO 2.2.2 Favorable traits are relative to their environment and subject to change.
a. Changes in the environment happen both naturally (e.g., floods, fires, climate change) and through human-induced activities (e.g., pollution, habitat destruction, climate change).

Pre AP Biology-Concise Summary Notes- All Topics

How Selective Pressures in the Environment Affect an Organism’s Fitness

🌱 Introduction

Organisms live in environments that constantly challenge survival and reproduction.
These challenges are called selective pressures.

Selective pressures determine which individuals survive, reproduce, and pass on their genes, thereby affecting an organism’s fitness.

🧬 Key Definitions

Selective Pressure

Any environmental factor that affects an organism’s ability to:

Survive
Reproduce

Selective pressures can favor some traits while disadvantaging others.

Fitness

Fitness is an organism’s reproductive success.

Measured by:

Ability to survive to reproductive age
Number of viable offspring produced

Fitness is relative, not absolute.
An organism is fit only in a specific environment.

🌍 Types of Selective Pressures

Abiotic Selective Pressures (Non-living Factors)

These are physical or chemical components of the environment.

Examples:

Temperature
Water availability
Nutrient levels
Light intensity
pH
Salinity

Abiotic pressures affect:

Metabolism
Growth
Survival

Biotic Selective Pressures (Living Factors)

These involve interactions with other organisms.

Examples:

Predators
Competitors
Parasites
Pathogens
Availability of food or mates

Biotic pressures affect:

Survival chances
Reproductive opportunities

🧠 How Selective Pressures Affect Fitness

Favorable Traits

Traits that improve survival or reproduction under specific conditions.

Individuals with these traits:

Survive longer
Reproduce more
Pass on these traits more frequently

Result → Higher fitness

Unfavorable Traits

Traits that reduce survival or reproductive success.

Individuals with these traits:

Die earlier
Produce fewer offspring
Contribute less to the next generation

Result → Lower fitness

Environment-Specific Fitness

A trait that increases fitness in one environment may reduce fitness in another.

Fitness changes if the environment changes.

There is no universally “best” trait.

🌿 Fitness and Natural Selection

Natural selection acts on existing variation
Selective pressures do not create new traits
They determine which traits are favored

Individuals do not evolve.
Populations evolve over generations.

🧬 Key Concept

Abiotic and biotic factors act as selective pressures
Favorable traits lead to:
- Differential survival
- Differential reproduction

This difference in reproduction = fitness differences

📊 Summary Table

Factor	Effect on Fitness
Favorable trait	Increases fitness
Unfavorable trait	Decreases fitness
Abiotic pressure	Affects survival
Biotic pressure	Affects survival & reproduction
Environmental change	Alters which traits are favored

📦 Quick Recap
✔ Selective pressures are environmental challenges
✔ Fitness = reproductive success
✔ Abiotic and biotic factors act as selective pressures
✔ Favorable traits increase fitness
✔ Unfavorable traits reduce fitness
✔ Fitness is environment-specific
✔ Natural selection acts on variation

How Selective Pressures Cause Shifts in Phenotypic and Allele Frequencies

🌱 Introduction

Populations show variation in traits.
When the environment applies selective pressures, some traits provide an advantage while others do not.

Over generations, this leads to changes in phenotypes and allele frequencies within a population.
This process is a core mechanism of natural selection.

🧬 Key Definitions

Phenotypic Frequency

The proportion of individuals showing a specific observable trait.

Example: Percentage of light-colored vs dark-colored individuals

Allele Frequency

The proportion of a particular allele in the population’s gene pool.

Calculated across all individuals, not just those expressing the trait.

Selective Pressure

Any abiotic or biotic environmental factor that affects survival or reproduction.

Selective pressures act on phenotypes, but evolution is measured as allele frequency change.

🧠 Core Principle

Natural selection changes allele frequencies by favoring phenotypes that increase fitness in a given environment.

🌿 How Selective Pressures Cause Phenotypic Shifts

Step 1: Variation Exists

Individuals differ in traits due to genetic variation
Different phenotypes exist in the population

Step 2: Environment Applies Pressure

Examples:

Predators
Limited food
Climate conditions
Disease

These pressures do not act equally on all phenotypes.

Step 3: Differential Survival and Reproduction

Individuals with advantageous phenotypes:

Survive longer
Reproduce more

Individuals with disadvantageous phenotypes:

Leave fewer offspring

Step 4: Phenotypic Frequency Shifts

Favorable phenotypes become more common
Unfavorable phenotypes become less common

This shift is observed across multiple generations, not immediately.

🧬 How Phenotypic Shifts Lead to Allele Frequency Changes

Important Link

Phenotypes are controlled by genes
Genes are inherited as alleles

Process of Allele Frequency Change

Favorable phenotype → associated allele passed on more often
Alleles linked to low fitness → passed on less often

Over time:

Frequency of advantageous alleles increases
Frequency of disadvantageous alleles decreases

This is evolution at the population level.

🌱 Adaptations and Allele Frequency

A heritable trait that increases fitness is an adaptation.

As selection continues:

Alleles responsible for adaptations accumulate
Population becomes better suited to its environment

🌿 Patterns of Natural Selection That Shift Frequencies

1. Directional Selection

One extreme phenotype is favored
Mean phenotype shifts in one direction
Alleles for favored trait increase

2. Stabilizing Selection

Intermediate phenotype favored
Extremes selected against
Reduces variation, maintains allele balance

3. Disruptive Selection

Both extremes favored
Intermediate selected against
Can increase variation and lead to speciation

🌍 Environment-Dependent Nature of Selection

What is favorable depends on the current environment
Environmental change can:
- Reverse selection
- Favor previously rare alleles
- Alter phenotypic distribution

There are no permanently superior traits.

🧠 Important Clarifications

Selection acts on phenotypes, not directly on alleles
Individuals do not evolve
Populations evolve through allele frequency changes
Traits must be heritable to affect evolution

📊 Summary Table

Process	Result
Selective pressure	Unequal survival & reproduction
Favorable phenotype	Increases in frequency
Unfavorable phenotype	Decreases in frequency
Advantageous allele	Becomes more common
Population over time	Evolves

📦 Quick Recap
✔ Selective pressures favor certain phenotypes
✔ Phenotypes linked to higher fitness increase
✔ Alleles controlling favorable traits increase in frequency
✔ Unfavorable alleles decrease over generations
✔ Evolution = change in allele frequencies
✔ Selection acts on populations, not individuals

Using Data to Describe How Environmental Changes Affect Phenotypes in a Population

🌱 Introduction

Populations show variation in phenotypes.
When the environment changes, certain phenotypes become more favorable, while others become less favorable.
Using data such as graphs, tables, and trends, scientists can describe how phenotype frequencies change in response to environmental change.

🧬 Key Terms

Phenotype

The observable characteristics of an organism.

Includes:

Physical traits (color, size, shape)
Physiological traits (tolerance, resistance)
Behavioral traits

Phenotypic Frequency

The proportion of individuals in a population showing a particular phenotype.

Often expressed as:

Percentage
Number of individuals
Height of bars on a graph

Environmental Change

Any alteration in abiotic or biotic factors.

Examples:

Temperature increase or decrease
Drought or flooding
Introduction or removal of predators
Disease outbreak
Pollution or habitat modification

📊 Types of Data Used

Environmental effects on phenotypes are commonly shown using:

Bar graphs → compare phenotype frequencies
Line graphs → show changes over time
Bell-shaped curves → show phenotype distribution

🧠 What “Use Data” Means

When given data, you must:

Identify what changed in the environment
Identify which phenotype is being measured
Describe how phenotype frequencies changed
Avoid explaining why unless asked

📌 Use clear descriptive terms:

Increased / decreased
More common / less common
Shifted left or right
Remained constant

🌿 Step-by-Step Data Interpretation Framework

Step 1: Identify the Environmental Change

From the question or graph, determine what factor changed.

Temperature
Predation
Resource availability
Pollution
Climate condition

Step 2: Identify the Phenotypes

Determine which phenotypes are shown:

Color variants
Size classes
Trait categories

Each bar or section represents a phenotype.

Step 3: Observe the Trend

Compare data before vs after the environmental change:

Which phenotype increased?
Which phenotype decreased?
Did one phenotype become dominant?

Step 4: Describe the Population-Level Change

Describe how the overall phenotypic distribution changed.

Always describe change at the population level.

🌱 Common Patterns Seen in Data

1. Directional Change

One phenotype increases steadily
Opposite phenotype decreases
Graph shows a shift in one direction

Indicates the environment favors one extreme phenotype.

2. Reduced Variation

Intermediate phenotype becomes most common
Extreme phenotypes decrease
Bell curve becomes narrower

Indicates selection against extremes.

3. Increased Variation

Both extreme phenotypes increase
Intermediate phenotype decreases
Distribution becomes split

Indicates environment favors multiple phenotypes.

🌍 Environmental Change and Phenotypic Advantage

A phenotype that is common before an environmental change may decrease in frequency after change
A previously rare phenotype may increase and become common

Favorable phenotypes depend on the environment.

🚫 Common Student Mistakes

Explaining allele changes instead of describing phenotypes
Talking about survival mechanisms instead of data trends
Describing individual organisms instead of populations
Ignoring units, axes, or labels on graphs

📊 Summary Table

Data Observation	Correct Description
Increase in bars	Phenotype frequency increased
Decrease in bars	Phenotype frequency decreased
Curve shifts	Population phenotype changed
One phenotype dominates	Environmental advantage
Distribution changes	Population response to environment

📦 Quick Recap
✔ Environmental changes affect phenotype frequencies
✔ Data shows population-level trends
✔ Identify environment change first
✔ Observe phenotype frequency shifts
✔ Use increase / decrease language
✔ Do not explain mechanisms unless asked
✔ Phenotypes, not individuals, are analyzed

Predicting How Allelic Frequencies Shift in Response to Environmental Change

🌱 Introduction

In a population, individuals differ genetically.
These genetic differences exist as different alleles of genes.
When the environment changes, it creates new selective pressures.
These pressures do not change individuals, but they change which alleles are passed on more often.

Over many generations, this leads to a shift in allele (allelic) frequencies in the population.
This shift is called evolution at the population level.

🧬 Important Definitions

Allele

An alternative form of a gene.
Example: allele A and allele a

Gene Pool

The total collection of all alleles present in a population.

Allele Frequency

The proportion of a specific allele in the gene pool.
Expressed as a fraction or percentage.

Example:
If allele A is present 60 times out of 100 alleles → frequency = 0.6

Environmental Change

Any change in biotic or abiotic factors such as:

Temperature change
Drought or flooding
New predators or diseases
Pollution or habitat destruction

🧠 Core Concept

Natural selection changes allele frequencies by favoring alleles that increase fitness in a given environment.

Selection acts on phenotypes
Evolution is measured by allele frequency change

🌿 How Environmental Change Causes Allele Frequency Shifts

Step 1: Genetic Variation Exists

Population already contains different alleles
These alleles produce different phenotypes

No new alleles are created by selection.

Step 2: Environment Changes

Examples:

Climate becomes hotter
Antibiotic is introduced
Predator pressure increases

This change creates new selection conditions.

Step 3: Differential Survival and Reproduction

Individuals with favorable phenotypes survive better and reproduce more
Individuals with unfavorable phenotypes leave fewer offspring

This difference in reproduction = fitness difference

Step 4: Allele Frequencies Shift

Alleles linked to favorable traits are passed on more frequently → frequency increases
Alleles linked to unfavorable traits are passed on less frequently → frequency decreases

This change occurs over many generations.

🧬 Predicting Allele Frequency Changes

Step 1: Identify the Environmental Change

What factor changed?
Is it abiotic or biotic?

Step 2: Identify the Favored Phenotype

Which trait improves survival or reproduction now?

Step 3: Link Phenotype to Allele

Allele A → advantageous trait
Allele a → disadvantageous trait

Step 4: Make the Prediction

Allele linked to advantageous trait → frequency increases
Other allele → frequency decreases

🌱 Common Selection Patterns Seen in Allele Frequencies

1. Directional Selection

One allele consistently favored
Its frequency increases generation after generation
Other allele decreases

Very common after sudden environmental change.

2. Maintenance of Multiple Alleles

Environment varies
Heterozygotes have advantage
Different conditions favor different alleles

Allele frequencies stabilize, not reach 100%.

3. Loss of an Allele

If an allele strongly reduces fitness
Its frequency may approach zero
Genetic variation decreases

🌍 Environment-Specific Nature of Alleles

An allele favorable today may become unfavorable if the environment changes

Fitness is relative to the environment.

📊 Summary Table

Situation	Allele Frequency Outcome
Allele increases fitness	Frequency increases
Allele decreases fitness	Frequency decreases
Stable environment	Frequencies may remain constant
Environmental shift	Direction of change may reverse

📦 Quick Recap
✔ Alleles are different forms of a gene
✔ Environmental change creates new selective pressures
✔ Favorable phenotypes reproduce more
✔ Alleles linked to favorable traits increase in frequency
✔ Unfavorable alleles decrease over generations
✔ Evolution = change in allele frequencies
✔ Individuals do not evolve, populations do

EVO 2.2 Selective Mechanisms- Pre AP Biology Study Notes - New Syllabus.