IB DP Biology Natural selection Study Notes

IB DP Biology Natural selection Study Notes at IITian Academy focus on specific topic and type of questions asked in actual exam. Study Notes focus on IB Biology syllabus with guiding questions of

• What processes can cause changes in allele frequencies within a population?
• What is the role of reproduction in the process of natural selection?

Standard level and higher level: 2 hours
Additional higher level: 2 hours

IBDP Biology 2025 -Study Notes -All Topics

D4.1.1 – Natural Selection: Mechanism Driving Evolutionary Change

🧠 What is Natural Selection?

Natural selection is the process where organisms better adapted to their environment tend to survive and reproduce more.
It drives evolutionary change by increasing the frequency of advantageous traits over generations.
This process happens continuously and has shaped life over billions of years, creating Earth’s biodiversity.

🌿 Key Features of Natural Selection

Variation: Individuals in a population differ genetically.
Inheritance: Traits are passed from parents to offspring.
Differential survival and reproduction: Some traits improve chances of survival and reproduction.
Adaptation: Over time, populations become better suited to their environment.

🔍 Historical Context: Darwin and Lamarck

Theory	Key Idea	Status
Lamarckism	Organisms can pass on traits acquired during their lifetime (e.g., giraffes stretching necks)	Rejected
Darwin’s Natural Selection	Traits that increase survival and reproduction are naturally selected and passed on	Accepted and supported by evidence

📌 What is a Paradigm Shift?

A paradigm shift is a major change in scientific thinking.
Darwin’s theory replaced the earlier Lamarckian view.
It transformed biology by providing a clear, evidence-based mechanism for evolution.
Example: shift from believing acquired traits are inherited to understanding natural selection.

🧠 Why Natural Selection Matters

Explains how species change over time.
Accounts for adaptations and biodiversity.
Foundation of modern biology and evolutionary science.

🧠 Summary Box:
Natural selection acts on genetic variation to drive evolution.
It has operated over billions of years, producing diverse life forms.
Darwin’s theory was a paradigm shift, replacing Lamarckism with a stronger explanation.
Understanding natural selection is essential for grasping evolutionary biology.

D4.1.2 – Roles of Mutation and Sexual Reproduction in Generating Variation

🧠 Why Is Variation Important?

Natural selection needs genetic variation to work.
Variation provides different traits that may help survival or reproduction.

🌿 Role of Mutation

A mutation is a random change in DNA sequence.
Mutations create new alleles (versions of a gene).
Most mutations are neutral or harmful, but some can be beneficial.
Mutation is the ultimate source of new genetic material in populations.

🔬 Role of Sexual Reproduction

Sexual reproduction shuffles alleles to create new genetic combinations.
Processes include:
- Independent assortment of chromosomes during meiosis.
- Crossing over between homologous chromosomes.
- Random fertilization of gametes.
This increases genetic diversity without changing the DNA itself.

📍 How Mutation and Sexual Reproduction Work Together

Process	Effect on Genetic Variation
Mutation	Introduces new alleles into the gene pool
Sexual reproduction	Produces new combinations of existing alleles

🧠 Why This Matters for Natural Selection

More variation means a greater chance some individuals have traits suited to environmental changes.
This fuels adaptation and evolution over generations.

🧠 Summary Box:
Mutations create new alleles, adding raw genetic material.
Sexual reproduction rearranges alleles to create diverse offspring.
Together, they provide the variation natural selection needs to shape populations.

D4.1.3 – Overproduction of Offspring and Competition for Resources in Natural Selection

🧠 Why Overproduction Matters

Many species produce more offspring than can survive.
This overproduction creates a struggle for survival.
Not all offspring get enough resources to live and reproduce.

🌿 Competition for Resources

Resources like food, water, space, and shelter are limited.
Organisms compete for these to survive and reproduce.
Only those best adapted to compete successfully will pass on their genes.

🔍 Examples of Limiting Resources

Resource	Example
Food	Plants competing for sunlight; animals hunting prey
Water	Desert animals competing for scarce water sources
Space	Plants competing for territory; animals competing for nesting sites
Mates	Animals competing to attract partners

📍 How These Factors Promote Natural Selection

Overproduction increases competition.
Competition causes differential survival and reproduction.
This leads to natural selection, where advantageous traits become more common.

🧠 Why This Is Important

Ensures populations adapt to their environment.
Controls population size near the carrying capacity (max population the environment can support).

🧠 Summary Box:
Overproduction creates more individuals than resources can support.
Competition for limited food, water, space, and mates drives survival struggles.
These pressures cause natural selection by favoring the best-adapted organisms.
Limits population growth to the carrying capacity of the environment.

D4.1.4 – Abiotic Factors as Selection Pressures

🧠 What Are Abiotic Factors?

Abiotic factors are non-living environmental factors that influence living organisms.
They can act as selection pressures by affecting survival and reproduction.

🌿 Abiotic Selection Pressures

These factors can limit population size and affect which individuals survive.
Unlike biotic factors (like predators), abiotic factors are often density-independent — they affect populations regardless of size.

🔍 Examples of Abiotic Selection Pressures

Abiotic Factor	Effect on Population	Example
Temperature extremes	Can kill or stress organisms not adapted to survive high or low temps	Frost killing unprotected plants; heatwaves reducing insect survival
Drought	Reduces water availability, stressing organisms	Drying ponds causing fish deaths
Flooding	Can destroy habitats or drown organisms	River flooding washing away small mammals
Salinity changes	Affects water balance in aquatic organisms	Increased salt in freshwater habitats killing fish
Pollution	Can poison or disrupt ecosystems	Chemical spills harming amphibians

📍 Why Abiotic Factors Matter

Abiotic factors influence which traits are advantageous (e.g., heat tolerance).
Populations evolve to cope with these non-living environmental challenges.
Their effects are often sudden and can drastically reduce populations.

🧠 Summary Box:
Abiotic factors like temperature, drought, and flooding act as selection pressures.
These are often density-independent, affecting populations regardless of their size.
Organisms with traits suited to these abiotic stresses are more likely to survive and reproduce.
Abiotic selection pressures shape the evolution and adaptation of populations.

D4.1.5 – Differences Between Individuals in Adaptation, Survival and Reproduction: Basis for Natural Selection

🧠 Key Concept: Intraspecific Competition

Intraspecific competition is competition within the same species for limited resources like food, mates, and space.
This competition leads to differences in survival and reproduction among individuals.

🌿 Differences in Adaptation

Individuals vary genetically, leading to differences in adaptations.
Adaptations are traits that improve an organism’s fitness — its ability to survive and reproduce.

🔍 Fitness: Survival Value and Reproductive Potential

Fitness measures how well a genotype contributes to the next generation.
It depends on:
- Survival value: How likely an individual is to survive to reproductive age.
- Reproductive potential: How many offspring an individual can produce.

📍 How Differences Lead to Natural Selection

Individuals with higher fitness survive better and leave more offspring.
These offspring inherit advantageous traits, increasing their frequency in the population.
Over time, this drives evolutionary change.

🧠 Why This Matters

Natural selection acts on variation in fitness within populations.
It is a key mechanism for populations to adapt to their environment.

🧠 Summary Box:
Differences between individuals in adaptation affect their survival and reproduction.
Intraspecific competition creates selection pressure favoring fitter individuals.
Fitness combines survival and reproductive success.
Natural selection increases the frequency of beneficial traits through fitness differences.

D4.1.6 – Requirement That Traits Are Heritable for Evolutionary Change

🧠 Key Concept: Heritability and Evolution

For evolution to occur, traits must be heritable — passed from parents to offspring through genes.
Only genetic changes (mutations, gene combinations) are inherited.

🌿 Acquired vs Heritable Traits

Trait Type	Description	Inheritance
Acquired traits	Changes during an individual’s life due to environment (e.g., muscle build, scars)	Not passed to offspring
Heritable traits	Traits encoded in DNA sequences (e.g., eye color, blood type)	Passed genetically

🔍 Why Acquired Traits Are Not Heritable

Acquired traits do not alter the DNA sequence in reproductive cells.
Therefore, they cannot be passed on to the next generation.
Only changes in the base sequence of genes affect offspring traits.

📍 Implications for Evolution

Evolution acts on heritable genetic variation, not acquired characteristics.
This distinction was a key part of understanding evolution after Lamarck’s incorrect theory.

🧠 Summary Box:
Traits must be encoded in genes to be inherited and drive evolution.
Acquired traits from environmental influences do not contribute to genetic change.
Evolution depends on heritable genetic variation passed through generations.

D4.1.7 – Sexual Selection as a Selection Pressure in Animal Species

🧠 What is Sexual Selection?

Sexual selection is a type of natural selection where individuals with certain traits are more successful at attracting mates.
These traits increase mating success, not necessarily survival.
It drives evolution by favoring traits that improve reproductive success.

🌿 Physical and Behavioural Traits in Sexual Selection

Traits can be:
- Physical: Bright colours, elaborate plumage, large size, antlers.
- Behavioural: Courtship dances, vocal calls, territorial displays.
These traits often signal overall fitness — health, strength, and good genes.

🔍 Example: Birds of Paradise

Male birds of paradise have bright, elaborate plumage and perform complex courtship dances.
Females prefer males with the most impressive displays.
Over generations, this preference has led to the evolution of extravagant plumage and behaviours.
These traits improve male reproductive success despite possible survival costs.

📍 How Sexual Selection Works

Trait Type	Effect	Example
Physical	Attract mates by appearance	Peacock’s colourful tail
Behavioural	Attract mates through actions	Bird of paradise dances

🧠 Why Sexual Selection Matters

Explains traits that don’t necessarily improve survival but increase mating chances.
Adds to genetic diversity by influencing mate choice.
Drives evolutionary changes alongside natural selection.

🧠 Summary Box:
Sexual selection favours traits that improve mating success.
These traits serve as signals of fitness to potential mates.
Birds of paradise are a classic example of sexual selection shaping extravagant traits.
Sexual selection is a powerful force in animal evolution.

D4.1.8 – Modelling Sexual and Natural Selection: John Endler’s Guppy Experiments

🧠 What Was the Aim of Endler’s Experiments?

To study how sexual and natural selection affect guppy populations.
To see how predation pressure and mate choice influence guppy coloration and survival.

🌿 Experimental Setup

Guppies were placed in streams with different predator densities:
- High predation areas
- Low predation areas
Observed changes in male guppy color patterns and survival.
Controlled for other variables to isolate effects of selection pressures.

🔍 Key Findings

Selection Pressure	Effect on Guppy Traits
Natural selection (predation)	Favoured duller, less conspicuous males to avoid predators
Sexual selection (mate choice)	Favoured brightly coloured males preferred by females

📍 Interpretation

There is a trade-off between survival (natural selection) and reproduction (sexual selection).
Selection pressures can work in opposition, shaping traits differently depending on environment.
Controlled experiments allow clear understanding of how selection drives evolution.

🧠 Why This Experiment Is Important

Demonstrated how environmental factors influence evolutionary outcomes.
Showed real-time evolutionary change in natural populations.
Provides a model for studying interactions of multiple selection pressures.

🧠 Summary Box:
Endler’s guppy experiments showed natural selection favours camouflage under predation.
Sexual selection favours bright colours for mate attraction.
Trade-offs between survival and reproduction shape traits.
Controlled experiments help us understand complex evolutionary processes.

D4.1.9 – Concept of the Gene Pool

🧠 What is a Gene Pool?

A gene pool is the total set of all genes and their different alleles present in a population.
It represents the genetic diversity available for natural selection and evolution.

🌿 Key Points About Gene Pools

Includes every allele variant for each gene in the population.
Larger gene pools mean more genetic variation.
Genetic variation is essential for populations to adapt to changing environments.

🔍 Why Gene Pools Matter

The gene pool is the source of heritable traits that natural selection acts upon.
Changes in allele frequencies in the gene pool lead to evolution.
Populations with small gene pools (low variation) are at risk of inbreeding and reduced adaptability.

📍 Example

In a population of butterflies, the gene pool includes alleles for wing color, pattern, and size.
If a new allele arises (e.g., a different wing color), it adds to the gene pool’s diversity.

🧠 Summary Box:
The gene pool contains all genes and allele variants in a population.
It reflects the genetic variation necessary for evolution.
Monitoring gene pools helps understand population health and evolutionary potential.

D4.1.10 – Allele Frequencies of Geographically Isolated Populations

🧠 What Are Allele Frequencies?

Allele frequency is how common a specific allele is in a population’s gene pool.
Expressed as a percentage or proportion of all alleles for a gene.

🌿 Geographical Isolation and Allele Frequencies

When populations are geographically isolated (e.g., separated by mountains, oceans), gene flow between them is limited.
This leads to differences in allele frequencies over time due to:
- Mutation
- Natural selection
- Genetic drift
- Sexual selection

🔍 Human Example: Sickle Cell Allele

The sickle cell allele (HbS) frequency varies geographically:
- High frequency in populations from malaria-prone regions (e.g., parts of Africa).
- Low or absent in populations from areas without malaria.
This reflects natural selection, where the sickle cell allele provides malaria resistance.

📌 Using Databases to Study Allele Frequencies

Databases like the 1000 Genomes Project or Ensembl provide data on human allele frequencies worldwide.
Students can:
- Search allele frequencies for specific genes.
- Compare between different populations.
- Analyze how geography affects genetic diversity.

🧠 Summary Box:
Allele frequencies measure how common alleles are in populations.
Geographic isolation limits gene flow, causing allele frequencies to diverge.
The sickle cell allele is a classic example linked to environment (malaria).
Online databases are useful tools for exploring allele frequencies across populations.

D4.1.11 – Changes in Allele Frequency Due to Natural Selection

🧠 Key Concept: Natural Selection and Allele Frequency

Natural selection changes the frequency of alleles in a population’s gene pool.
Individuals with advantageous heritable traits are more likely to survive and reproduce.
Their alleles increase in frequency over generations.

🌿 Neo-Darwinism: Integrating Genetics and Natural Selection

Darwin proposed natural selection but didn’t know about genes.
Neo-Darwinism combines:
- Darwin’s natural selection
- Genetic inheritance (Mendelian genetics)
This modern synthesis explains evolution through changes in allele frequencies.

🔍 How Natural Selection Changes Allele Frequencies

Step	Effect on Allele Frequency
Variation in traits	Different alleles exist in the gene pool
Differential survival and reproduction	Alleles for beneficial traits become more common
Reproduction of fitter individuals	Pass on advantageous alleles
Allele frequency shifts in population	Population evolves over time

📍 Why This Matters

Evolution is genetic change in populations over time.
Tracking allele frequency helps understand how populations adapt.
Neo-Darwinism provides the scientific basis for modern evolutionary biology.

🧠 Summary Box:
Natural selection causes allele frequencies to change based on fitness differences.
Neo-Darwinism merges Darwin’s ideas with genetics to explain evolution.
Evolution is the result of heritable trait differences affecting survival and reproduction.

D4.1.12 – Differences Between Directional, Disruptive, and Stabilizing Selection

🧠 Overview

All three types of natural selection cause changes in allele frequencies.
They differ in which traits are favored and how the population’s traits shift over time.

🌿 Types of Selection

Type	Description	Effect on Population	Example
Directional Selection	Favors one extreme phenotype over others	Shifts population trait distribution toward one extreme	Peppered moth color change during industrial revolution
Disruptive Selection	Favors both extreme phenotypes, against intermediates	Population splits towards two or more distinct forms	Beak sizes in birds specialized for different seeds
Stabilizing Selection	Favors intermediate phenotypes, against extremes	Reduces variation, population centered around average	Human birth weights close to average for survival

🔍 How Each Changes Allele Frequencies

Directional: Increases frequency of alleles for one extreme trait.
Disruptive: Increases alleles for both extremes, reduces intermediate alleles.
Stabilizing: Increases alleles for intermediate traits, reduces extremes.

📌 Why These Types Matter

They explain different ways populations adapt and evolve depending on environmental pressures.
Help maintain or increase genetic diversity in populations.

🧠 Summary Box:
Directional shifts traits toward one extreme.
Disruptive favors extremes at both ends.
Stabilizing favors average traits.
All change allele frequencies, shaping population evolution.

D4.1.13 – Hardy-Weinberg Equation and Allele/Genotype Frequency Calculations

🧠 What is the Hardy-Weinberg Principle?

A mathematical model predicting allele and genotype frequencies in a population.
Assumes no evolution occurs (no mutation, selection, migration, random mating, large population).

🌿 Key Variables

p = frequency of one allele (e.g., dominant allele)
q = frequency of the other allele (e.g., recessive allele)

🔍 Basic Equations
Since there are only two alleles:
p + q = 1

Genotype frequencies predicted by:
p² + 2pq + q² = 1

Where:
    p² = frequency of homozygous dominant genotype
    2pq = frequency of heterozygous genotype
    q² = frequency of homozygous recessive genotype

📍 Using the Equation

If you know one genotype frequency (usually q² from recessive phenotype), you can:
- Calculate q by taking the square root of q².
- Find p using p = 1 − q.
- Calculate other genotype frequencies using the equations above.

🔢 Example Calculation

Suppose 9% of a population show a recessive trait (genotype frequency q² = 0.09):
- q = √0.09 = 0.3
- p = 1 − 0.3 = 0.7
- Frequency of heterozygotes 2pq = 2 × 0.7 × 0.3 = 0.42 (42%)
- Frequency of homozygous dominant p² = 0.7² = 0.49 (49%)

🧠 Why Hardy–Weinberg Matters

Provides a baseline to detect if evolution is occurring.
Helps estimate genetic variation in populations.
Useful for studying disease allele frequencies, conservation genetics, and evolutionary biology.

🧠 Summary Box:
p + q = 1 for allele frequencies.
p² + 2pq + q² = 1 for genotype frequencies.
Knowing one genotype frequency lets you calculate allele and other genotype frequencies.
Hardy–Weinberg predicts genetic makeup in a stable, non-evolving population.

D4.1.14 – Hardy-Weinberg Conditions for Genetic Equilibrium

🧠 What is Genetic Equilibrium?

A population is in genetic equilibrium when allele and genotype frequencies remain constant over generations.
Under these conditions, no evolution occurs.

🌿 Hardy-Weinberg Conditions

For genetic equilibrium, all these must be true:

Large population size
Minimizes effects of genetic drift (random allele frequency changes).
No mutations
No new alleles introduced or lost.
No gene flow (no migration)
No alleles added or removed by migration.
Random mating
All individuals have equal chance of mating; no genotype-based mate selection.
No natural selection
All genotypes have equal survival and reproductive success.

🔍 What If These Conditions Are Not Met?

Violations cause allele/genotype frequencies to change, indicating evolution.
Examples:
- Non-random mating (e.g., inbreeding) alters genotype frequencies.
- Natural selection favors some genotypes, changing allele frequencies.
- Mutation introduces new alleles.
- Migration changes allele composition.
- Small population size leads to genetic drift.

📍 Why This Matters

Testing Hardy–Weinberg equilibrium helps detect evolutionary forces.
Useful in population genetics, conservation, and medical genetics.

🧠 Summary Box:
Genetic equilibrium requires:
  – Large population
  – No mutation
  – No migration
  – Random mating
  – No natural selection

Deviations from equilibrium mean evolutionary change is occurring.
Hardy–Weinberg provides a baseline model for studying populations.

D4.1.15 – Artificial Selection by Deliberate Choice of Traits

🧠 What is Artificial Selection?

Human-driven breeding of plants or animals for specific desirable traits.
Humans choose which individuals reproduce, unlike natural selection driven by the environment.

🌿 Applications of Artificial Selection

Crop plants: Selecting for higher yield, disease resistance, or taste.
Domesticated animals: Selecting for size, temperament, milk production, coat colour.
Examples:
- Breeding cows producing more milk.
- Growing wheat varieties resistant to drought.

🔍 Unintended Consequences and Natural Selection

Some human activities cause natural selection unintentionally.
Example: Antibiotic use leads to evolution of antibiotic-resistant bacteria.
This is natural selection (environment-driven), not artificial (human-driven).

📍 Difference Between Artificial and Natural Selection

Aspect	Artificial Selection	Natural Selection
Who decides selection	Humans	Environment and survival pressures
Selection criteria	Desirable traits chosen by breeders	Traits that improve survival and reproduction
Timescale	Often faster due to human control	Usually slower, over many generations

🧠 Why Artificial Selection Matters

Crucial for agriculture and domestication.
Allows rapid development of beneficial traits.
May reduce genetic diversity and cause issues like inbreeding.

🧠 Summary Box:
Artificial selection involves humans choosing traits to breed.
Common in farming and animal breeding.
Antibiotic resistance evolves by natural, not artificial selection.
Artificial selection can rapidly change species but may have drawbacks.

IB DP Biology Natural selection Study Notes

IB DP Biology Natural selection Study Notes