Key Principles:

🦒 Classic Example: Giraffe Necks

Lamarck's Explanation:

1. Ancestral giraffes had short necks and stretched to reach leaves on tall trees
2. Through constant stretching during their lifetime, their necks became slightly longer
3. These acquired longer necks were passed to their offspring
4. Each generation stretched more, inheriting the previous generation's longer necks
5. Over many generations, giraffes developed very long necks

❌ Why Lamarckism Was Rejected:

• No genetic mechanism: Acquired characteristics don't change DNA in gametes (sex cells)
• Contradicts observations: If someone builds muscles through exercise, their children aren't born muscular
• Cannot explain simple organisms: Theory suggests all organisms become more complex, but bacteria and simple organisms still exist
• Lack of evidence: No experimental support for inheritance of acquired traits

Key Principles:

🦒 Classic Example: Giraffe Necks

Darwin's Explanation:

1. Ancestral giraffe populations had natural variation in neck length (some longer, some shorter)
2. During times of food scarcity, giraffes with longer necks could reach higher leaves
3. Longer-necked giraffes had better nutrition, survived longer, and produced more offspring
4. The genetic trait for longer necks was passed to offspring
5. Over many generations, the frequency of long-neck genes increased in the population
6. Eventually, all giraffes had long necks through natural selection

✓ Why Darwin's Theory Was Accepted:

• Observable variation: We can see natural variation within populations
• Supported by evidence: Fossil record, comparative anatomy, biogeography all support it
• Genetic basis: Modern genetics confirms traits are inherited through DNA
• Testable predictions: Can observe natural selection in action (e.g., antibiotic resistance)
• Explains adaptation: Accounts for how organisms become suited to their environments

Comparing DNA sequences and protein structures between species reveals evolutionary relationships. The more similar the sequences, the more recently species shared a common ancestor.

Key Evidence:

• Universal genetic code: All living organisms use the same genetic code (DNA/RNA bases coding for amino acids), suggesting common ancestry
• DNA sequence similarity: Humans share ~98.8% DNA with chimpanzees, ~85% with mice, ~60% with fruit flies
• Cytochrome c protein: This essential protein shows greater similarity between closely related species
• Molecular clocks: Mutations accumulate at relatively constant rates, allowing scientists to estimate divergence times
• Pseudogenes: Non-functional DNA sequences shared between species indicate common ancestry

🔍 Example: Cytochrome c Differences

Number of amino acid differences in cytochrome c compared to humans:

• Chimpanzee: 0 differences (identical)
• Rhesus monkey: 1 difference
• Dog: 11 differences
• Chicken: 13 differences
• Tuna: 21 differences
• Yeast: 44 differences

This pattern matches evolutionary relationships predicted from other evidence.

Selective breeding (artificial selection) demonstrates that species can change dramatically over relatively short time periods. Humans select organisms with desired traits and breed them, demonstrating evolution in action.

Examples of Selective Breeding:

💡 Key Point:

If humans can cause such dramatic changes through artificial selection in just thousands of years, imagine what natural selection can achieve over millions of years. Selective breeding demonstrates the mechanism of evolution and proves that species are not fixed.

The pentadactyl limb (five-fingered limb) is found in all tetrapod vertebrates (amphibians, reptiles, birds, and mammals). Despite serving different functions, all these limbs share the same basic bone structure:

Pentadactyl Limbs in Different Species:

🔍 Evolutionary Significance:

Despite vastly different functions (flying, swimming, running, grasping), all these limbs share the same fundamental bone pattern. This strongly suggests these animals inherited this structure from a common tetrapod ancestor approximately 370 million years ago. Through natural selection, the basic structure was modified for different purposes—a process called adaptive radiation.

🦈 🐬 Example 1: Dolphins (Mammals) vs. Sharks (Fish)

🌵 Example 2: Cacti (Americas) vs. Euphorbia (Africa)

🦋 🦇 🐦 Example 3: Wings in Insects, Bats, and Birds

1. Single species population exists with genetic variation
2. Isolation occurs - populations become separated (geographically, ecologically, or behaviorally)
3. Gene flow stops - populations can no longer interbreed
4. Different environments - each population experiences unique selection pressures
5. Mutations arise independently - random mutations occur in each isolated population
6. Natural selection acts differently - different traits are favored in each population
7. Genetic drift occurs - random changes accumulate, especially in small populations
8. Genetic differences accumulate - over many generations, populations become genetically distinct
9. Reproductive incompatibility develops - populations can no longer produce fertile offspring together
10. New species formed - two distinct species now exist where there was once one

Two main pathways to speciation:

💡 Important Note:

Speciation increases biodiversity by adding new species. Extinction decreases biodiversity by removing species. Net biodiversity = Speciation rate − Extinction rate.

1. Geographic (Spatial) Isolation

Definition: Populations are separated by physical distance or geographic barriers, preventing them from meeting.

Examples:

• Fish populations separated by land masses
• Squirrels on opposite sides of the Grand Canyon
• Birds on different islands

2. Temporal (Time-Based) Isolation

Definition: Species breed at different times (seasons, times of day, or years), preventing mating.

Examples:

• Two plant species that flower in different seasons
• Frogs that breed at different times of year
• Corals that release gametes at different times
• Insects that emerge as adults in different months

3. Habitat (Ecological) Isolation

Definition: Species occupy different habitats or ecological niches in the same area, reducing encounters.

Examples:

• One snake species lives in water, another on land
• Insects that feed on different host plants
• Fish living at different depths in a lake
• Parasites that infect different host species

4. Behavioral (Ethological) Isolation

Definition: Differences in courtship behaviors, mating rituals, or signals prevent attraction between species.

Examples:

• Bird species with different songs or mating dances
• Fireflies with species-specific light flash patterns
• Frogs with distinct mating calls
• Peacocks with different tail displays

5. Mechanical Isolation

Definition: Anatomical incompatibility prevents successful mating; reproductive structures don't fit together.

Examples:

• Insects with incompatible genitalia shapes
• Flowers with different shapes preventing pollination by same species
• Snails with shells that spiral in opposite directions

6. Gametic Isolation

Definition: Sperm and egg are incompatible; fertilization cannot occur even if gametes meet.

Examples:

• Sperm cannot penetrate egg membrane of different species
• Biochemical incompatibility between gametes
• Sea urchin sperm recognized only by species-specific egg receptors
• Pollen tubes cannot grow through different species' styles

1. Hybrid Inviability

Definition: Hybrid embryos fail to develop properly or die before reaching reproductive maturity.

Causes:

• Incompatible gene combinations from different species
• Developmental abnormalities
• Chromosomal mismatches during cell division

Examples:

• Sheep-goat hybrids die as embryos
• Some frog species produce tadpoles that fail to mature
• Hybrid seeds that germinate but seedlings die young

2. Hybrid Sterility

Definition: Hybrid offspring survive to adulthood but are sterile (unable to produce functional gametes and reproduce).

Causes:

• Chromosomes from different species cannot pair properly during meiosis
• Abnormal gamete production
• Incompatible chromosome numbers

Examples:

• Mule: Hybrid of horse (64 chromosomes) and donkey (62 chromosomes); mules have 63 chromosomes and are sterile
• Liger: Lion-tiger hybrid; usually sterile
• Hinny: Horse-donkey hybrid (opposite cross from mule); also sterile

3. Hybrid Breakdown

Definition: First-generation (F₁) hybrids are viable and fertile, but second generation (F₂) or later generations are weak, sterile, or inviable.

Examples:

• Cotton species hybrids produce healthy F₁, but F₂ shows reduced fitness
• Some rice species hybrids have fertile F₁ but sterile F₂
• Incompatible gene combinations segregate in later generations

🔑 Key Insight:

Prezygotic barriers are more common and evolutionarily advantageous because they prevent wasted energy on producing inviable or sterile offspring. Natural selection favors prezygotic mechanisms because they're more efficient.

✓ Darwin vs. Lamarck

Darwin's natural selection (random variation + differential survival) is supported by evidence, while Lamarck's inheritance of acquired characteristics was disproven.

✓ Evidence for Evolution

DNA/amino acid sequences, selective breeding, and anatomical structures all provide compelling evidence for evolutionary change over time.

✓ Homologous Structures

Similar underlying anatomy from common ancestry (divergent evolution); pentadactyl limb exemplifies shared heritage despite different functions.

✓ Convergent Evolution

Unrelated species independently evolve similar traits (analogous structures) due to similar environmental pressures—dolphins and sharks exemplify this.

✓ Speciation

New species arise when populations split, experience reproductive isolation and differential selection, accumulating genetic differences over time.

✓ Reproductive Isolation

Prezygotic barriers (before fertilization) and postzygotic barriers (after fertilization) prevent gene flow between populations, maintaining species boundaries.

Adam Kumar

Co-Founder @ RevisionTown

Math Expert specializing in various international curricula including IB, AP, GCSE, IGCSE, and more. Dedicated to providing comprehensive, accessible educational resources that help students excel in their academic journey across multiple subjects and examination systems.