NEET Biology - Unit 7- Evolution- Study Notes - New Syllabus
NEET Biology – Unit 7- Evolution- Study Notes – New Syllabus
Key Concepts:
- Evolution: Origin of life; Biological evolution and evidences for biological evolution from Paleontology, comparative anatomy, embryology and molecular evidence); Darwin’s contribution, Modern Synthetic theory of Evolution; Mechanism of evolutionVariation (Mutation and Recombination) and Natural Selection with examples, types of natural selection; Gene flow and genetic drift; Hardy-Weinberg’s principle; Adaptive Radiation; Human evolution.
Evolution: Origin of Life
🌱 Introduction
Evolution explains how life forms changed over generations, giving rise to today’s biodiversity.
It focuses on how new traits arise, how populations change genetically, and how species diverge over time.
What is Evolution?
Evolution is the change in heritable traits (genes, allele frequencies) of a biological population over generations.
These continuous changes create diversity at multiple levels:
- Molecules
- Individuals
- Species
- Entire ecosystems
In simple words: Populations evolve, not individuals.
Example of Evolution (Peppered Moth Story)
- Before Industrial Revolution: Light-coloured moths blended with lichen-covered trees → less predation by birds
- After Industrial Revolution: Smoke blackened tree trunks → light moths easily spotted by birds
- Dark-coloured moths (mutation) survived and reproduced more
Result: Population shifted from light → dark form.
Classic example of natural selection in action.
🌿Microevolution
Microevolution: Small genetic changes within a population affecting allele frequencies over time.
Four major forces behind microevolution
- Natural Selection: Environment favours organisms with advantageous traits → survive & reproduce more
- Genetic Drift: Random changes in allele frequencies in small populations
Example: Founder effect, bottleneck effect - Mutation: Sudden random DNA changes → create new alleles
- Gene Flow: Movement of individuals/genes between populations → introduces new variation
What drives evolution more: Mutation or Genetic Recombination?
- Mutation: Random DNA changes; introduces new alleles; slow but essential for long-term evolution
- Genetic Recombination: During gamete formation; shuffles existing alleles to form new combinations; creates massive variation every generation
Both contribute to evolution:
Genetic recombination → primary source of variation
Mutation → provides raw material (new alleles)
Together → diversity for natural selection.
💡Summary Table
| Concept | Meaning | NEET Point |
|---|---|---|
| Evolution | Genetic change over generations | Populations evolve |
| Microevolution | Small allele frequency changes | Driven by 4 forces |
| Natural Selection | Favors better-adapted individuals | Peppered moth example |
| Mutation | Random DNA changes | Source of new alleles |
| Recombination | Shuffling of alleles during gamete formation | Major source of variation |
🧾 Quick Recap
Evolution = heritable genetic change across generations
Peppered moth = example of natural selection
Microevolution caused by natural selection, genetic drift, mutation, and gene flow
Mutation = new alleles
Recombination = reshuffles alleles, main source of variation
Variation allows populations to adapt and evolve
Biological Evolution & Evidences
🦴Biological Evolution
Biological evolution is the process by which species change over time through heritable variations.
It explains the diversity of life on Earth and is driven by natural selection, mutation, recombination, and genetic drift.
Leads to adaptation, speciation, and formation of new species.
Key Point: Evolution acts at the population level, not on individuals.
Evidences for Biological Evolution
Evolution is supported by multiple lines of evidence from fossils to molecular studies.
A. Paleontology (Fossil Evidence)
Fossils = preserved remains or traces of ancient organisms.
Show gradual changes over geological time.
- Horse evolution: small, 3-toed ancestor (Eohippus) → modern horse (Equus)
- Whale evolution: terrestrial ancestors (Pakicetus) → aquatic whales (Basilosaurus → Modern whales)
Transitional fossils link ancestral and modern species.
Tip: Fossils show sequence of evolutionary changes → “descent with modification.”
B. Comparative Anatomy
- Homologous Structures: Same structure, different function → indicate common ancestry
Example: Human, bat, whale, cat forelimbs - Analogous Structures: Different structure, same function → convergent evolution
Example: Wings of birds and insects - Vestigial Structures: Reduced/non-functional organs → ancestral traits
Example: Human appendix, tailbone; hind limb bones in whales
C. Embryology
Early embryos of vertebrates show similar patterns → evidence of common ancestry.
- Example: Human, chick, fish embryos → gill slits (pharyngeal arches), tail, somites
Tip: “Ontogeny recapitulates phylogeny” → early development reveals evolutionary history.
D. Molecular Evidence
DNA, RNA, and protein sequences show genetic similarities among species.
Closely related species have more similar sequences.
- Humans & chimpanzees → ~98–99% DNA similarity
- Hemoglobin sequences in vertebrates show gradual changes over evolution
Key Point: Molecular data confirms anatomical and fossil evidence.
💡 Summary Table
| Evidence Type | Basis | Example | Importance |
|---|---|---|---|
| Paleontology | Fossils | Horse, Whale evolution | Shows gradual changes & transitional forms |
| Comparative Anatomy | Homology/Analogy/Vestigial organs | Forelimb of human, bat; appendix | Common ancestry / adaptive evolution |
| Embryology | Early developmental similarity | Pharyngeal arches in vertebrates | Reveals ancestral traits |
| Molecular | DNA/RNA/protein sequences | Human-chimpanzee DNA similarity | Confirms evolutionary relationships |
🧾 Quick Recap
Biological evolution = heritable changes in populations → adaptation, speciation
Fossils → show transitional forms
Comparative anatomy → homologous & vestigial structures indicate common ancestry
Embryology → early developmental similarities reveal ancestral traits
Molecular evidence → DNA/protein sequences confirm evolutionary relationships
Conclusion: All evidences support “descent with modification”
Darwin’s Contribution & Modern Synthetic Theory of Evolution
🌱 Charles Darwin & Natural Selection
Definition of Natural Selection:
Process by which individuals with favorable traits survive and reproduce more successfully, passing those traits to the next generation.
Darwin’s Major Contributions:
| Contribution | Explanation | Example / Note |
|---|---|---|
| Theory of Evolution by Natural Selection | Populations evolve over time because individuals with advantageous traits survive and reproduce | Peppered moth, Galapagos finches |
| Descent with Modification | All species descend from common ancestors but acquire modifications over time | Horses: small, multi-toed ancestors → modern single-toed form |
| Variation is Important | Natural variations exist in populations; only advantageous ones are selected | Beak size in finches based on food availability |
| Struggle for Existence | Individuals compete for limited resources | Food, mates, space |
| Survival of the Fittest | Only best-adapted individuals survive to reproduce | Dark-colored moths survived pollution-darkened forests |
Tip: Darwin observed variation, competition, and adaptation as key to evolution.
Note: Darwin did not know the mechanism of inheritance (later explained by Mendel).
🌱 Modern Synthetic Theory of Evolution
Also called Neo-Darwinism or Synthetic Theory.
Combines Darwin’s natural selection with Mendelian genetics.
Explains evolution as change in allele frequency in populations over time.
Key Points / Principles:
| Principle | Explanation |
|---|---|
| Genetic Variation | Variation arises via mutation and genetic recombination |
| Natural Selection | Favors individuals with advantageous alleles → adaptation |
| Speciation | Populations diverge → new species via drift, selection, migration |
| Population Genetics | Focuses on gene pools, allele frequency changes, evolution at population level |
| Integration of Disciplines | Combines paleontology, systematics, embryology, molecular biology with Darwinism |
Example: Dark moths in polluted areas → allele for dark color increases → population evolves.
🍀Comparison Table: Darwin vs Modern Synthetic Theory
| Feature | Darwin’s Theory | Modern Synthetic Theory |
|---|---|---|
| Variation | Observed but not explained | Explained by Mendelian genetics, mutation, recombination |
| Mechanism of Inheritance | Unknown | Mendelian inheritance (gene-based) |
| Level of Evolution | Population & species | Population genetics & allele frequencies |
| Source of Evolutionary Change | Natural selection | Natural selection + mutation + gene flow + genetic drift |
| Supporting Evidence | Fossils, comparative anatomy | Fossils, embryology, molecular biology, genetics |
🧾 Quick Recap
Darwin: Natural selection, “survival of the fittest”, descent with modification
Variation in populations is key
Modern Synthetic Theory: Combines Darwin + Mendel + genetics
Evolution = change in allele frequencies in populations
Supported by fossils, molecular biology, embryology, comparative anatomy
Mechanism of Evolution: Variation & Natural Selection
Introduction
Evolution is driven by changes in the genetic makeup of populations over generations.
Main mechanisms include: Mutation, Genetic Recombination, Natural Selection, Migration (Gene Flow), Genetic Drift.
Natural selection is the primary driver shaping adaptive evolution.
🌿Variation
Variation provides the raw material for natural selection.
Sources of Variation
- Mutation: Random DNA changes, natural or induced by mutagens (radiation, chemicals). Can be beneficial, neutral, or harmful.
- Genetic Recombination: Occurs during meiosis → gametes. Shuffles alleles to produce new combinations. Generates genetic diversity each generation.
Tip: Mutation = creates new alleles, Recombination = reshuffles alleles → variation within population.
🦎Natural Selection
Definition: Process where organisms better adapted to their environment survive and reproduce more successfully.
Variations that increase survival/reproduction are retained; less advantageous traits diminish.
Example: Tree frogs
Grey frogs blend with dark bark → survive
Green frogs blend with green leaves → survive
Frogs on contrasting backgrounds → eaten by predators
Key Point: Natural selection acts on phenotypes but changes allele frequencies in populations.
Factors Affecting Natural Selection
Gene frequencies remain stable if undisturbed.
Disturbing factors include:
- Migration / Gene flow → introduces new alleles
- Mutation → creates new alleles
- Genetic drift → random changes in small populations
🌱 Types of Natural Selection
| Type | Definition | Example | Effect on Population |
|---|---|---|---|
| Stabilizing Selection | Selects against both extremes → favors average trait | Medium-height plants: short → cannot compete, tall → wind damage | Maintains intermediate traits |
| Directional Selection | Selects against one extreme → shifts trait distribution | Giraffes: short necks cannot reach leaves → population shifts to long necks | Shift in population mean |
| Disruptive Selection | Selects against the middle → favors extremes | Plants: pollinators disappear for medium plants → only short & tall survive | Creates bimodal / polymorphic population |
Tip:
Stabilizing → preserves average
Directional → favors one extreme
Disruptive → favors both extremes
Darwin’s Theory of Evolution
- All life is connected → common ancestry
- Diversity arises from population modifications via natural selection
- Evolution occurs through behavioral and physical changes
- Limitation: Did not know mechanism of inheritance → later explained by Mendel
Modern Understanding: Modern Evolutionary Synthesis
- Combines Darwin’s natural selection with genetics & molecular biology
- Mutations in DNA → source of new traits
- Natural selection acts on phenotypes → changes allele frequencies
- Mutations may be beneficial, neutral, or harmful
- Key Point: Evolution is change at DNA level reflected in population traits
🧾 Quick Recap
Variation → mutation + genetic recombination → raw material for evolution
Natural Selection: survival of the fittest → retention of advantageous traits
Types of Natural Selection:
Stabilizing → favors average
Directional → favors one extreme
Disruptive → favors both extremes
Modern Evolutionary Synthesis: Darwin + Mendel + genetics → evolution acts via allele frequency changes
Gene Flow and Genetic Drift
🌍 Gene Flow (Migration)
Definition: Gene flow is the transfer of alleles from one population to another due to movement of individuals or gametes.
- Increases genetic similarity between populations
- Can introduce new alleles into a population
- Reduces genetic differences between populations
- Occurs in plants (pollen/seed dispersal) and animals (migration)
Example: Bees carry pollen from one flower population → alleles transferred
Human migration introduces new genes into populations
Tip: Gene flow prevents populations from diverging completely.
🎲 Genetic Drift
Definition: Genetic drift is the random change in allele frequencies, especially significant in small populations.
- Occurs by chance, not selection
- Can fix or eliminate alleles randomly
- Stronger effect in small populations
- Leads to reduced genetic variation
Types of Genetic Drift:
- Bottleneck Effect: Population drastically reduces due to catastrophic events → only surviving alleles remain.
Example: Cheetah population bottleneck → low genetic diversity - Founder Effect: Small group colonizes new area → only alleles of founders present.
Example: Amish population → higher frequency of rare genetic disorders
🌱 Comparison: Gene Flow vs Genetic Drift
| Feature | Gene Flow | Genetic Drift |
|---|---|---|
| Cause | Movement of individuals / gametes | Random chance events |
| Population size effect | Any size, more in large populations | Strong in small populations |
| Effect on variation | Increases similarity between populations | Decreases variation within population |
| Example | Pollen transfer, human migration | Bottleneck, founder effect |
🌱 Significance in Evolution
- Gene Flow: Maintains genetic connectivity, slows speciation
- Genetic Drift: Rapid allele frequency changes, may fix or lose alleles, accelerates divergence in small populations
🧾 Quick Recap
Gene Flow: movement of alleles between populations → increases similarity
Genetic Drift: random changes in allele frequencies → stronger in small populations
Special cases of drift: Bottleneck & Founder effects
Both are mechanisms of evolution along with mutation and natural selection
Hardy-Weinberg Principle
Definition
The Hardy-Weinberg principle states that in a large, randomly mating population, with no evolutionary forces acting (mutation, migration, selection, genetic drift), the allele and genotype frequencies remain constant from generation to generation.
Key Point: Represents a theoretical equilibrium and provides a baseline to study evolution.
Assumptions of Hardy-Weinberg Equilibrium
- Large population (avoids genetic drift)
- Random mating (no sexual selection)
- No mutation (allele frequencies stable)
- No migration (gene flow absent)
- No natural selection (all genotypes equally fit)
Tip: Violation of any assumption → population evolves.
Allele and Genotype Frequencies
- Let p = frequency of dominant allele (A), q = frequency of recessive allele (a)
- p + q = 1
- Genotype frequencies:
- AA (homozygous dominant) = p²
- Aa (heterozygous) = 2pq
- aa (homozygous recessive) = q²
- Check: p² + 2pq + q² = 1
Applications / Example
Population of 1000 individuals, 160 show recessive trait (aa):
- q² = 160 / 1000 = 0.16 → q = √0.16 = 0.4
- p = 1 – q = 0.6
- Genotype frequencies:
- AA = p² = 0.36 → 360 individuals
- Aa = 2pq = 0.48 → 480 individuals
- aa = q² = 0.16 → 160 individuals
Tip: Useful for estimating carrier frequency of genetic diseases like cystic fibrosis or sickle cell anemia.
Importance of Hardy-Weinberg Principle
- Provides a mathematical model to study evolution
- Helps estimate allele frequencies in populations
- Detects forces of evolution if population is not in equilibrium
- Applied in population genetics and medical genetics
🧾 Quick Recap
Hardy-Weinberg Principle: Allele & genotype frequencies remain constant without evolution
Equations: p + q = 1 → allele frequency, p² + 2pq + q² = 1 → genotype frequency
Assumptions: Large population, random mating, no mutation, no migration, no selection
Application: Estimate carriers & study population evolution
Adaptive Radiation
Definition
Adaptive radiation is the rapid evolution of diversely adapted species from a common ancestor in response to different environmental conditions and ecological niches.
Key Idea: One ancestor → multiple species with specialized adaptations.
Conditions for Adaptive Radiation
- Common Ancestry → species share a recent common ancestor
- Diverse Habitats / Niches → ecological opportunities exist
- Natural Selection → favors traits suited for specific niches
- Geographical Isolation → populations adapt independently
Mechanism
- Single ancestral species colonizes a new area
- Populations face different ecological pressures (food, predators, climate)
- Variation arises via mutation and recombination
- Natural selection favors beneficial traits in each environment
- Over generations, populations diverge into multiple species
Examples
| Example | Ancestor | Outcome / Adaptation |
|---|---|---|
| Darwin’s Finches (Galapagos) | Small seed-eating finch | Beak variations: thick (seeds), slender (insects), cactus-eating finch |
| Hawaiian Honeycreepers | Single ancestral bird | Diverse feeding adaptations: nectar, seeds, insects |
| Cichlid Fishes (African Great Lakes) | Single ancestor | Mouth adaptations: algae scrapers, insectivores, predators |
| Marsupials in Australia | Common marsupial ancestor | Specialized forms: kangaroo (herbivore), koala (leaf eater), Tasmanian devil (carnivore) |
Significance
- Explains high species diversity in specific regions
- Demonstrates role of natural selection in shaping morphology
- Provides evidence for evolution from a common ancestor
🧾 Quick Recap
Adaptive radiation: Rapid diversification of species from a common ancestor
Drivers: Ecological opportunities + natural selection
Classic Example: Darwin’s finches – beak adaptations
Significance: Demonstrates evolution, adaptation, and biodiversity
Human Evolution
🌱Definition
Human evolution is the process by which modern humans (Homo sapiens) evolved from apelike ancestors over millions of years through changes in morphology, behavior, and genetics.
Key Idea: Combination of fossil evidence, comparative anatomy, and molecular studies showing gradual adaptation and diversification.
Important Stages of Human Evolution
| Stage | Approx. Time | Key Features | Significance |
|---|---|---|---|
| Sahelanthropus tchadensis | ~7 mya | Small brain (~350 cc), bipedal traits | Earliest known hominin; transitional ape-human features |
| Australopithecus afarensis | ~3.9–2.9 mya | Brain ~400–500 cc, bipedal, small jaw, prognathous face | Famous fossil: “Lucy”; clear evidence of bipedalism |
| Australopithecus africanus | ~3–2 mya | Brain ~450–550 cc, upright posture | Adapted to walking, partially arboreal |
| Paranthropus robustus / boisei | ~2.7–1 mya | Brain ~500–550 cc, large jaws, sagittal crest | Specialized for chewing tough vegetation |
| Homo habilis | ~2.4–1.4 mya | Brain ~600–750 cc, used stone tools, smaller teeth | “Handy man” – first tool users |
| Homo erectus | ~1.9 mya–0.1 mya | Brain ~900–1200 cc, long legs, used fire | First to leave Africa, controlled fire, more complex tools |
| Homo neanderthalensis | ~0.4–0.04 mya | Brain ~1600 cc, stocky, adapted to cold, wore clothes | Europe/West Asia; buried dead, used advanced tools |
| Homo sapiens | ~0.3 mya–present | Brain ~1350 cc, high forehead, small jaws, advanced tools, language | Modern humans; global spread, cultural & technological evolution |
Key Features of Human Evolution
- Bipedalism → upright walking; freed hands for tool use
- Increase in Brain Size → complex behavior, problem-solving, language
- Reduction in Jaw & Teeth Size → shift to cooking and tools for food processing
- Tool Use & Technology → from Oldowan to Acheulean to modern tools
- Cultural Evolution → language, art, rituals, social organization
- Migration & Global Spread → out of Africa → colonization of Asia, Europe, Americas
Theories / Patterns
- Out of Africa Theory: Modern humans originated in Africa (~0.3 mya) and replaced archaic humans elsewhere
- Multiregional Continuity Theory: Homo erectus populations in different regions evolved simultaneously into modern humans; supported by some regional fossil traits
Fossil Evidence
| Fossil / Site | Species | Key Feature |
|---|---|---|
| Lucy (Ethiopia) | Australopithecus afarensis | Bipedal, 3.5 ft tall |
| Turkana Boy (Kenya) | Homo erectus | Nearly complete skeleton, upright posture |
| Neanderthals (Europe) | Homo neanderthalensis | Stone tools, burial, adapted to cold |
| Cro-Magnon (France) | Homo sapiens | Cave art, advanced tools |
Significance of Human Evolution
- Shows progressive adaptation from apes to modern humans
- Demonstrates role of natural selection and environment in shaping human traits
- Explains origin of intelligence, language, and culture
- Supports Out of Africa theory as most accepted model
🧾 Quick Recap
Earliest hominin: Sahelanthropus tchadensis (~7 mya)
Lucy: Australopithecus afarensis → evidence of bipedalism
Tool use: Homo habilis (“handy man”)
Fire & migration: Homo erectus
Modern humans: Homo sapiens → brain ~1350 cc, culture, language
Theories: Out of Africa (most accepted), Multiregional