Chapter 1 — Heredity & Evolution (Detailed Notes)
Can You Recall?
- Chromosomes & DNA in the nucleus carry hereditary characters via genes.
- Transfer of parental traits to offspring is Heredity.
- DNA is built from nucleotides (sugar–phosphate backbone + bases A, T, G, C).
1) Heredity & Hereditary Changes — Historical Milestones
- Gregor J. Mendel — Father of modern genetics; basic laws of inheritance (not recognized widely until later).
- 1901: Hugo de Vries — Mutation theory: sudden heritable changes can occur.
- 1902: W. Sutton — Chromosome theory: chromosomes occur in pairs and carry genes.
- 1944: Avery–MacLeod–McCarty — DNA is the genetic material in all organisms except some viruses.
- 1961: Jacob & Monod — Model of gene regulation/protein synthesis in bacteria (operon concept).
- Recombinant DNA — Genetic engineering arises; huge impact in medicine, agriculture & industry.
2) DNA ↦ RNA ↦ Protein — Transcription, Translation & Translocation
DNA & RNA Basics
- DNA bases: A, T, G, C with base pairing \(A=T, \; G\equiv C\).
- RNA bases: A, U, G, C (Uracil replaces Thymine).
- RNA types: mRNA (message), tRNA (amino acid carrier; anticodon), rRNA (ribosome & peptide bond formation).
Transcription
RNA polymerase reads one DNA strand (template) and synthesizes complementary mRNA. The mRNA sequence is complementary to the DNA template (U pairs with A).
Translation & Translocation
- mRNA codons are triplets of nucleotides, each coding one amino acid.
- tRNA carries specific amino acids; its anticodon pairs with mRNA codon.
- Ribosome catalyzes peptide bond formation and translocates one codon at a time along mRNA.
Mutations
Sudden heritable changes in DNA sequence (e.g., base substitution, insertion/deletion) are mutations. Effects range from silent to severe. Example: Sickle-cell anaemia (β-globin gene point mutation).
3) Evolution — Big Picture
Evolution is the gradual change in living organisms over long periods, leading to diversity and new species under natural selection.
- From simple molecules → macromolecules (proteins/nucleic acids) → first cells → diversification.
- Cells differed in traits; fitter variants survived & reproduced (natural selection).
- Today’s biodiversity spans unicellular to complex multicellular organisms across all habitats.
4) Evidences for Evolution
4.1 Morphological Evidence
Superficial similarities in external features within groups (leaf shapes/venation in plants; facial hair/nostrils/mouth placement in mammals) suggest common ancestry.
4.2 Anatomical (Homologous Structures)
Different functions but similar internal bone plans (human hand, ox foreleg, whale flipper, bat wing) → homology → descent from a common ancestor.
4.3 Vestigial Organs
- Degenerate/underdeveloped organs with little/no function in some species: appendix, coccyx (tail-bone), wisdom teeth, ear muscles, body hair in humans.
- Often functional in related species (e.g., ruminant appendix; ear muscles in monkeys).
4.4 Palaeontological Evidence (Fossils)
Remnants/impressions preserved in sedimentary rocks reveal past life and transitions. Fossils support gradual change and branching from simpler forms.
Decay law: \(N(t)=N_0 e^{-\lambda t}\), so \( \displaystyle t=\frac{1}{\lambda}\ln\!\left(\frac{N_0}{N(t)}\right)\).
Ratio \(^{14}\text{C}/^{12}\text{C}\) changes with time → age determination (Willard Libby; Nobel 1960).
4.5 Connecting Links
- Peripatus — annelid-like segmentation + arthropod-like tracheae/open circulation → link between Annelida & Arthropoda.
- Duck-billed platypus — lays eggs (reptile-like) but has mammary glands & hair → mammal–reptile link.
- Lungfish — fish that breathes with lungs → link between fishes & amphibians.
4.6 Embryological Evidence
Early embryos of vertebrates (fish, amphibian, reptile, bird, mammal) appear strikingly similar, diverging later — evidence for common origin.
5) Major Theories of Evolution
Darwin’s Natural Selection
- Organisms reproduce prolifically; resources are limited → struggle for existence.
- Variation exists; those with advantageous traits survive & reproduce (“survival of the fittest”).
- Over generations, populations change; new species can arise.
Lamarckism (Use & Disuse; Inheritance of Acquired Characters)
- Organs strengthen with use and weaken with disuse (e.g., giraffe neck lengthening, strong blacksmith shoulders, flightless bird wings).
- Claimed such acquired changes pass to offspring; later evidence did not support inheritance of acquired traits as a general rule.
6) Speciation
Species: group of naturally interbreeding organisms producing fertile offspring.
Drivers of New Species Formation
- Genetic variation (mutations, recombination).
- Isolation — geographical (mountains, rivers, distance) or reproductive (timing, behavior, anatomy).
- Natural selection in differing environments → divergence.
Over time, separated populations accumulate differences → reproductive isolation → speciation.
7) Human Evolution — A Brief Timeline
- ~70 million years: Decline of dinosaurs; primate-like ancestors evolve (lemur-like).
- ~40 million years: African monkeys lose tails; brain enlargement begins → apes (gibbon, orangutan later in Asia).
- ~25 million years: Gorilla & chimpanzee lineages in Africa.
- ~20 million years: Early human-like ancestors adopt more erect posture; hands free for use.
- ~10–7 million years: Early hominins diversify in Africa.
- ~4 million years: Australopithecines; bipedalism advances.
- ~2 million years: Homo lineage with tool use — “skilled human”.
- ~1.5 million years: Homo erectus — fully upright, wider range (Africa → Asia); control of fire emerges later.
- ~150,000 years: Early Homo sapiens (Neanderthals in some regions).
- ~50,000 years: Cro-Magnon/modern human behavior & art flourish.
- ~10,000 years: Agriculture, animal domestication, settlements, culture.
- ~5,000 years: Writing; recorded history begins. ~400–200 years: Science & industry accelerate change.
8) Quick Reference Tables
Types of Evolutionary Evidence
| Evidence | Key Idea | Examples |
|---|---|---|
| Morphological | External similarity within groups | Leaf venation; mammal facial traits |
| Anatomical (Homology) | Similar internal structure, different function | Human arm, bat wing, whale flipper |
| Vestigial Organs | Reduced/unused in one species; functional in another | Appendix, coccyx, ear muscles, wisdom teeth |
| Fossils (Palaeontology) | Past life forms preserved in rocks | Transitional fossils; sedimentary layers |
| Connecting Links | Bridge characters across two groups | Peripatus, Lungfish, Duck-billed platypus |
| Embryology | Similar early embryos | Vertebrate embryos converge early, diverge later |
Central Dogma Cheat-Sheet
9) Key Terms (Exam-friendly)
Gene Chromosome Mutation Transcription Translation Translocation (ribosome) Codon/Anticodon Homologous Structures Vestigial Organs Fossils Connecting Link Natural Selection Lamarckism Speciation Homo sapiens
Heredity & Evolution — Exercise Solutions
1) Complete the diagram
(Filled-in concept map for “Evidences of Evolution”)
External similarities (leaf venation, facial traits in mammals)
Similarities in bone structure (homologous limbs: human hand, ox foreleg, whale flipper, bat wing)
Appendix, coccyx, ear muscles, wisdom teeth
Fossils in sedimentary rocks; carbon dating
Peripatus, Lungfish, Duck-billed platypus
Early vertebrate embryos look alike
2) Justify the following statements (with examples)
a) Human evolution began approximately 7 crore (70 million) years ago.
After dinosaurs declined (~7 crore years ago), primate-like ancestors (lemur-like) diversified. Over tens of millions of years: tailless apes with larger brains evolved; later Australopithecus (∼4 million yrs), then Homo line (∼2 million yrs), leading to Homo erectus, Neanderthals, and finally modern Homo sapiens. The long primate timeline supports the ~7-crore-years beginning for the human line.
b) Geographical and reproductive isolation leads to speciation.
Barriers (mountains, rivers, islands) stop gene flow, so separated populations accumulate different mutations and adaptations. Over time, they become reproductively incompatible → new species. Examples: Darwin’s finches on Galápagos; sister species of squirrels across the Grand Canyon; African cichlid fishes in different lakes.
c) Study of fossils is an important aspect of evolution.
Fossils provide direct records of past life, show transitional forms, reveal sequences of appearance/extinction, and allow age estimation (e.g., carbon dating using \(^{14}\mathrm{C}\)). This evidence corroborates gradual change and branching descent.
d) There is evidence of foetal (embryological) similarity among chordates.
Early embryos of fish, amphibians, reptiles, birds, and mammals display similar features (pharyngeal arches, tail), diverging later. Such embryological homology indicates common ancestry of vertebrates.
3) Complete the statements (choose correct option)
- a) The causality behind the sudden changes was understood due to the principle of Mutation (Hugo de Vries).
- b) The proof that protein synthesis is governed through the Gene was given by George Beadle & Edward Tatum (one gene–one enzyme).
- c) Transfer of information from DNA to mRNA is called Transcription.
- d) Evolution means Gradual development.
- e) Vestigial organ Appendix in human is a proof of evolution.
4) Short Notes
a) Lamarckism
Lamarck proposed that use strengthens organs and disuse weakens them; such acquired characters were thought to be inherited (e.g., giraffe’s long neck from stretching; weak wings in ostrich/emus). While use/disuse can affect individuals, inheritance of acquired traits is generally not supported; hence Lamarckism is historically important but limited.
b) Darwin’s Theory of Natural Selection
Populations show variation; organisms overproduce; resources are limited → struggle for existence. Individuals with advantageous traits survive/reproduce more (“survival of the fittest”). Over generations, favorable traits accumulate → adaptation and speciation.
c) Embryology
Comparative study of embryos reveals early similarities among vertebrates (pharyngeal arches, tails), supporting common ancestry. Differences arise later as development proceeds.
d) Evolution
Evolution is long-term, heritable change in populations, leading to biodiversity and new species via mechanisms such as mutation, recombination, genetic drift, and natural selection. Evidence: morphological, anatomical (homology), vestigial organs, fossils, connecting links, embryology.
e) Connecting Link
Organisms showing features of two groups, bridging them evolutionarily. Examples: Peripatus (Annelida–Arthropoda), Lungfish (fishes–amphibians), Duck-billed platypus (reptile-like egg laying + mammalian hair & mammary glands).
5) Definitions & Explanations
a) Define heredity. Explain the mechanism of hereditary changes.
Heredity is transmission of genetic traits from parents to offspring via genes on DNA.
Mechanism (overview)
- Genes (DNA segments) replicate during cell division; meiosis creates gametes with variation (independent assortment, crossing-over).
- Fertilization restores diploidy, combining parental genes.
- Gene expression: Transcription (DNA → mRNA) and Translation (mRNA → protein) produce traits.
- Mutations and recombination introduce new variation; selection filters them → evolutionary change.
b) Define vestigial organs. Give human examples and animals where they’re functional.
Vestigial organs are reduced, non-functional (or weakly functional) structures retained from ancestors.
| Human Vestigial Organ | Functional In | Function There |
|---|---|---|
| Appendix (vermiform) | Ruminants (e.g., herbivores with large caecum) | Cellulose digestion/fermentation support |
| Ear muscles | Monkeys & many mammals | Orient pinnae to localize sounds |
| Coccyx (tail-bone) | Tailed mammals/primates | Supports tail movement/balance |
| Wisdom teeth | Primates/herbivores with coarse diets | Grinding tough plant material |
| Body hair (reduced) | Mammals | Thermal insulation, sensation |
6) (Not asked)
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7) Long Answers
a) How are hereditary changes responsible for evolution?
Hereditary changes (mutations, recombination during meiosis, gene flow) create variation. Natural selection favors advantageous variants; harmful ones are removed. Over generations, allele frequencies shift, adaptations arise, and populations diverge → speciation. Thus, small heritable changes accumulate into large-scale evolutionary change.
b) Explain the process of formation of complex proteins.
- Transcription: DNA template strand is copied into pre-mRNA → processed to mRNA.
- Translation initiation: Ribosome binds mRNA; first tRNA pairs at start codon (AUG).
- Elongation: tRNAs deliver amino acids; peptide bonds form; ribosome translocates one codon each step.
- Termination: Stop codon reached; polypeptide released.
- Folding & assembly: Polypeptide folds (often assisted by chaperones); multiple chains assemble into complex proteins.
5'-AUG-3' pairs with tRNA anticodon 3'-UAC-5'.c) Explain the theory of evolution and mention the proof supporting it.
Natural selection (Darwin): variation + overproduction + struggle → survival of the fittest → descent with modification. Evidence: morphological patterns, anatomical homology (pentadactyl limb), vestigial organs (appendix), fossils & carbon dating, connecting links (Peripatus, Lungfish, Platypus), embryological similarities among vertebrates.
d) Importance of anatomical evidences in evolution (with examples)
Homologous organs share internal structure (bone arrangement) but differ in function, indicating common ancestry and adaptive radiation.
- Human hand (grasping), bat wing (flight), whale flipper (swimming), ox foreleg (support) — same basic bone plan.
- Shows divergence from a common ancestral limb into different adaptive functions.
e) Define fossil. Explain importance as proof of evolution.
Fossils are preserved remains/impressions of ancient organisms in sedimentary rocks. Importance: reveal past biodiversity, transitional forms, chronological sequences; enable age dating (e.g., \(^{14}\mathrm{C}\) for recent samples) and show directional change, supporting descent with modification.
f) Write the evolutionary history of modern man (concise).
- ~70 million yrs: primate-like ancestors diversify after dinosaur decline.
- ~40–25 million yrs: apes with larger brains; gorilla/chimp line in Africa.
- ~4 million yrs: Australopithecus — habitual bipedalism.
- ~2 million yrs: early Homo (tool users).
- ~1.5 million yrs: Homo erectus — upright walker; wider range; control of fire later.
- ~150,000 yrs: early Homo sapiens (incl. Neanderthals regionally).
- ~50,000 yrs: behaviorally modern humans; art, culture.
- ~10,000 yrs: agriculture & settlements; ~5,000 yrs: writing & recorded history.