Chapter 9: Evolution

Introduction

Overview

• Evolutionary biology studies the history of life on Earth.
• Evolution means an orderly change from one condition to another.
• It involves changes in flora and fauna over millions of years.

Memory Aids

• Evolution = Orderly change in life forms.

Quick Checkpoints

• What does evolutionary biology study?
• Define evolution in simple terms.

Origin of Life

Key Points

• Universe is about 13.8 billion years old.
• Big Bang theory: Universe began with a huge explosion called Big Bang.
• Hydrogen and helium formed after expansion and cooling.
• Earth formed about 4.5 billion years ago in the solar system.
• Life originated on Earth from non-living matter (biopoiesis) about 4000-4200 million years ago.
• Several theories explain origin of life: special creation, cosmic panspermia, biogenesis, Oparin-Haldane theory.

Memory Aids

• Biopoiesis = Life from non-living matter.

Quick Checkpoints

• What is biopoiesis?
• When did Earth form?

Theories for Origin of Life

Main Theories

• Theory of special creation: Life created by supernatural power; organisms formed suddenly and unchanged.
• Cosmic panspermia theory: Life came from spores transferred from space.
• Theory of spontaneous generation: Life arose from decaying matter; disproved later.
• Theory of biogenesis: Life arises only from pre-existing life; supported by Pasteur's experiments.

Memory Aids

• Biogenesis = Life from life only.
• Spontaneous generation = Disproved old idea.

Quick Checkpoints

• What does biogenesis state?
• Who disproved spontaneous generation?

Modern Hypothesis or Chemical Theory

Oparin-Haldane Theory

• Life originated from non-living organic molecules via chemical evolution.
• Early Earth had different atmosphere and conditions.
• Spontaneous generation under current conditions is impossible.
• Published in Oparin's book The Origin of Life.

Memory Aids

• Chemical evolution = Life from organic molecules.

Quick Checkpoints

• What is chemical evolution?
• Who proposed the chemical theory?

Chemical Evolution

Key Experiments

• 1953: S.L. Miller simulated early Earth conditions in lab.
• Electric discharge passed through gases (CH4, H2, NH3, water vapor) at 800°C.
• Amino acids and other organic compounds formed.
• Supports idea of "hot dilute soup" where life building blocks formed.

Memory Aids

• Miller experiment = Lab simulation of early Earth.

Quick Checkpoints

• What gases were used in Miller's experiment?
• What was the significance of electric sparks?

Biological Evolution

Evolution of Life Forms

• First non-cellular life forms appeared ~3 billion years ago (RNA, proteins, polysaccharides).
• Macromolecules formed protobionts (protein, lipid, nucleic acid clusters) with simple metabolism.
• Artificial protobionts: Oparin's coacervates (no lipid membrane), Fox's microspheres (lipid coat).
• Nucleic acid mutation led to self-duplication; nucleoproteins formed first life signs.
• Protocells (nucleoproteins + lipid coat) were first life forms.
• Protocells evolved into prokaryotic cells.
• First living beings: single-celled prokaryotes with naked DNA, mostly anaerobic chemoheterotrophs.
• Some evolved into chemoautotrophs using chemosynthesis (e.g., iron bacteria).
• Development of bacteriochlorophyll led to photosynthesis.
• True chlorophyll evolved; oxygenic photosynthetic bacteria (cyanobacteria) released free oxygen.
• Development of nucleus, mitochondria, and organelles led to metabolically active eukaryotic cells (~1.5 billion years ago).

Memory Aids

• Protobionts = Precursors to cells.
• Protocell = First true life form.
• Cyanobacteria = First oxygen producers.

Quick Checkpoints

• What are protobionts?
• How did photosynthesis evolve?
• When did eukaryotic cells appear?

Evolution of Life Forms

Key Observations

• Charles Darwin observed during H.M.S Beagle voyage that living forms share similarities with each other and with extinct life forms.
• Extinctions occurred over time; new life forms appeared in different geological periods.
• Populations have built-in variations in traits.
• Traits aiding survival in natural conditions lead to better reproduction.

Memory Aids

• "Survival of the fittest" means better reproductive success.
• Variation + Environment = Natural Selection.

Quick Checkpoints

• What did Darwin conclude about similarities among life forms?
• How do variations affect survival?
• What is the role of natural selection?

Fitness and Natural Selection

Concepts

• Fitness refers to reproductive success, not just survival.
• Individuals better adapted to environment leave more offspring.
• Natural selection is the process where nature selects fitter individuals.
• This process drives evolution over time.

Memory Aids

• Fitness = Reproductive success.
• Natural selection = Nature's filter.

Quick Checkpoints

• Define fitness in evolutionary terms.
• How does natural selection affect populations?
• What is the mechanism of evolution according to Darwin?

Contributions of Alfred Wallace

Insights

• Alfred Wallace independently reached similar conclusions as Darwin.
• New organism types appear over time.
• All life forms share common ancestors from different geological periods.
• Geological and biological histories of Earth are closely linked.

Memory Aids

• Wallace = Co-discoverer of natural selection.
• Common ancestors link all life forms.

Quick Checkpoints

• Who was Alfred Wallace?
• What is the significance of common ancestors?
• How are geological and biological histories related?

What are the Evidences for Evolution?

Palaeontological Evidences

• Fossils are remains of hard parts of life-forms found in sedimentary rocks.
• Different-aged rock layers contain fossils of different life-forms indicating their existence in specific geological periods.
• Fossil records show a chronological succession: fishes → amphibians → reptiles → birds → mammals.
• Unaltered fossils preserve original form (e.g., woolly mammoths in ice, insects in amber).
• Petrified fossils form by mineral replacement of hard parts (bones, teeth, shells).
• Geological time is divided into eras, periods, and epochs based on fossil study.
• Mass extinction events caused large-scale disappearance of species, possibly due to meteor impacts or continental shifts.

Memory Aids

• Fossils show life’s timeline in rocks.
• Unaltered = original form; petrified = mineral replaced.
• Mass extinction = sudden large-scale species loss.

Quick Checkpoints

• What do fossils indicate about past life?
• Difference between unaltered and petrified fossils?
• What causes mass extinction?

Evidences from Comparative Anatomy and Morphology

Homology

• Homologous organs share common ancestry and embryonic origin but perform different functions.
• Example: Forelimbs of whale, bat, cheetah, and human have similar bone structure (humerus, radius, ulna, carpals, metacarpals, phalanges).
• Divergent evolution/adaptive radiation leads to homologous structures.
• Heart structure in vertebrates shows gradual complexity from fishes (2-chambered) to mammals (4-chambered), confirming evolutionary changes.

Memory Aids

• Homology = same structure, different function.
• Forelimb bones pattern is key homology example.
• Heart chambers increase complexity in evolution.

Quick Checkpoints

• What defines homologous organs?
• Examples of homologous structures?
• How does vertebrate heart complexity support evolution?

Analogy

Analogous Organs

• Analogous organs differ in structure and origin but perform similar functions.
• Example: Wings of butterflies and birds look alike but differ anatomically.
• Other examples: Octopus eye vs mammal eye; penguin flippers vs dolphin flippers; sweet potato (root) vs potato (stem).
• Analogy results from convergent evolution or adaptive convergence.
• Biochemical similarities in proteins and genes also indicate common ancestry.
• Connecting links show traits of two taxonomic groups (e.g., Protopterus between fish and amphibians).
• Vestigial organs are reduced, non-functional remnants (e.g., human nictitating membrane).
• Atavism is reappearance of ancestral traits (e.g., moving pinna in some humans).

Memory Aids

• Analogy = different structure, same function.
• Convergent evolution creates analogous organs.
• Vestigial organs = evolutionary leftovers.

Quick Checkpoints

• Define analogous organs.
• Examples of analogy in animals and plants?
• What are vestigial organs and atavism?

Evidences from Embryology

Embryological Evidence

• All multicellular organisms start development from a single fertilized egg forming blastula and then gastrula with ectoderm and endoderm layers.
• Early embryos of vertebrates show remarkable similarity, hard to distinguish between species.
• Recapitulation theory: Ontogeny recapitulates phylogeny (development repeats evolutionary history).
• Retrogressive metamorphosis: loss of chordate features during development (e.g., ascidian tadpole).
• Neoteny: retention of larval traits in adults (e.g., cartilaginous skeleton in adult chondrichthyes, larval gills in salamanders).

Memory Aids

• Embryos show common early development.
• Recapitulation links development to evolution.
• Neoteny retains juvenile traits in adults.

Quick Checkpoints

• What is homology in embryonic development?
• Explain recapitulation theory.
• Examples of retrogressive metamorphosis and neoteny?

Natural Selection

Industrial Melanism

• Example of natural selection studied in moths by R.A. Fischer, E.B. Ford, and H.B.D. Kettlewell.
• Dark melanic moths appeared in industrial areas with soot-darkened trees.
• Peppered moth has two forms: light grey (typica) and dark melanic (carbonaria) due to mutation.
• Before industrialization, white moths camouflaged on lichen-covered trees and survived better.
• After industrialization, soot covered trees, white moths were preyed upon more, dark moths survived better.
• Result: increase in dark moth population, decrease in white moths.

Memory Aids

• Natural selection favors traits suited to environment.
• Industrial melanism shows color adaptation to pollution.
• Mutation creates variation for selection.

Quick Checkpoints

• What is industrial melanism?
• How does environment affect moth coloration?
• Role of mutation in natural selection?

Adaptive Radiation

Definition and Examples

• Evolution of different species from a common ancestor in a geographical area, spreading to various habitats.
• Darwin's finches of Galapagos Islands: Different beak types evolved due to varied feeding habits from a common seed-eating ancestor.
• Australian marsupials: Diverse species evolved from a single ancestral stock within Australia.
• Multiple adaptive radiations in one area lead to convergent evolution.
• Example of convergent evolution: Australian marsupials and placental mammals (e.g., placental wolf and Tasmanian wolf-marsupial).
• Parallel evolution: Convergent evolution in closely related species (e.g., running habit in deer and horse with vestigial splint bones).

Memory Aids

• Adaptive radiation = species radiate from one ancestor to many habitats.
• Darwin's finches = beak diversity from seed-eating ancestor.
• Convergent evolution = similar traits in different lineages.
• Parallel evolution = similar traits in closely related species.

Quick Checkpoints

• What defines adaptive radiation?
• Examples of adaptive radiation in nature?
• Difference between convergent and parallel evolution?
• Role of common ancestor in adaptive radiation?

Biological Evolution

Lamarck's Theory

• Lamarck published his theory in 1809 as 'The inheritance of acquired characters.'
• Changes acquired during an individual's life due to environment are inherited by offspring.
• Four main propositions:
  • Internal vital force: Organisms grow due to an internal force.
  • Effect of environment and new needs: Environmental changes create new needs, producing new structures (appetency).
  • Use and disuse of organs: Used organs develop; unused organs degenerate.
  • Inheritance of acquired characters: Acquired traits are passed to offspring, accumulating over generations to form new species.

Memory Aids

• Appetency = Desire for new structures.
• Use organs = Grow; Disuse organs = Shrink.
• Internal vital force drives growth.

Quick Checkpoints

• What triggers new needs in Lamarckism?
• How does use and disuse affect organs?
• What is inherited according to Lamarck?

Evidences in favour of Lamarckism

• Giraffe's long neck evolved by gradual elongation due to food scarcity in dry deserts.

Criticism of Lamarckism

• August Weismann disproved inheritance of acquired characters.
• Only germ cell traits are inherited (continuity of germplasm theory).
• Experiment: Rats' tails cut for 22 generations; offspring still had tails.

Darwin's Theory

Natural Selection

• Charles Darwin studied nature extensively during H.M.S. Beagle voyage.
• Observed variations in Galapagos Islands, called it "living laboratory of evolution."
• Theory announced in 1858 by Darwin and Alfred Russel Wallace.
• Key steps:
  • Rapid multiplication: Many offspring produced.
  • Struggle for existence: Competition for resources.
  • Survival of the fittest: Best adapted survive.
  • Limited resources: Food and space are limited.
  • Inheritance of useful variations: Favorable traits passed on.
  • Formation of new species: Accumulated changes create new species.

Memory Aids

• Survival of the fittest = Best traits survive.
• Struggle for existence = Competition for limited resources.
• New species form by accumulated useful changes.

Quick Checkpoints

• What causes struggle for existence?
• How are useful variations inherited?
• What leads to formation of new species?

Evidences in favour of Darwinism

• Variations, mimicry, protective coloration, and flower-insect correlations support Darwinism.
• Lederberg's replica experiment with penicillin supports adaptation.

Criticism against Darwinism

• Did not distinguish somatic and germinal variations.
• Cannot explain unchanged organisms over millions of years.
• Discontinuous variations and survival of the fittest have limitations.
• Use and disuse effects and vestigial organs unexplained.

Mechanism of Evolution

Hugo de Vries' Mutation Theory

• Proposed by Dutch botanist Hugo de Vries based on experiments on Oenothera lamarckiana (evening primrose).
• Mutations or discontinuous variations are the raw materials of evolution.
• Mutations appear suddenly and become effective immediately.
• Mutations are random and directionless, unlike Darwinian small, directional variations.
• Mutations do not revolve around the species' mean character, unlike continuous variations.
• Same mutation types can appear in multiple individuals of a species.
• All mutations are inheritable.
• Mutations occur in all possible directions.
• Useful mutations are naturally selected; lethal ones are eliminated; harmless or useless mutations may persist.
• Evolution by mutation is called saltation (single-step large mutation), contrasting Darwin's gradualism.

Neo-Darwinism

• Modified Darwinism incorporating research by Weismann, de Vries, Stebbins, Dobzhansky, Wright, Mayr, etc.
• Factors for new species formation: genetic variation, isolation, heredity, natural selection, and speciation.

Memory Aids

• Mutation = sudden, inheritable change.
• Saltation = big jump in evolution.
• Neo-Darwinism = Darwin + genetics + natural selection.

Quick Checkpoints

• What is the basis of mutation theory?
• How do mutations differ from Darwinian variations?
• Define saltation in evolution.
• List factors responsible for speciation in Neo-Darwinism.

Hardy-Weinberg Principle

Genetic Equilibrium and Formulae

• Genetic equilibrium: gene frequencies remain constant across generations in a large, randomly mating population without mutation, selection, or gene flow.
• Alleles: D (dominant) with frequency p, d (recessive) with frequency q.
• Genotype frequencies: DD = p², Dd = 2pq, dd = q².
• Equations: p + q = 1 (allele frequencies), p² + 2pq + q² = 1 (genotype frequencies).
• p² = homozygous dominant %, q² = homozygous recessive %, 2pq = heterozygous %.

Factors Affecting Equilibrium

• Gene migration: movement of individuals changes gene frequencies in original and new populations.
• Gene flow: repeated gene migration causing gene exchange between populations.
• Genetic drift: random changes in allele frequencies by chance.
• Founder effect: new population with different allele frequencies due to small group migration.
• Mutation and genetic recombination introduce new variations.
• Natural selection alters gene frequencies based on survival advantage.

Natural Selection Types

• Stabilising selection: favors average traits, reduces extremes.
• Directional selection: favors one extreme trait, shifts population trait distribution.
• Disruptive selection: favors both extremes, reduces average traits.

Memory Aids

• p + q = 1: total allele frequency.
• p² + 2pq + q² = 1: genotype frequency sum.
• Gene flow = repeated gene migration.
• Founder effect = small group starts new population.
• Natural selection types: Stabilising (average), Directional (one extreme), Disruptive (both extremes).

Quick Checkpoints

• What conditions maintain Hardy-Weinberg equilibrium?
• How do gene migration and gene flow differ?
• What causes genetic drift and founder effect?
• How do mutation and recombination affect allele frequencies?
• What are the three types of natural selection and their effects?

A Brief Account of Evolution

Evolution Timeline and Key Events

• About 2000 million years ago (mya), first cellular life appeared on Earth.
• Single-celled organisms evolved into multicellular forms.
• By 500 mya, invertebrates were active.
• Jawless fish evolved around 350 mya.
• Seaweeds and early plants appeared around 320 mya.
• Plants were the first to invade land, becoming widespread before animals.
• Fish with strong fins (lobefins) evolved into first amphibians around 350 mya.
• Amphibians gave rise to reptiles with thick-shelled eggs resistant to drying.
• Reptiles dominated for 200 million years; some returned to water (e.g., Ichthyosaurs).
• Dinosaurs, including Tyrannosaurus rex, existed until about 65 mya before sudden extinction.
• Small reptiles survived; mammals evolved as viviparous, intelligent animals.
• Continental drift influenced mammal distribution; pouched mammals survived in Australia.
• Some mammals adapted fully to aquatic life (whales, dolphins, seals, sea cows).

Evolution of Plants and Vertebrates

• Plants evolved from Chlorophyte ancestors to land plants.
• Major plant groups: ferns, conifers, cycads, ginkgos, horsetails.
• Seed plants evolved from progymnosperms, splitting into seed ferns and gymnosperms.
• Flowering plants (angiosperms) appeared in the Cretaceous period, including monocots and dicots.
• Vertebrates evolved into Sauropsids (reptiles, birds) and Synapsids (mammals).
• Extinct groups: Thecodonts, Therapsids, Pelycosaurs.
• Modern reptiles include turtles, lizards, snakes, tuatars, crocodiles.
• Birds and mammals evolved and continue to exist today.

Memory Aids

• "2000 mya first life, 500 mya invertebrates thrive"
• "Lobefins to amphibians, then reptiles lay tough eggs"
• "Dinosaurs extinct 65 mya, mammals rise and survive"
• "Plants first on land, then animals follow"
• "Sauropsids = reptiles & birds, Synapsids = mammals"
• "Continental drift shapes mammal distribution"

Quick Checkpoints

• When did first cellular life appear?
• What are lobefins and their significance?
• Which group evolved first on land, plants or animals?
• What caused dinosaur extinction?
• How did mammals differ from reptiles in reproduction?
• What are the main vertebrate evolutionary groups?
• How did continental drift affect mammal evolution?

Origin and Evolution of Man

Human Evolution Overview

• Human evolution (anthropogenesis) is the biological evolution of Homo sapiens sapiens from other hominids and mammals.
• Primates like Dryopithecus and Ramapithecus existed ~15 million years ago (mya); hairy, walked like apes.
• Ramapithecus was more man-like; Dryopithecus more ape-like.
• Fossils in Ethiopia and Tanzania (~3-4 mya) show hominid features; upright walking, height under 4 feet.
• Australopithecus lived ~2 mya in East African grasslands; hunted with stone tools, mainly frugivorous.
• Homo habilis (brain 650-800 cc) was the first hominid, likely non-meat eater.
• Homo erectus (~1.5 mya) had ~900 cc brain, likely ate meat.
• Neanderthal man (brain 1400 cc) lived 100,000-40,000 years ago in Near East and Central Asia; used hides, buried dead.
• Homo sapiens arose in Africa, spread globally, developed races.
• Modern Homo sapiens appeared during Ice Age (75,000-10,000 years ago).
• Prehistoric cave art (~18,000 years ago) found at Bhimbetka rock shelter, Madhya Pradesh.
• Agriculture and human settlements began ~10,000 years ago.

Memory Aids

• Dryopithecus = "Dry ape" (more ape-like).
• Ramapithecus = "Rama man" (more man-like).
• Australopithecus = "Southern ape" (early upright walker).
• Homo habilis = "Handy man" (first tool user).
• Homo erectus = "Upright man" (fire user, meat eater).
• Neanderthal = "Near East man" (buried dead, used hides).
• Modern humans = "Wise man" (advanced culture and technology).

Quick Checkpoints

• What distinguishes Ramapithecus from Dryopithecus?
• Brain size range of Homo habilis?
• Key features of Neanderthal man?
• When did modern Homo sapiens appear?
• Significance of Bhimbetka rock shelter?

Human Evolution

Evolutionary Stages

• Australopithecus (5 mya): Ape-like, upright walking, small brain, plant eater, climbed trees.
• Homo habilis (1.5-2 mya): "Skillful man," used simple tools, larger brain than Australopithecus.
• Homo erectus (1.8-1.7 mya): Taller, human-like, used fire, lived in groups, omnivorous.
• Homo sapiens neanderthalensis (400,000-30,000 years ago): Strong, used tools and fire, cared for sick.
• Homo sapiens fossilis (50,000-10,000 years ago): Early modern humans (Cro-Magnon), skilled tool makers, artists, complex communities.
• Homo sapiens sapiens (25,000 years ago to present): Modern humans, larger brains, advanced culture and technology, originated near Caspian and Mediterranean seas.

Memory Aids

• Australopithecus = "Southern ape," early upright walker.
• Homo habilis = "Handy man," first tool user.
• Homo erectus = "Upright man," fire user.
• Neanderthals = "Near East man," cared for sick.
• Homo sapiens fossilis = Early modern humans, artists.
• Homo sapiens sapiens = "Wise man," modern culture.

Quick Checkpoints

• Order of human ancestors from Australopithecus to modern humans?
• Which species first used fire?
• Brain size progression in human evolution?
• Characteristics of Neanderthals?
• Significance of Cro-Magnon humans?