Evolutionary Biology BZO5T11

Syllabus topics (RTMNU)

• Theories of origin of life – Chemical evolution, Oparin–Haldane concept, Miller–Urey experiment
• Evolution of prokaryotes and eukaryotes – endosymbiotic theory
• Geological time scale - Precambrian, Paleozoic Era, Mesozoic Era, Cenozoic Era
• Overview of major evolutionary events (Origin of prokaryotes, Evolution of eukaryotic cells (endosymbiotic theory), Transition from water to land, Evolution of amniotic egg)
• Fossils and fossilization – types, dating methods, and significance

Theories of Origin of Life: Chemical Evolution, Oparin–Haldane Concept

Older ideas such as special creation and spontaneous generation (life from non-living matter like mud) were rejected after experiments of Francesco Redi and Louis Pasteur (swan-neck flask, 1860s) showed that life comes only from pre-existing life (biogenesis).

Chemical evolution means the gradual formation of complex organic molecules from simple inorganic molecules on the early earth, which finally led to the first living cell.

Oparin–Haldane concept: A. I. Oparin (Russia, 1924) and J. B. S. Haldane (Britain, 1929) independently proposed:

1. The early earth had a reducing atmosphere (CH₄, NH₃, H₂, water vapour) with no free O₂.
2. Energy from lightning, UV rays, volcanic heat caused these gases to react and form simple organic compounds (sugars, amino acids, nitrogen bases).
3. These collected in the oceans to form a "hot dilute soup" (primordial soup – Haldane's term).
4. They joined into polymers (proteins, nucleic acids) and formed aggregates – coacervates (Oparin) or microspheres (Sidney Fox).
5. Such aggregates got a membrane, a self-copying molecule and metabolism and became the first protocell – an anaerobic heterotroph.

Miller–Urey Experiment

In 1953, Stanley Miller and Harold Urey tested the Oparin–Haldane idea in a closed glass apparatus.

• A flask of boiling water (ocean) sent vapour into a chamber with CH₄, NH₃ and H₂ (atmosphere).
• Electric sparks from electrodes (lightning) passed through the gases.
• A condenser cooled the gases (rain), and the liquid collected in a U-tube trap.
• After about a week, the liquid turned brown and contained amino acids (glycine, alanine), simple sugars and other organic compounds.

Significance: proved that organic molecules can form from inorganic ones under early-earth conditions, supporting chemical evolution.

Evolution of Prokaryotes and Eukaryotes – Endosymbiotic Theory

The first cells (about 3.5–3.8 billion years ago) were simple anaerobic prokaryotes. Later, cyanobacteria evolved oxygenic photosynthesis and released O₂, changing the atmosphere into an oxidising one (Great Oxidation Event, about 2.4 billion years ago) and allowing aerobic life and the ozone layer.

Endosymbiotic theory (Lynn Margulis, 1967): mitochondria and chloroplasts were once free-living prokaryotes that were engulfed by a larger host cell and lived inside it as symbionts; over time they became permanent organelles. Aerobic bacteria (like α-proteobacteria) → mitochondria; cyanobacteria → chloroplasts.

Evidences:

1. They have their own circular DNA, like bacteria.
2. They have 70S ribosomes, like prokaryotes.
3. They divide by binary fission.
4. They have double membranes (inner one is like a bacterial membrane).
5. Their size is similar to bacteria; antibiotics affecting bacteria also affect their protein synthesis.

Geological Time Scale and Major Evolutionary Events

The geological time scale divides the history of the earth (about 4.6 billion years) into eras, periods and epochs based on rock layers and fossils.

Transition from water to land: lobe-finned fishes (like Tiktaalik, Eusthenopteron) with fleshy fins and lungs gave rise to early tetrapods (Ichthyostega) in the Devonian. Needs met: lungs, limbs, protection from drying, new sense organs.

Evolution of amniotic egg: reptiles evolved a shelled egg with membranes (amnion, chorion, allantois, yolk sac). It protects the embryo from drying, so reproduction no longer needed water – full conquest of land.

Fossils and Fossilisation: Types, Dating and Significance

Fossils are preserved remains, impressions or traces of organisms of past geological ages, found mostly in sedimentary rocks. Fossilisation is the process by which they form – quick burial, absence of decay and replacement by minerals.

Types: petrified (minerals replace tissue, e.g., petrified wood), moulds and casts, impressions/compressions (leaves), whole-body preservation (insects in amber, mammoths in ice), trace fossils (footprints, burrows), coprolites (fossil dung).

Dating methods: relative dating – order of rock layers (deeper is older); absolute (radiometric) dating – based on half-life of radioactive elements: carbon-14 (half-life about 5730 years, for remains up to about 50,000 years), potassium–argon and uranium–lead (for very old rocks).

Significance: direct evidence of evolution; show sequence of life; connecting links (Archaeopteryx); help in finding oil and coal; show past climates.

Likely exam questions

1. (Long, 8 marks) Explain the Oparin–Haldane theory of origin of life and describe the Miller–Urey experiment.
2. (Long, 8 marks) Describe the geological time scale with major evolutionary events of each era.
3. (Short, 4 marks) Give evidences supporting the endosymbiotic theory.
4. (Short, 4 marks) What are fossils? Describe their types.
5. (Short, 4 marks) Write a note on radiometric dating of fossils.

Syllabus topics (RTMNU)

• Sources of variation – mutation and recombination
• Population genetics – gene pool, allele frequency, Hardy–Weinberg equilibrium
• Forces of evolution – natural selection, genetic drift, gene flow
• Types of Evolution – Divergent evolution, Convergent evolution and Adaptive radiation

Sources of Variation: Mutation and Recombination

Variation means differences among individuals of the same species. Heritable variation is the raw material of evolution.

Mutation is a sudden, heritable change in the genetic material. It is the ultimate (original) source of new alleles.

• Gene (point) mutations: substitution (transition, transversion), insertion, deletion (frameshift). Example: sickle-cell anaemia – one base change in the β-globin gene (glutamic acid → valine).
• Chromosomal mutations: deletion, duplication, inversion, translocation.
• Genomic mutations: change in chromosome number – aneuploidy (Down syndrome, trisomy 21) and polyploidy (common in plants).

Mutations are random; most are harmful or neutral; a few are useful.

Recombination reshuffles existing alleles into new combinations: (1) crossing over between homologous chromosomes in meiosis, (2) independent assortment of chromosomes, (3) random fertilisation. It does not make new alleles but creates huge numbers of new genotypes in each generation.

Population Genetics: Gene Pool, Allele Frequency, Hardy–Weinberg Equilibrium

Population genetics studies the genetic make-up of populations and how it changes. A population is a group of interbreeding individuals of one species in one area.

Gene pool: the total of all alleles of all genes present in a population at one time.

Allele frequency: the proportion of a particular allele among all alleles of that gene in the population. Evolution (at the population level) is a change in allele frequencies over generations.

Hardy–Weinberg principle (G. H. Hardy and W. Weinberg, 1908): in a large, randomly mating population, allele and genotype frequencies remain constant from generation to generation if there is no evolutionary force.

If p = frequency of allele A and q = frequency of allele a, then p + q = 1 and genotype frequencies are p² + 2pq + q² = 1 (AA, Aa, aa).

Conditions: (1) large population, (2) random mating, (3) no mutation, (4) no migration (gene flow), (5) no natural selection. In nature these are rarely all met, so populations evolve.

Forces of Evolution: Natural Selection, Genetic Drift, Gene Flow

1. Natural selection (Darwin): individuals with useful variations survive and reproduce more ("survival of the fittest"), so their alleles increase. Types: stabilising (favours average, e.g., human birth weight), directional (favours one extreme, e.g., industrial melanism in peppered moth Biston betularia, antibiotic resistance), disruptive (favours both extremes).
2. Genetic drift (Sewall Wright effect): random change in allele frequencies by chance, strongest in small populations. Special cases: founder effect (a few individuals start a new population) and bottleneck effect (population size suddenly crashes, e.g., cheetah).
3. Gene flow (migration): movement of alleles between populations by migration of individuals or gametes. It brings new alleles and makes populations more similar.

Mutation and non-random mating also change gene frequencies.

Types of Evolution: Divergent, Convergent and Adaptive Radiation

Adaptive radiation (H. F. Osborn): the rapid evolution of many different species from one ancestral form, each adapted to a different ecological niche. Examples: Darwin's finches of the Galapagos (beaks for seeds, insects, cactus); Australian marsupials (kangaroo, koala, marsupial mole); radiation of placental mammals after dinosaurs died out; limbs of mammals adapted for running, flying, swimming, burrowing and climbing.

Likely exam questions

1. (Long, 8 marks) State and explain the Hardy–Weinberg principle. What factors disturb the equilibrium?
2. (Long, 8 marks) Describe the forces of evolution: natural selection, genetic drift and gene flow.
3. (Short, 4 marks) Differentiate between divergent and convergent evolution.
4. (Short, 4 marks) Write a note on adaptive radiation.
5. (Short, 4 marks) In a population 9% individuals are recessive (aa). Calculate the frequencies of AA and Aa.

Syllabus topics (RTMNU)

• Concept of species and isolating mechanisms
• Modes of speciation - Allopatric, Peripatric, Parapatric and Sympatric
• Patterns of evolution – Phyletic gradualism and Punctuated equilibrium
• Mass extinction - Concept of Mass Extinction, Big Five Mass Extinction Events (Overview Only), Causes of Mass Extinction and evolutionary radiations

Concept of Species

A species is the basic unit of classification and evolution. Main concepts:

1. Typological (morphological) species concept: a species is a group of individuals that look alike (Linnaeus). Weakness: males and females or larval stages may look different; sibling species look the same.
2. Biological species concept (Ernst Mayr, 1942): "Species are groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups." This is the most accepted definition for sexually reproducing animals.
3. Evolutionary and phylogenetic concepts: a species is a lineage with its own evolutionary history / the smallest group sharing a common ancestor.

Isolating Mechanisms

Isolating mechanisms are biological barriers that prevent interbreeding between species and keep their gene pools separate.

A. Pre-zygotic (prevent mating or fertilisation):

• Habitat (ecological) isolation: live in different habitats in the same area.
• Seasonal (temporal) isolation: breed at different times or seasons.
• Behavioural (ethological) isolation: different courtship songs, dances or signals (e.g., flashing patterns of fireflies).
• Mechanical isolation: genitalia do not fit.
• Gametic isolation: sperm cannot fertilise eggs of other species.

B. Post-zygotic (after fertilisation):

• Hybrid inviability: hybrid embryo dies early.
• Hybrid sterility: hybrid lives but is sterile (mule = horse × donkey).
• Hybrid breakdown: F₁ fertile but F₂ weak or sterile.

Modes of Speciation: Allopatric, Peripatric, Parapatric and Sympatric

Speciation is the process by which new species arise from an existing species.

Patterns of Evolution: Phyletic Gradualism and Punctuated Equilibrium

Macroevolution is large-scale evolution above species level – origin of new genera, families and major groups over long time. Microevolution is small change within a species (allele frequency).

Mass Extinction: Concept, the Big Five, Causes and Evolutionary Radiations

Extinction is the complete disappearance of a species. Background extinction happens slowly all the time. A mass extinction is a geologically short event in which a large proportion (often over 50–75%) of species on earth die out.

Big Five mass extinctions (overview):

1. End-Ordovician (about 444 mya) – glaciation and fall in sea level; many marine invertebrates lost.
2. Late Devonian (about 372–359 mya) – loss of reef organisms and many fishes; low ocean oxygen.
3. End-Permian (about 252 mya) – the largest ("Great Dying"); about 90% or more of marine species; Siberian volcanic eruptions, global warming, ocean acidification.
4. End-Triassic (about 201 mya) – volcanic activity; opened the way for dinosaurs.
5. End-Cretaceous (K–Pg) (about 66 mya) – asteroid impact (Chicxulub crater, Mexico) plus Deccan Trap volcanism in India; dinosaurs (except birds) and ammonites died out.

Causes: asteroid impact, massive volcanic eruptions, climate change (cooling or warming), sea-level change, loss of oxygen in oceans, ocean acidification. Today human activities (habitat loss, pollution, climate change) are causing what many scientists call a "sixth extinction".

Evolutionary radiations after extinction: Mass extinctions empty many ecological niches. Surviving groups then undergo rapid adaptive radiation to fill them. Example: after the K–Pg extinction, mammals radiated into many forms (whales, bats, primates, ungulates); after the Permian event, reptiles and later dinosaurs radiated.

Likely exam questions

1. (Long, 8 marks) What is a species? Describe the isolating mechanisms in animals.
2. (Long, 8 marks) Describe the modes of speciation with suitable examples.
3. (Short, 4 marks) Differentiate between phyletic gradualism and punctuated equilibrium.
4. (Short, 4 marks) Write a note on the Big Five mass extinctions.
5. (Short, 4 marks) What is allopatric speciation?

Syllabus topics (RTMNU)

• Primate Characteristics and Major Primate Groups - Diagnostic features of primates, Evolutionary origin of primates (brief overview), New World monkeys, Old World monkeys and apes
• Evolution of humans – Australopithecus, Homo habilis, Homo erectus, Homo neanderthalensis, Homo sapiens
• Molecular evolution – molecular clock (Definition, Concept that mutations accumulate at a relatively constant rate over time, Use of molecular clock to estimate divergence time)
• Phylogeny (Phylogenetic trees - cladograms vs phylograms, Nodes, branches, common ancestor, Monophyletic, paraphyletic, and polyphyletic groups, Concept of homology vs analogy)
• Applications of evolutionary biology – medicine, agriculture, and environment

Primate Characteristics and Major Primate Groups

Primates are an order of mammals including lemurs, tarsiers, monkeys, apes and humans. They are mainly tree-dwelling (arboreal), and their features are adaptations to life in trees.

Diagnostic features:

1. Pentadactyl limbs with grasping hands and feet; opposable thumb (and big toe in many).
2. Flat nails instead of claws; sensitive finger pads.
3. Forward-facing eyes giving stereoscopic (3-D) vision; colour vision; reduced snout and sense of smell.
4. Large brain compared to body size; high intelligence.
5. Usually one young at a time; long parental care and long childhood.
6. Complex social behaviour; clavicle present; mobile shoulder joint.

Evolutionary origin: primates arose from small insect-eating, tree-dwelling mammals around the end of the Cretaceous/early Palaeocene (about 65 mya). Early forms like Plesiadapis led to prosimians (lemurs, lorises, tarsiers) and later to anthropoids.

Evolution of Humans

Humans (family Hominidae, tribe Hominini) shared a common ancestor with chimpanzees about 6–7 million years ago in Africa. Main trends: bipedal (upright) walking, larger brain, smaller jaws and teeth, tool making, language and culture.

Molecular Evolution: Molecular Clock

Molecular clock: the idea that mutations in DNA (and amino acid changes in proteins) accumulate at a roughly constant rate over time, so the number of differences between two species tells how long ago they shared a common ancestor.

• Proposed by Emile Zuckerkandl and Linus Pauling (1962) from haemoglobin studies.
• Molecules used: cytochrome c, haemoglobin, rRNA, mitochondrial DNA.
• Use: the clock is calibrated with a fossil of known age. Then divergence time = number of differences ÷ rate of change. Example: if two species differ by 20 substitutions and the rate is 2 substitutions per million years along the two lineages combined, they diverged about 10 million years ago.
• Limitation: rates differ between genes and groups, so results are estimates.

Phylogeny: Trees, Groups, Homology and Analogy

Phylogeny is the evolutionary history of a group. It is shown as a phylogenetic tree.

• Branches = lineages; nodes = branching points representing a common ancestor; tips = living or studied species; root = oldest ancestor.
• Cladogram: shows only the branching order (relationships); branch lengths have no meaning.
• Phylogram: branch lengths show the amount of change (or time).

Homology vs analogy: homologous structures have the same origin (forelimbs of vertebrates) and indicate common ancestry – used to build trees; analogous structures have similar function but different origin (wings of insects and birds) – result of convergence and can mislead tree building.

Applications of Evolutionary Biology

• Medicine: understanding antibiotic resistance in bacteria and drug resistance in malaria parasites; tracking virus evolution (influenza, SARS-CoV-2 variants) to update vaccines; evolutionary origin of genetic diseases.
• Agriculture: crop and animal breeding uses artificial selection; managing insecticide resistance in pests (rotation of pesticides, refuges for Bt cotton); using wild relatives of crops for disease-resistance genes.
• Environment and conservation: identifying evolutionarily distinct species for protection; keeping genetic diversity to avoid bottleneck effects; predicting how species respond to climate change; biological control.

Likely exam questions

1. (Long, 8 marks) Give the diagnostic features of primates and compare New World monkeys, Old World monkeys and apes.
2. (Long, 8 marks) Trace the evolution of man from Australopithecus to Homo sapiens.
3. (Short, 4 marks) What is a molecular clock?
4. (Short, 4 marks) Differentiate between monophyletic, paraphyletic and polyphyletic groups.
5. (Short, 4 marks) Write any four applications of evolutionary biology.