Evolutionary Biology

Evolution is the one idea that makes the whole of biology hang together. Every animal you meet in the next unit is a modified version of something older. This unit asks three questions in order: how did life begin, what is the evidence that species have changed, and by what mechanism did they change? It ends with our own case, the descent of Homo sapiens from a Miocene ape.

7.1 Life and its Origin

The Earth formed about 4.6 billion years ago; the oldest undisputed fossils, cyanobacterial mats preserved as stromatolites, are about 3.5 billion years old. Somewhere in that billion years, non-living chemistry became living chemistry.

Theories that do not work

Special creation holds that all forms of life were made at once and have not changed. It makes no testable prediction and is contradicted by the different faunas of different rock layers. Spontaneous generation (abiogenesis) held that life arises repeatedly from non-living matter — maggots from rotting meat. Three experiments killed it.

Worker	Experiment	Conclusion
Redi (1668)	meat in open, sealed and gauze-covered jars	maggots only where flies can lay eggs
Spallanzani (1767)	boiled broth in sealed flasks stayed sterile	microbes come from the air
Pasteur (1864)	broth in a swan-necked flask stayed sterile until the neck was broken	air could enter, dust could not — biogenesis

Biogenesis — omne vivum ex vivo — is therefore the rule now, but it cannot explain the first life, and panspermia (life arrived as spores from space) only moves the problem elsewhere. The surviving explanation is chemical evolution.

Chemical evolution — the Oparin–Haldane theory

A. I. Oparin (1924) and J. B. S. Haldane (1929) proposed independently that life arose by slow chemical change under conditions that no longer exist. The primitive atmosphere was reducing and had no free oxygen — methane (CH₄), ammonia (NH₃), hydrogen (H₂) and water vapour — with ultraviolet light, lightning and volcanic heat as energy sources and no ozone layer to block the UV.

1. Inorganic gases → monomers (amino acids, sugars, nitrogen bases, fatty acids) accumulating in the warm sea — Haldane's "hot dilute soup".
2. Monomers → polymers: proteins, polysaccharides, nucleic acids.
3. Polymers aggregated into membrane-bounded droplets — Oparin's coacervates, Fox's microspheres — the first protobionts.
4. A self-copying nucleic acid appeared (the RNA world), giving heredity and the first true cell, an anaerobic heterotroph feeding on the soup.
5. As the soup ran out, chemo- and then photoautotrophs evolved. Photosynthesis released O₂, which built the ozone layer and ended chemical evolution for ever.
6. Free O₂ allowed aerobic respiration and so the larger eukaryotes and multicellular life.

The Miller–Urey experiment (1953)

Stanley Miller and Harold Urey tested step 1 in a sealed glass apparatus. A flask of boiling water fed vapour to an upper chamber charged with CH₄, NH₃ and H₂; tungsten electrodes sparked for a week at about 800 °C; a condenser returned the products to a U-trap. The trap collected amino acids — glycine, alanine, aspartic acid — with urea, simple sugars and fatty acids.

The geological time scale

Geologists divide Earth history into eras, periods and epochs, with boundaries drawn where the fossil fauna changes sharply — often at a mass extinction. Learning the sequence is worth marks: almost every fossil you must name belongs to a named period.

7.2 Evidences of evolution

Darwin could not show evolution happening, so he built his case the way a lawyer does — from converging independent lines of evidence.

Evidence from morphology and anatomy

Compare the forelimb of a man, a whale, a bat and a horse. The jobs could hardly differ more — grasping, swimming, flying, running — yet the bones are the same bones in the same order: humerus, radius and ulna, carpals, metacarpals and phalanges. Only their relative lengths differ. Such organs are homologous: one ancestral pentadactyl limb remodelled for four ways of life, which is divergent evolution. A bird's wing and an insect's wing are the opposite case: both fly, but one is a bony forelimb with feathers and the other a double fold of cuticle with no bones. These are analogous — unrelated lines meeting the same problem the same way, or convergent evolution.

Feature	Homologous organs	Analogous organs
Basic structure and embryonic origin	same	different
Function	different	same
Ancestry	common ancestor	unrelated ancestors
Process shown	divergent evolution	convergent evolution
Animal example	forelimbs of man, whale, bat, horse	wing of bird and wing of insect
Plant example	thorn of Bougainvillea, tendril of Cucurbita	sweet potato (root tuber), potato (stem tuber)
Meaning	proves common descent	proves adaptation, not relationship

Vestigial organs are reduced, functionless remnants of organs that worked in an ancestor. Man has about 90: the vermiform appendix (a functional caecum in herbivores), the coccyx (four fused tail vertebrae), the plica semilunaris (a full third eyelid in frogs and birds), the auricular muscles that let a horse swivel its ear, body hair and the wisdom teeth. A designer would not fit a useless organ; an ancestor that used it explains it at once.

Evidence from embryology

Von Baer's law: the general characters of a large group appear in the embryo before the special characters of the species. Early embryos of a fish, a chick and a human can be confused — each has pharyngeal (gill) slits, a notochord, a two-chambered heart and a post-anal tail — and the later the differences appear, the closer the relationship. Haeckel's stronger theory of recapitulation ("ontogeny repeats phylogeny", the embryo passing through the adult stages of its ancestors) is wrong. What is true is that related embryos inherit the same early developmental programme, so useless leftovers persist: gill slits in a human, teeth in a baleen-whale foetus, a tail at six weeks.

Evidence from palaeontology (fossils)

A fossil is any preserved trace of a past organism, and fossils are the only direct evidence of evolution, being the extinct organisms themselves. Age is read from the law of superposition (the lower stratum of undisturbed sedimentary rock is older) and measured by radiometric dating — carbon-14 below about 50,000 years, uranium–lead and potassium–argon for older rocks. Fossils occur in a definite sequence — invertebrates, fish, amphibians, reptiles, then birds and mammals — none out of order. Some are connecting links, carrying characters of two groups.

Organism	Links	Mixed characters
Archaeopteryx (Jurassic)	reptiles ↔ birds	feathers, wings, beak; but teeth, clawed digits, long bony tail
Seymouria (Permian)	amphibians ↔ reptiles	amphibian skull; reptilian limb girdles
Lungfish (Protopterus)	fish ↔ amphibians	gills and lungs; fins with bones
Peripatus	annelids ↔ arthropods	annelid body wall; arthropod tracheae
Platypus (Ornithorhynchus)	reptiles ↔ mammals	lays eggs, cloaca; but hair and milk

A living fossil is a species almost unchanged from its fossil ancestors — Latimeria, Limulus, Sphenodon, Ginkgo biloba, Cycas. The best-documented lineage is the horse, worked out by Marsh and Osborn from an almost unbroken North American series.

The trends are: increase in body size; lengthening of limbs, head and neck; reduction of the toes from four to one with the weight on the enlarged third digit; increase in tooth-crown height with cement filling the folds; and enlargement of the brain. All follow from one change — Miocene forest gave way to grassland, so a small hiding browser became a large running grazer, and grass silica wore teeth down fast enough to make tall crowns pay.

Evidence from biogeography

Animal distribution fits history, not climate. Australia has been isolated since the Mesozoic, so almost all its mammals are marsupials, radiated into grazers (kangaroo), burrowers (wombat), gliders (flying phalanger) and carnivores (Tasmanian wolf) — a marsupial version of each placental mammal elsewhere, with no placentals to compete. Darwin's decisive observation came from the Galápagos Islands in 1835: 13 finch species found nowhere else, all close relatives of one South American ground finch but differing chiefly in the beak, each beak matching a different food. One immigrant population spread to different islands and foods and diverged — adaptive radiation.

Evidence from taxonomy, biochemistry and artificial selection

Only common descent explains why living things can be arranged in nested groups at all. Biochemistry says the same at the molecular level: all organisms use nucleic acid as the genetic material, the same genetic code, ATP and largely the same 20 amino acids — and the degree of similarity measures relationship. Cytochrome c differs from the human protein by 1 amino acid in the rhesus monkey, about 12 in the horse and about 45 in yeast; human and chimpanzee DNA are 98–99 % identical.

Artificial selection makes the argument practical: if man can change a species in centuries, nature can do it in millions of years. From the one wild species Brassica oleracea breeders produced cabbage, cauliflower, kohlrabi, Brussels sprouts, broccoli and kale, and all dogs descend from the wolf.

Evolution happening now: industrial melanism

The peppered moth Biston betularia rests by day on tree trunks in England in two forms — speckled grey typica and black carbonaria. Before industrialisation the trunks were pale with lichen and carbonaria was almost unknown, first recorded in Manchester in 1848. Soot killed the lichens and blackened the bark, and within fifty years about 98 % of Manchester moths were black. Birds ate whichever form they could see, as Kettlewell proved by mark–release–recapture in 1955. After Britain's Clean Air Act of 1956 the lichens returned and the melanic form collapsed again — the same allele favoured and then disfavoured within 150 years.

The same process runs around us: Staphylococcus resistant to penicillin, Anopheles resistant to DDT in the Terai, Plasmodium falciparum resistant to chloroquine. Each time a rare pre-existing resistant variant was the only one to survive the new chemical, and so left all the offspring.

7.3 Theories of evolution

By 1859 everyone agreed that species had changed. The argument was about how. Three answers were given in turn; the third is the one now accepted.

Lamarckism — inheritance of acquired characters

Jean Baptiste Lamarck (Philosophie Zoologique, 1809) rested his theory on four propositions: (i) a changed environment creates new needs; (ii) an organ used more enlarges and one not used degenerates; (iii) characters acquired in life are inherited; (iv) such changes accumulate into a new species. His examples are still quoted — the giraffe stretching its neck for high foliage, snakes losing limbs by creeping, webbed feet in ducks, the blind cave salamander Proteus, the reduced wings of ostrich and kiwi.

The first two propositions are true; the third is false, and everything depends on it. August Weismann cut the tails off mice for five generations (901 mice) and every litter was born with a normal tail. His germplasm theory explains why: the body is somatoplasm, the gametes germplasm, and only germplasm is transmitted — a change in the body cannot write itself back into the DNA of the gametes.

Darwinism — natural selection

Charles Darwin sailed as naturalist on HMS Beagle (1831–36), read Malthus on population growth, and published On the Origin of Species on 24 November 1859 — after Alfred Russel Wallace reached the same theory independently and the two read a joint paper in 1858. The argument has six steps.

1. Prodigality of reproduction. Every organism produces far more offspring than can survive — a pair of elephants would leave 19 million descendants in 750 years — yet populations stay roughly constant.
2. Struggle for existence. Therefore most offspring die: intraspecific (severest, since needs are identical), interspecific and environmental.
3. Variation. No two individuals of a species are exactly alike.
4. Natural selection. Individuals whose variations fit the environment better survive and reproduce more — "survival of the fittest", where fittest means best able to leave offspring, not strongest.
5. Inheritance of useful variations by the survivors.
6. Origin of species by accumulation of these changes in different directions in different places.

Darwin knew nothing of Mendel, so he could not explain how variations arise nor separate inherited from acquired ones; he could not account for over-specialisation (the antlers of the Irish elk); and a rare favourable variation seemed likely to be diluted by intercrossing (the swamping effect).

Mutation theory (Hugo de Vries, 1901)

Working on the evening primrose Oenothera lamarckiana, de Vries found individuals that differed sharply from the parents, bred true at once and had not arisen gradually. He called these mutations and argued that evolution proceeds by large, sudden, discontinuous, heritable changes which are random and pre-adaptive — they occur before, and independently of, any need for them. Mutation is indeed the ultimate source of the variation Darwin lacked, but most mutations are harmful and mutation alone has no direction.

Neo-Darwinism — the modern synthesis

Neo-Darwinism is Darwin's selection combined with Mendelian genetics, mutation theory and population mathematics, built in the 1930s and 40s by Fisher, Haldane, Wright, Dobzhansky, Huxley and Mayr.

Factor	What it does	Direction
Mutation	creates new alleles	random
Recombination	reshuffles existing alleles	random
Genetic drift	random change of frequency in small populations	random
Gene flow	moves alleles between populations	random
Natural selection	sorts variants by reproductive success	the only directional one
Isolation	stops gene pools re-mixing	fixes divergence

Point of comparison	Lamarckism (1809)	Darwinism (1859)	Neo-Darwinism (1940s)
Source of variation	environment, new needs	not explained	mutation, recombination
Nature of variation	acquired, directed	small, continuous, random	random, genetic
Role of environment	creates the character	selects among characters	selects among characters
Acquired characters inherited?	yes — central claim	no	no
Unit of evolution	the individual	the individual	the population
Mechanism	use and disuse	natural selection	selection, drift, gene flow, isolation
Present status	rejected	accepted but incomplete	accepted

Types of natural selection

Selection acts on a continuously varying character — height, seed size, beak depth — and reshapes its distribution in one of three ways.

Type	Which survive	Effect on the curve	Example
Stabilising	the intermediates	narrower, mean unchanged	human birth weight: 3–4 kg babies survive best
Directional	one extreme	mean shifts that way	industrial melanism; DDT resistance
Disruptive	both extremes	splits in two; can start speciation	seedcracker beaks: large crack hard seeds, small soft ones, intermediates neither

The Hardy–Weinberg principle

If evolution is a change in allele frequency, we need a baseline for what frequencies do when nothing changes them. It was given independently in 1908 by G. H. Hardy and Wilhelm Weinberg.

Let p be the frequency of the dominant allele A and q that of the recessive a. These are the only alleles, so p + q = 1, and random mating is equivalent to drawing two alleles at random from the gene pool, giving the terms of (p+q)^2:

with p^{2} the frequency of AA, 2pq of Aa and q^{2} of aa.

Each of the five conditions, when broken, names an agent of evolution: the population is very large (else drift); mating is random; there is no mutation, no migration and no natural selection.

Genetic drift, founder effect and bottleneck

Genetic drift is random change in allele frequency caused by sampling error in a small population. Toss a coin ten times and you may get seven heads; toss it ten thousand times and you will not get 70 %. In a population of 20 an allele can be lost or fixed by chance alone, useful or not — hence the first Hardy–Weinberg condition. Two cases are named. In the founder effect a few colonists' small sample of alleles becomes the gene pool of the new population, as in the Galápagos finches. In the bottleneck effect a population is cut down by catastrophe and recovers from the few survivors, permanently losing variation: Nepal's greater one-horned rhinoceros fell below 100 animals in Chitwan in the late 1960s and has recovered to about 750, but carries only the alleles those hundred happened to have.

Isolation and speciation

Category	Mechanism	Example
Geographical	mountain, river, sea or desert separates them	the Himalaya; Galápagos channels
Pre-zygotic — ecological	different habitat, so they never meet	Rana of ponds and of streams
Pre-zygotic — temporal	different breeding season or time of day	two pines shedding pollen a month apart
Pre-zygotic — ethological	different courtship, song or pheromone	fireflies with different flash patterns
Pre-zygotic — mechanical	genitalia or flower structure do not fit	flowers with different pollinators
Pre-zygotic — gametic	sperm cannot fertilise the egg	many marine invertebrates
Post-zygotic — hybrid inviability	the hybrid zygote dies early	sheep × goat
Post-zygotic — hybrid sterility	the hybrid lives but cannot breed	the mule (horse × donkey)
Post-zygotic — hybrid breakdown	F_1 fertile but F_2 weak	some cotton crosses

	Allopatric speciation	Sympatric speciation
Populations are	geographically separated	in the same area
First barrier	geographical isolation	reproductive, from the start
Speed	usually slow	can be immediate
Typical cause	mountain uplift, river change, islands	polyploidy, host shift, disruptive selection
Example	Darwin's finches; Grand Canyon squirrels	polyploid wheat; apple maggot flies
Frequency	the common route in animals	common in plants

The Himalaya is a working example: the range rose through the Miocene and Pliocene, cutting one fauna into northern and southern halves and stacking habitats from Terai to alpine desert within 150 km. Nepal's diversity for its size — over 200 mammal species in 147,000 km² — is a result of that repeated allopatric isolation.

7.4 Human evolution

Our place among the primates

Man is classified: Animalia → Chordata → Mammalia → Order Primates → Family Hominidae → Homo → Homo sapiens. Three words are confused: hominoid = apes plus man; hominid = great apes plus man; hominin = man and his extinct bipedal ancestors.

With all primates we share grasping hands and feet with an opposable thumb, nails instead of claws, forward-facing eyes giving stereoscopic colour vision, a large brain for body size and prolonged parental care. What separates man from the apes: habitual bipedal gait with an S-shaped vertebral column, broad pelvis, arched foot and non-opposable big toe; the foramen magnum central in the skull base, so the head balances on the column; cranial capacity 1350–1450 cm³ with a vertical forehead and no brow ridge; a chin, small canines and an evenly arched tooth row; and a precision grip, speech and learned culture.

Dryopithecus and Ramapithecus — the Miocene apes

Dryopithecus ("oak ape") lived about 22 to 9 Mya in the forests of Africa, Europe and Asia: limbs of about equal length, semi-erect with knuckle-walking, a U-shaped jaw with large canines and roughly 400 cm³ of brain. It is regarded as ancestral to the great apes and possibly to the African ape–human line.

Ramapithecus ("Rama's ape") matters in Nepal. The first jaws came from the Siwalik (Churia) Hills in 1932, and the Nepali specimen — molars dated to about 9.0–9.5 million years — from Tinau Khola, Palpa district. It was smaller than Dryopithecus, with thickly enamelled molars, reduced canines and a short deep jaw: a ground-dwelling eater of hard seeds. For twenty years it was called our first direct ancestor, but more complete specimens found in 1975–76 showed this was a mistake; it is now referred to Sivapithecus and placed on the orangutan branch. Learn it as the best-known Miocene ape of South Asia, not as our ancestor.

Australopithecus — the first biped

Australopithecus ("southern ape") lived in eastern and southern Africa from about 4.2 to 2 Mya. A. africanus was described by Raymond Dart in 1924 from the Taung skull; A. afarensis is known from "Lucy", a 40 %-complete skeleton found at Hadar, Ethiopia, in 1974 and dated to 3.2 Mya. The pelvis and knee are decisive: it walked upright on two legs while its brain was still ape-sized, only 400–500 cm³, and the 3.6-million-year-old Laetoli footprints show the stride directly. It stood 1.0–1.5 m, had large molars and heavy brow ridges, and used broken stones but made no tools. Bipedalism came before the brain, and freeing the hands made everything afterwards possible.

Homo habilis, Homo erectus and the Neanderthals

Homo habilis ("handy man", 2.4–1.5 Mya) was found by Louis and Mary Leakey at Olduvai Gorge, Tanzania, in 1960. Cranial capacity rose to 650–800 cm³ and the jaws grew lighter, but the name carries its importance: it is the first hominin that certainly made stone tools, the flaked pebble choppers of the Oldowan industry.

Homo erectus appeared about 1.9 Mya and survived to roughly 300,000 years ago, the longest run of any Homo. Cranial capacity 800–1100 cm³ (average about 950), height 1.6–1.8 m, thick brow ridge, sloping forehead and no chin. Java man (Pithecanthropus erectus, Dubois, 1891) and Peking man (Sinanthropus pekinensis, 1927) are the same species under older names. H. erectus was the first to use fire, to make the Acheulian hand axe, to hunt large game cooperatively and to leave Africa.

Homo neanderthalensis lived in Europe, the Near East and Central Asia from about 400,000 to 40,000 years ago, the first skull coming from the Neander valley, Germany, in 1856. Cranial capacity 1300–1600 cm³ (average ~1450) was larger than the modern average. They were short and heavily built, with a receding forehead, heavy brow ridge and weak chin; they made fine Mousterian flake tools, wore skins and buried their dead with grave goods, the first evidence of ritual. They disappeared about 40,000 years ago after modern humans reached Europe, leaving 1–2 % of their DNA in living non-African populations.

Homo sapiens

Anatomically modern Homo sapiens arose in Africa about 300,000 years ago and spread worldwide from roughly 70,000 years ago. The best-known early European population is Cro-Magnon man, 40,000 to 10,000 years ago, about 1.8 m tall with a cranial capacity near 1600 cm³, a vertical forehead, no brow ridge and a well-marked chin; Cro-Magnons made blade and bone tools, sewed clothing and painted Altamira and Lascaux. With Homo sapiens sapiens (1350–1450 cm³) biological evolution was overtaken by cultural evolution: agriculture from about 10,000 years ago, then writing and science.

Stage	Age	Cranial capacity	Where found	Key advance
Dryopithecus	22–9 Mya	~400 cm³	Africa, Europe, Asia	Miocene ape, semi-erect
Ramapithecus (Sivapithecus)	14–9 Mya	~400 cm³	Siwaliks, Nepal and India	thick molars; orangutan branch
Australopithecus	4.2–2 Mya	400–500 cm³	East and South Africa	fully bipedal, no tools
Homo habilis	2.4–1.5 Mya	650–800 cm³	Olduvai Gorge, Tanzania	first stone tools (Oldowan)
Homo erectus	1.9–0.3 Mya	800–1100 cm³	Java, China, Africa, Europe	fire, hand axes, left Africa
Homo neanderthalensis	0.4–0.04 Mya	1300–1600 cm³	Europe, West and Central Asia	burial of the dead, Mousterian
Homo sapiens (Cro-Magnon)	40,000–10,000 yr	~1600 cm³	Europe	cave art, blade tools
Homo sapiens sapiens	300,000 yr – now	1350–1450 cm³	worldwide	agriculture, language, culture

Chapter summary

• Life arose once, by chemical evolution under a reducing, oxygen-free atmosphere (Oparin 1924, Haldane 1929): gases → monomers → polymers → protobionts → anaerobic heterotroph → photoautotroph → aerobe. Miller and Urey (1953) obtained amino acids, not life. Pasteur's swan-necked flask had already established biogenesis.
• Homologous organs = same structure, different function → divergent evolution → common ancestry. Analogous = different structure, same function → convergent evolution → adaptation only. Vestigial organs are functionless remnants (appendix, coccyx, plica semilunaris, ear muscles). Early vertebrate embryos share gill slits, notochord and a post-anal tail (von Baer's law).
• Fossils are the only direct evidence and include connecting links — Archaeopteryx, lungfish, Peripatus, platypus, Seymouria. Horse series Eohippus → Mesohippus → Merychippus → Pliohippus → Equus: 0.3 → 1.6 m, toes 4 → 1, browsing → grazing teeth.
• Lamarckism (use, disuse, inheritance of acquired characters) was disproved by Weismann's germplasm theory. Darwinism = over-production → struggle → variation → natural selection → new species, but could not explain variation. Neo-Darwinism = mutation + recombination + selection + drift + gene flow + isolation, acting on a population.
• Selection is stabilising, directional or disruptive; industrial melanism in Biston betularia is directional selection recorded in real time.
• Hardy–Weinberg: p + q = 1 and p^{2} + 2pq + q^{2} = 1, with p^2 = AA, 2pq = Aa, q^2 = aa; it needs a large, randomly mating population with no mutation, migration or selection, and deviation from it is evolution. For a recessive trait q = \sqrt{q^{2}} from the phenotype frequency, and the carrier : affected ratio is 2p/q. Speciation needs isolation — geographical, pre-zygotic or post-zygotic; allopatric dominates in animals, sympatric in plants.
• Human line: Dryopithecus (400 cm³) → Australopithecus (450 cm³, first biped) → H. habilis (650–800 cm³, first tools) → H. erectus (950 cm³, fire) → H. neanderthalensis (1450 cm³, burial) → H. sapiens (1350–1450 cm³). Ramapithecus of the Nepali Siwaliks belongs to the orangutan branch.

Practice questions

Group A — Multiple choice (1 mark each)

1. The theory of inheritance of acquired characters was proposed by [1]aCharles DarwinbJ. B. LamarckcHugo de VriesdAugust Weismann
2. The wing of a bird and the wing of an insect are [1]ahomologous organsbanalogous organscvestigial organsdatavistic organs
3. The connecting link between reptiles and birds is [1]aSeymouriabPeripatuscArchaeopteryxdOrnithorhynchus
4. In a population in Hardy–Weinberg equilibrium the frequency of the recessive allele is 0.2. The frequency of heterozygotes is [1]a0.04b0.16c0.32d0.64
5. The first hominin to make stone tools was [1]aAustralopithecusbHomo habiliscHomo erectusdRamapithecus

Answers to Group A

1. (b) — Lamarck, Philosophie Zoologique, 1809.

2. (b) — same function (flight), completely different structure and origin.

3. (c) — Archaeopteryx: feathers and wings, but reptilian teeth, claws and a long bony tail.

4. (c) — q = 0.2, p = 0.8, so 2pq = 2(0.8)(0.2) = 0.32.

5. (b) — Homo habilis, the "handy man", made the Oldowan pebble tools.

Group B — Short answer (4 marks each)

1. Differentiate between homologous and analogous organs, giving one animal and one plant example of each. [4]
2. Describe the Miller–Urey experiment and state precisely what it proved. [4]
3. In a population of 2000 people, 320 cannot roll the tongue, a recessive character. Assuming Hardy–Weinberg equilibrium, calculate both allele frequencies and the expected number of heterozygotes. [4]
4. What is industrial melanism, and how does it demonstrate natural selection? [4]
5. State the Hardy–Weinberg principle and the conditions under which a population stays in genetic equilibrium. [4]

Answers to Group B

1. Homologous — same basic structure and embryonic origin, different function, by divergent evolution from a common ancestor: forelimbs of man, whale, bat and horse; thorn of Bougainvillea and tendril of Cucurbita. Analogous — different structure and origin, same function, by convergent evolution: wing of a bird and of an insect; sweet potato (root tuber) and potato (stem tuber). Homology proves relationship, analogy only adaptation.

2. Sealed glass system: boiling water feeds vapour into a chamber charged with CH₄, NH₃ and H₂, tungsten electrodes spark for a week at about 800 °C, a condenser returns the products to a U-trap. Result: amino acids (glycine, alanine, aspartic acid) with urea, sugars and fatty acids. It proved that the monomers of life form spontaneously from inorganic gases — it did not produce a cell, a protein or a nucleic acid.

3. q^{2} = 320/2000 = 0.16, so q = 0.4 and p = 0.6. Then 2pq = 2(0.6)(0.4) = 0.48, so heterozygotes = 0.48 \times 2000 = 960. (Check: p^2 \times 2000 = 720 homozygous rollers; 720 + 960 + 320 = 2000. ✓)

4. Industrial melanism is the rise in frequency of a dark form in a polluted area. In Biston betularia the melanic carbonaria was almost unknown before 1848; soot killed the lichens and by 1895 about 98 % of Manchester moths were black. Kettlewell's 1955 mark–recapture recovered 27.5 % of carbonaria against 13.0 % of typica in sooty Birmingham and the reverse in unpolluted Dorset: birds ate whichever form was conspicuous. After the Clean Air Act of 1956 the melanic form fell back. It is directional natural selection, because a pre-existing heritable variation changed in frequency through differential survival in a direction set by the environment.

5. In a large, randomly mating population the allele and genotype frequencies at a locus stay constant across generations, with p + q = 1 and p^{2} + 2pq + q^{2} = 1. Conditions: very large population, random mating, no mutation, no migration, no natural selection. Breaking any one makes the population evolve.

Group C — Long answer (8 marks each)

1. Describe the evolution of the horse from Eohippus to Equus with the help of a labelled diagram, and state four evolutionary trends shown by the series. [8]
2. (a) State the postulates of Darwin's theory of natural selection. [4](b) Explain the long neck of the giraffe first on Lamarck's theory and then on Darwin's, and say why only one explanation is accepted. [4]
3. Give an account of human evolution from Dryopithecus to Homo sapiens sapiens with the approximate age and cranial capacity of each stage, and list four trends shown by the series. [8]

Answer outlines to Group C

1. Diagram as in the figure above. Eohippus (Eocene, 0.3 m, 4 toes in front and 3 behind, low-crowned browsing teeth, forest) → Mesohippus (Oligocene, 0.6 m, 3 toes) → Merychippus (Miocene, 1.0 m, 3 toes with the weight on the enlarged middle toe, first high-crowned grazing teeth with cement) → Pliohippus (Pliocene, 1.2 m, one functional toe with two splints) → Equus (Pleistocene to now, 1.5–1.6 m, single hoof). Trends: body size increases; toes reduce from four to one with the weight on the third digit; crown height increases with cement in the folds, browser → grazer; limbs, head and neck lengthen and the brain enlarges. Cause: Miocene forest gave way to grassland, favouring speed and abrasion-resistant teeth.

2. (a) Prodigality of reproduction; struggle for existence (intraspecific, interspecific, environmental); variation; natural selection, or survival of the fittest; inheritance of the useful variations; origin of new species by their accumulation. (b) Lamarck: the short-necked ancestor stretched its neck for high foliage, the stretch was acquired and passed on, and the offspring stretched further — the environment created the long neck. Darwin: the population already varied in neck length; when low foliage grew scarce the longer-necked individuals fed better and left more offspring, so mean neck length rose each generation — the environment only selected among existing variations. Darwin's version is accepted because acquired somatic characters are not inherited (Weismann), whereas heritable variation plus differential survival is directly observed, as in industrial melanism.

3. Stages, ages and cranial capacities as in the table of section 7.4, in that order, with the branching of the figure. Trends: erect posture and complete bipedalism, which came before the large brain; progressive increase in cranial capacity; shortening of the jaws and face with reduced canines, loss of the brow ridges and appearance of a chin; forward shift of the foramen magnum with an S-shaped spine, broad pelvis and arched foot; and tool making, fire and speech, so that cultural evolution replaces biological.