Mechanism of Evolution and Human Evolution — Notes

Charles Darwin and Alfred Russel Wallace proposed the theory of natural selection; Darwin set it out in On the Origin of Species (1859). Thomas Malthus (Essay on Population) argued that populations increase geometrically while food supply increases arithmetically; Darwin applied this to all organisms.

Postulates: (1) Overproduction (prodigality) — organisms produce many more offspring than can survive. (2) Limited resources and struggle for existence — intraspecific, interspecific and environmental. (3) Variation — no two individuals are alike, and many variations are heritable. (4) Natural selection / survival of the fittest (Herbert Spencer’s phrase) — individuals with favourable variations survive and reproduce more; fitness is reproductive fitness. (5) Origin of species — favourable variations accumulate over generations, forming new species.

Examples of natural selection:

• Industrial melanism in the peppered moth Biston betularia: before industrialisation in England, light moths were camouflaged on lichen-covered trees; after soot darkened the trunks, light moths were picked off by birds and dark (melanic) moths increased (for example from 2 to 90 percent). In unpolluted areas light moths remained common.
• Giraffe’s neck: Lamarck explained it by use and stretching (inheritance of acquired characters); Darwin by selection of giraffes that already had longer necks.
• DDT resistance in mosquitoes and antibiotic resistance in bacteria: resistant variants were present before exposure and were selected.

Lederberg’s replica plating (1952): Joshua and Esther Lederberg transferred colonies from a master plate (no penicillin) using sterile velvet to plates with and without penicillin. Resistant colonies grew on the penicillin plate at the same positions as colonies on the master plate, so resistance mutations arose before exposure; penicillin only selected them — pre-adaptation.

Criticism of Darwinism: it did not explain the origin or inheritance of variations; it did not distinguish heritable from non-heritable variations; it explained the survival but not the arrival of the fittest; it could not explain vestigial and over-specialised organs well.

Hugo de Vries, studying the evening primrose Oenothera lamarckiana, observed sudden, large, heritable changes — mutations. He proposed that new species arise by saltation (single large steps), and that mutations are random and directionless, unlike Darwin’s small, continuous variations.

Modern synthetic theory (neo-Darwinism) — developed by Dobzhansky, Fisher, Haldane, Huxley, Mayr and Stebbins — combines Darwin’s natural selection with genetics. Evolution is a change in allele frequencies in a population. Factors:

1. Gene mutation and chromosomal mutation — new alleles and gene arrangements.
2. Genetic recombination — crossing over, independent assortment and random fertilisation give new combinations.
3. Gene flow (gene migration) — movement of alleles between populations.
4. Genetic drift — random changes in allele frequencies, strongest in small populations:
   • Founder effect: a few individuals colonise a new area; the new population carries the founders’ (possibly unusual) allele frequencies (for example, high frequency of some inherited disorders in isolated communities).
   • Bottleneck effect: a disaster drastically reduces a population; survivors’ frequencies, not the original ones, are passed on (for example, northern elephant seals, cheetahs with very low genetic variation).
5. Natural selection — favours alleles that increase reproductive success.
6. Reproductive isolation — prevents interbreeding, so diverging populations become new species.

Variation is the raw material of evolution. Heritable (genetic) variations arise from mutation, recombination (crossing over and independent assortment in meiosis), random fertilisation and gene flow. Non-heritable (environmental) variations — such as muscles developed by exercise — are not inherited and play no part in evolution. Variations may be continuous (height, skin colour) or discontinuous (blood groups).

Types of natural selection (for a trait with a bell-shaped distribution):

• Stabilising selection favours the average phenotype and removes both extremes; the curve becomes narrower (for example, human birth weight: very light and very heavy babies survive less well).
• Directional selection favours one extreme; the curve shifts in that direction (for example, dark peppered moths, antibiotic resistance, longer necks in giraffes).
• Disruptive selection favours both extremes over the average; the curve develops two peaks, which may lead to two new species (for example, birds with small and large beaks where only small and large seeds are available).

The Hardy–Weinberg principle (G. H. Hardy and W. Weinberg, 1908): in a large, randomly mating population with no mutation, migration, genetic drift or natural selection, allele and genotype frequencies remain constant from generation to generation — genetic equilibrium. The total gene pool stays the same.

For a gene with alleles A (frequency p) and a (frequency q):

• p + q = 1
• p² + 2pq + q² = 1, where p² = frequency of AA, 2pq = Aa and q² = aa.

Solving problems: start from the recessive phenotype (q²), take the square root to get q, then p = 1 − q. Example: 36 percent recessive → q² = 0.36, q = 0.6, p = 0.4, carriers 2pq = 0.48. From genotype counts, p = (2 × number of AA + number of Aa) ÷ (2 × total).

Factors that disturb the equilibrium (and so cause evolution): gene migration (gene flow), genetic drift, mutation, genetic recombination and natural selection. A change in frequencies from one generation to the next is a measure of evolution.

Human evolution is traced through fossils from the Miocene onward (dates and brain sizes approximate):

• Dryopithecus (about 15 million years ago; Africa, Europe): ape-like, hairy, walked like gorillas and chimpanzees; ate soft fruits and leaves; a common ancestor of apes and humans.
• Ramapithecus (about 14 million years ago; Shivalik hills of India, Africa): more man-like; small canines and thick enamel; possibly semi-erect.
• Australopithecus (about 2 million years ago; East African grasslands): upright, bipedal; brain about 450–600 cc; hunted with stone weapons; mainly ate fruit.
• Homo habilis (about 2 million years ago): the “handy man”, first human-like being; brain 650–800 cc; made stone tools; probably did not eat meat.
• Homo erectus (about 1.5 million years ago): brain about 900 cc; probably ate meat; used fire; Java man and Peking man.
• Neanderthal man (Homo neanderthalensis; 100 000–40 000 years ago; east and central Asia): brain about 1400 cc; used hides to protect the body; buried their dead.
• Cro-Magnon man (about 30 000 years ago): brain about 1650 cc; fully erect; made cave paintings and fine tools.
• Homo sapiens (modern man): arose in Africa and spread across continents; during the ice age between 75 000 and 10 000 years ago modern humans spread widely; brain about 1350–1450 cc; agriculture began about 10 000 years ago with settlements; pre-historic cave art (such as at Bhimbetka) about 18 000 years ago.

Trends: increase in brain size, upright posture and bipedal gait, reduction of jaws and canines, use of tools, fire, language and culture.