Chapter 6: Evolution (Genetics and Evolution)

Darwin's theory says that individuals in a population vary, some variations are heritable, and those with useful variations survive and reproduce more (natural selection).

In a large bacterial population, a few bacteria, by chance mutation, carry genes that let them resist an antibiotic. When the antibiotic is used, the sensitive bacteria die but the resistant ones survive and multiply. Within a short time, most of the population is resistant. The antibiotic did not create the resistance; it only selected the already-present resistant variants. This is evolution by natural selection seen within a human lifetime.

Resistant variants already exist by mutation; the antibiotic kills the rest, so the resistant ones survive and multiply (natural selection).

This is a project activity. Some well-known examples you can read about:

• Homo floresiensis, the small "hobbit" human found on the island of Flores, Indonesia (2003).
• Homo naledi, found in the Rising Star cave in South Africa (2015).
• Tiktaalik, a fish with features of four-legged animals, a link between fish and tetrapods (2004).
• Feathered dinosaur fossils from China, supporting the link between dinosaurs and birds.

Collect news reports on such finds and note what they tell us about evolution and the debates around them.

A project: examples include Homo floresiensis, Homo naledi, Tiktaalik and feathered dinosaurs.

A species is a group of natural populations whose members are similar in structure and function and can interbreed freely to produce fertile offspring, and which are reproductively isolated from other such groups.

A group of similar organisms that can interbreed in nature to produce fertile offspring and are reproductively isolated from others.

The trends: brain size and intelligence increased; upright (bipedal) posture developed; the skeleton changed (shorter arms, S-shaped spine, smaller jaws and teeth); diet moved from fruits to include meat; tool-making, use of fire, language and culture developed.

Brain size grew from about 500 cc to about 1350 cc, upright walking developed, the jaw and teeth reduced, diet broadened to meat, and tools, fire and culture appeared.

This is a project activity. Studies such as the mirror test (an animal recognises a mark on its own body in a mirror) suggest that some animals have self-awareness: great apes (chimpanzees, orangutans), dolphins, elephants and magpies have passed this test. Self-consciousness in its full human sense is still debated.

A project: apes, dolphins, elephants and magpies pass the mirror self-recognition test.

For example, horse–Eohippus, elephant–Moeritherium, whale–Pakicetus, birds–Archaeopteryx and human–Australopithecus (full list in the table).

This is a practice activity. Draw neat, labelled diagrams of the plants and animals studied in your syllabus (e.g. Archaeopteryx, the evolution of the horse's foot, homologous organs such as the forelimbs of a whale, bat, cheetah and human).

A practice activity: make neat labelled drawings of the organisms and structures in the chapter.

Adaptive radiation is the evolution of different species from a common ancestor, spreading into different habitats (geographical areas) and adapting to them.

Example: Darwin's finches of the Galapagos Islands. From an original seed-eating finch that reached the islands, many species evolved with different beak shapes suited to different foods: some kept seed-eating beaks, others developed beaks for eating insects, cacti and so on.

(Another example: Australian marsupials, which radiated from a common ancestor into forms like kangaroo, koala, wombat and marsupial mole.)

Darwin's finches: from one seed-eating ancestor, many species evolved with different beaks for different foods.

No. Adaptive radiation produces many different species from one ancestor, living side by side in different habitats. Human evolution is mainly a single line of change over time (from Australopithecus through Homo habilis and Homo erectus to Homo sapiens), producing one surviving species. So it is better called anagenesis (linear evolution), not adaptive radiation.

No; it is mostly a single line leading to one surviving species, not many species radiating into different habitats.

The horse evolved over about 50–60 million years in North America:

• Eohippus (Eocene): size of a fox (about 30 cm); four toes on forelimbs, three on hindlimbs; lived in forests; ate soft leaves.
• Mesohippus (Oligocene): larger; three toes on each foot, the middle one larger.
• Merychippus (Miocene): about 100 cm tall; three toes, but only the middle toe touched the ground; teeth suited for grazing grass.
• Pliohippus (Pliocene): one-toed (side toes reduced to splints); fast runner.
• Equus (modern horse): large; single hoofed toe; long legs; high-crowned grinding teeth for grass.

Trends: increase in body size; reduction in the number of toes to one hoof; lengthening of legs for fast running on grassland; teeth adapted from browsing to grazing.

Eohippus → Mesohippus → Merychippus → Pliohippus → Equus: bigger size, fewer toes (one hoof), longer legs and grazing teeth.