Evolution — Notes

This chapter is assessed through internal (school-based) assessment, not the board theory paper.

Evolution is the change in the inherited characteristics of a population over many generations. Individuals develop; populations evolve.

• Acquired traits develop during an individual’s life through use, training, injury or surroundings (muscles built by exercise, a scar, a language learnt). They do not change the DNA of the germ cells, so they are not inherited.
• Inherited traits come from the DNA received from the parents (blood group, natural eye colour) and can be passed on.
• Experiments in which the tails of mice were cut for several generations still produced young with normal tails: body changes are not inherited.

Only variations in germ-cell DNA can be the raw material for evolution.

Natural selection: individuals vary; more are born than can survive; those with variations that suit their surroundings survive and reproduce more; they pass these variations on; over generations, the useful variations become common.

• Example: on dry brown grass, birds catch green grasshoppers more easily, so brown ones become common.
• The variations already exist; animals do not change because they "need" to.

Genetic drift: a random change in how common a variation is, caused by chance events (which individuals happen to survive or breed). It gives no survival advantage and has the greatest effect in small populations.

A species is a group whose members can interbreed and produce fertile offspring. Speciation is the formation of new species.

How it happens:

1. A barrier (river, mountain range, sea) splits a population; gene flow between the parts stops (geographical isolation).
2. Each part changes separately by genetic drift and by natural selection in its own surroundings.
3. Over many generations, changes in DNA (sometimes even in chromosome number) or in behaviour (mating calls, breeding seasons) make the two groups unable to interbreed: reproductive isolation.
4. The two groups are now separate species.

The more characteristics two organisms share, the more recently they had a common ancestor. So classification also traces evolutionary relationships, shown as a family tree.

• Homologous organs: same basic structure and origin, possibly different functions. Forelimbs of a human (grasping), cat (walking), whale (swimming) and bird (flying) all have one upper arm bone, two forearm bones, wrist bones and finger bones. They show common ancestry.
• Analogous organs: different structure and origin, similar function. Wings of an insect and a bird; a potato (stem) and a sweet potato (root) both storing food. They show that similar ways of life can lead to similar solutions, not close relationship.

Fossils are the preserved remains or traces of organisms that lived long ago (bones, shells, leaf impressions, footprints, insects in amber). Most form when remains are buried in sediment that slowly turns to rock.

Finding the age of a fossil

• Relative dating (depth): in undisturbed layers of sedimentary rock, deeper layers are older, so deeper fossils are older.
• Radioactive dating: comparing the amounts of different isotopes of an element (such as carbon) in the fossil gives its approximate age in years. Carbon-14 halves about every 5730 years.

Archaeopteryx had feathers (like birds) but also teeth, clawed fingers and a long bony tail (like reptiles): a link between reptiles (dinosaurs) and birds. Feathers probably first evolved for warmth and were later used for flight.

Complex organs evolve in stages, each stage being useful. The eye: light-sensitive patches (eyespots of planaria) → a cup that shows the direction of light → a narrow opening that gives a rough image → a lens eye with a sharp image. Eyes evolved separately in several groups (octopuses, insects, vertebrates).

Artificial selection shows how powerful selection is. From wild cabbage, farmers selected different features over many generations to produce cabbage (short stem, packed leaves), broccoli (flower-bud clusters), cauliflower (dense head of undeveloped flowers), kohlrabi (swollen stem) and kale (large leaves) — all still one species.

Molecular phylogeny: comparing DNA shows relationships directly: the more similar the DNA, the closer the relationship.

• Humans did not evolve from chimpanzees. Humans and chimpanzees evolved from a common ancestor that lived several million years ago; their DNA is close to 99% alike.
• Modern humans, Homo sapiens, arose in Africa a few hundred thousand years ago. Groups later spread out of Africa to Asia, Europe, Australia and the Americas.
• All humans today are one species. Differences in skin colour, height or hair are small variations, not separate species or "races" in the biological sense.
• Evolution is not progress towards a goal: it produces diversity suited to surroundings. Bacteria and humans are both products of the same long history.