Natural Selection

Explained

Natural selection, and the order the marks come in

Darwin's idea rests on things that are individually unremarkable. Organisms vary. More offspring are produced than can survive. The ones that happen to be better suited to their surroundings are more likely to survive and reproduce. Their offspring inherit what made them suited.

None of those statements is surprising on its own. Put together and repeated over many generations, they change a population, and that is evolution.

The chain

Examiners mark this as a sequence, so it is worth learning as one.

Variation exists within the population, and it arises from mutation. A selection pressure acts, which might be a predator, a disease, a shortage of food or a change in climate. Individuals with the useful variation are more likely to survive. Those survivors reproduce. They pass the allele to their offspring. Over many generations the allele becomes more common in the population.

Six steps, and each one is usually a mark. Miss out reproduction and the chain breaks, because surviving changes nothing on its own. An organism that survives and never breeds leaves the population exactly as it found it.

The word that causes most of the lost marks

Organisms do not adapt because they need to. The variation is already there, produced at random by mutation before the selection pressure arrived, and the pressure merely decides which of the existing variants does well.

So a population does not develop resistance in response to a poison. Some individuals were resistant already, by chance, and those are the ones left alive to breed. The population changes; no individual within it changed at all.

Sentences beginning they evolved so that, or they needed to, describe a mechanism Darwin argued against. They cost marks reliably, and rewriting them is usually a matter of moving the variation to the start of the sentence.

Using an example

Antibiotic resistance is the example most questions use, because it happens fast enough to observe.

A random mutation in one bacterium gives resistance to a particular antibiotic. When the antibiotic is used, the non resistant bacteria are killed and that one survives. Bacteria divide roughly every twenty minutes, so the survivor's descendants quickly become the whole population, and the antibiotic no longer works.

Peppered moths make the same argument over a longer period. Dark and pale forms both existed before the industrial revolution; soot darkened the tree trunks, the pale moths became easier for birds to see, and the proportion of dark moths rose. When the air cleaned up, the proportions moved back.

That reversal is useful. It shows the population tracking its environment rather than improving, and it shows that whether a variation is an advantage depends entirely on where the organism happens to be living.

Evolution and speciation

Evolution is the change in the inherited characteristics of a population over time. It is a property of populations, not of individuals, and that distinction is worth guarding.

If two populations of one species become separated, natural selection acts on each in its own conditions. Different variations are favoured in each place, and the two populations drift apart. Given enough time they can no longer interbreed to produce fertile offspring, and at that point they are separate species.