Evolution by Natural Selection, Grade 12: Darwin, Speciation and Five Corrections

This is the topic where the printed account argues with itself, and one of the two sides is right.

Unit 1 tells learners that over time "only the most suitable traits would be present in individuals of the species". Unit 5, twenty pages later, explains antibiotic resistance by saying that bacteria already carry the alleles that make them resistant before the antibiotic arrives.

Both cannot be true, and the second one is correct. If selection really removed every trait but the best, there would be no resistant bacteria, no variation left to select from, and no further evolution of anything. This page sorts that out, along with four more.

Where this fits in the curriculum

Subject Life Sciences
Grade 12
Term 3
Topic Topic 10, Evolution by natural selection
Units Unit 1 origins of ideas about evolution. Unit 2 artificial selection. Unit 3 Darwin's theory. Unit 4 speciation. Unit 5 evolution in present times
Status No formally assessed practical. The class activity in this topic carries no marks
Marks Our worksheet is 70

This page sells you nothing

There is no apparatus in this topic and we are not going to invent any. Nothing to pick, nothing to refill. Evolution is taught with a board, a discussion and a worksheet, and anyone selling you a natural selection kit is selling you coloured paper.

Unit 1: where the ideas came from

Kind of evidence What it shows
The fossil record How organisms changed over time, and the relationship between extinct and living species
Structural similarity Shared characteristics inherited from a common ancestor, called modification by descent
Embryo development Vertebrate embryos share a nerve cord, a notochord, gill slits and a tail
Genetic similarity Close similarity in DNA means a recent common ancestor
Chemical similarity Cytochrome C is identical in humans and chimpanzees and differs from the rhesus monkey by a single amino acid
Biogeography Where species are distributed, and what that says about how they formed

Hypothesis against theory, which is worth a mark every year

Term What it means here
Hypothesis An explanation that can be tested but may turn out not to be true
Theory An explanation built on observation, experiment and reasoning that has not been disproved

Evolution is a theory, not a hypothesis. In everyday speech "theory" means a guess. In science it means close to the opposite, and that difference is the whole of the confusion outside the classroom.

Finding 1: vestigial does not mean useless

You will read that vestigial structures are "small, poorly developed structures that are inherited from a common ancestor but have no real use", with the human appendix given as the example.

Vestigial means reduced from something larger in an ancestor. It does not mean the structure does nothing. Those are two different claims, and only the first one is what the word means.

The appendix, specifically
It carries lymphoid tissue and takes part in immune function
It is understood to act as a reservoir of gut bacteria, which helps repopulate the gut after illness clears it out
It is still vestigial, because it is a much reduced caecum compared with the large fermenting caecum of a plant-eating ancestor

Both things are true at once, and that is the point. A structure can be reduced from its ancestral form and still be doing a job. If a learner is asked to define vestigial, reduced and no longer serving its original function is the answer. "Useless" is not.

Finding 2: punctuated equilibrium is not about jumps

You will read that Eldredge and Gould's theory of punctuated equilibrium "suggests that evolution occurs as big jumps rather than small steps".

That describes a different idea, and one Gould spent years arguing against.

What punctuated equilibrium actually says What the sentence describes
Species stay much the same for long stretches, called stasis, then change over a geologically short period New forms appearing in single large leaps
"Short" here means thousands of years. Each step is still a small one, generation by generation Steps large enough to be visible between parent and offspring
It is a claim about rate and about stasis, not about the size of each change It is a claim about the size of each change

The useful exam answer compares the two models on tempo. Gradualism has change spread evenly through time. Punctuated equilibrium has long flat stretches with change concentrated into short bursts, which is what produces the gaps in the fossil record.

Unit 3: Darwin, and the sentence that undoes him

Darwin's theory as the topic states it: organisms with genetic variations best adapted to their environment tend to survive and pass those characteristics to the next generation, and organisms less adapted probably do not.

That is sound. The content summary then adds a sentence that is not.

Finding 3: the claim that selection removes all variation

"Over time, only the most suitable traits would be present in individuals of the species."

This is the most consequential error in the topic, because the rest of the topic refutes it.

If that sentence were true What the same topic says
A population would end up with no variation left Bacteria already carry resistance alleles before the antibiotic ever arrives. That is Unit 5
There would be nothing further for selection to act on, so evolution would stop Evolution is described as continuing in the present, in insects, bacteria and viruses
Nobody could be heterozygous for anything The genetics topic is built entirely on heterozygotes

Unit 5 is the one to believe. Selection shifts the proportions of alleles in a population. It does not sweep the others away, and if it did, the next change in the environment would wipe the population out because nothing would be left to select.

Put both sentences on the board together. This is the single best five minutes available in Grade 12 Life Sciences, because the contradiction is sharp, it is in the learner's own material, and resolving it teaches what natural selection actually does.

Finding 4: individuals do not adapt

You will read that "species and individuals that cannot adapt to the changes quickly enough become extinct", and that "those that do survive evolve into new species".

An individual cannot adapt and cannot evolve. It is born with whatever alleles it has and it dies with them. It either survives and reproduces or it does not.

Level What happens there
The individual Survives or does not. Reproduces or does not. Does not change
The population This is where adaptation happens, as allele proportions shift across generations
The species Where speciation and extinction are counted

This matters more than it looks. "Individuals adapt" is the Lamarckian idea the same unit sets out to replace, and it is the single most common wrong answer in this topic. A learner who writes that the giraffe stretched its neck has made exactly the mistake that sentence invites.

Finding 5: adaptations do not happen in order to help

"Adaptations are genetic changes that lead to a change in the structure or functioning of an organism to make it better suited to its environment."

The words "to make it" put the purpose before the change. Mutations happen without reference to what would be useful. Most are neutral or harmful. The environment then decides which of the ones that already exist get passed on.

The honest order is: variation first, selection second. An adaptation is a heritable feature that happens to work, not a change made in order to work. In an exam, describing a mutation as occurring "so that" the organism could survive loses the mark.

Unit 2: artificial selection

Where Example given
Domesticated animals Speed in racehorses, increased milk production in cows
Crop plants New varieties of maize and wheat, bred from the seed of individual plants with the wanted traits

The disadvantage is the one worth understanding, because it is natural selection and artificial selection pulling against each other. Breeding hard for one trait narrows the gene pool, and a narrow gene pool is a fragile one.

The topic states that selectively bred organisms "do not have enough genetic variation to make them resistant to diseases or changes in the environment". The accurate version is that selective breeding reduces genetic variation, which raises the risk. It does not switch resistance off, and bred populations are not uniformly defenceless. Say reduced rather than absent and the answer is both correct and more useful.

Unit 4: speciation

The biological species concept: a group of individuals that can breed among themselves and produce fertile offspring. The word that earns the mark is fertile. A horse and a donkey produce a mule, the mule is sterile, and so they remain two species.

Mechanism of reproductive isolation What keeps the two groups apart
Geographic A physical barrier separates two populations and genetic changes build up in each. This is the one the exam asks about most
Temporal They breed at different times
Behavioural Species-specific courtship, so the signal is not recognised
Mechanical Physical or structural differences prevent mating
Gametic Mating happens, the gametes are chemically incompatible, no fertilisation follows
Ecological Different habitats, different food, different ways of life

A smaller point for the board. The topic lists discontinuous variation as "gender, number of blood groups or eye colour". The biological characteristic is sex, and the characteristic is blood group rather than the number of them. Eye colour is the weakest of the three examples, because it is controlled by several genes and does not fall into neat categories the way blood group does.

Unit 5: evolution you can watch

This unit is the strongest writing in the topic and it is where the page agrees with the material rather than correcting it.

Case What it shows
Insects and DDT Resistant individuals were already present. The insecticide removed the rest
Bacteria and antibiotics Same mechanism, which is why finishing a course matters
HIV and antiretroviral drugs Same again, and the reason treatment uses several drugs at once
Galapagos finches Bill and body size shifting measurably with food supply

Short life cycles are what makes this visible. Bacteria, viruses and insects turn over generations fast enough that allele proportions shift inside a human lifetime, which is why evolution stops being a thing that happened and becomes a thing that is happening.

Use this unit to settle Finding 3. Resistance only works as an explanation if the resistant alleles were already in the population. That is the proof that selection does not remove all variation, and it is in the learner's own material.

What you should be able to do by the end

  • Name the kinds of evidence for evolution and give an example of each
  • Distinguish a hypothesis from a theory, and say which evolution is
  • Define vestigial correctly, as reduced rather than useless
  • Compare gradualism with punctuated equilibrium on tempo, not on step size
  • State Darwin's theory and keep adaptation at the level of the population
  • Explain artificial selection and why it narrows a gene pool
  • Give the biological species concept, with the word fertile in it
  • Name the mechanisms of reproductive isolation and explain geographic speciation
  • Explain antibiotic or insecticide resistance with the alleles present beforehand

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