Organisms do not choose to adapt. Natural selection works on differences that are already there in a population, and it takes place over generations. A change that one individual shows during its own life is not passed on through alleles.
This lesson corrects a very common wrong explanation. It builds on the steps of natural selection.
How are the two ideas different?
| Natural selection | Changing within one lifetime | |
|---|---|---|
| Who changes | A population | One individual |
| Timescale | Many generations | One lifetime |
| Cause | Different alleles, and the environment favours some | A response to conditions, such as exercise |
| Inherited | Yes, through alleles | No |
| Intention | None | None needed |
Notice that neither column involves the organism wanting anything. The wrong wording is usually “the giraffes stretched to reach higher leaves, so their necks got longer and they passed it on”.
Worked example
A fictional lizard species eats leaves from tall shrubs. The data below are invented.
The wrong explanation: “The lizards needed longer necks to reach the leaves, so they stretched and their necks grew. Their young then had longer necks.”
Step 1, find the faulty claims: there are two. First, “needed” treats the need as the cause. Second, “stretched and grew, so their young had longer necks” treats an acquired change as inherited.
Step 2, rebuild with the selection chain: in the population, neck length varies because of different alleles. Shorter-necked lizards reach fewer leaves and are less likely to survive when food is short. Longer-necked lizards survive and reproduce more, so they pass on the alleles for longer necks.
Step 3, add evidence across generations: suppose the mean neck length in the population is 20 mm at the start and 26 mm after many generations. The increase is 26 − 20 = 6 mm. As a percentage of the start, 6/20 = 0.3, so 30%.
Step 4, write the final answer: “There was variation in neck length due to alleles. Longer-necked lizards reached more food, so survived and reproduced more. They passed on the allele for long necks, so the proportion of long-necked lizards increased over generations.”
Notice the final answer never uses “needed”, “wanted” or “tried”. That avoids the most common penalty wording.
The mistake to watch for
Mistaken answer: “The bacteria became resistant because the antibiotic made them adapt.”
The antibiotic did not cause resistance. The resistance allele was already present in a few bacteria, or arose by chance mutation. The antibiotic killed the non-resistant ones, so the resistant ones survived and reproduced.
Correction: replace “made them adapt” with “selected for bacteria that already had the allele”. Selective breeding is a related case, but there it is humans who choose which individuals breed, not the organism.
Check yourself
All examples are invented.
1. A student writes: “The fictional frogs grew webbed feet because they needed to swim.” Identify the error and rewrite the sentence.
Show answer
The error is treating need as the cause. A correct version: “Frogs varied in foot webbing because of alleles. Those with more webbing swam better, so survived and reproduced more and passed on the allele.”
2. A man trains at a gym and gains muscle. Explain why his children are not born with larger muscles because of this.
Show answer
The muscle gain is an acquired characteristic that happened in his lifetime. It does not change the alleles in his gametes, so it is not inherited.
3. State two differences between natural selection and selective breeding.
Show answer
In natural selection the environment decides which individuals survive and reproduce, and it takes place over many generations. In selective breeding humans choose which individuals breed, for a feature they want.
Where this leads next
After this, practise reading data without over-claiming in interpret a distribution without biological stereotypes. Afterwards, test your wording on the module practice set. For a check on how a single allele passes on in a model, the inheritance model board is useful.
Some students know the correct idea but let “needs” and “wants” slip back in under time pressure. A teacher in online one-to-one Biology tuition can listen to your explanations and flag each slip as it happens.