Natural selection is the process by which individuals with alleles that suit their environment survive and reproduce more, so those alleles become more common in the next generation. The process acts on a population, over generations. It needs variation that is inherited.
You can already describe sources of genetic variation. This lesson shows how the environment then favours some of the variation.
What is the chain to write out?
Use this chain for any organism. Each link must appear in a full answer.
- Variation: individuals in the population differ because they carry different alleles.
- Overproduction: more offspring are born than the environment can support.
- Selection pressure: a factor such as predators, disease or food supply causes some to die.
- Survival of the better adapted: individuals with helpful alleles are more likely to survive.
- Reproduction: the survivors breed and pass on the alleles.
- Change in the population: the proportion of those alleles is higher in the next generation.
Step 5 is the one students leave out. Survival alone changes nothing in the next generation.
Worked example
On a fictional island, pale sand is replaced by dark volcanic ground. A population of 1000 invented mice has 800 pale and 200 dark.
A predator hunts by sight. On the dark ground, the survival rate to breeding age is 25% for pale mice and 60% for dark mice.
Step 1, survivors among the pale mice: 800 × 0.25 = 200.
Step 2, survivors among the dark mice: 200 × 0.60 = 120.
Step 3, total survivors: 200 + 120 = 320.
Step 4, change in proportion: before selection, dark mice were 200/1000 = 20% of the population. After selection they are 120/320 = 37.5%. Check: 0.375 × 320 = 120.
Step 5, next generation: suppose the survivors breed and the new population is again 1000 with the same proportions as the parents. Then dark mice number 0.375 × 1000 = 375, and pale mice 625.
Step 6, write the explanation: “The mice varied in colour because of different alleles. Predators ate more pale mice on the dark ground. Dark mice survived and reproduced more, so they passed on the allele for dark fur. The proportion of dark mice rose.”
This is a simplified model with assumed survival rates. Real populations change by smaller steps, and repeating the same selection over many generations can move the proportion much further.
The mistake to watch for
Mistaken answer: “The strongest mice survived because they were fittest.”
Fittest does not mean strongest. Here the mice that survived were simply better camouflaged. The answer also stops at survival, so the link to the next generation is missing.
The correction: name the feature (dark fur), say why it helps (camouflage from the predator), and finish with “reproduced and passed on the allele”. A quick test is to check that your answer ends with alleles, not with individuals.
Check yourself
All populations are invented.
1. Put these steps in order: (a) survivors reproduce; (b) individuals vary; (c) proportion of the allele rises; (d) some individuals die because of a predator.
Show answer
(b), (d), (a), (c). Variation comes first, then selection pressure, then reproduction, then the change in the population.
2. After selection on a population of insects, 300 survivors remain and 210 of them carry an allele for green colour. What percentage of survivors carry the allele?
Show answer
210/300 = 0.7, so 70%.
3. A culture of 2000 bacteria contains 2 cells with a resistance mutation. An antibiotic kills every non-resistant cell. Explain what happens over the next generations.
Show answer
Only the 2 resistant cells survive. They reproduce and pass on the resistance allele. After a few divisions, all of the population descends from resistant cells, so the proportion of resistant bacteria is much higher.
Where this leads next
Next, compare selection with an organism choosing to adapt, which tackles the most common wrong explanation. You can practise counting outcomes in the probability tree and counting board, and check a simple cross in the inheritance model board. When you are ready, test the chain in the module practice set.
Students often find that the steps make sense on the page but fall apart when a new organism appears. A teacher in online one-to-one Biology tuition can give you fresh organisms to trace until the chain becomes automatic.