AP Biology · Unit 7 · Topic 7.1
Natural selection is usually explained. Here it is run: give a population heritable variation, apply an environment, and count what is left after fifty generations.
Learning objective 7.1.A Describe the causes of natural selection, and 7.1.B explain how natural selection affects populations.
A new variant appears in one individual in ten. Choose how much better it does than the alternative, then watch fifty generations of counting — nothing else happens, and nothing else needs to.
| Advantage per generation | Generations to 95% | Frequency at generation 50 |
|---|
Look at the graph again. No organism in it changed in any way. Every individual was born with its variant and died with it. The only thing that moved was the proportion — which is why evolution is defined as a change in a population across generations, and why asking how an individual evolved is asking the wrong question.
Each case gives two variants a survival rate and a number of offspring. Predict which one takes over before you look — then see whether survival was any guide at all.
| Case | Survives better | Fitness A | Fitness B | Wins | Did survival predict it? |
|---|
The same allele, the same population, the same rules. The only difference is whether the environment stays put — and that turns out to decide not just the speed but the direction.
| Generations | Environment | Favors | Allele frequency at the end of the spell |
|---|
An allele at zero cannot come back — there is nothing left for selection to act on. That is why a fluctuating environment can preserve variation that a constant one would have destroyed: the environment turns over before selection finishes the job. It is also why the same population, watched for fifty generations, can look like it is evolving steadily in one direction and be doing nothing of the kind.
Answer these in your own words before you open the answers.
1.
2.
3.
4.
5.
6.
7.
8.