AP Biology · Unit 5 · Topic 5.5
One genotype does not specify one phenotype. Move the environment and watch the phenotype move with it — and watch two genotypes swap places on the way.
Learning objective 5.5.A Explain how the environment affects phenotype.
Pick a system and drag the environment. The genotype does not change — you are looking at the same DNA the whole time — and the phenotype moves anyway. The line each genotype traces is called its norm of reaction.
Drag the environment from one end to the other with a single genotype in mind. That whole range of phenotypes belongs to one genotype. A cloned hydrangea in two pots, a rabbit's ear and its shoulder, two genetically identical Daphnia in two ponds — each pair differs visibly with no genetic difference at all.
Two genotypes, one environmental gradient. Ask which genotype gives the bigger phenotype and the honest answer is usually another question: at what environment?
Where the two lines cross, the two genotypes produce exactly the same phenotype. Measure there and you cannot tell them apart at all. Measure to the left and one is ahead; measure to the right and the other is. None of those three measurements is wrong, and none of them generalises.
Step to the rabbit system. The full-color allele makes more pigment than the Himalayan allele at every temperature, so the lines never cross and the ranking holds everywhere. A reversal is a property of a particular pair of norms of reaction — it is a thing that can happen, not a thing that must.
Which is why it has to be checked rather than assumed, in either direction.
Set a genotype and an environment for each of two individuals, then read what their phenotype difference is evidence for. Most of the time the answer is: less than you would expect.
Set both individuals to the same genotype and different environments: any difference you see is environmental, because there is no genetic difference to explain it. Set them to the same environment and different genotypes: now the difference is genetic. Change both at once and the difference is uninterpretable — it could be either, or both, or one offsetting the other.
Try it deliberately: give the two individuals the same genotype and put them at opposite ends of the environment. They differ, sometimes by more than two different genotypes would in the same environment. If a phenotype difference by itself proved a genetic difference, this arrangement would be proving something false.
Set the two individuals to different genotypes and find the environments that make their phenotypes equal. They now look identical and are genetically different — so a shared phenotype is not evidence of a shared genotype either. The inference fails in both directions.
Answer these in your own words before you open the answers.
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| What you held constant | What a difference means | Can you conclude anything? |
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Only the first two rows support a conclusion, and each supports a different one. The third row is the situation most real observations are actually in.