5.5 Environmental Effects on Phenotype

Genes are instructions, but whether and how much they're used depends on conditions. Temperature, light, nutrients, pH, chemicals and even other organisms can switch genes on or off or change how well their proteins work. The result is phenotypic plasticity: one genotype producing different phenotypes in different environments.

This doesn't change the DNA sequence. If conditions change back, many plastic traits change back too. It's the environment acting on gene expression, which links this topic to Unit 6.

• Hydrangea flower color: the same plant makes blue flowers in acidic soil and pink flowers in alkaline soil. Acidic soil makes aluminum available to the plant, and aluminum changes the color of the flower pigments.
• Seasonal coat color: arctic foxes and snowshoe hares grow white coats in winter and brown or gray coats in summer, triggered mainly by changing day length.
• Temperature-dependent sex determination: in many turtles, the incubation temperature of the egg, not sex chromosomes, decides sex. In many turtle species, warmer nests produce mostly females and cooler nests mostly males.
• Melanin and UV light: more UV exposure causes skin cells to make more melanin (a tan), which helps protect DNA from UV damage.
• Height and weight in humans: genes set a range, but nutrition, illness and activity affect where a person ends up in that range.
• Fur color in Himalayan rabbits and Siamese cats: an enzyme for pigment works only at cooler temperatures, so the cooler ears, nose, paws and tail are dark while the warm body is light.
• Mating pheromones in yeast: yeast cells step up their production of mating pheromones when cells of the opposite mating type are nearby, so the same cell's behavior depends on its neighbors.
• Phenylketonuria (PKU): people with two nonfunctional alleles of a gene for breaking down the amino acid phenylalanine can develop serious brain damage, but a low-phenylalanine diet from birth prevents most symptoms. Same genotype, very different outcome.

To prove the environment causes a difference, you need to hold genotype constant. Biologists use clones (cuttings from one plant), identical twins, or inbred lines in which every individual has the same genotype. Then they raise groups in different conditions and compare. If genetically identical organisms develop different phenotypes, the difference must come from the environment.

The flip side: if two organisms differ and you don't control for genotype, you can't tell whether genes, environment or both are responsible. That's a common experimental-design point on the exam.

Plasticity lets an organism match its phenotype to conditions it can't predict at birth. A white coat helps a fox hide in snow but would stand out in summer, so switching colors with the seasons improves survival in both. The ability to be plastic is itself a trait that can be shaped by natural selection, because individuals that respond well to changing conditions tend to leave more offspring.