TOEFL Speaking Task 3

In biology, the concept of 'phenotypic plasticity' refers to the ability of a single genotype to produce different phenotypes in response to environmental conditions. This means that an organism’s physical traits are not fixed by its genes alone; instead, the environment can trigger changes in how genes are expressed. Phenotypic plasticity allows species to adapt to varying habitats without genetic mutation, often enhancing survival. A classic example involves temperature-dependent sex determination in reptiles, where the same clutch of eggs can yield different sexes based on incubation temperature.

Professor: Today, let’s look at a real case of phenotypic plasticity. Consider the red-eared slider turtle. When females lay eggs, the temperature of the sand where the eggs are buried determines the sex of the hatchlings. If the nest is warm, around 30 degrees Celsius, all the babies are female. If it’s cooler, around 24 degrees, they are all male. Now, the turtles in a single clutch share nearly identical genes—they are siblings. But because the mother chooses different nesting sites with varying temperatures, some eggs develop into males and others into females. This is not a genetic difference; it’s the environment acting on the same set of genes. As a result, the species can balance sex ratios in a population without any mutation. So, the same genotype can give rise to very different physical outcomes—here, the sex—just by adjusting to the surrounding temperature.