Final answer:
A negative slope is expected for a trade-off between traits, while positive slope indicates a positive relationship and zero slope indicates no relationship.
Step-by-step explanation:
A negative slope is expected for the line describing a trade-off between traits. This means that as one trait increases, the other trait decreases. Graphically, a negative slope appears as a line that moves from the upper left to the lower right on a coordinate plane. A positive slope, on the other hand, means that as one trait increases, the other trait also increases. Graphically, a positive slope appears as a line that moves from the lower left to the upper right on a coordinate plane. Finally, a zero slope would indicate that there is no trade-off between the two traits, meaning that changes in one trait do not affect the other at all.