Final answer:
In directional selection, the mean of the trait shifts towards the extreme values, while the variance of the trait generally decreases.
Step-by-step explanation:
Directional selection, one type of natural selection, has a specific effect on the mean and variance of a trait in the next generation. In directional selection, individuals with extreme values of the trait have a higher fitness than those with average values. This causes the mean of the trait to shift towards the extreme values.
For example, if in a population of birds, birds with longer wings are better suited for a new environment, they have a higher chance of survival and reproduction compared to birds with shorter wings. Over time, the mean wing length of the bird population will increase as the genes for longer wings are passed on to the next generation. On the other hand, the variance of the trait generally decreases in directional selection. This is because extreme values become more common in the population, reducing the range of variation in the trait.