Paleontologists observed changes in the sizes of black bears as their environment changed. During warmer periods between ice ages, most black bears were small. During ice ages, most black bears were large. Which type of selection is this, and how would a graph of the distribution curve of black bear size change as Earth moved out of an ice age?

Question

1. stabilizing; the curve would become taller in the middle and shorter on each side
2. directional; the curve would shift to the left
3. directional; the curve would shift to the right
4. disruptive; the curve would break into two peaks with a dip in the middle

Answer 1

The scenario described suggests that during warmer periods, smaller black bears are favored, while during ice ages, larger black bears are favored. This indicates that the population experiences changes in size depending on the environmental conditions, which reflects directional selection rather than stabilizing or disruptive selection.

In this case, as Earth moves out of an ice age, the selection pressure would favor smaller black bears, leading to a shift in the size distribution curve toward the left (indicating a decrease in size as smaller bears become more common).

Thus, the correct answer is:

2. directional; the curve would shift to the left.