This is an example of stabilizing selection.
During ice ages, larger black bears have an advantage because their size helps them withstand the harsh conditions. On the other hand, during warmer periods, smaller black bears have an advantage because their smaller size allows them to better navigate the environment and obtain food resources.
If the Earth moved out of an ice age, the distribution curve of black bear size would shift towards smaller sizes. This is because the smaller bears would have a higher reproductive success and survival rate in the new environmental conditions. Over time, this would lead to a decrease in the average size of black bears.