When a group of deer migrated to a cold mountain, the thin-furred deer and average-furred deer died. The thick-furred deer survived and eventually the population became thicker-furred.

Small turtles are too weak to protect themselves from predators and large turtles are too slow to hide from predators. Average-sized turtles have high fitness.

Occurs when changing environment favors one extreme phenotype

Tall kelp have a distinct advantage over shorter kelp in receiving needed sunlight in their ocean environment

Aspen trees that are over 25 m tend to blow over in high winds; aspen that are under 15 m don't get enough sun

On average, human newborns weigh 7.2 pounds.

Bacteria that are too large cannot obtain nutrients efficiently and bacteria that are too small cannot compete for those nutrients successfully

Producing 1000 cactus spines requires too much energy, but producing <500 leaves the plant open to predation

Ground finches born following a severe drought tend to have larger beaks (on average) than the previous generation.

Light-colored oysters would blend into the rocks in the shallows, and the darkest would blend better into the shadows. The ones in the intermediate range make them easier prey.

A fire has wiped out the vegetation that produces medium sized seeds. As a result, the finches left can feed on seed that are available in two sizes: small and large. What type of selection is likely to take place in this population?

People have overfished the salmon in a particular river. They catch the larger salmon and throw the smaller salmon back into the river. Over time, what type of selection is likely to take place?

In a forest in Germany, there is a rat who has three fur coat colors: a light creamy color, a medium tan color, and a dark brown color. In this particular forest, the floor is a medium brown and the intermediate tan color mouse is better camouflaged.

A population of rabbits has three fur colors: white, gray, and black. After volcanic activity, the terrain changed and now the rabbits now live on white rocks and black rocks. What type of selection is likely to occur?

In interglacial periods in Europe, black bears body sizes got smaller. In the glacial periods in Europe, black bears body sizes got larger. What is this type of selection?

Starlings produce an average of five eggs. If there are more than five the parents cannot adequately feed the young. Fewer than five and predators may destroy them. As a result, five eggs becomes the most common number.

A scientist measures the circumference of acorns in a population of oak trees and discovers that the most common circumference is 2 cm. There are very few acorns with circumferences of 3 cm or circumferences of 1 cm. Only the 2 cm circumference acorns survive.

A population of Madagascar hissing cockroaches lives in a woodpile. The cockroaches are eaten by lizards. Because the lizards have small heads, the lizards are unable to eat the very largest adult cockroaches, and instead prey upon small and medium sized adults. Over time, only the large headed lizards survive.

A population of birds with various size beaks eats seeds. Small seeds can be eaten by birds with small beaks. Larger, thicker seeds can only be eaten by birds with larger, thicker beaks. During a drought, only large thick seeds exist so only the large, thick-beaked birds survive.

Black rabbits and white rabbits are both able to survive because they can camouflage into the white and black rocks in their environment. However, the intermediate gray rabbits do not survive. This results in only white and black rabbits.

Small lizards have difficulty defending territories so they end up dying out. Large lizards are more likely to be preyed upon by owls. Over time, only medium sized lizards end up surviving.