The beta fish population of over 10,000 fish is 50% long tailed and 50% short tailed. Because of an oil spill, most of the beta fish are killed and only 500 remain. After the survivors reproduce, 75% are long tailed and 25% are short tailed.

In the 1700s, there were no cows in Hawaii. When cows were brought to the island by boat, smaller American cows were chosen for the voyage to save space on the ship. After 300 years, most Hawaiian cows are significantly smaller than American cows.

Budgies are a type of parrot not native to North America. Breeders get their original breeding pairs from Australia, and pet stores buy birds from those breeders. However, because many Americans accidentally let their budgies loose, in St. Petersburg Florida, there are flocks of budgies. Wild Florida budgies are more likely to be green than budgies found in Australia.

After birds eat fruits, they drop seeds through their droppings, often in different locations. For example, a migrating bird could carry an apple seed from Georgia to an orchard in Tennessee.

The population of North Americans looks very different than it did in the 1400s because European Americans and many other groups immigrated here and have been reproducing for generations.

Field mice are usually found in shades of brown. Lab mice are the same species with white with red eyes. Imagine that a rogue lab member sets a box of lab mice free in a population of field mice that was 100% brown. Two generations later, about 25% of the wild mice are white with red eyes.

When you start antibiotics, but don't finish the bottle, some bacteria that are more resistant to drugs survive. These bacteria then reproduce, passing the genes for resistance on to the next generation.

In Scotland in the middle ages, food was hard to come by. Those who could get energy by digesting lactose found in cow's milk were more likely to survive. The ability to digest lactose, or "lactose tolerance" is genetic. Today's Scottish population has much higher lactose tolerance than countries like Japan, where milk was not an important food source.

Roosters have brightly colored faces. These are only helpful for attracting hens, whose feathers are much more dull. Dull coloring gives hens camouflage. The only benefit that roosters get from their bright faces is more success reproducing! It actually attracts predators.

Although the large, twister antlers of a buck may get it caught and hinder its survival, this risk is "worth it" evolutionarily, because the buck with the biggest antlers wins a mate, and the big-antler genes are passed on to his offspring.

In a pride of lions, young females remain, but males are driven away as they approach adulthood, and they must find another pride to join. Which of the following does this behavior ensure?

The goldfinch is a brightly colored bird. Its brilliant yellow color may make it more vulnerable to predators, but bright males have greater breeding success than their duller counterparts. Which of the following accounts for this apparent paradox?