A grove of aspens grew entirely from root sprouts of one tree, so every trunk is genetically identical. Each year far more sprouts appear than the soil water can support, and most of them die. Which requirement for natural selection is missing?

Mosquitofish differ in inherited body size. Far more fry hatch than the pond can feed, and most die before breeding. Which fry die is decided entirely by which are swept into a drainage pipe; size makes no difference. What is missing here?

A zoo breeds Arabian oryx that differ in inherited horn length. Each pair is allowed one calf per year, keepers hand-rear every calf on unlimited feed, and no calf is ever lost. Which requirement for natural selection is missing in this herd?

Rotifers in a large tank differ in inherited feeding rate, and every female produces many eggs. Algae are pumped in faster than the rotifers can consume them, so no rotifer is ever short of food. Which requirement for natural selection is missing?

A bacterial culture is grown from a single cell, so at first every cell is genetically identical. After many generations, a few cells survive an antibiotic that kills the rest. Where did the variation that let them survive most likely come from?

Clover plants in a field differ in inherited growth form, and far more seeds germinate each spring than the field can hold. A mower cuts the field every week. Which finding would show that mowing is acting as natural selection?

Soil mites in a lab dish differ in inherited jaw size, and each female lays hundreds of eggs. Food is kept scarce, and mites with larger jaws feed faster and leave more young. What is expected over many generations?

A single female eastern oyster releases tens of millions of eggs in one spawning season, and only a handful of the resulting oysters ever reach adulthood. Which requirement for natural selection does this fact illustrate?

In one lawn, some dandelions hold their flowers on tall stalks and others on very short ones. Seeds taken from tall-stalked plants and grown in the same soil produce tall-stalked plants. Which requirement for natural selection does this show?

About 200 finches on a small island feed on the seeds of one grove of shrubs, and by midsummer the seed crop is stripped bare while birds are still searching. Which requirement for natural selection does this describe?

During a drought on Daphne Major, the small soft seeds ran out and mainly large hard seeds remained. Ground finches with deeper, stronger beaks cracked those seeds and survived at a much higher rate than shallow-beaked birds. Which requirement is this?

In a cricket population, males with an inherited longer chirp attract more females and father far more offspring than males with a short chirp, though both kinds live equally long. Which requirement for natural selection does this illustrate?

A patient's throat is full of bacteria, a few of which carry an inherited gene that lets them survive a common antibiotic. The patient takes that antibiotic for ten days. What is expected in the bacterial population over the following generations?

Grasshoppers in a meadow range from bright green to dull brown, and the color is inherited. A wildfire leaves the ground bare and brown for several seasons, and birds hunting by sight take grasshoppers from it. What is expected over many generations?

Land snails differ in inherited shell color, and far more young hatch than the woodland can support. Snails of every color are eaten by thrushes at the same rate. What is expected in this population over many generations?

Cod mature at different body sizes, and that size is inherited. For decades, nets with a large mesh have taken only the biggest cod and let the smaller ones pass through and breed. What is expected in this cod population over many generations?

Sunflowers in one plot vary a great deal in height. An investigation shows the differences come entirely from how much nitrogen each plant happened to get, and seeds from the tallest plants grow no taller than seeds from the shortest. What follows?

Mice given a running wheel from a young age develop noticeably larger leg muscles than caged mice. When both groups are bred, their pups are born with legs of the same size. Can natural selection make large leg muscles more common this way?

Some grasses on a salted roadside survive better in salty soil than others. Seedlings raised from the survivors in identical clean pots still tolerate salt better than seedlings from the other plants. Can salt tolerance spread in this population?

Peppered moth wing color is set by alleles, and far more caterpillars hatch than survive. On soot-blackened trunks near industrial cities, birds hunting by sight took pale moths more often than dark ones. Could selection change this population?