Definition
Selection pressure is any environmental factor that makes it easier or harder for individuals with a particular trait to survive and reproduce in a population. Predators, climate change, and disease are examples. It decides which way natural selection will push.
It works like shaking a sieve to sort grains
Picture a kitchen scene: coarse salt and fine sand are mixed together, and you shake them in a fine sieve. The sand grains smaller than the holes slip right through, while the coarse salt stays on top. The size of the holes decides which grains pass through.
Nature does the same sorting around the clock. Say sharp-eyed hawks start breeding nearby. Field mice with eye-catching fur get caught easily and disappear. Mice whose fur matches the soil are harder to spot, so they survive and are more likely to have young.
The hawk's hunting skill and sharp eyesight, or any condition in the environment that threatens or helps the survival of organisms with a certain trait, is what we call selection pressure applied by nature. Just as a sieve sorts grains, the environment coldly sorts organisms by how well they reproduce.
When the environment changes, the sieve changes too
Selection pressure is not a fixed rock. It is more like a wind that keeps changing direction with the environment. A wing that was a great advantage in one place can become a heavy burden in another.
The classic example is the peppered moth that lived near England's industrial cities. When tree trunks in the old woods were covered in pale lichen, moths with white wings were better at escaping the eyes of birds. But when factory smoke turned the trunks black, everything flipped.
On dark trees, white moths became far too easy for birds to spot, a fatal weakness. Now it was moths with black wings that survived, so the direction of selection pressure reversed completely, and in the end black moths made up most of the woodland population.
To be more precise: organisms don't change themselves
When organisms face selection pressure, it is easy to think they "change their bodies on purpose to survive." Like a giraffe stretching its neck to reach high leaves, or bacteria getting used to a drug in order to survive.
But organisms cannot rewrite their genes at will. Genetic mutations happen first, by chance and at random. A giraffe with a slightly longer neck, or a mutant bacterium that doesn't react to the drug, was already mixed into the original population by luck.
Antibiotics used in hospitals are also a very strong selection pressure made by humans. Resistance is not the human body getting used to the drug. It is a change on the microbe side. The human body doesn't change. Instead, only the resistant bacteria survive in that environment and multiply.
Common misconceptions
When organisms face selection pressure from the environment, they change their own genes in order to survive.
Mutations appear first, by chance and at random. Selection pressure just acts as a filter that lets only the individuals with traits suited to the environment survive among all those varied individuals.
If you take antibiotics often, your body gets used to the drug and becomes resistant.
What changes is the germs, not the person. Under the strong selection pressure of antibiotics, only the mutant bacteria that withstand the drug survive and multiply.
Where you meet it
Under the selection pressure of Antarctica's harsh cold, penguins with a thick layer of fat survived and reproduced better.
A common explanation is that long-necked giraffes, better at eating leaves up high, left more offspring. But some say that male-versus-male neck fighting mattered more, so there are several views.
Under the strong selection pressure of pesticide spraying, only the pests that happened to be resistant survived and reproduced.
In one sentence
The environment is a sieve that affects the survival and reproduction of organisms, and it sets the direction in which traits are passed on to the next generation.