2.6 Adaptations

An adaptation is a heritable trait, meaning one passed from parents to offspring through genes, that increases an organism's chance of surviving and reproducing in its environment. Adaptations can be physical (a polar bear's thick fur), physiological (a desert plant's way of saving water) or behavioral (birds migrating south for winter).

Fitness, in biology, means how well an organism survives and reproduces compared with others in the same population. It's not about strength. An organism with more surviving offspring has higher fitness.

Natural selection acts on individuals, but it's populations that change. A single organism can't evolve a new trait during its life because it needs one. Instead, organisms that already happen to have a helpful trait leave more offspring.

• Variation: individuals in a population differ in their traits.
• Inheritance: many of those differences are passed on through genes.
• Overproduction: organisms produce more offspring than can survive, so they compete.
• Selective pressure: something in the environment, like a predator, disease, climate or food supply, makes some traits better than others.
• Differential survival and reproduction: individuals with helpful traits survive and reproduce more, passing those traits on. Over many generations, the helpful trait becomes more common.

Peppered moths in England come in light and dark forms. In the 1800s, factory soot darkened tree trunks, so light moths stood out to birds while dark moths were hidden. Dark moths became much more common. After clean air laws reduced soot, light moths became common again.

Rock pocket mice in the southwestern U.S. live on both light sandy ground and dark lava rock. On the dark rock, dark-furred mice are far more common, because owls more easily spot light mice there.

Bacteria evolve resistance to antibiotics, and insects evolve resistance to pesticides, in exactly the same way: the few resistant individuals survive and pass on their genes.

Natural selection takes many generations. Species with short generations and many offspring, like insects and bacteria, can adapt quickly. Species that reproduce slowly, like elephants, whales and many trees, adapt slowly.

If an environment changes faster than a population can adapt, individuals have to respond some other way: change their behavior, move to a better area, or die. Rapid human-caused changes, such as habitat destruction and fast climate warming, are a big reason so many species are declining today. For example, coral reefs are struggling because the oceans are warming faster than corals can adapt.