How Traits Help Species Survive

By the end of this lesson, you will be able to describe how small differences in traits within a population lead to adaptations that help a species survive and reproduce over many generations.

Every group of the same kind of organism living in one area, called a population, contains variation, which means the individuals are not exactly alike. Some are taller, some have thicker fur, some run faster, and some behave a little differently. These differences come from the information stored in genes and passed from parents to offspring. Variation is normal, and it turns out to be very useful.

Think about black-tailed jackrabbits in West Texas, where summer afternoons can be brutally hot. Within that population, some jackrabbits are born with slightly larger ears than others. Large ears have many blood vessels close to the surface, so heat leaves the body faster and the animal stays cooler. During a long, hot, dry summer, the rabbits that stay cool are more likely to survive, and survivors are the ones that have babies. Their offspring often inherit those larger ears, so after many generations, large ears become common in the population. A trait that spreads this way because it helps an organism survive in its environment is called an adaptation.

Adaptations come in three flavors. Structural adaptations are body parts, like the jackrabbit's big ears or a mesquite tree's deep roots. Behavioral adaptations are actions, like jackrabbits resting in shade during the hottest hours. Physiological adaptations are internal body processes, like producing very concentrated urine so the body loses less water.

Notice that individual rabbits do not choose to change. The environment simply favors certain traits that already exist. Over many generations, the whole population shifts, and the species becomes better matched to where it lives.

Try it: If a cold, wet climate lasted many years, would jackrabbits with larger or smaller ears likely become more common, and why?