reflect Giraffes are unique animals because they have such long necks. The neck of a modern day giraffe is much longer than the neck of an ancient giraffe that lived long ago. How and why did the giraffe change in such a way? What are the advantages of having a long neck? Natural Selection and Species Survival Structures of living organisms demonstrate adaptations that allow for specific functions. For example, the giraffe s long neck is an adaptation because it allows the giraffe to reach leaves further up on trees. This provides an advantage because the giraffe can reach food that other adaptations: structures animals cannot reach. Since a longer neck provides an or behaviors in organisms advantage, giraffes with this trait are more likely to survive, that help them survive in reproduce, and pass the trait to their offspring. This is their environment called natural selection. It is the process by which, over generations, favorable traits become more common in a species or population and unfavorable traits become less common. Plants also have adaptations that help them survive. Water lilies, for example, have large, thin leaves. The structure of these leaves helps water lilies float so they can get enough sunlight for photosynthesis. Because floating has an advantage, plants that have this leaf structure can produce more food during photosynthesis. The extra food helps the plants make more seeds and pass on this trait to the next generation. Some plants, like tulips, store food in structures called bulbs. The bulbs grow underground. During winter (or other harsh conditions like a drought), the above-ground structures of the plant (stem, fl ower, leaves) die, but the bulb stays alive. It produces a new stem, new leaves, and new flowers in the spring. This adaptation helps the plant survive diffi cult environmental conditions. One challenge for animals that live in water is getting enough oxygen. Animals that live on land use lungs to get oxygen from the air. Fish have gills to help them take in enough oxygen from the water. Fish gills can be diffi cult to see because they are inside the animal. 1
Amphibians also have gills during the time that they live in water. These gills can be found on the outside of the animal, as shown in the picture of the salamander on the previous page. They look like feathery structures on each side of the body. Gills are adapted to have many fringes to increase the area that can absorb oxygen from the water. An internal adaptation in plants is the xylem. The xylem is a structure that moves water through the plant. Plants that do not have xylem, like the mosses shown in the image on the right, are limited to wet climates. Xylem is an adaptation that allows plants to grow in drier climates because the xylem can draw water up from deep in the soil. what do you think? If you examined a chicken or turkey bone, you would notice that the bone is lightweight and hollow. In fact, most birds have hollow bones. Why do you think this is so? What adaptive advantage do hollow bones provide to a bird? Behavioral Adaptations The adaptations discussed so far have been structural, or physical, adaptations. In the example above of hollow bones in birds, the hollow structure of the bones allows birds to fly. Therefore, birds can fulfi ll a role in the environment that most other organisms cannot fill. Animals also have behavioral adaptations. For example, humpback whales use unique songs to communicate with one another. These songs can help a mother fi nd her lost calf. They can also be used to claim a territory and keep other whales at a distance. Or, the songs can be used to fi nd a mate. Whales that are better at communicating have many advantages. A whale that is better able to attract a mate through song is more likely to breed and pass that trait on to the next generation. Squirrels and some other animals hibernate to help them get through times of the year during which temperatures are cold and food is limited. Animals that hibernate do not eat and rely on stored body fat for energy. Thus, the timing of hibernation is important. An animal that hibernates too long may run out of fat reserves. An animal that awakes from hibernation too hibernate: a process during which an organism undergoes a slowed physiological state to survive long periods of cold temperature or reduced food supplies 2
soon may not fi nd food. Producing enough fat reserves is also an essential component of this adaptation. Animals that have more fat reserves when they enter hibernation are more likely to survive the winter. look out! Monarch butterflies and many species of birds have a different behavioral adaptation for surviving cold winters. They leave the area. These animals migrate from northern areas in the summer to southern areas in the winter. migrate: to move to a different geographical or climatic region where food is more available Natural selection does not have a goal. Organisms cannot plan to have better offspring. Organisms that are born with more favorable traits are more likely to survive and reproduce than organisms born with less favorable traits. Those organisms that survive and reproduce pass their traits on to the next generation. What makes a trait more or less favorable? Many times it depends on the environment. A trait that is favorable for living in the arctic may not be favorable for living in the desert. Genetic Variation Natural selection is possible because of genetic variation among organisms within a population. In a population of giraffes, for example, individuals vary in height (as well as other traits.) Some of these individuals have more favorable sets of traits; others have less favorable sets of traits. The individuals with the most favorable set of traits will survive and reproduce to pass on those traits. It is the variation between individuals that allows for natural selection. population: members of the same species that live in the same geographical area; may be distinctly different than populations living elsewhere, due to the spread of local adaptations within a population The process of natural selection has changed nearly every species found on Earth. For example, the Galapagos Medium Ground Finch has developed a unique beak shape allowing it to fi ll a specifi c role in its environment and improve its ability to survive. The ground fi nches of the Galapagos (a group of islands in the Pacifi c Ocean) feed mainly on seed. Several different species of fi nches evolved from a common ancestor that was a seed-eating ground fi nch with a relatively small, pointy beak. Each species specializes in feeding based on beak size. For example, the large ground fi nch (Geospiza magnirostris) has a large beak and feeds mainly on large seeds. The small ground fi nch (Geospiza fuliginosa) has a small beak and feeds mainly on small seeds. The medium ground fi nch (Geospiza fortis) as a medium-sized beak. This allows the bird to be more of a generalist; it is able to eat either large or small seeds. The mediumsized beak provides an advantage because it has increased options for feeding, so its chances of survival are greater. 3
Populations and species demonstrate unique variations and inherited traits, which have gradually changed over time as they were passed from one generation to the next. Changes in external features, internal features, and behaviors of organisms, through natural selection can enhance the survival of a species or produce desired genetic traits. Scientists in the Spotlight: Peter and Rosemary Grant Peter and Rosemary Grant are biologists who studied the Galapagos fi nches. They were able to observe natural selection over the course of a few generations of the fi nches when the environment dramatically changed. For example, in 1977 a drought resulted in more large seeds and fewer small seeds. The average beak size of medium ground fi nches was larger after the drought than before the drought. After a particularly wet year in 1985, the average fi nch beak was smaller. Smaller seeds were more plentiful and the smaller-beaked fi nches were more successful at survival and reproduction that year. This data is important because it helps scientists measure and describe natural selection. Obtaining the data took years of careful measuring and weighing. Peter and Rosemary Grant spent a few months each year on the Galapagos Islands. They measured the fi nches and collected weather data for many years. This dedication resulted in a long-term study that showed natural selection in action. Everyday Life: Selective Breeding In selective breeding, the favorable traits to be passed on to offspring are chosen by a breeder. Selective breeding of domestic animals, like dogs, has led to incredible diversity from their wolf ancestor. Dogs are as diverse as the Great Dane and Chihuahua; yet they had a common ancestor. People who selectively breed animals select for particular traits and breed only the animals that have those traits. After many domestic: related to the household generations, each breed has highly specific traits that have been selected for. Some breeds are the result of breeding for a good sense of smell, a good protective instinct, unique fur patterns, or a particular body shape. 4
People also selectively breed plants. Wild corn, for example, has very small kernels. Over many years of breeding, humans have created modern corn with the large kernels like we expect to see in the grocery store. People prefer to purchase the larger kernels. Therefore, farmers plant the seeds that produce the desired traits. Other crops, like wheat, have also been selectively bred to produce desirable traits. Modern wheat crops are more resistant to harsh weather and produce more wheat per growing season. Some modern crops are even bred to contain more vitamins! What do you know? The chart on the next page lists examples of adaptations in different organisms. For each example, state whether the adaptation is likely the result of natural selection or selective breeding. Then, describe at least one advantage for the adaptation. Be accurate, but creative! Many items in the chart have more than one answer. 5
Example of Adaptation Apples with large, sweet fruit Natural Selection or Selective Breeding? Advantage(s) of Adaptation Cacti with sharp spines Bats that can fl y Crabs that can hide in the sand Tomato plants that can grow in salty environments 6
connecting with your child Artificial Selection at the Grocery Store To help your child learn more about adaptations, investigate selective breeding (also called artificial selection) by taking a trip to the grocery store or farmer s market. If you are unable to travel to the market together, try to fi nd samples of fruits and vegetables in your refrigerator or search for pictures on the Internet. Selective breeding is common in food crops; these crops are often called genetically modifi ed crops. Breeders can select for a variety of traits, such as leaf size in lettuce, fruit size in tomatoes, root size and vitamin content in carrots, and flower size in cauliflower. Choose at least six different fruits and/or vegetables to discuss. Most of the traits selected for by breeders can be benefi cial in the wild within the limits of a particular range of variability. For example, larger leaves help a plant by increasing photosynthesis, but they can also increase the potential for frost or herbivore damage. Larger fruit can be advantageous by attracting dispersing animals, but it also requires a lot of energy from the fruitproducing plant. Some traits, however, are entirely of benefi t to humans and not the plant. Seedless plants, for example, would not be selected for by natural selection. Here are some questions to discuss with the student: Which traits do you think were artificially selected for in each fruit or vegetable? Why do you think these traits were selected? Would similar adaptations be benefi cial or harmful in the wild? Which type of environmental conditions would be most favorable for these plants? 7