What do you know about evolution? Evolution is a population s change in inheritable traits over time. One of the most common examples of evolution is an ape walking and evolving into an animal that stands in an upright position. One mechanism of evolution is natural selection. Natural selection is a popular theory proposed by Charles Darwin which suggests that species change over time because organisms with traits well suited to their environments have greater success surviving and reproducing. natural selection: theory that organisms with traits that are well suited to their environment survive and reproduce more successfully Charles Darwin s theory of evolution by natural selection changed the way we understood the diversity of life. Factors that Contribute to Natural Selection Genetic variation: There are differences among individuals within the same species. Traits that are more favorable to a species are more likely to be passed on from generation to generation, increasing the species chances of survival. Overproduction: If a species produces more offspring than the environment can support, many of the offspring will not survive into adulthood. Competition: Since resources such as food, water, and space are limited, offspring must compete for the resources for survival. Individuals with a trait that gives them a competitive advantage are more likely to reproduce. 1
Adaptation: An adaptation is an inherited trait that improves the survival of a species. For example, the giraffe underwent mutations that caused some offspring to have longer necks. The giraffes with longer necks ultimately had a better rate of survival because they could stretch their necks to reach leaves on tall trees. Over time, more and more giraffes inherited the longer necks, increasing the survival of the species. How does a species change over time? Adaptation: an inherited trait that improves the survival of a species. Many birds migrate in the winter. An environment meets the needs of the organisms that live there. Environments are always changing. Adaptations can be behavioral traits that are inherited over generations as well as physical traits. Think about the seasons; it is warm in the summer and cold in the winter. Organisms must find ways to deal with these changes. Some plants lose their leaves and stop growing when it is cold. Some animals grow thick fur coats. Animals like birds migrate to warmer areas to make it easier to find food during the winter season. Plants also have adaptations that help them survive. For example, water lilies have large thin leaves. The structure of the leaves help water lilies float so they can get enough sunlight for photosynthesis. Floating is beneficial because the leaves can produce more food. More food produces more seeds, and naturally the trait is passed to the next generation. Genetic variation: differences in physical traits as well as behavioral traits among individuals competition: contest between individuals or species for resources and space 2
Galapagos Finches Natural Selection Prime examples of adaptation are seen in the Galapagos Medium Ground Finches. A study in the 1970s revealed that variation in beak size changed over time. During times of drought, finches with large beaks had a higher survival rate because they were able to eat the seeds available during the drought that those with smaller beaks had a harder time eating. Therefore, the generations of finches after the drought had more birds with larger beaks. After the drought, more seeds were available to the the finches with smaller beaks. The larger beak was no longer a beneficial trait, so the number of birds with larger beaks stopped increasing. Artificial Selection Artificial selection is another mechanism by which evolution can occur. Artificial selection is the intentional breeding of plants or animals to produce desirable traits. It can increase diversity by creating new variants and also decrease diversity by eliminating other variants. Artificial selection is also called selective breeding. Selective breeding of domestic animals, thoroughbred horses, and meats we eat such as cows, pigs, and chickens. Common examples of artificial selection include cabbage, broccoli, cauliflower, collards, and kale, which are all members of the same species, Brassica oleracea. These common vegetables were cultivated from the wild mustard plant. There is a downside to artificial selection because it removes genetic variation from a species, making it susceptible to disease and environmental changes. 3
The chart below list examples of adaptations of different organisms. For each example, state whether the adaptation is the result of natural selection or artificial selection. Describe at least one advantage of the adaptation. Example of Adaptation Apples with large, sweet fruit Natural Selection or Selective Breeding? Advantage(s) of Adaptation Cacti with sharp spines Bats that can fly Crabs that can hide in the sand Tomato plants that can grow in salty environments 4
Artificial Selection Find samples of fruits and vegetables in your refrigerator, or search for pictures on the Internet. Selective breeding is common in food crops; these are often called genetically modified crops. Breeders can select for a variety of traits, such as root size, leaf size in lettuce, fruit size in tomatoes, vitamin content in carrots, and flower size in cauliflower. Choose at least six different fruits and/or vegetables to discuss. Here are some questions to discuss with your child: 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 beneficial or harmful in the wild? Which type of environmental conditions would be most favorable for these plants? 5