Comparative Anatomy Comparative Anatomy is the study of the similarities and differences in the anatomy of difference species. It has long served as one of the main evidences for evolution, due to the fact that it is very concrete, and does not require extensive technology. REMEMBER THIS Evolution is a theory that states that species change over time in response to natural selection. There is evidence that shows evolutionary relationships between organisms. Some organisms are more alike than others. For instance a dog is more closely related to a wolf than it is to a fish. Sometimes, biologists use fossils of organisms that lived in the past to determine how closely certain organisms are related. They also use evidence in the organisms that are alive today. Homologous structures, analogous structures and vestigial structures all are used by biologist to study which species are more closely related to each other. Homologous Structures Homologous structures have similar structures, but function differently in different organisms. For example, your arm is made of one bone between your shoulder and elbow, two bones between your elbow and wrist, and many bones in your hands in fingers. Other animals, like birds, turtles, frogs, and bats have the same appearance in their forearms (one bone, two bones, and many bones from the shoulder down). In all of these organisms the job of the forearm is a little bit different. Yet they look the same. Structures that show similar design are called homologous structures. You can see that even though some bones are slightly different lengths and shapes, all limbs above share the same bone structure, yet we know that they are used for many different functions such as: walking, swimming, flying, etc. This points to the fact that even though each animal shown has a specialized limb used for different things, all of these mammals have descended from one organism. Since genes produce the structures in organisms, biologists assume that organisms with homologous structures share similar genes. The more similar the genes are in different organisms, the more closely related they are. Birds, turtles, frogs, humans, and bats all have a similar bone structure in their forearm. Therefore, we share similar genes with birds, turtles, frogs and bats because our forearms are homologous to theirs. This makes us more closely related to them than to other organisms, like a jellyfish, that doesn’t have similar homologous structures. REMEMBER THIS Animals that have homologous structures are more closely related than animals that do not have homologous structures. Homologous structures between animals indicate that they have some DNA in common; therefore they are more closely related. Analogous Structures Analogous structures are structures function similarly, but are not made up of similar structures. For example, the wing of a butterfly and the wing of a bat are analogous structures. They both do the same job they help the organism to fly. However, the wings of a bat have bones inside them. The wings of the butterfly do not have bones. Therefore, the genes that produced the wings in bats and butterflies are not the same. As a result, butterflies and bats are not very closely related. You can see that even though the wings look very similar and function the same way (to aide in flight), the structure of the bones inside the wings are very different. This points to the fact that even though each animal shown appears to be of similar descent, they do not come from a common ancestor. REMEMBER THIS Analogous structures do not indicate a relationship. They have no common DNA. Vestigial Structures Vestigial structures are structures that have no function in an organism. They are there because the organisms still has some genes that produced structures that its ancestors needed. For example, snakes are believed to have evolved from lizards that found safety by burrowing. Their legs became less and less important for survival, so natural selection allowed populations of legless lizards (snakes) to survive and reproduce. Some snakes still have a pelvis, the bone that attaches the legs to the body. Snakes have no need for a pelvis, but they still have the genes that produce it. It is an example of a vestigial structure. In humans, the appendix is an example of a vestigial structure. Take a look at an example of a vestigial structure in whales in the pictures to the left. As you know, land mammals have hind (back) legs. Whales do not have hind legs. What they do have though, are small skeletal structures that look like shrunken hind legs. As ancestral cetaceans began to transition from land to water, their legs were pushed back further for swimming, and eventually reduced in size due to lack of use. Note the transition of the hind legs: Note that the presence of an unused structure may point to the theory that some organisms may have evolved from ancestors of a different species. (Ex: land vs. water animal) REMEMBER THIS Vestigial structures are present but not needed and usually do not function. They may indicate a degree of kinship.
