Similarities and differences: understanding homology and analogy http://evolution.berkeley.edu/evolibr ary/article/0_0_0/similarity_hs_01 Inherited Traits People look like one another for different reasons. Two sisters, for example, might look alike because they both inherited dark eyes and brown hair from their father. Environmental Traits On the other hand, two people attending an Elvis impersonators' convention may look alike because they are both wearing rhinestone studded suits and long sideburns. Biological Similarity • Biological similarity: It works the same way in biology. Some traits shared by two living things were inherited from their ancestor, and some similarities evolved in other ways. These are called homologies and analogies. Homology Homology: traits inherited by two different organisms from a common ancestor Analogy Analogy: similarity due to convergent evolution (often occurs when both lineages face similar environmental challenges and selective pressures) The Tale of the Limb Look at the following diagrams of front limbs presented. Each is from a different animal. In each grouping you see, you should select the limb that does not belong, and take note of the similarities that the others share. Cross out the limb that does not fit with the group. Explain why you removed that limb in the box beneath the group. Set 1: Set 2 Set 3 The Tetrapod Limb These four limbs all belong to tetrapods — animals with four limbs. Notice the similarities: 1) They are all built from many individual bones. 2) They are all spin-offs of the same basic bone layout. What animals possess these homologous limbs? Inheriting homologies Whales, lizards, humans, and birds all have the same basic limb layout. But how did such different animals wind up with the same sort of limb? Inheriting homologies The answer is that they inherited it from a common ancestor, just as cousins might inherit the same trait from their grandfather. This evolutionary tree shows the relationships between different tetrapod lineages, all of which evolved from a single common ancestor. This 350 million year old animal, the first tetrapod, had limbs with one long bone (the humerus) attached to two other long bones (the radius and ulna). Its descendants, including whales, lizards, humans, and birds, as well as many others, inherited the tetrapod limb from this ancestor. Structures inherited from a common ancestor are called homologous structures, or homologies. Other Homologies The beaver uses its teeth for chewing through tree trunks, and the elephant uses its tusks for a number of tasks including digging, peeling bark from trees, and fighting. But if you examine these two structures closely, you will see that each is a modification of the basic incisor tooth structure. Over time, evolution adapted each of these animals' incisors to perform different functions. Not all similarity is homology Not all similarity is homology In the beginning of this module, you decided that some legs were "not like the others." The matching limbs had bones, and you eliminated the limbs that did not have bones. Since the octopus, sea star and grasshopper limbs do not have bones, you concluded that they are probably not homologous to tetrapod limbs. Analogies Elvis impersonators have many similarities, such as long black sideburns and rhinestone-studded suits, that are not inherited. In the same way, biological similarities are not always due to inheritance. Sometimes the environment around the organism plays a role. Analogies Often, two species face a similar problem or challenge. Evolution may then shape both of them in similar ways — resulting in analogous structures. In contrast, the wing of a bird and the wing of a bat are classified as being analogous structures since their structures are unrelated to any previous ancestor. Why Flight? The most amazing fact about the evolution of flight is the extent of convergent evolution between the three main groups that evolved it (again, the pterosaurs, birds, and bats). Evolution has no sense of future; the here and now is the only place where evolution occurs.