Each organism develops as a result of a fertilized egg cell. These laws can be explained in the following manner. In comparison, other quadrupeds like monkeys, have deep and narrow chests with the shoulder blades present at the sides. These features imply that both humans and apes have descended from the same ancestor who could suspend itself with the use of its upper limbs. The chest cavity in both species is broader than it is deep, and has flat shoulder blades at the back. The structure of the bones forming the chest cavity in humans and apes is yet another example of a skeletal homologous structure. Hence the bone structures in these animals can be called homologous structures. For example, in case of man the front limb takes the shape of upper and lower arm and hand, in case of a bat it has transformed into a wing, and in case of a porpoise it forms a flipper/fin. The differences in the length and position of the different bones in the forelimb are a result of evolutionary adaptation. As shown in the figure above, the basic skeletal structure is similar enough, and consists of the same type of bones and joints so as to be evident of being derived from a common ancestor. While all three appendages share the same function, the varied bone placement and the differing characteristic features of each bone imply that they are not variations of a common primitive morphological feature.Ī common example of morphological homology is evident in the skeletal structure of the front limbs of vertebrates like humans, cats, whales, bats, porpoises, horses, frogs, etc. An example for such a case would be that of the wings of a butterfly, a bat, and a bird. However, this concept should not be confused with that of analogous structures, which are structures that serve the same function in various organisms, however, are not derived from a common ancestor. Hence, evolution can be considered as a remodeling process, that is driven by the force of natural selection in order to allow the most adapted species to develop and proliferate. This disparity is due to the phenomenon of natural adaptation of organisms to their surroundings. However, the function or use of that structure may or may not be the same in all the organisms. The common structure arises due to the evolution of the different species from a common ancestor. It describes anatomical and skeletal similarities with respect to shape, placement, and/or function between organisms belonging to the same taxonomic category. Based on these, homology across organisms can be categorized and into three types. Homologous structures do not merely point towards physical features of organisms, but also refer to the conserved genetic sequences, and the dichotomy observed between different organs arising from the same type of embryonic tissue. Homoplasty is also referred to as analogy. ‘Homogeny’ implies homology due to the inheritance of a feature from a common ancestor, and ‘homoplasty’ implies to the occurrence of homology due to any and all other factors. This term was further improved upon by Ray Lankester, a British zoologist, who provided two more terms describing the type of homology observed. It is derived from the Greek words homos, meaning ‘same’ and logos, meaning ‘relation’. The term homology was coined in the year 1656. Homologous structures are structurally and functionally similar and derived from a common ancestor whereas, analogous structures have similar functions but are not descended from a common ancestor.
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