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By the end of this section, you will be able to:
  • Compare homologous and analogous traits
  • Discuss the purpose of cladistics

Scientists collect information that allows them to make evolutionary connections between organisms. Similar to detective work, scientists must use evidence to uncover the facts. In the case of phylogeny, evolutionary investigations focus on two types of evidence: morphologic (form and function) and genetic.

Two measures of similarity

Organisms that share similar physical features and genetic sequences tend to be more closely related than those that do not. Features that overlap both morphologically and genetically are referred to as homologous structures; the similarities stem from common evolutionary paths. For example, as shown in [link] , the bones in the wings of bats and birds, the arms of humans, and the foreleg of a horse are homologous structures. Notice the structure is not simply a single bone, but rather a grouping of several bones arranged in a similar way in each organism even though the elements of the structure may have changed shape and size.

Photo A shows a bird in flight, with a corresponding drawing of bird wing bones. Photo B shows a bat in flight with a corresponding drawing of bat wing bones. Photo C shows a horse, with a corresponding drawing of front leg bones. Photo D shows a beluga whale, with a corresponding drawing of flipper bones. Photo E shows a human arm, with a corresponding drawing of arm bones. All the limbs share common bones, analogous to the bones in the arms and fingers of humans. However, in the bat wing, finger bones are long and separate and form a scaffolding on which the wing’s membrane is stretched. In the bird wing, the finger bones are fused together. In the horse leg, the ulna is shortened and is fused to the radius. The hand bones are reduced to one long thick bone and the finger bones are reduced to one long thick finger with a modified nail or hoof. In the whale flipper, the humerus, ulna, and radius are very short and thick.
Bat and bird wings, the foreleg of a horse, the flipper of a whale, and the arm of a human are homologous structures, indicating that bats, birds, horses, whales, and humans share a common evolutionary past. (credit a photo: modification of work by Steve Hillebrand, USFWS; credit b photo: modification of work by U.S. BLM; credit c photo: modification of work by Virendra Kankariya; credit d photo: modification of work by Russian Gov./Wikimedia Commons)

Misleading appearances

Some organisms may be very closely related, even though a minor genetic change caused a major morphological difference to make them look quite different. For example, chimpanzees and humans, the skulls of which are shown in [link] are very similar genetically, sharing 99 percent Gibbons, A. (2012, June 13). Science Now . Retrieved from http://news.sciencemag.org/sciencenow/2012/06/bonobo-genome-sequenced.html of their genes. However, chimpanzees and humans show considerable anatomical differences, including the degree to which the jaw protrudes in the adult and the relative lengths of our arms and legs.

Photo A is of a chimpanzee skull. There is a prominent ridged brow, the eye and nose area is quite flat, and the maxilla and mandible (the jaw) protrude. Photo B is of a human skull. The cranium is proportionately larger than the chimpanzee, the brow is smooth, the nose and cheekbones are more prominent and the mandible and maxilla protrude only slightly.
(a) The chimpanzee jaw protrudes to a much greater degree than (b) the human jaw. (credit a: modification of work by "Pastorius"/Wikimedia Commons)

However, unrelated organisms may be distantly related yet appear very much alike, usually because common adaptations to similar environmental conditions evolved in both. An example is the streamlined body shapes, the shapes of fins and appendages, and the shape of the tails in fishes and whales, which are mammals. These structures bear superficial similarity because they are adaptations to moving and maneuvering in the same environment—water. When a characteristic that is similar occurs by adaptive convergence (convergent evolution), and not because of a close evolutionary relationship, it is called an analogous structure    . In another example, insects use wings to fly like bats and birds. We call them both wings because they perform the same function and have a superficially similar form, but the embryonic origin of the two wings is completely different. The difference in the development, or embryogenesis, of the wings in each case is a signal that insects and bats or birds do not share a common ancestor that had a wing. The wing structures, shown in [link] evolved independently in the two lineages.

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Source:  OpenStax, Concepts of biology_biol 101 dacc. OpenStax CNX. Jan 13, 2014 Download for free at http://legacy.cnx.org/content/col11614/1.1
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