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Figure 3.17 The challenge of biological classification. (a) Old World vulture; (b) New World vulture; and (c) a stork. The evolutionary relationship between New World vultures and storks is not evident from their appearance.

similar anatomically, physiologically, and genetically to a stork. An evolutionary classification can be quite useful in the study of living organisms; for instance, if scientists wish to know more about the basic biology of New World vultures, they might start by learning what is known about the biology of storks.

Evolutionary classifications are based on the principle that all of the descendant species of a common ancestor will share any biological trait that first appeared in that ancestor. Let us create an imaginary scenario: Imagine a mite species found in college student bedrooms that evolves the ability to use stale corn chips as a food source. We could identify all the descendant species of the corn-chip-eating mite by their ability to eat old nachos. Now imagine that one of the nacho-eating descendant species evolves a coloration that allows it to blend in perfectly against a background of rarely washed socks. All of its descendant species can be identified by this trait. Finally, imagine that one of the sock-colored corn-chip eaters' descendants began reproducing in the bindings of unopened textbooks. Again, any of its descendants will have this trait as well. Because every speciation event involves an evolutionary change, scientists can use these modified traits in modern organisms to reconstruct its phylogeny, the evolutionary history of a group of organisms (Figure 3.18).

Of course, in the real world, reconstructing evolutionary relationships is not as simple as our scenario suggests. Descendant species may lose a trait that evolved in their ancestor, or unrelated species may acquire identical traits via a different evolutionary pathway, a process called convergent evolution. These occurrences complicate attempts to determine the accurate evolutionary classification of organisms.

Any classification developed by a biologist can be considered to be a hypothesis of the evolutionary relationship among organisms. It is difficult to test this hypothesis directly—scientists have no way of observing the actual speciation events that gave rise to distinct organisms. However, scientists can test their hypotheses by using information from both living organisms and fossils. Among living organisms, closely related species should have similar DNA: If the pattern of DNA similarity matches the hypothesized evolutionary relationship, the hypothesis is strongly supported. This is the case with the hypothesized relationship between New World vultures and storks; DNA sequence comparisons indicate that the DNA of New World vultures is more similar to the DNA of storks than to Old World vultures. By examining the fossils of extinct organisms,

Figure 3.18 Reconstruction of an evolutionary history. This diagram illustrates our imaginary phylogeny. Species B, C, and D all share the corn-chip eating trait, so they share a more recent common ancestor with each other than with species A. Species C and D both have the color of dirty socks, but B does not, indicating that C and D share an even more recent common ancestor.

Figure 3.18 Reconstruction of an evolutionary history. This diagram illustrates our imaginary phylogeny. Species B, C, and D all share the corn-chip eating trait, so they share a more recent common ancestor with each other than with species A. Species C and D both have the color of dirty socks, but B does not, indicating that C and D share an even more recent common ancestor.

scientists can deduce the genealogy of related species. For example, fossils of vulture-like birds clearly indicate that this lifestyle evolved independently in both the Old World and the New World. These data allow scientists to strongly infer that the superficial similarities between New World vultures and Old World vultures are a result of convergent evolution.

Once a hypothesis of evolutionary relationship is reasonably well supported by additional data, bioprospectors can use the information gathered about one species in a classification group to predict the characteristics of other species in that group. This helps them determine the likelihood that related species could be additional sources of biological gold.

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