2. FUTURE TRENDS
3. NO. OF SPECIES?
4. OTHER DEBATES
WANT MORE DETAIL? CHECK OUT KING & BURKE 1997
READ THE DEBATE ABOUT CAIMAN CLASSIFICATION
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An emerging trend, while still controversial, favours dropping the Linnean rank system described on the previous page for something less subjective: phylogenetic taxonomy. The problem with the Linnean system is that an organism's position within the ranking system depends upon possession of certain characters (e.g. scales, legs, etc), yet this may be flawed when evolutionary relationships are considered. For example, crocodilians are normally thought of as reptiles because, let's face it, they look a lot like lizards. However, crocodilians are actually more closely related to birds, even though on the surface they appear to be quite different both in their appearance and behaviour. More importantly, though, crocodilians and birds are both archosaurs, and share a recent ancestor within that group. The evolutionary distance between crocodilians and lizards is much greater. This comes as a bit of a shock to most people, but it's a classic example of phylogenetic taxonomy at work. In the same way, birds are actually dinosaurs (albeit somewhat evolved) because it is their evolutionary relationships which are important and not their physical similarities (or lack thereof).
As a result of this new thinking in phylogenetic taxonomy, the Linnean system is starting to fall out of favour with many taxonomists, although we've been using it for so long that it likely won't disappear easily. The abandonment of Linnean ranks doesn't mean that a hierarchical classification is not still relevant, but simply that the name of those ranks should not be important.
Therefore, instead of classifying the American alligator and the various caiman species in the Family Alligatoridae simply because they both share similar morphologies, instead we can define the Alligatoridae as the last common ancestor of all those species - i.e. any shared characters are derived, and not plesiomorphic.
"If we define Alligatoridae as the last common ancestor of Alligator, Caiman, Paleosuchus, and Melanosuchus and all of its
descendants, we are recognizing a real group defined by a completely natural event - a speciation event. This definition will remain stable in the face of new discoveries - a newly-discovered animal somewhere in the Amazon will be an alligatorid if it belongs phylogenetically to this group, even if it convergently has all the features conventionally thought to mark a crocodylid." Chris Brochu
Compare the following classification. The list on the left is the Linnean rank system, whereas the list on the right is the phylogenetic taxonomy system. Both mean exactly the same thing, yet the list on the right makes no assumptions about shared characteristics.