Understanding the Importance of Phylogenetic Classification

Scientists in general like to classify things, and they've been doing it for centuries. Why? Not because it makes them feel good (although it does), but because it helps keep things neat while at the same time providing a wealth of information. The simple process of sorting reveals patterns and relationships, and gives clues to past events — revelations that are absolutely fundamental to the study of evolution. This information is so important to evolutionary biologists, in fact, that they've come up with a way to show how related organisms are, evolution-wise, and to classify them accordingly. This method is called phylogenetic classification.

The advantage of a phylogenetic classification is that it shows the underlying biological processes that are responsible for the diversity of organisms. Through phylogenetics, scientists have been able to trace the genetic history of different species and, in doing so, have proved that the process of specia-tion — whereby ancestral species gives rise to descendent species — is real. (For more information on speciation, head to Chapter 8.) In fact, they've shown, as far as available data allows, that all species existing today descended from a single common ancestor.

Other classification systems, courtesy of Aristotle and Linneaus

The field of biological classification began with Aristotle (384-322 BCE). He concerned himself primarily with the classification of animals and recognized two major groups: those with blood and those without blood. In the "with blood" group, Aristotle recognized five subgroups: birds, things with four legs that lay eggs, things with legs that don't lay eggs, fish, and whales. The things-with-legs-that-lay-eggs group included animals such as crocodiles, lizards, frogs. The things-with-legs-that-don't-lay-eggs group corresponded mostly with what we now call mammals, except that Aristotle's group didn't include whales. (He recognized that whales are different from fish but didn't realize that they're mammals.) The main thing to know about Aristotle's system is that it used nested groups; he divided organisms into two main groups and then created subdivisions within those groups. This structure set the standard for later classification systems.

Carolus Linnaeus (1707-1778) developed Aristotle's idea of a nested classification scheme more completely. He divided all of life into two kingdoms: animal and vegetable. Then he subdivided those kingdoms into classes, divided classes into orders, divided orders into genera (singular: genus), and divided genera into species. Linnaeus brought a huge amount of order to the study of biology. The diversity of life is far more manageable if it can be broken down into smaller groups, and Linnaeus was the guy who really got the ball rolling on that front.

Today, scientists have modified the Linnean classification system to incorporate new discoveries and understanding. Instead of Linnaeus's two kingdoms (plants and animals), scientists have proposed additional kingdoms corresponding to such things as single-celled organisms and fungi, and even "grab bag" kingdoms for organisms that don't fit into one of the other kingdoms. The modern version of the Linnean classification system, which scientists use today and is probably the one you learned in high school, looks like this:

1

Domain

1

Kingdom

1

Phylum

1

Class

1

Order

1

Family

1

Genus

1

Species

Classify humans according to this system and you get

I Domain: Eukarya (organisms with one or more cells with a nucleus)

I Kingdom: Animalia (other kingdoms are plants and fungi)

I Phylum: Chordata (animals having a dorsal nerve tube, like the one that runs down your spine)

I Class: Mammalia (animals that have hair, nurse their young, and so on. )

I Order: Primates (yes, you're a primate — so are orangutans, apes, chimps, and others)

I Family: Hominidae (modern man and extinct ancestors of man)

I Genus: Homo (species of humans, both extinct and currently living)

I Species: sapien (wise — proving scientists do have senses of humor)

Here's a little tip: A fun way to remember each level of this system in order is to use the mnemonic "Did King Phillip Come Over For Good Sex?" And if you leave out Domain, which some systems do, the mnemonic changes only slightly: "King Phillip Came Over For Good Sex."

The evidence that all life descends from a single common ancestor includes such things as the unity of the genetic code. Organisms use a simple code to determine how to make proteins from DNA sequences, and all organisms use the same code (although some minor exceptions exist; go to Chapter 15 for details.) To find out more about the genetic code and its importance to evolution, refer to Chapter 3.

Beyond enabling scientists to trace genetic connections back through time, phylogenetics lets scientists better predict what's to come. Being able to anticipate future mutations is an especially important function in areas like health care; virologists and epidemiologists use info gleaned from phylogenetics to stay one step ahead of the bugs that are trying to stay one step ahead of the human immune system. (You can read more about viruses and the race for vaccines in Chapter 19.)

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Responses

  • anastasio
    Why is it important to classify things?
    8 months ago
  • giada lori
    What is the advantage ofphylogenetic classification?
    19 days ago

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