Discovering order in the natural world

Around us there are consistent patterns that obviously constitute order in nature. To cite two simple examples, all plants with flowers have leaves, and all birds have feathers. Indeed, the correlation between birds and feathers is so consistent in our modern world that we might go so far as to identify a bird as such because it has feathers. Going further: can we use features such as leaves and feathers to discover underlying patterns of organization among all organisms? In other words, is there some kind of organization that pertains to the reputedly infinite diversity of life?

Hierarchy Perhaps the most significant pattern applicable to all living organisms is the fact that their attributes - that is, all their features, from eyes, to hair, to chromosomes, to bones - can be organized into a hierarchy. Hierarchy refers to the rank or order of features. Indeed, the hierarchical distribution of features is the most fundamental property of the biota. For this reason, a great deal of attention will be devoted here to the business of hierarchies and to their implications.

Take the group that includes all living organisms. A subset of this group possesses a backbone. We call this subset "vertebrates." Within the vertebrates, some possess fur, while most do not. All members of the group that possesses fur are called "mammals." We choose features that characterize smaller and smaller groups within larger groups, the largest of which is the biota. This arrangement is hierarchical, because those creatures possessing fur are a subset of all animals possessing a backbone, which are in turn a subset of all living organisms (Figure 3.1). Although so far we have limited this discussion to backbones and fur, all features of living organisms, from the possession of DNA -which is ubiquitous - to highly restricted features such as the possession of a brain capable of producing a written record of culture, can be arranged hierarchically.

Although life is commonly referred to as infinitely diverse (indeed, we earlier used such a phrase), this is not correct: life's diversity is most assuredly finite. It is profoundly connected by a hierarchical array of shared features. Diversity actually takes the form of many variations on ultimately the most primitive body plan, with modifications upon modifications that take us to the present. Always, however, unmodified or slightly modified vestiges of the original plan remain, and these provide the keys to revealing the fundamental hierarchical relationships that underpin the history of life.

Characters Identifying the features themselves is a prerequisite to establishing the hierarchy of life's history, so we need to look more closely at what we mean by "features." Features of organisms are termed characters. Characters acquire their meaning not as a single feature on a particular organism, but when their distribution among a selected group of organisms is considered. For example, the group Felidae - cats - is generally linked on basis of distinctive features of the skull. Thus not only is the cartoon character Garfield a felid, but so are cats of all stripes, including bobcats, lions, jaguars, and saber-toothed tigers. And by the same token, if someone told us that some mammal is a felid, we could be confident in the prediction that it has those same unique skull features.

In living organisms, there is a wealth of characters: the macroscopic structure of the organism (skin, feathers, fur, muscles, bones, teeth, organs, etc.), genetic composition (chromosomal structure, DNA and amino acid sequences, aspects of transcription and translation), embryo-logical and developmental stages and patterns, and even ecology and behavior. Many of these features are obviously no longer available to paleontologists, and as a result, we are obliged to rely upon the hard skeletal material provided by the fossils themselves.

Because characters are distributed hierarchically, their position in the hierarchy is obviously a function of the groups they characterize. Consider again the simple example of fur in mammals. Since all mammals have fur, it follows that if one wanted to tell a mammal from a non-mammal (any other organism), he need only observe that the mammal is the one that has the fur. On the other hand, the character "possession of fur" is not useful for distinguishing, say, a bear from a dog; both have fur. To distinguish one mammal from another, characters that identify subsets within mammals must be used.

Figure 3.1 .The natural hierarchy illustrated as a wooden puzzle.The different organisms represent the larger groups to which they belong. For example, the mouse, representing Mammalia, and the lizard, representing Reptiiia, together fit within the puzzle to represent Vertebrata, itself a subset of bilaterally symmetrical organisms (Biiateralia), which would include invertebrates such as the lobster Biiateralia and other groups constitute the group of organisms that we call Animalia.

Figure 3.1 .The natural hierarchy illustrated as a wooden puzzle.The different organisms represent the larger groups to which they belong. For example, the mouse, representing Mammalia, and the lizard, representing Reptiiia, together fit within the puzzle to represent Vertebrata, itself a subset of bilaterally symmetrical organisms (Biiateralia), which would include invertebrates such as the lobster Biiateralia and other groups constitute the group of organisms that we call Animalia.

These distinctions are extremely important in establishing the hierarchy appropriately, and for this reason, characters may function in two ways: as "general" characters and as "specific" characters. A character is specific when it characterizes (or is diagnostic1 of) all members of a group, while a character is general when it is nondiagnostic of that group. The same character may be specific in one group but general in a smaller subset of that group (because it is now being applied at a different position in the hierarchy).

Suppose as a description to help you find someone you had never met, you were told, "He has two eyes." This would obviously be of little help, since all humans have two eyes. The character of possession of two eyes is a general character that is found not only among humans but in many other groups of organisms. Indeed, it is a general character among

I The word "diagnostic" has the same meaning here as in medicine. Just as a doctor diagnoses a malady by distinctive and unique properties, so a group of organisms is diagnosed by distinctive and unique characters.

all vertebrates, and the character of two eyes alone would not distinguish a human from a guppy. But at a much deeper level in the hierarchy, the character would be specific: possession of two eyes would distinguish a vertebrate (two eyes) from an earthworm (no eyes) or a spider (four eyes). Likewise, consider yet again the example of fur in mammals. The presence of fur is specific when mammals and non-mammals are considered together (because the presence of fur diagnoses mammals) but is general within mammals (it wouldn't be useful in telling one mammal from another).

Cladograms Evolutionary biologists, including paleontologists, commonly use a "cladogram" to visualize the hierarchies of characters in the biota. A cladogram (pronounced cla-do-gram; clados - branch; gramma - letter) is a hierarchical, branching diagram that can be used to depict the hierarchies of shared characters. Its implications, however, are far greater than those of a mere visual aid, for it and the methods behind it have become the single most important tool for understanding the evolutionary history of organisms.

To understand how a cladogram is constructed, we begin with two things to group; say, a car and a pick-up truck. Notice that anything can be grouped; it does not necessarily apply only to living (and extinct) organisms. Cars and trucks may be linked by any number of characters. The characters must, of course, be observable features of each. For example, "used for hauling lumber" is not appropriate, because hauling lumber is what it does, and is not an observable character. Note, though, that a pick-up truck could have characters that make hauling lumber easier than in a car. A cladogram of a car and a truck is shown in Figure 3.2.

Since it is the characters that are distributed hierarchically in the natural world, it is characters that we must choose to diagnose groups.2 Here, we choose (1) the presence of four wheels, (2) an engine, (3) chassis, (4) seats, and (5) lights. The cladogram simply connects these two separate objects (the car and the pick-up truck) based upon the characters that they share. The features are identified (and itemized) on the cladogram adjacent to the "node," which is a bifurcation (or two-way splitting) point in the diagram. Figure 3.2 shows this relationship.

The issue becomes more complicated (and more interesting) when a third item is added to the group (Figure 3.3). Consider a motorcycle. Now, for the first time, because none of the three items is identical, two of the three items will share more in common with each other than either does with the third. It is in this step that the hierarchy is established. The group that contains all three vehicles is diagnosed by certain features shared by all three. Notice that a subset containing two vehicles has been established. Because the two are linked together on the cladogram, not only do they share the characters pertaining to all three, but above and

2 The motor vehicles in this example are not from the natural world, and thus the distribution oftheir characters may not really be hierarchical. Nonetheless, this example serves as an effective illustration to show how characters function to unite groups on cladograms.

Car Cladogram

chassis seats lights

Figure 3.2. A cladogram.The car and pick-up truck are linked by the characters listed at the ban just below the node.The node itself defines the things to be united; commonly a name is attached to the node that designates the group. Here, such a name might be four-wheeled vehicles.

chassis seats lights

Figure 3.2. A cladogram.The car and pick-up truck are linked by the characters listed at the ban just below the node.The node itself defines the things to be united; commonly a name is attached to the node that designates the group. Here, such a name might be four-wheeled vehicles.

beyond these they share further characters that link them exclusive of the third vehicle. Lights and seats would be general when one is discussing the subset composed of two vehicles, since those characters are diagnostic only at a higher level in the hierarchy.

It should be clear by now that how these characters, and even the vehicles, are arranged on the cladogram is controlled by the choice of characters. Suppose that instead of "seats" we had specified bucket seats, and instead of "four wheels" we had simply specified "wheels." Bucket seats would then no longer be a general character diagnosing all vehicles, but instead would unite only the car and the truck. Likewise, the presence of wheels would be a general condition pertaining to all three, instead of uniting trucks and cars.

Now suppose that instead of the characters that we listed above, we had chosen wheels, engine, lights, seat, and non-passenger space less than passenger space. These characters produce a cladogram quite different from that in Figure 3.3, in which cars and motorcycles are linked more closely to each other than either is to a pick-up truck (Figure 3.4). This arrangement is counterintuitive, and contradicts the cladogram in Figure 3.3.

How do we choose? We must choose the characters, and order them so that the resultant cladogram doesn't change when other characters are added. Most of the characters that apply to these motor vehicles support the cladogram in Figure 3.3 and not that in Figure 3.4; they suggest that, in its design, a car shares much more in common with a pick-up truck than it does with a motorcycle. Moreover, those features that a car shares with a motorcyle are also present in the pick-up truck;

engine lights

Figure 3.3. One possible distribution of three motor vehicles. Members of the group designated by node I are united by the possession of wheels, lights, and an engine; that group could be called MotorVehicles. Within the group Motor Vehicles is a subset united by possession of bucket seats and a chassis.That subset is designated at node 2.

engine lights

Figure 3.3. One possible distribution of three motor vehicles. Members of the group designated by node I are united by the possession of wheels, lights, and an engine; that group could be called MotorVehicles. Within the group Motor Vehicles is a subset united by possession of bucket seats and a chassis.That subset is designated at node 2.

they are general features for cars, trucks, and motorcycles, rather than specific features that clearly diagnose a car-motorcycle subset within the three vehicles. Distinguishing among cladograms is an important subject that will be discussed below.

engine lights

Figure 3.4. An alternative distribution of three motor vehicles.The characters selected suggest that the car and motorcycle share more in common with each other than either does with the pick-up truck engine lights

Figure 3.4. An alternative distribution of three motor vehicles.The characters selected suggest that the car and motorcycle share more in common with each other than either does with the pick-up truck

So far, we have presented the cladogram only as a graphic method of showing hierarchies. Obviously, it must be far more than this, or its relevance to this book would be difficult to fathom. In fact, cladograms are powerful tools for studying the evolutionary relationships among organisms. Their use in the past 25 years has completely revolutionized our understanding of the interrelationships of organisms.3 Terms as fundamental as "reptile," "dinosaur," and "bird" have startling new meanings as a result of "cladistic analysis," or analysis using a cladogram. For this reason, cladograms play a profound role in this book.

A monkey's uncle Fundamental to evolutionary biology and paleontology is the recovery of who is related to whom. Before we can understand the great events and rhythms of biotic history, we need a way to discover the pattern of descent of the Earth's creatures. Considered individually, extinct and extant organisms are a myriad of apparently disconnected individuals, but considered as evolving groups (lineages), striking patterns emerge that enrich our view of ourselves and the world around us. It is for this reason that evolution is considered the unifying principle of biology: evolution is the basis of the fundamental genealogical connections among organisms. Accordingly, we are interested in "phylogeny": the history of the descent of organisms. It is in this respect that cladistic analysis makes a key contribution. Using character hierarchies portrayed on cladograms, we can establish "clades" or "monophyletic groups" (to add to the nomenclature, these are sometimes termed "natural groups" as well; here, these terms are all considered to be synonymous). These are groups of organisms that have evolutionary significance because the members of each group are more closely related to each other than they are to any other creature. For example, it is probably no surprise that humans are a monophyletic group: all members of Homo sapiens are more closely related to each other than they are to anything else. The idea that a group is monophyletic has a second, more subtle ramification: it implies that all members of that group share a more recent common ancestor with each other than with any other organism.

0 0

Post a comment