Wristwatches when is a watch a watch

We've used cladistic techniques to infer the history of the biota. Here we'll try something different: we'll use cladistic techniques to infer the evolutionary history of watches. Analog and digital timepieces are comonly called "watches." Implicit in the term "watches" is some kind of evolutionary relationship: that these instruments have a common heritage beyond merely post-dating a sundial. But is this really so?

Consider three types of watch: a wind-up watch, a digital watch, and a watch with a quartz movement. Six cladograms are possible for these instruments (Figure B3.1.1), but it can be seen that, by the definition of a cladogram, a and b for each type are identical. This is because the groups at a node share the characters listed at that node, regardless of order. For this reason, we really have only three cladograms to consider (Figure B3.1.2).

One might at first wish to place the digital watch in the smallest subset, in the most derived position (as in types I and II), since it is the most modern, technologically advanced, and sophisticated ofthe three. Remember, however, how the cladogram is established: on the basis of shared, derived characters. Cladograms types I and II say that the digital watch shares the most characters in common with either a wind-up watch (type I) or a quartz watch (type II). A look at the characters themselves suggests that this is not correct: wind-up and quartz watches are both analog watches (have a dial with moving, mechanical hands) and their internal mechanisms consist of complex gears and cogs to drive the hands at an appropriate speed. The digital watch, on the other hand, consists of microcircuitry and a microchip, with essentially no moving parts. It is apparently something very different and, from its characters, bears little relationship to the other "watches."

What is the digital watch? In an evolutionary sense, it is really a computer masquerading (or functioning) as a timepiece. The computer has been put in a case, and a watchband has been added, but fundamentally this "watch" is really a computer. In our hypothesis of relationship, the watchbands and cases ofwatches have evolved independently two times (once in computers and once in watches), rather than the guts of the instrument, itself, having evolved twice. That the watchbands and cases evolved independently two times is a more parsimonious hypothesis than arguing that the distinctive and complex internal mechanisms (themselves consisting of many hundreds of characters) of the watches evolved independently twice.

What, then, is a watch? If the term "watch" includes digital watches as well as the other two more conventional

Figure B3.1.1. Six possible arrangements of three timepieces on cladograms. Note that each pair is redundant: the order in which the objects on each "V" is presented is irrelevant.

varieties, then it should also include computers, since a digital watch has the shared, derived characters of computers. The cladogram suggests that the term "watch" does not describe an evolutionarily meaningful (monophyletic) group, in the sense that a cladogram that includes digital watches, wind-up watches, and quartz watches must also include computers, as well as a variety of more conventional mechanical timing devices (such as stop watches). Rather, the term "watch" may be thought of as some other kind of category: it describes a particular function (time-keeping) in conjunction with size (relatively small).

Figure B3.1.2. Because each pair of cladograms in Figure B3.1.1 is redundant, there are really only three cladograms under consideration.

In this example, we are fortunate in that, should we so choose, we can test the cladogram-based conclusions by studying the historical record and find out about the evolution of wrist watches, digital watches, and quartz watches. Obviously this is not possible to do with the record ofthe biota, because there is no written or historical record with which to compare our results. The characters of each new fossil find, however, can be added to existing cladograms and the hypothesis of relationship that shows the least complexity will be favored according to the principle of parsimony. In our discussions of the biota, we attempt to establish categories that are evolutionarily significant (monophyletic groups), and avoid groups that have less in common with each other than with anything else.

relationships, what we call mammals were invented when that character - among others -first evolved. And, the cladogram tells you that sometime thereafter - the cladogram does not specify when - a character that unites both humans and gorillas evolved, a character that we now recognize diagnoses a new group within Mammalia3.

Here is a fundamental difference between a cladogram and the more familiar tree of life that we discussed above. The cladogram does not incorporate time, nor does it tell you who the ancestors were. Instead, it can indicate the sequence of the evolutionary events and, more importantly, specify the characteristics that the ancestor possessed. So, in the case of the cladogram in Figure 3.9, we aren't told who was the ancestor of bears, gorillas, and humans, but we infer that the earliest mammal was fur-bearing (among the other characters that diagnose Mammalia). In Box 3.1 cladograms are used to reconstruct the evolution of wristwatches,

Cladogram Gorillas

MAMMALIA

warm-bloodedness fur- or hair-bearing

Figure 3.9. Addition of the genus Gorilla. The addition of gorillas to the cladogram does not alter the basic relationships outlined on the cladogram shown in Figure 3.8.

MAMMALIA

warm-bloodedness fur- or hair-bearing

Figure 3.9. Addition of the genus Gorilla. The addition of gorillas to the cladogram does not alter the basic relationships outlined on the cladogram shown in Figure 3.8.

3. That new group is called "Hominoidea," as it happens, and is diagnosed by lots of characters, among which are a series of specializations in the arms and trunk associated with walking bipedally and swinging through trees.

demonstrating its power to reveal the underlying evolutionary relationships of even inanimate objects.

Used as a tool to reconstruct evolution, then, a cladogram is actually a hypothesis of relationship; that is, a hypothesis about how closely (or distantly) organisms are related, and about what the sequence of the appearance of different diagnostic characters must have been.

As we have seen, it is possible to construct several possible cladograms which would represent different evolutionary sequences. Which to choose? We choose using the principle of parsimony. Parsimony, a sophisticated philosophical concept first articulated by the fourteenth-century English theologian William of Ockham, states that the explanation with the least necessary steps is probably the best one. Why resort to complexity when simplicity is equally informative? Why suppose more steps took place when fewer can provide the same information?

Figure 3.10 shows two cladograms that are possible with birds, a human, and a bat and the characters of wings, fur, feathers, and mammary glands. In (b), the bird has to lose ancestral mammary glands and it has to replace fur with feathers. In (a), wings must be invented by evolution twice. Cladogram (a) is the simpler of the two because it requires fewer evolutionary events or steps. It is uncomplicated by the addition of more characters. In contrast to cladogram (a), the addition of virtually any other characters that are shared by humans and bats to cladogram (b) (for example, the arrangement, shape, and number of bones, particularly those in the skull and forelimbs, the structure of the teeth, the biochemistry of each organism) requires that each of these shared characters evolved independently: once in bats and once in humans. That considerably complicates the number of evolutionary steps, leaving us with the conclusion that cladogram (b), the hypothesis that birds and bats are more closely related to each other than either is to a human, is a less parsimonious alternative.

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