Cladistics. What is it? Why is it used? How does it work? You'll have some of those questions answered in the pages of In Search of Deep Time, Beyond the Fossil Record to a new History of Life by Henry Gee. This is a Free Press publication of 267 pages from 1999 that retails for $26.00 in hardcover. Henry Gee has his doctorate in zoology from Cambridge University and is Chief Science Writer for the British Journal Nature.
Cladistics is a technique for comparing various traits from one group (family, genus, species) with other similar groups. The output of the analysis yields one or more "family trees" of possible relationships between these groups. The best of these family trees is so chosen if it is deemed to be the simplest explanation that fits.
In Search of Deep Time begins with a simple narrative that demonstrates the logic behind the development and use of cladistics in evolutionary science. The book's subtitle gives away the impact that the use of cladistics is having on the "family tree" of life on earth. Cladistic approaches have challenged the way scientists have looked at evolutionary relationships between animals to the point that similar looking animals may not be as closely related as previously thought. This chapter is a "what it is and how it works" description not a "how to do it" method. The remainder of the book presents numerous examples of cases in which this technique is changing our thinking.
My personal disappointment in this first chapter came during Gee's discussion of outgroups. I know from my reading of professional papers that the choice of an outgroup is very important in this analysis. Gee acknowledges this importance but fails to explain why one outgroup is chosen over another. Changing the outgroup choice changes the output of the analysis. I would have defined the term outgroup for you but without the rules for choosing one, I can't get my arms around a good explanation. In general, it appears to be a group (family, genus, species) that is similar to the group that is being studied.
Since cladistics deals with evolutionary relationships, Gee includes a historical chapter concerned with great thinkers in evolution both before and after Darwin. This chapter contains understandable explanations of the ideas these men proposed to interpret the living world around them. The history begins with Linnaeus and culminates with George Gaylord Simpson. Even if you have read other historical accounts, Gee's dialog may provide slightly different interpretations.
In Search of Deep Time contains some interesting conflicts on the validity of the cladistic approach between noted scientists. One such argument centered on fish - lungfish in particular - the result of which is a little confounding for non-specialists: the "fishes do not form a natural group . . . the term 'fish' has no zoological meaning, as a category". Historically, fish represent a group of equal status with reptiles and amphibians. This may indicate the level of changes that cladistics brings to the present hierarchical organization of living things that we are familiar with.
Gee makes the point that "fossils weren't buried with their birth certificates" and that it is virtually impossible to "link fossils into chains . . . of ancestry or descent". This point is stated in the first and fourth chapters. Beyond that it is repeated by example after example after example and on and on through most of the book. By the tenth example one pretty well has the drift assuming one's not tired of hearing this theme. The point is a valid one and quite frankly, if the repetitious examples hadn't been used, In Search of Deep Time would have been a very short book indeed.
I must confess to being a bit disappointed with In Search of Deep Time. Unfortunately, my expectations from the book were not in concert with the content. This does not mean that the book is deficient in any way. It merely means that I expected something other than what I got. My expectation was that In Search of Deep Time would teach me how to perform a cladistic analysis. Looking back on this, I should have known that a popular book would not supply a technical step-by-step procedure.
In Search of Deep Time will be most informative to general readers curious about the what and why of cladistics. To that purpose I can certainly recommend it.