ENHS Nature Trails Feb 99 Article 2

Natural History and You - The President's Forum
by Nathan Tublitz

Slow and steady wins again: Turtles leap-frog up the reptilian tree

What comes to mind when you think of turtles? Slow? Ponderous? Toothless? Venerable? A shell? All are apt descriptions of these cumbersome animals which biologists have traditionally placed as the most ancient of the reptiles, the ancestor of snakes, lizards, and crocodiles. Yet new evidence suggests that turtles may be younger and more advanced than first thought. This evidence, obtained from molecular studies, is but another skirmish in the on-going intellectual war between molecular evolutionists and their more traditional evolutionary biological brethren.

To put this battle into context, one must start at the beginning. In this modern world of overstatement and hyperbole, it is not too extreme to say that all of modern biology descends from Darwin's theory of evolution, which states that all living organisms evolved from ancestral forms as a result of natural selection.

Darwin's ideas on evolution came in part from his analysis of the finches of the Galapagos Islands. While there Darwin collected 13 different species of finches, each with a unique set of morphological features. After determining their habitats and which islands they inhabited (each lived on a small subset of islands), he concluded that it was easier to assume that all 13 species descended from a common ancestor rather than that they all arose independently. He based this conclusion primarily on differences in overt anatomical characteristics like beak shape, which varied among each finch species. Darwin's ability to determine evolutionary relatedness on the basis of shared characteristics, deduced by comparing the external anatomy of the organisms under study, was the template for thousands of subsequent studies. These investigations have demonstrated relatedness between species by drawing "evolutionary trees", tree-like stick diagrams where ancestral species are placed at the base of the "tree" and more advanced species are put into the tree's crown.

Reptiles are a prime example of this type of analysis. Using morphological information from living and extinct species, including the dinosaurs, classical evolutionary biologists convincingly deduced that turtles are at the base of the reptilian "tree" and have given rise to all other reptiles, including snakes, lizards and dinosaurs as well as to the lineage that includes their feathered cousins, the birds.

However, new molecular evidence -presented in this week's issue of the journal Science (12 Feb 1999, p. 998-1001)-- has upset the reptilian "tree" by placing turtles up in the "branches" along side the crocodiles and above the other squamate reptiles (reptiles with scales). The basis for the conclusion is a comparison of the structures of 33 different genes found in all reptilian groups (turtles, snakes, lizards, crocodiles) as well as in birds. How can these molecular results be at such odds with the anatomical data used by classical evolutionary biologists?

Like their classical counterparts, molecular evolutionists determine "relatedness" between species on the basis of similarities. The more similarities two groups have, the more closely related they are. However, instead of comparing anatomical features, the molecular evolutionists study molecules, primarily proteins and DNA because of their simple structures. Proteins are comprised of only 20 building blocks (amino acids) and DNA is even less complicated, having only 4 different building blocks. Let's assume the 4 building blocks of DNA are A, B, C, and D. If three species have a specific DNA sequence of AABBCCDD and a fourth species has a sequence of AAAACCCD, then molecular evolutionists conclude that the three species are more closely related to each other than to the fourth species. From data like this, an evolutionary tree can be drawn.

The classic reptilian "tree", based on morphological and paleontological evidence, has turtles at the base, followed by lizards and snakes who evolved next. Highest on that tree are the crocodiles and the birds. But the tree drawn with molecular evidence looks strikingly different: these data show that lizards and snakes are the earliest reptiles, followed next by the birds who branch off before the turtles and the crocodiles. The most intriguing result from the molecular study in Science is that none of the individual genes studied support the classical view of reptile evolution.

These new molecular data has thrown the reptile evolution world into a tizzy. The classicists insist that their "tree" is based on hundreds of anatomical characteristics and that the molecular data just doesn't fit in. For example, the eggs of crocodiles and birds - groups at the top of the classical "tree'-- are each formed from the mineral calcite and are similar in structure whereas turtle eggs are different in structure and are made from a different mineral, aragonite. Other classicists argue that turtles have been placed at the bottom of the tree because many of their features seem to be the basis for the appearance of more advanced characteristics seen in lizards, snakes and birds.

This is but one of many skirmishes being waged in the war between classical and molecular evolutionists for intellectual supremacy. Many old evolutionary questions, thought to have been answered using classical methods, are being re-examined in light of new molecular data. Did insects arise from a single ancestral species? How did vertebrates evolve? Did marine mammals arise once or several times? How did flowering plants evolve? One particularly intriguing issue being fought on several fronts is the relationship between humans and the great apes, with recent AIDS virus and other sequences raising new questions about whether chimpanzees are indeed our closest relative.

In his seminal work on the structure of scientific revolutions, Kuhn stated that science is by its very nature a conservative discipline where major changes in thought - the so-called paradigmatic shift occur only in response to new data viewed initially as heretical. In the case of evolutionary biology, we are seeing such a shift, from relying exclusively on morphological evidence to utilizing a variety of different data including those obtained from molecular methods. Obviously, the old guard classicists are uncomfortable since the status quo has been shaken to its roots. But the old guard will in time give way to a new generation of evolutionary biologists who are more comfortable with molecular data and more willing to reconcile it with traditional anatomical data. Until then, the battle between classical and molecular evolutionists will continue to rage on many fronts. And yet, the slow and steady turtle will go about its business, the same as it has for millions of years, unfazed by it all.

Nathan Tublitz

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