Arthropod Evolution

and, increasingly, molecular biology of living members of a group provide clues about the 15

evolutionary trends that have occurred within that group. It is, however, only the fossil record that can provide the direct evidence for such processes. Unfortunately, in the case of arthropods the early fossil record is poor. By the time the earth's crust became suitable for preservation of dead organisms, in the Cambrian period (about 600 million years ago), the arthropods had already undergone a wide adaptive radiation. Trilobites, crustaceans, and eurypterids were abundant at this time. Even after this time the fossil record is incomplete mainly because conditions were unsuitable for preserving rather delicate organisms such as myriapods and insects. The remains of such organisms are only preserved satisfactorily in media that have a fine texture, for example, mud, volcanic ash, fine humus, and resins (Ross, 1965). Therefore, arthropod phylogeneticists have had to rely almost entirely on comparative studies. Their problem then becomes one of determining the relative importance of similarities and differences that exist between organisms and whether apparently identical, shared characters are homologous (synapomorphic) or analogous (see Chapter 4, Section 3). Evolution is a process of divergence, and yet, paradoxically, organisms may evolve toward a similar way of life (and hence develop similar structures). A distinction must therefore be made between parallel and convergent evolution. As we shall see below, the difficulty in making this distinction led to the development of very different theories for the origin of and relationship between various arthropod groups.

3.2. Theories of Arthropod Evolution

As Manton (1973, p. 111) noted, "it has been a zoological pastime for a century or more to speculate about the origin, evolution, and relationships of Arthropoda, both living and fossil." Many zoologists have expounded their views on this subject. Not surprisingly, for the reasons noted above, these views have been widely divergent. Some authors have suggested that the arthropods are monophyletic, that is, have a common ancestor; others have proposed that the group is diphyletic (two major subgroups evolved from a common ancestor), and yet others believe that each major subgroup evolved independently of the others (a polyphyletic origin). Within the last 50 years, much evidence has been accumulated in the areas of functional morphology and comparative embryology but especially in paleontology and molecular biology, which has been brought to bear on the matter of arthropod phylogeny. This does not mean, however, that the problem has been solved! On the contrary, vigorous debate continues, with the proponents of each viewpoint pressing their claims, typically by using a particular methodology or a specific kind of evidence (for examples, see authors in Gupta, 1979; Edgecombe, 1998; Fortey and Thomas, 1998; also Emerson and Schram, 1990; Kukalova-Peck, 1992). Only rarely have authors attempted to marshall all of the evidence in order to arrive at an overall conclusion. Even then, there may be no agreement! For example, the analyses of Boudreaux (1979) and Wheeler etal. (1993) led them to favor a monophyletic origin whereas Willmer (1990) concluded that, for the present, a polyphyletic origin for the arthropods is more likely. In outlining the pros and cons of these theories it is useful to separate the mono- and diphyletic theories from the polyphyletic theory and to present them in a historical context showing the gradual development of evidence in support of one view or the other.

3.2.1. Mono- and Diphyletic Theories

In a nutshell, proponents of the monophyletic theory simply point to the abundance of features common to arthropods (Section 2) and argue that so many similarities could

not have been achieved other than through a common origin. However, their argument goes beyond simply noting the presence of these features; rather, as a result of improved technology and knowledge, the monophyleticists can now point to the highly conserved nature of key arthropod structures and the processes by which they are formed, for example, cuticle chemistry and molting, the development and fine structure of compound eyes, and embryonic head development (see Gupta, 1979). To this can be added ever-increasing evidence from molecular biology, most (but not all) of which supports monophyly. This should not be interpreted to mean that there is agreement among the monophyleticists as to a general scheme for arthropod evolution. On the contrary, there are quite divergent views with respect to the relationships of the various arthropod groups (Figure 1.8).

Space does not permit a detailed account of the early history of monophyletic proposals and readers interested in this should consult Tiegs and Manton (1958). Nevertheless, a few very early schemes should be noted to show how ideas changed as new information became available. The first monophyletic scheme for arthropod evolution was devised by Haeckel (1866).* Though believing that arthropods had evolved from a common ancestor, he divided them into the Carides (Crustacea, which included Xiphosura, Eurypterida, and Trilobita) and the Tracheata (Myriapoda, Insecta, and Arachnida). After recognizing that Peripatus (Onychophora) had a number of arthropodan features (including a tracheal system), Moseley (1894) envisaged it as being the ancestor of the Tracheata, with the Crustacea having evolved independently. Here, then, was the first diphyletic theory for the origin of arthropods.

At about the same time, after the realization that Limulus is an aquatic arachnid, not a crustacean, it was proposed that the aquatic Eurypterida were the ancestors of all terrestrial arachnids. As a result the eurypterid-xiphosuran-arachnid group emerged as an evolutionary line entirely separate from the myriapod-insect line and having perhaps only very slight affinities with the crustaceans. Thus emerged the first example of convergence in the Arthropoda, namely, a twofold origin of the tracheal system.

Handlirsch (1908, 1925, 1937)* saw the Trilobita as the group from which all other arthropod classes arose separately. Peripatus was placed in the Annelida, its several arthropod features presumed to be the result of convergence. The greatest difficulty with Handlirsch's scheme is the idea that the pleura of trilobites became the wings of insects. This means that the apterygote insects must have evolved from winged forms, which is contrary to all available evidence.

It was at about this time that the Cambrian lobopod fossil Aysheaia pedunculata (Figure 1.10) was discovered. This Peripatus-like creature had a number of primitive features (six claws at the tip of each leg, a terminal mouth, first appendages postoral, second and third appendages are legs). The associated fauna suggested that this creature was from a marine or amphibious habitat. This and other discoveries led Snodgrass (1938) to suggest another monophyletic scheme of arthropod evolution (Figure 1.8). In this scheme the hypothetical ancestral group were the lobopods (so-called because of the lobelike

* Cited from Tiegs and Manton (1958).

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Beekeeping for Beginners

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