Chapter

18 is a fairly common phenomenon in evolution and, on theoretical grounds alone, it could be expected that two unrelated groups of animals would evolve toward the same highly desirable situation and, as a result, develop almost identical structures serving the same purpose. (Even the monophyleticists have to accept some degree of convergence, for example, among the tracheae of insects and those of some arachnids and crustaceans.) Polyphyleticists argue that the similar features of arthropods are interrelated and interdependent; that is, they all result from the evolution of a rigid exoskeleton. Thus, in order to grow, arthropods must periodically molt; to move around, they must have articulated limbs and body; tagmosis is a logical consequence of segmentation and results in changes to the nervous and muscular systems; the presence of the cuticle demands changes in the gas exchange, sensory, and excretory systems; and the open circulatory system (hemocoel) is the result of an organism no longer requiring a body cavity with hydrostatic functions. In a sense, then, all the polyphyleticists need to demonstrate is the polyphyletic nature of the cuticular exoskeleton. As noted in Section 2.1, the onychophorans, tardigrades, and pentastomids have such an outer covering (and some other arthropod features) yet are generally considered distinct from true arthropods (their very existence tends to make life "uncomfortable" for the members of the monophyletic camp).

The second approach taken by the polyphyletic supporters is to criticize the evidence or the methodology used by those who favor monophyly. For example, they argue that the processes of cuticular hardening used by the three major arthropod groups are quite distinct; quinone-tanning in insects, disulfide bridges in arachnids, and impregnation with organic salts in crustaceans. Likewise, they claim that there are great differences in the structure of the compound eyes among the major arthropod groups. However, they also point out that the ommatidium (the eye's functional unit—see Chapter 12, Section 7.1) of some polychaete worms and bivalve mollusks is highly similar to its counterpart in arthropods, emphasizing the ease with which convergence occurs. Ironically, even the monophyleticists disagree over the number and homologies of the segments that make up the arthropod head.

The early polyphyleticists, including Tiegs and Manton (1958), Anderson (1973), and Manton (1973, 1977), also presented direct evidence to support their theory, largely from comparative embryology and functional morphology. More recently, proponents of poly-phyly have added information from paleontology (see authors in Gupta, 1979, and Fortey and Thomas, 1998). According to these authors, the evidence weighs heavily in support of the division of the arthropods into at least three natural groups, each with the rank of phylum (Figure 1.11). The phyla are the Chelicerata, the Crustacea, and the Uniramia

FIGURE 1.11. A scheme showing a possible polyphyletic origin for the major arthropod groups and related phyla. Hatched lines ending in a question mark indicate arthropod fossils not easily assigned to existing taxa.

FIGURE 1.11. A scheme showing a possible polyphyletic origin for the major arthropod groups and related phyla. Hatched lines ending in a question mark indicate arthropod fossils not easily assigned to existing taxa.

Beekeeping for Beginners

Beekeeping for Beginners

The information in this book is useful to anyone wanting to start beekeeping as a hobby or a business. It was written for beginners. Those who have never looked into beekeeping, may not understand the meaning of the terminology used by people in the industry. We have tried to overcome the problem by giving explanations. We want you to be able to use this book as a guide in to beekeeping.

Get My Free Ebook


Post a comment