Diversity Of Anteriorposterior Body Organization Within Arthropods And Vertebrates

The arthropods are the most successful animal taxa, with the insects alone accounting for roughly 75% of all known animal species. All arthropods share a body plan made up of repeated units—the segments that bear the paired, jointed appendages for which the phylum is named. Different arthropod classes have particular body plans characterized by the subdivision of the body into distinct regions (for example, head, thorax, abdomen) containing specific numbers of segments and by the distribution of appendages on those segments. Thus the dominant theme in the evolution of the various arthropod body plans has been the diversification of segment and appendage number, organization, and morphology (see Chapter 1, Fig. 1.6).

Extant arthropods include the following groups:

• The myriapods, which have many similar (homonomous) trunk segments

• The crustaceans, which display dramatic diversity in segment number and appendage shape and function

• The insects, which possess a stereotypical body organization consisting of a complex head, a thorax bearing six walking legs and two pairs of wings, and a limbless adult abdomen

• The chelicerates, which have a unique, two-part body subdivision into prosoma and opisthosoma

By contrast, the sister phylum to the arthropods, the onychophora, consists of animals that are much less diverse and have only a small number of segment and appendage morphologies. These characteristics likely reflect the primitive condition of the arthropod/onychophoran clade.

The arthropod fossil record is rich with diverse arthropod forms, and distinct arthropod and onychophoran body plans are readily apparent in the Cambrian period. The early divergence of this clade remains cryptic, as the segmental morphology and complexity of the earliest (Precambrian) arthropod ancestors have not been documented.

The evolution of different vertebrate body plans is also characterized by the divergence of repeated units—namely, the vertebrae and paired appendages (see Chapter 1, Figs 1.6 & 1.7). The mesodermal segments of vertebrates (that is, the somites) originate as identical, serially homologous fields that will eventually give rise to the vertebrae and associated processes of the axial skeleton (see Chapter 3). The number and morphology of the vertebrae that constitute each region of the tetrapod vertebral column (for example, cervical, thoracic, lumbar, sacral) differ among the major tetrapod groups. For instance, mammals typically have seven cervical (neck) vertebrae, as opposed to 13 or more in birds. Further, snakes can have hundreds of thoracic (rib-bearing) vertebrae. These differences in the number of specific vertebrae are significant in terms of the evolutionary adaptation of the various groups of tetrapods. The elongation of the thorax of snakes and some lizards, coupled with the loss of limbs, allowed them to exploit unique ecological niches. Likewise, the variations in the number of cervical vertebrae in birds and the loss of tail vertebrae in some primates are tied to their unique evolutionary histories.

Vertebrate limbs also exhibit a wide range of patterns and functions. The serially homologous forelimbs and hindlimbs of a single animal can have dramatically different morphologies, such as the wings and legs of birds. The shapes and functions of homologous limbs of different animals range from seal flippers to bat wings to human arms.

The evolution of different body plans within a phylum includes both the morphological diversification of repeated body parts within a single animal and the morphological diversification of homologous structures between animal lineages. In genetic terms, serially homologous body parts evolve in the context of the genome of a single species. By contrast, homologous structures that diverge among lineages, such as the forelimbs of different tetrapods or the hindwings of diverse insects, evolve in the context of independently evolving genomes.

0 0

Post a comment