General Features Of Animal Design And Diversity

One of the most outstanding features of animal design, particularly of larger bilaterians, is their construction from repeating structures (or modules). The segments of arthropods and annelids and the vertebrae (and associated processes) of vertebrates are the basic units of body plan organization in these phyla (Fig. 1.5a-c). Similarly, many body parts such as the insect wing (Fig. 1.5d) and the tetrapod hand (Fig. 1.5e) are composed of repeated structures.

An important trend in the morphological evolution of animals has been the individualization of modular elements. For example, among the arthropods, we observe a large number of different segment types in crustaceans and insects. This diversity far exceeds that found in the onychophora, a phylum closely related to the arthropods. Thus the evolution of the onychophoran/arthropod clade has been marked by increased diversity of segment types from the more uniform patterns found in earlier forms. Similarly, in some mammals, teeth are differentiated into molars, premolars, canines, and incisors, whereas in the ancestral condition exhibited by most reptiles, the teeth are of uniform shape. Because the diversification of the

Diversity Animal Shape

Figure 1.5

The modularity of body plans and body parts

The body plans of many major phyla, including the annelids (a), arthropods ( b), and chordates (c), are composed of many repeating parts. Some of these parts are similar or identical in appearance to other parts; others are individuated. Sets of serially homologous structures are shaded a unique color. Body parts, such as a butterfly wing (d), or a fossil tetrapod limb from the amphibian fossil Acanthostega (e), are also composed of repeating structures or patterns, some of which are differentiated from others. For example, Acanthostega has eight digits, but like its modern descendants, only five distinct types of digits can be distinguished. Source: Parts a-c from Weatherbee SD, Carroll SB. Selector genes and limb identity in arthropods and vertebrates. Cell 1999; 97: 283-286; part e from Michael Coates.

Figure 1.5

The modularity of body plans and body parts

The body plans of many major phyla, including the annelids (a), arthropods ( b), and chordates (c), are composed of many repeating parts. Some of these parts are similar or identical in appearance to other parts; others are individuated. Sets of serially homologous structures are shaded a unique color. Body parts, such as a butterfly wing (d), or a fossil tetrapod limb from the amphibian fossil Acanthostega (e), are also composed of repeating structures or patterns, some of which are differentiated from others. For example, Acanthostega has eight digits, but like its modern descendants, only five distinct types of digits can be distinguished. Source: Parts a-c from Weatherbee SD, Carroll SB. Selector genes and limb identity in arthropods and vertebrates. Cell 1999; 97: 283-286; part e from Michael Coates.

number, morphology, and function of these repeated units characterizes many of the large-scale differences that distinguish related taxa, understanding how repeated structures form and become individualized is a prerequisite for understanding the developmental basis of large-scale morphological evolution.

The modular organization of animal bodies and body parts has long been recognized by comparative biologists. William Bateson, in his classic treatise Materials for the Study of Variation (1894), identified several kinds of organization found among animals. More importantly, he was the first to bring a Darwinian perspective to the question of how different body patterns may have evolved. Bateson focused particularly on the repetition of parts, cataloguing a large number of rare, but naturally occurring, variants that differed from the norms within various species with regard to either the number or individualization of characters. He suggested that these variations within species could provide insight into the evolution of the large-scale morphological discontinuities between species. For example, variations in the number of body segments within onychophora and centipede species, and of vertebrae in humans and pythons, suggested to Bateson that such discontinuities arose at some frequency in populations and therefore represented plausible steps in the morphological diversification of species.

The question of whether evolution may progress in large, discrete steps remains controversial (we will address this issue in Chapter 8). Nevertheless, these sorts of variants and the organizational concepts espoused by Bateson have been enormously helpful in understanding the genetics and developmental logic underlying the modularity of animal design. In fact, they led to the discovery of genes that play key roles in morphological evolution, albeit not in the fashion Bateson first imagined.

Four fundamental kinds of large-scale, evolutionary differences in morphology are most prevalent in modularly organized animals and are the most significant in terms of adaptation:

1. Changes in the number of repeated parts Bateson referred to this type of change as meristic variation when describing differences within species. Differences in segment number and vertebral number are some of the most obvious characteristics that distinguish classes of arthropods and various classes and orders of vertebrates, respectively (Fig. 1.6).

2. Diversification of serially homologous parts A series of reiterated parts are termed serially homologous. The individualization of repeated parts in an animal reflects the diversification of serially homologous structures. For example, arthropod appendages are serially homologous structures. In the course of arthropod evolution, ancestrally similar appendages have evolved into antennae, various mouthparts, walking legs, and genital structures. In vertebrates, serially homologous vertebrae have evolved into distinct cervical, thoracic, lumbar, and sacral vertebral types.

3. The diversification of homologous parts One of the most prevalent trends in animal evolution is the morphological diversification of homologous parts between lineages. The same structures in different lineages are termed "homologous" when they share a common history, even if they no longer serve the same function. For example, all tetrapod forelimbs are homologous (Fig. 1.7). Despite their differing appearances and functions, bird wings, bat wings, and human forelimbs have all conserved the basic architecture of the tetrapod forelimbs.

Figure 1.6

Meristic differences among arthropods and among vertebrates

Figure 1.6

Meristic differences among arthropods and among vertebrates

Among arthropods such as this trilobite (a), crustacean (b), centipede (c), and insect (d), the number of body segments differs, as does the diversity of segment morphology. Among vertebrates, the number of vertebrae and associated processes differs considerably between a fish (e), frog (f), python (g), and chimpanzee (h).

Frog Lizard Bird

Human Cat Whale Bat

Figure 1.7

The diversification of homologous parts

Figure 1.7

The diversification of homologous parts

All vertebrate forelimbs are homologous structures whose anatomy has undergone considerable diversification in the evolution and adaptation of these various vertebrate lineages. Not to scale.

Source: Redrawn from Ridley M. Evolution, 2nd edn. Malden, MA: Blackwell Science, 1996.

4. The evolution of novelties New characters or "novelties" may evolve from a preexisting structure or arise de novo and become adapted to a new purpose. The evolution of feathers, fur, teeth, antlers, and butterfly wing eyespots are examples of such morphological novelties.

Considering that modularly organized animals are among the most diverse groups (in terms of both the number and morphology of species), could there be a correlation between body design and evolutionary diversity? One possible explanation for this relationship is that modular organization allows one part of the animal to change without necessarily affecting other parts. The evolution of genetic mechanisms that control the individualization of parts would allow for the uncoupling of developmental processes in one part of the body from the developmental processes in another part of the body. In this fashion, for example, vertebrate forelimbs can evolve into wings while hindlimbs remain walking legs. Dissociation of the forelimb and hindlimb developmental programs allows further modifications to occur selectively in either structure, such as the development of feathers in the forelimb of birds and scales in the hindlimb.

EVOLUTION AND DEVELOPMENT: DNA AND DIVERSITY

To understand the major trends in animal diversity and the various kinds of morphological evolution, we must first understand how animal form is generated. Morphology is the product of development, the process through which a single fertilized egg cell gives rise to an entire organism. The physical basis of animal diversity has been viewed since Darwin's time as the outcome of development. Until very recently, however, the developmental principles underlying animal design remained unknown. Although experimental embryologists of the late 1800s and the first half of the 1900s had identified many fascinating phenomena concerning the organization of embryos and the formation of particular structures, the mechanisms responsible for these properties were beyond their reach.

With better understanding of the nature of genes and the process of gene regulation, development has been increasingly viewed as a process orchestrated by the products of genes. Thus the puzzles of embryology, such as how cells come to know their position and identity within a developing animal, have become rephrased in genetic terms. Given that the DNA of (most) all cells in an animal is identical, how do different cells acquire the unique morphologies and functional properties required in the diverse organs and tissues of the body? We now understand that this process occurs through the selective expression of distinct subsets of the many thousands of genes in any animal's genome in different cells. How genes are turned on and off in different cells over the course of animal development is an exquisitely orchestrated regulatory program whose features are only now coming into detailed view.

If morphological diversity is all about development, and development results from genetic regulatory programs, then is the evolution of diversity directly related to the evolution of genetic regulatory programs? Simply put, yes. But to understand how diversity evolves, we must first understand the genetic regulatory mechanisms that operate in development. In other words, what is the genetic toolkit of development and how does it operate to build animals? In the next two chapters, we will examine some of the general features of the genetic and regulatory logic of animal development.

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