Complex multicellular organisms account for a large proportion of all eukaryotic species. Thus, one evolutionary consequence of complex multicellularity was an increase in biological diversity. Let’s compare the diversity of complex multicellular groups with their simpler phylogenetic sisters. Choanoflagellates, for example, number about 150 species; simple animals such as sponges and jellyfish about 20,000; and complex animals with circulatory systems perhaps as many as 10 million. Plants and their relatives show a comparable pattern. There are about 6000 species of green algae on the branch that includes land plants, but about 400,000 species of anatomically complex land plants capable of bulk flow. Similarly, complex fungi and red algae include more species by an order of magnitude than their simpler relatives.

How did this immense diversity arise? At one level, the answer is functional and ecological. Complex multicellular organisms can perform a range of functions that simpler organisms cannot, and so they have evolved many specific types of interaction with other organisms and the physical environment. Carnivory, for example, requires both sophisticated sensory systems and jaws or limbs to locate and capture prey, but once established, carnivorous animals radiated throughout the oceans and, eventually, on land. This explanation, however, prompts another question: What is the genetic basis for the bewildering range of sizes and shapes displayed by complex multicellular organisms?

The answer has to do with the network of genes that guide development. Since the 1950s, biologists have learned a remarkable amount about this genetic network and its host of interacting genes (Chapter 20). Many of the genes involved in development are what we might consider middle managers: A molecular signal induces the expression of a gene, and the protein product, in turn, prompts the expression or repression of another gene. It is the complex interplay of these genetic switches—turning specific genes on in one cell and off in another, depending on where those cells occur in the developing body—that results in crabs with large pincers or butterflies with patterned wings (Fig. 28.16).

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It makes sense that if regulatory genes guide development of the body in each complex multicellular species, then mutations in regulatory genes may account for many of the differences we observe among different species. Our growing understanding of how developmental genes underpin evolutionary change has given rise to a whole new field of biology called evolutionary-developmental biology, or evo-devo for short.

In evo-devo research, scientists compare the genetic programs for growth and development in species found on different branches of phylogenetic trees. In addition to wanting to discover the molecular mechanisms that guide development in individual species, these scientists aim to understand the genetic differences associated with differences in form and function among species. Evo-devo is an exciting and rapidly expanding field of research because it is helping to illuminate the long-suspected relationship between the development of individuals and patterns of evolutionary relatedness among species. Continuing research promises to shed new light on the similarities and differences among plants, animals, and other complex multicellular organisms.