Evolutionary developmental biology (evo-devo) has undergone dramatic transformations since its emergence as a distinct discipline. This paper aims to highlight the scope, power, and future promise of evo-devo to transform and unify diverse aspects of biology. We articulate key questions at the core of eleven biological disciplines-from Evolution, Development, Paleontology, and Neurobiology to Cellular and Molecular Biology, Quantitative Genetics, Human Diseases, Ecology, Agriculture and Science Education, and lastly, Evolutionary Developmental Biology itself-and discuss why evo-devo is uniquely situated to substantially improve our ability to find meaningful answers to these fundamental questions. We posit that the tools, concepts, and ways of thinking developed by evo-devo have profound potential to advance, integrate, and unify biological sciences as well as inform policy decisions and illuminate science education. We look to the next generation of evolutionary developmental biologists to help shape this process as we confront the scientific challenges of the 21st century.

Recognition that the transformation of one form into another is caused by both internal and external factors is the foundation and driving philosophy underlying all research in the field of biology. 10,11 In practice, however, studies of the internal (proximate) causes of biological transformation within the lifetime of an organism and of the external (ultimate) causes of transformation from one generation to the next (evolutionary transformation) have been pursued independently in two sub-disciplines, developmental and evolutionary biology, respectively. This situation has changed with the emergence of evolutionary developmental biology or "evo-devo," which seeks, by means of a comparative approach, to explain the evolution and development of morphological characters as well as the evolution of their underlying genetic and developmental mechanisms. 12 Essential to this approach is a well-defined and strongly supported phylogeny from which homology and the direction of morphological transformations can be accurately assessed. [13][14][15] Exciting and significant findings have been made in evo-devo. One is that the metazoan body plan is established by a surprisingly small set of highly conserved patterning genes. These homeobox (Hox) genes (Fig. 2), which originated early in metazoan evolution, are distributed throughout the animal kingdom. 16 -18 Another important finding is that homology at the genetic level is not necessarily correlated with homology at the morphological level. For example, the compound eyes of insects and the camera eyes of vertebrates evolved independently, but in both the initiation of eye formation requires expression of the same gene, Pax-6. 19 It also has been demonstrated that morphological novelties such as butterfly eye spots 20 or limbless tetrapods (snakes 21 ) result from alterations in the molecular mechanisms that control the development of major anatomical structures. Moreover, the evolution of developmental systems, which mediate morphological evolution, can occur by slight changes in the regulation of otherwise conserved patterning genes (Fig. 3). [22][23][24] While evo-devo research has focused primarily on broad taxonomic comparisons and major morphological transitions such as that from fish fins to tetrapod limbs, its potential for explaining phenotypic differences between and among closely related taxa is clearly recognized. [25][26][27] In this context, understanding how molecular evolution shapes genetic variation of patterning and growth genes in different primate lineages can be a powerful method for linking genetic and developmental variation to phenotypic (morphological) variation at both the microevolutionary and macroevolutionary scales. 28 -31 Indeed, the focus of evo- The order Primates is composed of many closely related lineages, each having a relatively well established phylogeny supported by both the fossil record and molecular data. 1 Primate evolution is characterized by a series of adaptive radiations beginning early in the Cenozoic era. Studies of th...

Conclusive evidence was provided that y', the upstream of the two linked simian y-globin loci (5'-y'-'y2-3'), is a pseudogene in a major group of New World monkeys. Sequence analysis of PCR-amplified genomic fragments of predicted sizes revealed that all extant genera of the platyrrhine family Atelidae [Lagothrix (woolly monkeys), Brachyteles (woolly spider monkeys), Ateles (spider monkeys), and Alouafta (howler monkeys)] share a large deletion that removed most of exon 2, all of intron 2 and exon 3, and much of the 3' flanking sequence of y. The fact that two functional 'y-globin genes were not present in early ancestors of the Atelidae (and that y1 was the dispensible gene) suggests that for much or even all of their evolution, platyrrhines have had ly2as the primary fetally expressed 'y-globin gene, in contrast to catarrhines (e.g., humans and chimpanzees) that have 'y' as the primary fetally expressed y-globin gene. Results from promoter sequences further suggest that all three platyrrhine families (Atelidae, Cebidae, and Pitheciidae) have y2 rather than y' as their primary fetally expressed y-globin gene. The implications of this suggestion were explored in terms of how gene redundancy, regulatory mutations, and distance of each 'y-globin gene from the locus control region were possibly involved in the acquisition and maintenance of fetal, rather than embryonic, expression.

Reverse phase chromatography of the globin chains of adult, newborn, and fetal erythrocytes from three species of New World monkeys (Cebus apella, Aotus azarae, and Callithrix jacchus) representing three of the seven platyrrhine clades showed that gamma-globin expression was fetal in these animals. The globins were identified by a combination of chemical sequencing and mass spectrometric analysis. Since gamma-globin expression is fetal in the other major simian branch, the catarrhines, but embryonic in prosimian primates and nonprimate placental mammals, the evolution of fetal recruitment can now be assigned to the period between the simian-prosimian divergence (55 million years ago) and the platyrrhine-catarrhine divergence (35 million years ago). The gamma-globin gene underwent tandem duplication during the same evolutionary epoch, in accord with a model that suggests that the downstream duplicated gamma-gene (gamma2) was free to acquire the mutations necessary for fetal recruitment. Mass spectrometric analysis of tryptic digests of the gamma-globins verified the amino acid sequences deduced from genomic sequencing. Detailed analysis of high performance liquid chromatography and matrix-assisted laser desorption/ionization mass spectrometry data showed that gamma2-globin in Cebus was expressed to a far greater extent than gamma1-globin, supporting inferences drawn from a study of the promoter sequences. A "pre-gamma"-globin was observed in C. apella and shown to be primarily the glutathionyl adduct. The other species, A. azarae and C. jacchus, also express only one gamma-globin polypeptide. This work provides biochemical evidence of an evolutionary trend in the platyrrhines to alter the duplicated gamma-globin gene locus so that only one gamma-globin polypeptide is expressed.