Concept 14.1 Development Involves Distinct but Overlapping Processes

• A multicellular organism begins its development as an embryo, and several embryonic stages precede the birth of an independent organism. Review Figure 14.1 and WEB ACTIVITY 14.1
   
• The processes of development are determination, differentiation, morphogenesis, and growth.
   
• The zygote is totipotent; it is capable of producing an entire new organism, with every type of cell in the adult body. Review INTERACTIVE TUTORIAL 14.1
   
• The ability to create clones from differentiated cells demonstrates the principle of genomic equivalence. Review Figure 14.3
   
• Multipotent stem cells occur in the growing regions of plants and in most tissues of mammals. They constantly divide and form a pool of cells that can be used for differentiation to specialized cells. Review Figure 14.5
   
• Pluripotent stem cells can form every cell type of a mammal, but not an entire organism. They occur in the embryo and can be induced to form in the laboratory. They may have medical uses. Review Figure 14.6 and ANIMATED TUTORIAL 14.1
   

Concept 14.2 Changes in Gene Expression Underlie Cell Differentiation in Development

• Differential gene expression results in cell differentiation. Transcription factors are especially important in regulating gene expression during differentiation.
   
• Cytoplasmic segregation — the unequal distribution of cytoplasmic determinants in the egg, zygote, or early embryo — can establish polarity and lead to cell fate determination. Review Figure 14.8 and ANIMATED TUTORIAL 14.2
   
• Induction is a process by which embryonic animal tissues direct the development of neighboring cells and tissues by secreting chemical signals called inducers. Review Figure 14.10
   

Concept 14.3 Spatial Differences in Gene Expression Lead to Morphogenesis

• During development, selective elimination of cells by apoptosis results from the expression of specific genes.
   
• In plants, organ identity genes encode polypeptides that associate to form transcription factors. These proteins determine the formation of flower organs. Review Figure 14.11
   
• Both plants and animals use positional information in the form of a signal called a morphogen to stimulate cell determination. Review Figure 14.12
   
• In the fruit fly Drosophila melanogaster, a cascade of transcriptional activation sets up the axes of the embryo, the development of the segments, and the determination of cell fate in each segment. Review Figure 14.13 and ANIMATED TUTORIAL 14.3
   
• Hox genes determine cell fate in the embryos of many animals. The homeobox is a DNA sequence found in Hox genes and other genes that code for transcription factors. The sequence of amino acids encoded by the homeobox is called the homeodomain.
   

Concept 14.4 Gene Expression Pathways Underlie the Evolution of Development

• Evolutionary developmental biology (evo-devo) is the modern study of the evolutionary aspects of development, and it focuses on molecular mechanisms.
   
• Hox genes have evolved from a common ancestor. Review Figure 14.15
   
• Genes such as Hox genes underlie evolutionary changes in morphology that produce major differences in body forms.
   
• Evolutionary diversity is produced using a modest number of regulatory genes. Review Figure 14.16
   
• The transcription factors and chemical signals that govern pattern formation in the bodies of multicellular organisms, and the genes that encode them, can be thought of as a genetic toolkit.
   
• The bodies of developing and mature organisms are organized into self-contained units that can be modified independently in space and time. Review ANIMATED TUTORIAL 14.4
   
• Changes in genetic switches that determine where and when a set of genes will be expressed underlie the transformation of an individual from egg to adult.
   

Concept 14.5 Developmental Genes Contribute to Species Evolution but Also Pose Constraints

• Evolutionary innovations are modifications of preexisting structures. Review Figure 14.20
   
• Because many genes that govern development have been highly conserved, similar traits are likely to evolve repeatedly, especially among closely related species. This process is called parallel phenotypic evolution. Review Figure 14.21