Chapter 25 (5)

An understanding of __, __ and _ provides a picture of how those changes came about.

__- reasearch at the interface between evolutionary biology and developmental biology- is illuminating how slight genetic divergences can produce major morphological differences between species. Genes that control development influence the rate, timing, and spatial pattern of change in a n organism's form as it develops from a zygote into an adult.

Many striking evolutionary transformations are the result of __ body features that were juvenile structures in an ancestral species- a condition called __. Such an evolutionary alteration of developmental timing can produce animals that appear very different from ttheir ancestors, even though the overall genetic cnage may be small. Indeed, recent evidence indiccates that a change at a single locus was prob sufficient to bring about __ in the axolotl salamander, although other genes may have contributed as well.

Substantial evolutionary changes can also result from alteration sin genes that control the placement and spatial organization of body parts, such as master regulatory called __., which such basic features as where a pair of wings and legs will develop on birds or how flower parts are arranged.

The products of one class of homeotic genes, called __, provide positional info in an animal embryo; it promts cells to develop into structures appropriate at a part location. Changes in teh _- or in how they are expressed has a profound impact on morphology.

THThe evolution of vertebrates from invertebrate animals was an even larger evolutionary change, and it, too may have been influenced by alterations in __ and genes that regulat them. Two duplications of __ have occurred in the vertebrate lineage, and all vertebrate genomes tested to date have both of these duplications. These duplications may have been instrucmental in the origin of novel characteristics in the vertebrates. THis hypothesis remains controversial, however, in part because in addition to the __, many other genes were duplicated int eh vertebrate ancestor. Thus, the origin of novel characteristics cannot be ascribed solely to new __.

The 565- mil year old fossils of __ animals suggest that a set of genes sufficient to produce complex animals existed at least 30 mil years before the _-.

If developmental genes have existed for so long, how can we explain teh astonishing increases in diversity seen during and since the Cambrian explosion?
__ by __ provides one answer to the question. By sorting among differences in the sequence of protein- encoding genes, selection can improve adaptations rapidly. In addition, new genes (by duplication events) may have taken on a wide range of new metabolic and structural functions. Thus, __ of new and existing genes may have played a key rold in shaping the great diversity of life.

New developmental genes arising after gene duplicatione vents very likely facilitated the oringin of new __ forms. It can be difficult to establish causal links between genetic and morphological changes that occurred in the past.

ThA change in the nucleotide sequence of a gene may affect its function wherever the gene is expessed. In contrast, changes in the regultaion of __ can be limited to single cell type. Thus, a change int he regulation of a developmental gene may have fewer harmful side effects than a change to the sequence of the gene. This line of reasoning has prompted researchers to suggest that changes in the form of organisms often may be caused by mutations that affect teh regulation of developmental genes- not their sequence.