Lab5W10Dogs - GE 70B: Evolution of the Cosmos and Life...

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1 GE 70B: Evolution of the Cosmos and Life LABORATORY/DISCUSSION 5 ONTOGENY AND PHYLOGENY: Evolution of breeds in the domestic dog, Canis familiaris INTRODUCTION Humans have changed species through controlled breeding programs, a process termed artificial selection . Usually it is easy to match the domesticated form with a wild relative. Occasionally, however, numerous different forms are selected to diverge from one progenitor. In today’s lab we will be focusing on the great diversity of dogs breeds in an attempt to understand how it was possible to select for such a wide array of dogs from a single ancestral form, and why comparable diversity is not seen among some other domesticated animals, and us! BIOLOGICAL CONCEPTS Heterochrony The entire life history of an organism from conception to death is referred to as its development or ontogeny . Any evolutionary change in the timing of events during development, or in the rate of development, is termed heterochrony . Development from fertilized egg to adult involves a host of complex processes and interactions among genes, molecules, and tissues. Even subtle alterations in the rate or time of onset of one of those developmental processes can have far-reaching consequences. The result may be a different final product, i.e. a different adult form. Thus, heterochrony can be an important evolutionary process, because even slight genetic changes in developmental timing can sometimes give rise to marked heritable differences. Perhaps the most celebrated example is the axolotl, a salamander found in Mexico. Salamanders undergo early development as gilled, aquatic animals, but most species lose their gills and live on land as adults. However, when conditions in ponds are particularly favorable, certain key aspects of morphological development slow down in the axolotl relative to those in ancestral salamanders, even though sexual maturation continues. This change in developmental timing, which has been traced to a simple change in the amount of a hormone released by the thyroid gland, results in a sexually mature salamander that, retaining gills and continuing its aquatic existences, looks like juvenile. We usually speak of heterochrony in terms of the change in form between an ancestral species and a descendant species. Heterochrony usually involves an evolutionary change in developmental timing or in the rate of development so that the shape, size, and/or behavior of the descendant no longer matches the ancestor at an equivalent age. That means development can either be reduced or increased in the descendant adult, relative to the ancestor. A developmental shift resulting in underdevelopment is called paedomorphosis (literally meaning “child formation”). A developmental shift resulting in overdevelopment is called peramorphosis (meaning “over formation”). Finally, it is important to realize that all organisms are a mosaic of heterochronies in terms of their
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This note was uploaded on 03/11/2010 for the course GE CLST 70B taught by Professor Morris,m.r./friscia,a.r./moldwin,m.b./vanvalkenburgh,b during the Winter '10 term at UCLA.

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Lab5W10Dogs - GE 70B: Evolution of the Cosmos and Life...

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