The basic principles of evo-
Organisms share similar molecular mechanisms for development that include a “toolkit” of regulatory molecules that control the expression of genes.
Toolkit regulatory molecules are able to act independently in different tissues and regions of the body, enabling modular evolutionary change.
Developmental differences can arise from changes in the timing of regulatory molecule action, the location of its action, or the quantity of its action.
Differences among species can arise from alterations in the expression of developmental genes.
Developmental changes can arise from environmental influences on developmental processes.
As you saw earlier in the chapter, the development of a multicellular organism from a fertilized egg involves sequential gene expression. The complexities for each organism led biologists to predict that the pathways that make a mouse, for example, would be very different from those that make a fruit fly. But they were in for a surprise. It turns out that the regulatory genes that control the formation of organs are similar in very different organisms.
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