Part Four - Molecular Biology: The Genome in Action 331

20. Development and Evo­lutionary Change 448

20.1 How Does a Molecular Tool Kit Govern Development? 449 Developmental genes in diverse organisms are similar, but have different results 449 20.2 How Can Mutations with Large Effects Change Only One Part of the Body? 451 Genetic switches govern how the molecular tool kit is used 451 Modularity allows differences in the timing and spatial pattern of gene expression 451 20.3 How Can Differences among Species Evolve? 453 20.4 How Does the Environment Modulate Development? 454 Organisms respond to signals that accurately predict the future 454 Some signals that accurately predict the future may not always occur 456 Organisms do not respond to signals that are poorly correlated with future conditions 456 Organisms may lack appropriate responses to new environmental signals 457 20.5 How Do Developmental Genes Constrain Evolution? 457 Evolution proceeds by changing what’s already there 457 Conserved developmental genes can lead to parallel evolution 458