Macroevolution Major innovations  separation at higher taxonomic levels Exaptation Existing feature modified to perform novel function Ex: Development of bird wings from reptiles forelimbs not designed for flight Some plasticity in biological structures Ex: Bird feathers evolved from reptile scales May have provided insulation for primitive birds Preadaptation for flight Ex: Middle ear bones in mammals evolved from jaw bones in reptiles Does not imply that structures developed in anticipation of future use

Macroevolution Developmental Rate and Timing Some evolutionary modifications result from minor genetic changes Heterochrony May affect regulatory genes Ex: Allometric growth – Changes in relative growth rates of body parts during development

Prolonged juvenile period Fig. 25.21

Macroevolution Developmental Rate and Timing Paedomorphosis – Retention of juvenile characteristics in adults Fig. 25.22

Macroevolution Changes in Spatial Pattern Some genes control spatial organization of body parts Ex: Homeotic genes regulate organism’s bauplan Hox genes provide positional information in animal embryos Purple: HoxC6 expression (limbless regions) Fig. 25.23

Macroevolution Changes in Spatial Pattern Regulatory changes may affect gene expression without changes in gene sequence Fig. 25.25

Macroevolution Directionality Macroevolution not goal-oriented

Fig. 25.27

Phylogeny and Systematics Evolutionary processes reflected in organismal phylogeny Genealogies often represented as phylogenetic trees that reflect relationships among species Systematics deals with classification of taxa according to evolutionary history

Fig. 26.4