Figure 1. MOSAIC EVOLUTION is exemplified by a fossil of Archeopteryx, a species that lived some 150 million years ago and is considered to be a transitional form between the dinosaurs and the birds. In spite of the intermediate position of Archeopteryx as a species its individual traits are not transitional in character: it unites a skeleton that remains dinosaur-like with feathers that closely resemble those of modern birds. The concept of mosaic evolution, which holds that different characteristics tend to evolve at different rates because they face different selective constraints, has recently enriched the synthetic theory of evolution.