 The toolkit can drive evolution in two ways.A toolkit gene can be expressed in a different pattern, as when the beak of Darwin's large ground-finch was enlarged by the "BMP" gene, or when snakes lost their legs as "distal-less" became under-expressed or not expressed at all in the places where other reptiles continued to form their limbs.Or, a toolkit gene can acquire a new function, as seen in the many functions of that same gene, "distal-less", which controls such diverse structures as the mandible in vertebrates, legs and antennae in the fruit fly, and eyespot pattern in butterfly wings.