In 1915, Mildred Hoge, studying the genetics of the fruit fly Drosophila mela-
nogaster, found a mutation that she designated eyeless, because it reduced
or eliminated the flies’ eyes [15]. The normal form of the eyeless gene, then,
must be necessary for eye development. Years later, similar mutations were
found in mice and humans, and eventually it was discovered that the DNA
sequences of the mammals’ genes were similar to that of the fly’s gene. In
1995, Georg Halder and his colleagues reported a remarkable experiment
[12]. By inserting an extra copy of the normal form of a fly’s eyeless gene into
Drosophila larvae, they induced the development of almost perfect minia-
ture extra eyes—on the wings, legs, and elsewhere on the body of the adult
fly. But more astonishingly, the researchers obtained exactly the same effect
when they inserted the mouse version of the gene. The mouse gene (now
called Pax6) caused the flies to develop eyes. Not mouse eyes, however, but
almost perfect fly eyes (FIGURE 2.1). The same result was later obtained with
the human gene.
Halder and his colleagues noted that more than 2000 different genes are
thought to contribute to developing a Drosophila eye. The normal eyeless gene
is near the start of a chain of command: it activates other genes, which acti-
vate yet others, and so on, to produce all the details of the eye. This experi-
ment shows that even though insect and vertebrate eyes are radically different
in structure, and are produced by somewhat different sets of genes, the sys-
tem that activates these genes is very similar in insects and vertebrates. The fly
genome responds to the signal from the mammalian Pax6 gene just as it does toThe Tree of Life
2
A great egret (Ardea alba) stands on an American alligator (Alligator mississippiensis).
Although it is certainly not obvious, birds and crocodilians are each others' closest
living relatives, having descended from a common ancestor that lived more than 200
million years ago.02_EVOL4E_CH02.indd 27 3/23/17 8:59 AM