A little-known species of Mexican lizard appears to be in the midst of evolving into several new species in a process never before documented by evolutionary biologists.

For reasons not yet understood, the mesquite lizard's chromosomes are undergoing such significant reorganizations that at least 45 different groups of mesquite lizards can be distinguished on the basis of chromosome patterns. Inheritors of some chromosomal arrangements appear unable to interbreed with inheritors of certain others.

Biologists commonly define a species as any group of organisms that interbreeds naturally to produce fertile offspring. If the chromosome differences prevent the lizards from interbreeding successfully, they are, by definition, separate species.

Even though the 45 groups of lizards still look alike, the possibility that any group is unable to interbreed suggests that any further changes altering the appearance of that group will be confined to its progeny and will eventually give rise to visible differences.

The research is being done by Jack Sites, a zoologist at Brigham Young University. Sites said the cause of the chromosomal variation is a mystery. Other lizard species show no such variation.

One possible cause is the phenomenon of "jumping genes," known in some plant species. Such genes readily break away from their home on one chromosome and take up residence on other chromosomes within the same cell, sometimes with visible effects. In corn, for example, the results can show up as the variably colored kernels of Indian corn.

One of the most debated problems in evolutionary biology is the method by which new species arise, and many mechanisms have been proposed. Most scientists believe several different processes may be operating in the natural world. One of the better known is geographic isolation, which prevents interbreeding of populations of a species long enough for minor mutations to accumulate into major differences that inhibit interbreeding if the two groups come together again.