Inspecting the tiny roundworms Caenorhabditis briggsae and Caenorhabditis nigoni through a microscope, you’d have trouble telling them apart. Both are about a millimeter long and transparent. On the evolutionary tree, they’re closer together than horses and donkeys.

The key distinction between the two nematodes is their sex lives. Sex in C. nigoni takes place between a male and female. But only a small minority of C. briggsae are males. The rest are hermaphroditic females that reproduce by self-fertilizing, or selfing. They have evolved the ability to produce sperm that merge with their own eggs.

This sexual switch may have caused profound changes at the genetic level for C. briggsae. In a study published last week in Science, biologists reported that C. briggsae lost thousands of genes — a staggering quarter of its genome — since it diverged from C. nigoni a million years ago.

“Many of these genes had been around, and were presumably needed, for tens of millions of years or longer,” said Eric Haag, a biology professor at the University of Maryland, College Park, and an author of the paper. “In the blink of an eye, they disappeared.” He and his co-authors believe that a large portion of the genes shed are related to male reproduction.