Scientists who study how organisms reproduce know that asexual reproduction is more efficient  for one thing, it’s about twice as fast as sexual reproduction, since every offspring can produce more.

But if the asexual way is so efficient, why do almost all animal species reproduce sexually, and why are most asexual reproducers on their last legs, evolutionarily speaking? One hypothesis is that asexual organisms have locked up their genome, while their pathogenic enemies are constantly evolving to defeat them.

Paul W. Sherman and Christopher G. Wilson of Cornell University have now come up with evidence supporting that hypothesis, by studying bdelloid rotifers, tiny invertebrates that for 30 million years have reproduced asexually.

Image A fungi-infected bdelloid rotifier. Credit... Kent Loeffler, Kathie T. Hodge and C.G. Wilson

“We wanted to know how they’ve managed to last so long without sex,” Mr. Wilson said, “how they managed to escape parasites and pathogens.”