Males? What For!?...

There is a general concept that sex is needed for organisms to diversify. Sexual reproduction would be necessary for speciation because interbreeding triggers variation.

This concept is challenged by a group of organisms that has never had sex in over 100 million years of existence and has nevertheless managed to evolve into 400 distinct species.

Asexual species like the bdelloid rotifers were thought to be all identical, and the differences might arise solely through random mutations that occur in the "cloning" process when a new rotifer is born. The bdelloid rotifers are microscopic aquatic animals that live in watery or wet zones including ponds, rivers, soils, and on mosses and lichens.

These tiny creatures reproduce through parthenogenesis: virgin females produce fertile eggs that are genetic clones of the mother and there are no males. Fossil records and molecular data show that bdelloid rotifers have been around for over 100 million years without producing males, and yet they evolved into a wide array of species.

A team from Imperial College London's Division of Biology Using employed DNA sequencing and jaw measurements (achieved by using a scanning electron microscope) for examining bdelloid rotifers from different aquatic environments across the UK, Italy and other parts of the world.

The analyses revealed that the rotifers evolved into distinct species by adapting to differences in their habitat. "We found evidence that different populations of these creatures have diverged into distinct species, not just because they become isolated in different places, but because of the differing selection pressures in different environments," said Dr Tim Barraclough, one of the researchers.

"One remarkable example is of two species living in close proximity on the body of another animal, a water louse. One lives around its legs, the other on its chest, yet they have diverged in body size and jaw shape to occupy these distinct ecological niches. Our results show that, over millions of years, natural selection has caused divergence into distinct entities equivalent to the species found in sexual organisms."

The research shows that these differences are not random but the result of "divergent selection", that causes the speciation in sexual organisms. "These really are amazing creatures, whose very existence calls into question scientific understanding, because it is generally thought that asexual creatures die out quickly, but these have been around for millions of years," said Barraclough.

"Our proof that natural selection has driven their divergence into distinct species is another example of these minuscule creatures surprising scientists – and their ability to survive and adapt to change certainly raises interesting questions about our understanding of evolutionary processes."

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